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 glossaryharvest.htm 



 



 A ...
AACS: Attitude & Articulation Control Subsystem.
AAF: Association Astronautique Francaise.
AAS: American Astronautical Society.
ABL: Allegheny Ballistics Laboratory.
Ablation: The erosion of a solid body by a high-temperature gas stream moving with high velocity, e.g. a reentry vehicle's heat shield which melts or chars under the effects of air friction.
ABMA: Army Ballistic Missile Agency (USA).
Abort: To cancel or cut short a mission.
Absolute zero: The temperature at which all heat action ceases, -273.16oC (-459.69oF).
Acceleration: A change in velocity, including changes of direction and decreases as well as increases in speed.
Accelerometer: A device that senses changes in speed along its axis.
ACS: Attitude Control System.
Active heating: The use of resistive electric heaters or radioisotope heaters to keep spacecraft components above their minimum allowable temperatures.
Active sun: The Sun during times of frequent solar activity such as sunspots, flares, and associated phenomena.
Actuator: A device which transforms an electric signal into a measured motion using hydraulic, pneumatic or pyrotechnic (explosive) action.
Aerobraking: The process of decelerating by converting velocity into heat through friction with a planetary atmosphere.
Aerodynamic heating: The heating of a body due to the passage of air or other gases over the body; caused by friction and compression processes.
Aeronautics: The science of building and operating vehicles for flight.
Aerozine 50: A storable liquid fuel: 50% hydrazine, 50% UDMH.
AIAA: American Institute of Aeronautics and Astronautics (USA).
AIDAA: Associazione Italiana di Aeronautica e Astronautica.
Aileron: A hinged surface on the wing of an aircraft or spacecraft used to adjust the craft's angle of flight.
Aimpoint: The point in the planet's plane that spacecraft aim for to either fly by or enter orbit.
Airglow: The visible light that appears at night in the upper atmosphere.
Air lock: A compartment separating areas a different environment, especially different air pressures, that is used for entry to and departure from a spacecraft.
Albedo: Reflectivity; the ratio of reflected light to incident light. The fraction of the sunlight that is reflected off a planet.
Algae: A group of simple organisms, mostly aquatic, which contain chlorophyll and thus provide a means of photosynthesis. They could be used to absorb carbon dioxide and provide nourishment in a spaceship.
ALSEP: Apollo Lunar Surface Experiment Package.
Altimeter: A device that measures altitude above the surface of a planet or moon. Spacecraft altimeters work by timing the round trip of radio signals bounced off the surface.
Ambient: Environmental conditions, such as pressure or temperature.
Analog computer: A computing machine that works on the principle of measuring, as distinct from counting, in which the measurements obtained (as voltages, resistances, etc.) are translated into desired data.
Angle of attack: The angle between the velocity vector and the longitudinal axis of a missile or rocket.
Angstrom: A unit for the measurement of wavelength. Equals one hundred millionth of a centimeter (0.003937 millionth of an inch).
Annular: Pertaining to, or having the form of a ring.
Anomaly: The angular distance between the position of a planet and its last perihelion, or between that of a satellite and its last perigee.
Anti-matter: A hypothetical form of matter of which the atoms are composed of anti-particles, as protons, electrons, etc. assumed to carry charges opposite to those associated with ordinary matter. Particles having such properties have been produced in particle accelerators.
Antipodal: Pertaining to, or located on, the opposite side of the Earth.
AOCS: Attitude & Orbit Control System.
Aperture: The diameter of an opening; the diameter of the primary lens or mirror of a telescope.
Apex: The point towards which a body is moving.
Aphelion: That point in a solar orbit which is farthest from the Sun.
Apoapsis: That point in an orbit which is farthest from the primary.
Apogee: That point in a terrestrial orbit which is farthest from the Earth.
Apolune: That point in a lunar orbit which is farthest from the Moon.
Argument: Angular distance.
Argument of periapsis: In an orbit, the angular distance between the point of periapsis and the ascending node.
Arianespace: A private limited company established for the purpose of producing, financing and marketing the ESA Ariane launch vehicle. Comprises European companies concerned in the rocket's development, CNES and several banks. US agent is Grumman Aerospace.
ARS: American Rocket Society (USA).
Artificial gravity: Use of centrifugal force to simulate weight reaction in a condition of free-fall. May be achieved by spinning the vehicle to make the centrifugal force of the outer periphery or bodies within the vehicle to replace the weight reaction experienced at Earth's surface.
ASAT: Anti-satellite.
Ascending node: The point at which an orbiting object or spacecraft, traveling from south to north, crosses the plane of the equator.
Ascent module: That part of a spacecraft that ascends from the surface of a planet or moon to rendezvous and dock with an orbiting spacecraft.
Asteroid: A small, usually irregularly shaped body orbiting the sun, most often at least partially between the orbits of Mars and Jupiter.
Asteroid belt: A 1/2 AU wide region between the orbits of Mars and Jupiter where most asteroids are found.
Astronaut: A person who flies in space, whether as a crew member or passenger.
Astronautics: The science and technology of space flight.
Astronomical unit: The mean distance of Earth from the Sun, i.e. 92,955,807 miles (149,597,870 km).
Astrophysics: Study of the physical and chemical nature of celestial bodies and their environs.
ATDA: Agena Target Docking Adapter.
Atmospheric balloon: An instrumented package suspended from a buoyant gas bag; deployed in a planet's atmosphere to study wind circulation patterns.
Atmospheric pressure: The weight of air on surfaces within Earth's atmosphere, about 14.7 PSI (101 kPa) at sea level. Such pressure is also supplied artificially in spacecraft and spacesuits.
Atmospheric probe: A small instrumented craft which separates from the main spacecraft prior to closest approach to a planet to study the gaseous atmosphere of the body as it drops through it.
Attenuation: The decrease of a propagating physical quantity, such as a radio signal, with increasing distance from the source, or from some obstruction.
Attitude: Orientation of a space vehicle as determined by the relationship between its axes and some reference plane, e.g. the horizon.
Attitude & articulation control subsystem: The onboard computer that manages the tasks involved in spacecraft stabilization via its interface equipment. For attitude reference, star trackers, star scanners, solar trackers, sun sensors, and planetary limb trackers are used.
Attitude control: The system that turns and maintains a spacecraft in the required direction as indicated by its sensors.
AU: Astronomical Unit.
Aurora: Arcs, rays or swaying curtains of green, yellow or white lights seen in latitudes of about 70o, such as Aurora Borealis or Northern Lights, and Aurora Australis or Southern Lights; caused by streams of electrified particles, emitted by the Sun, trapped in the Earth's magnetic field.
Autopilot: A system or device that controls a vehicle's flight at a preset course and altitude.
Azimuth: The angular position of an object measured in the observer's horizontal plane, usually from north through east. Bearing or direction in the horizontal plane. As one of the coordinates expressing celestial location, it is sometimes used in tracking spacecraft.

B ...
Backscattering: Reflecting light back in the direction of the source.
Back-up: An item kept available to replace an item which fails to perform satisfactorily.
Ballistics: The science that deals with the motion, behavior, appearance or modification of missiles acted upon by propellants, rifling, wind, gravity, temperature or other modifying conditions of force.
Ballute: An aerodynamic braking device which is both balloon and parachute.
Bar: A unit of pressure equal to one million dynes per square centimeter, or 0.99 atmospheres.
Barycenter: The common center of mass about which two or more bodies revolve.
Basin: A large, >200 km, circular depression from the explosive impact of an asteroid or similar sized body on a planet surface, usually rimmed by mountains.
Battery: A device with two or more connected cells that produce a direct current by converting chemical energy into electrical energy.
Big Bang theory: The theory that the universe was once clustered and at the 'beginning' it exploded out, as shown by the fact that objects are still moving out from the center.
Binary star: Two stars revolving around a common center of gravity.
Bi-propellant: A rocket propellant consisting of two unmixed or uncombined chemicals (fuel and oxidizer) fed separately into the combustion chamber.
BIS: British Interplanetary Society.
Bit: A basic unit of computer information; abbreviation of binary digit.
Black hole: An object whose gravity is so strong that the escape velocity exceeds the speed of light.
Blackout (physiological): A temporary loss of vision and/or consciousness when a person is subjected to high accelerations.
Blackout (radio): A temporary loss of radio communications which occurs between a spacecraft reentering the atmosphere and ground stations due to an ionized sheath of plasma which develops around the vehicle.
Black powder: A mixture of saltpeter (potassium nitrate), sulfur, and charcoal, used in explosives and as an early propellant for rockets.
Boilerplate: A metal replica of the flight model (e.g. of a spacecraft) but usually heavier and cruder for test purposes.
BOL: Beginning Of Life.
Boost: The extra power given to a rocket or space vehicle during liftoff, climb or flight, as with a booster rocket.
Booster: The first stage of a missile or rocket.
Bow shock wave: The compressed wave that forms in front of a spacecraft or satellite as it moves rapidly through Earth's atmosphere; more generally, any such wave that forms between an object and a fluid medium.
Burn: Combustion action in rockets. Propulsion in space is achieved through a sequence of burns.
Bus: A major part of the structural subsystem of a spacecraft which provides a place to attach components internally and externally, and to house delicate modules requiring a measure of thermal and mechanical stability. The bus also establishes the basic geometry of the spacecraft.

C ...
Calibration: Setting a measuring instrument before measuring for accurate results.
Carrier: The main frequency of a radio signal generated by a transmitter prior to application of modulation.
CAS: Chinese Academy of Sciences.
CAST: Chinese Academy of Space Technology.
CAT: Capsule Ariane Technologique.
Catalytic decomposition engine: A mono-propellant engine in which a liquid fuel decomposes into hot gas in the presence of a catalyst. The fuel is most commonly hydrazine.
C-band: A range of microwave radio frequencies in the neighborhood of 4 to 8 GHz, used for spacecraft communications on Mercury and Gemini flights (~5 Ghz).
CCD: Charged Coupled Device.
CDS: Command & Data Subsystem.
Celestial sphere: The apparent sphere of sky that surrounds the Earth; used as a convention for specifying the location of a celestial object.
Centrifugal force: A force which is directed away from the center of rotation.
Centripetal force: A force which is directed towards the center of rotation.
CEO: Close Earth Orbit.
CETI: Communication with Extra-Terrestrial Intelligence.
Chaff: Metallic foil ejected by a reentry module to enhance its radar image.
Charged coupled device: An imaging device consisting of a large-scale integrated circuit which has a two-dimensional array of hundreds of thousands of charge-isolated wells, each representing a pixel.
Cholorella: A genus of unicellular green algae, proposed for converting carbon dioxide into oxygen for use in spacecraft.
Chromosphere: A reddish layer in the Sun's atmosphere, the transition between the photosphere and the corona.
Cislunar: Relating to the space between the Earth and the orbit of the Moon.
CM: Command Module.
CNES: Centre National d'Etudes Spatiales (France).
CO: Circular Orbit.
Coherence: The property of being coherent, e.g. waves having similar direction, amplitude and phase that are capable of exhibiting interference.
Coma: The cloud of diffuse material surrounding the nucleus of a comet.
Combustion: A chemical reaction between two or more substances that releases heat, light, and gases.
Combustion chamber: The chamber in a rocket where the fuel and oxidizer are ignited and burned. By common usage the expansion nozzle is included as part of the combustion chamber, particularly for liquid-propelled rocket engines.
Comet: A body of small mass but large volume, compared to a planet, often developing a long luminous and partly transparent tail when close to the Sun.
Command & data subsystem: The onboard computer responsible for overall management of a spacecraft's activity.
Command module: The compartment of a spacecraft which contains the crew and main controls, and is used as the reentry vehicle.
Composites: Structural materials of metal alloys or plastics with built-in strengthening agents, e.g. carbon fibers.
Constellation: A group of stars that make a shape, often named after mythological characters, people, animals, and things.
Control rocket: A vernier or other rocket used to control the attitude of, or slightly change the speed of, a spacecraft.
Coolant: A medium, usually a fluid, which transfers heat from an object.
Core: The innermost layer of a planet or star.
Coreolis effect: Dizziness or nausea experienced when an astronaut in a spinning spacecraft moves his head in the opposite direction.
Corona: The Sun's outer layer. The corona's changing appearance reflects changing solar activity.
Coronal mass ejection : A huge cloud of hot plasma, expelled sometimes from the Sun. It may accelerate ions and electrons, and may travel through interplanetary space as far as the Earth's orbit and beyond it, often preceded by a shock front. When the shock reaches Earth, a magnetic storm may result.
Cosmic ray: An extremely energetic (relativistic) charged particle.
Cosmic year: The time it takes the Sun to revolve around the center of the galaxy, approximately 225 million years.
Cosmonaut: The Russian term for an astronaut. A space traveler.
COSPAR: The Committee on Space Research (established October 1958).
Countdown: A count in inverse numerical order, in hours, minutes and finally seconds, of time remaining before the launch of a rocket.
Crater: A round impression left in a planet or satellite from a meteoroid.
Crust: The outer layer of Earth and other terrestrial planets.
Cryogenic: A rocket fuel or oxidizer which is liquid only at very low temperatures, e.g. liquid hydrogen which has a boiling point of -217.2oC (-423oF).
CSA: Chinese Society of Astronautics.
CSAA: Chinese Society of Aeronautics and Astronautics.
CSM: Command/Service Module.
C-stoff: A rocket fuel used by Germany in World War II: 30% hydrazine hydrate, 57% methanol, 13% water with traces of potassium cuprocyanate. Used in conjunction with T-stoff oxidizer: 80% hydrogen peroxide with 1 to 2% oxiquinoline as a stabilizer.
Current: The amount of electric charge flowing past a specified circuit point per unit time.
Cut-off: The action of stopping a process abruptly, such as shutting off the flow of propellant to a rocket engine.

D ...
Dark matter: A form of matter which has not been directly observed but whose existence has been deduced by its gravitational effects.
Data reduction: Conversion of observed values into useful, ordered and simplified information.
DC: Direct Current.
Decay: The action of air drag upon an artificial satellite causing it to spiral back into the atmosphere, eventually to disintegrate or burn up.
Deceleration: Negative acceleration, slowing.
Declination: One of the coordinates, measured in degrees, used to designate the location of an object on the celestial sphere. Declination is a north-south value similar to latitude on Earth.
Decompression: The relief of pressure. Explosive decompression would occur if the cabin of a spacecraft was punctured in space.
Delta V: Difference or change in velocity.
Demodulation: To extract information from a modulated carrier wave.
Density: Amount of matter per unit volume.
Density Wave: A kind of wave induced in a flat plane of a resisting medium (such as the rings of Saturn) by gravitational forces, often assuming the form of a tightly wound spiral.
Descending node: The point at which an orbiting object or spacecraft, moving from north to south, crosses the plane of the equator.
Descent engine: The rocket used to power a spacecraft as it makes a controlled landing on the surface of a planet or moon.
Descent module: That part of a spacecraft that descends from orbit to the surface of a planet or moon.
DGLR: Deutsche Gesellschaft fur Luft-und Raumfahrt (German Company for Air and Space Travel).
Digital computer: An electronic device for solving numerically a variety of problems.
Dipole: A compact source of magnetic force, with two magnetic poles. A bar magnet, coil or current loop, if their size is small, create a dipole field. The Earth's field, as a first approximation, also resembles that of a dipole.
Direct current: Electrical current flowing in one direction and substantially constant in value.
Direct sensing: Instruments that interact with phenomena in their immediate vicinity, and register characteristics of them.
Dish: A reflector for radio waves, usually a paraboloid.
Docking: The technique of connecting two or more spacecraft in space.
DoD: Department of Defense (USA).
DOF: Degrees Of Freedom.
Doppler effect: A phenomenon in which waves appear to compress as their source approaches the observer or stretch out as the source recedes from the observer.
Dose: A quantity of radiation delivered at a position. In the context of space energetic particle radiation effects, it usually refers to the energy absorbed locally per unit mass as a result of radiation exposure.
Dose equivalent: A dose normally applied to biological effects and including scaling factors to account for the more severe effects of certain kinds of radiation.
Downlink: The radio signal transmitted from a spacecraft to Earth.
Drag: The resistance offered by a gas or liquid to a body moving through it.
Drogue: A small parachute used to slow and stabilize a spacecraft returning to the atmosphere, usually preceding deployment of a main landing parachute.
DSN: Deep Space Network.
Dust: Particulates which have a direct relation to a specific solar system body and which are usually found close to the surface of this body (e.g. Lunar, Martian or Cometary dust).
Dust detector: A device for measuring the velocity, mass, charge, flight direction and number of dust particles striking the instrument.
Dynamo process: The generation of an electric current by the flow of an electrically conducting fluid through a magnetic field. For instance, the magnetic field originating inside the Earth is believed to come from a dynamo process involving the flow of molten iron in the Earth's hot core. The energy required by the current is obtained from the motion of the flow.
Dyne: A unit of force equal to the force required to accelerate a 1 g mass 1 cm per square second.

E ...
Earth: Third planet from the Sun, a terrestrial planet.
Earth radius: The average radius of the Earth, a convenient unit of distance in describing phenomena and orbits in the Earth's neighborhood in space. 1 RE = 6371.2 km approximately.
Earth-sensor: A light-sensitive diode which seeks the direction of Earth and then informs the attitude control system of a spacecraft.
Eccentric: Noncircular; elliptical (applied to an orbit).
Eccentricity: The amount of separation between the two foci of an ellipse and, hence, the degree to which an elliptical orbit deviates from a circular shape.
Eclipse: The obscuring of one celestial body by the passage of another in front of it.
Ecliptic: The great circle on the celestial sphere which traces the path of the Sun during the year.
ECM: Electromagnetic Countermeasures.
EDT: Eastern Daylight Time.
EELV: Evolved Expendable Launch Vehicle.
Ejection seat: A seat fitted with an explosive charge and designed to eject the occupant clear of an aircraft during an in-flight emergency.
ELDO: European Launcher Development Organization.
Electric propulsion: A form of rocket propulsion which depends on some form of electric acceleration of propellant to achieve low thrust over long periods of time. E.g. an ion or magnetohydrodynamic engine.
Electromagnetic: Relating to the interplay between electric and magnetic fields.
Electromagnetic waves: A wave propagated through space by simultaneous periodic variation in the electric and magnetic field intensity at right angles to each other and to the direction of propagation. The electromagnetic spectrum includes radio waves, microwaves, infrared, visible and ultraviolet radiation, X-rays, gamma rays and cosmic rays.
Elevation: The angular measure of the height of an object above the horizon; with azimuth, one of the coordinates defining celestial location and sometimes used in tracking spacecraft.
ELV: Expendable Launch Vehicle.
EMU: Extravehicular Mobility Unit.
Energetic particle: Particles that can penetrate outer surfaces of spacecraft. For electrons, this is typically above 100 keV, while for protons and other ions this is above 1 Mev. Neutrons, gamma-rays and X-rays are also considered energetic particles in this context.
Engine: In spacecraft, a rocket or thruster that burns liquid propellants and can be throttled to adjust thrust.
EOL: End Of Life.
Ephemeris: Table of predicted positions of bodies in the solar system.
Ephemeris time: A measurement of time defined by orbital motions. Equates to Mean Solar Time corrected for irregularities in Earth's motions.
Epoch: An instant in time that is arbitrarily selected as a point of reference, e.g. for a set of orbital elements.
Equator: An imaginary circle around a body which is everywhere equidistant from the poles, defining the boundary between the northern and southern hemispheres.
Equatorial orbit: An orbit in the plane of the equator.
ESA: European Space Agency.
Escape tower: A rocket-powered framework designed to separate spacecraft modules from their booster rockets in case of accident. Escape towers are mounted atop the spacecraft and jettisoned after launch.
Escape velocity: The precise velocity necessary to escape from a given point in a gravitational field. A body in a parabolic orbit has escape velocity at any point in that orbit. The velocity necessary to escape from the Earth's surface is 6.95 miles/sec. (11.2 km/sec.).
ESMC: Eastern Space and Missile Center.
EST: Eastern Standard Time.
Eurospace: Non-profit-making industrial association with headquarters in Paris (founded September 1961).
EVA: Extravehicular Activity.
Exhaust velocity: The velocity of the exhaust leaving the nozzle of a rocket.
Exosphere: The part of the Earth atmosphere above the thermosphere which extends into space. H and He atoms can attain escape velocities at the outer rim of the exosphere.
Extravehicular activity: Action performed by an astronaut or cosmonaut outside a vehicle in space; a spacewalk.

F ...
Fairing: A structure whose main function is to streamline and smooth the surface of an aircraft or space vehicle..
Fault: A crack or break in the crust of a planet along which slippage or movement can take place.
Fault protection: Algorithms, which reside in a spacecraft's subsystems, that insure the ability of the spacecraft to both prevent a mishap and to reestablish contact with Earth if a mishap occurs and contact is interrupted.
Ferret: Satellite using electromagnetic surveillance techniques.
Fission: The release of energy through splitting atoms.
Fluorescence: The phenomenon of emitting light upon absorbing radiation of an invisible wavelength.
Flux: The amount of radiation crossing a surface per unit of time, often expressed in "integral form" as particles per unit area per unit time.
Flyby: Space flight past a heavenly body without orbiting.
Flyby spacecraft: A spacecraft which follows a continuous trajectory past a target object, never to be captured into an orbit. It must carry instruments that are capable of observing passing targets by compensating for the target's apparent motion.
FOBS: Fractional Orbit Bombardment System. A Soviet method of delivering a warhead from partial satellite orbit and thus approaching from any direction.
Force: A vector quantity that tends to produce an acceleration of a body in the direction of its application.
Forward scattering: Reflecting light approximately away from the source.
FOV: Field Of View.
Free-fall: The motion of any unpowered body moving in a gravitational field.
Free-return trajectory: Path of a spacecraft that provides for a return to Earth.
Frequency: The number of oscillations per second of an electromagnetic (or other) wave.
Fuel: A substance that when combined with an oxidizer burns to produce thrust in rockets.
Fuel cell: A cell in which chemical reaction is used directly to produce electricity.
Fusion: The release of nuclear energy through the uniting of atoms.
FY: Fiscal Year.

G ...
g: The symbol for the acceleration of a freely moving body due to gravity at the surface of the Earth. Alternatively, 1 g.
Galaxy: A very large system of stars, gas and dust isolated from its neighbors by an immensity of space; an "island universe".
GALCIT: Guggenheim Aeronautical Laboratory of the California Institute of Technology.
Gamma rays: Very short, highly-penetrative electromagnetic radiation with a shorter wavelength than X-rays; produced in general by emission from atomic nuclei.
Gas generator: A chamber in which propellant is burned to produce high pressure gas that is then used to drive a turbine, e.g. turbopump.
Gas giant: A large planet composed mostly of gas, e.g. the Jovian planets.
GATV: Gemini-Agena Target Vehicle.
Gauss: CGS unit of magnetic induction (after the German mathematician Karl F. Gauss).
GE: General Electric Company of the USA.
GEO: Geostationary Orbit. Also abbreviated GO.
Geo-: Prefix referring to the Earth.
Geocentric: Earth centered.
Geodesy: The science of the Earth's shape.
Geomagnetic storm: A worldwide disturbance of the Earth's magnetic field, distinct from regular diurnal variations.
Geospace: Also called the solar-terrestrial environment, geospace is the domain of Sun-Earth interactions. It consists of the particles, fields, and radiation environment from the Sun to Earth's space plasma environment and upper atmosphere. Geospace is considered to be the fourth physical geosphere (after solid earth, oceans, and atmosphere).
Geostationary orbit: A geosynchronous orbit with an inclination of zero degrees. A spacecraft in such an orbit appears to remain fixed above one particular point on the Earth's equator.
Geostationary transfer orbit: An elliptical orbit used to transfer a space vehicle from low earth orbit to geostationary orbit.
Geosynchronous orbit: A prograde, circular, low inclination orbit about Earth having a period of 23 hours 56 hours 4 seconds. A spacecraft in such an orbit appears to remain above Earth at a constant longitude, although it may seem to wander north and south.
g-Force: A force caused by acceleration expressed in g's.
GH2: Gaseous Hydrogen.
GHz: Gigahertz, equal to one billion hertz.
Gimbal: A mechanical frame for a gyroscope or power unit, usually with two perpendicular axes of rotation.
GMT: Greenwich Mean Time.
GN&C: Guidance, Navigation and Control.
GO: Geostationary Orbit. Also abbreviated GEO.
GOX: Gaseous Oxygen.
Grain: The rubber-like mass of chemical propellant that provides propulsion in solid fuel rockets. The shape of the grain determines the rate and pattern of burn and thus controls thrust.
Gravitational waves : Einsteinian distortions of the space-time medium predicted by general relativity theory (not yet detected as of November 1995).
Gravity: The force responsible for the mutual attraction of separate masses.
Gravity assist trajectory: A trajectory in which angular momentum is transferred from an orbiting planet to a spacecraft approaching from behind. The result is an increase in the spacecraft's velocity.
Gravity field survey: The mapping of a planet's mass distribution by studying variations in the in the planet's gravity field strength made evident by minute Doppler shifts in an orbiting spacecraft's radio signal.
Gravity waves: Certain atmospheric waves within a planet's atmosphere.
Great circle: An imaginary circle on the surface of a sphere whose center is at the center of the sphere.
Greenwich mean time: See universal time.
GSFC: Goddard Space Flight Center (Greenbelt, Maryland).
GSO: Geosynchronous Orbit.
GTO: Geostationary Transfer Orbit.
Guillotine: A device equipped with explosive blades used to cut cables, water lines, wires, etc. during separation of spacecraft modules.
Gyration: The circular motion of ions and electrons around magnetic field lines.
Gyroscope: A spinning, wheel-like device that resists any force that tries to tilt its axis. Gyroscopes are used for stabilizing the attitude of rockets and spacecraft in motion.

H ...
H2O2: Hydrogen Peroxide.
Hatch: Door or doorway, usually hermetically sealed.
Heat shield: A device which protects people or equipment from heat, such as a shield in front of a reentry capsule.
Helio-: Prefix referring to the Sun.
Heliocentric: Centered on the Sun.
Heliopause: The boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence, perhaps 100 AU from the Sun.
Heliosphere: The space within the boundary of the heliopause, containing the Sun and solar system.
HEO: Highly Elliptical Orbit.
Hertz: A unit of frequency equal to one cycle per second, named after Heinrich Hertz.
Heterosphere: The Earth atmosphere above 105 km altitude where species-wise concentration profiles establish due to diffusive equilibrium, with N2 dominance below 200 km, O dominance from 200 to 600 km, and He dominance as of 600 km altitude.
HGA: High-Gain Antenna.
High-energy particle detector: A device for measuring the energy spectra of trapped energetic electrons, and the energy and composition of atomic nuclei.
High-gain antenna: A dish-shaped spacecraft antenna principally used for high rate communication with Earth This type of antenna is highly directionally and must be pointed to within a fraction of a degree of Earth.
Hohmann transfer orbit: An interplanetary trajectory in which a spacecraft is launched into an elliptical solar orbit whose perihelion (inner planet) or aphelion (outer planet) reaches the orbit of the target planet on the opposite side of the Sun. Uses least propellant.
Homosphere: The Earth atmosphere below 105 km altitude where complete vertical mixing yields a near-homogeneous composition of about 78.1% N2, 20.9% O2, 0.9% Ar, and 0.1% CO2 and trace constituents. The homopause (or turbopause) marks the ceiling of the homosphere. The homosphere can be broadly divided into three distinct regimes: the troposphere (0 to 12 km), the stratosphere (12 to 50 km) and the mesosphere (50 to 90 km)
Horizon: The line marking the apparent junction of Earth and sky.
Horizon scanner: A scanner which automatically seeks the horizon for purposes of a spacecraft's orientation and control, e.g. one that detects the sharp discontinuity in infrared intensity at the outer edge of the Earth's tropopause.
Hour angle : The angular distance of a celestial object measured westward along the celestial equator from the zenith crossing.
HTP: High Test Peroxide.
HTPB: Hydroxy-terminator polybutadiene. A polymeric fuel binder.
Hydrazine: A rocket fuel which burns spontaneously with nitric acid or nitrogen tetroxide. Can also be used as a mono-propellant: when passed through an iridium-bearing catalyst, it decomposes at high temperature into constituent gases of ammonia, nitrogen and hydrogen. Used in small thrusters for orbit modification and attitude control of spacecraft. Also see MMH and UDMH.
Hydrosphere: The water on or around the surface of a planet.
Hydyne: A rocket fuel comprised of 60% UDMH and 40% diethylene-triamine.
Hyperbolic: A trajectory path to a planet shaped like a hyperbola.
Hypergolic: A term applied to an oxidizer and a fuel which ignite spontaneously with each other.

I ...
IAA: Indian Astronautical Association.
IAA: International Academy of Astronautics (established August 1960).
IAF: International Astronautical Federation (formally inaugurated 1951).
IC: Integrated Circuit.
ICBM: Inter-Continental Ballistic Missile (range >5,500 km).
ICO: Intermediate Circular Orbit.
IGY: International Geophysical Year (1957-58).
Impulse: The product of the average force acting on a body and the interval of time during which it acts, being a vector quantity equal to the change of momentum of the body during the same time interval.
IMU: Inertial Measurement Unit.
Inclination: The angular distance between a satellite's orbital plane and the equator of its primary.
Inertial guidance: An on-board system for launch vehicles and spacecraft where gyroscopes, accelerometers and other devices satisfy guidance requirements.
Inertial measurement unit: An on-board instrument system that measures the attitude of a spacecraft. It includes accelerometers and gyroscopes.
Inferior conjunction: Alignment of Earth, Sun, and an inferior planet on the same side of the Sun.
Inferior planets: Planets whose orbits are closer to the Sun than Earth's, i.e. Mercury and Venus. Also called inner planets.
Infrared: Electromagnetic radiation of wavelengths between 7500 A, the limit of the visible light spectrum at the red end, and centimetric radio waves.
Infrared radiometer: A telescope based instrument that measures the intensity of infrared energy radiated by the targets.
Injection angle: The angle at which a spacecraft's return trajectory intersects the Earth's atmosphere.
Injector: Typically, a perforated plate through which liquid fuel and oxidizer are injected into the combustion chamber at a controlled rate.
Intelsat: Organization of 105 countries (July 1980) owning or operating systems of satellites used by 144 countries and territories around the world for international communications, and by 16 countries for domestic communications.
Interferometer: Any of several optical, acoustic, or radio frequency instruments that use interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to determine wavelengths and wave velocities, measure very small distances and thicknesses, and measure indices of refraction.
Interplanetary magnetic field: The weak magnetic field filling interplanetary space, with field lines usually connected to the Sun. The IMF is kept out of the Earth's magnetosphere, but the interaction of the two plays a major role in the flow of energy from the solar wind to the Earth's environment.
Interplanetary probe: Unmanned instrumented spacecraft capable of reaching the planets.
Interplanetary shock: The abrupt boundary formed at the front of a plasma cloud (e.g. from a coronal mass ejection) moving much faster than the rest of the solar wind, as it pushes its way through interplanetary space.
Interstellar ark: Hypothetical space colony capable of transporting human intelligence to the stars.
Interstallar probe: Unmanned instrumented spaceship with artificial intelligence capable of reaching the nearer stars.
Inverse-square law: The mathematical description of how the strength of some forces, including gravity, changes in inverse proportion to the square of the distance from the source.
Ion: An atom that has lost or acquired one or more electrons.
Ion engine: A rocket engine, the thrust of which is obtained by the electrostatic acceleration of ionized particles.
Ionization: Formation of electrically charged particles. Can be produced by high-energy radiation such as light or UV rays, or by collision of particles in thermal agitation.
Ionosphere: An atmospheric layer dominated by charged, or ionized, atoms that extend from about 38 to 400 miles above the Earth's surface.
IR: Infrared.
IRBM: Intermediate Range Ballistic Missile (range 2,400-5,500 km).
IRFNA: Inhibited Red Fuming Nitric Acid; RFNA + 0.6% HF as corrosion inhibitor.
ISAS: Institute of Space and Aeronautical Science; University of Tokyo (Japan).
Isotropic: A property of a distribution of particles where the flux is constant over all directions.
Isp: Specific Impulse. Also abbreviated SI.
ISRO: Indian Space Research Organization.
IWFNA: Inhibited White Fuming Nitric Acid; WFNA + 0.6% HF as corrosion inhibitor.

J ...
Jansky: Unit used to express flux. 1 Jansky = 10-26 W m-2 Hz-1 Bandwidth.
JAXA: Japan Aerospace Exploration Agency (formed in 2003 by merger of ISAS, NAL and NASDA).
Jet propulsion: Reaction propulsion in which the propulsion unit obtains oxygen from the air as distinguished from rocket propulsion in which the unit carries its own oxygen-producing material.
Jovian planet: Any of the four biggest planets: Jupiter, Saturn, Uranus, and Neptune.
JPL: Jet Propulsion Laboratory (Pasadena, California).
JSC: Johnson Space Center (Houston, Texas).
Jupiter: Fifth planet from the Sun, a gas giant or Jovian planet.

K ...
K-band: A range of microwave radio frequencies in the neighborhood of 12 to 40 GHz, used for high speed data transmission on shuttle flights (~15 Ghz).
Kelvin: Scale of temperature named after the English physicist Lord Kelvin, based on the average kinetic energy per molecule of a perfect gas. Absolute zero is equivalent to -273.16oC (-459.69oF).
Kerosene: A mixture of hydrocarbons distilled from crude petroleum; see RJ-1, RP-1.
KHz: Kilohertz, equal to 1,000 Hertz.
Kilogram: The standard unit of mass in the metric system.
Kinetic energy: An object's energy of motion; for example, the force of a falling body.
Kosmobuksir: Russian name for "space tug".
Kosmolyot: Russian name for "spaceplane".
KSC: Kennedy Space Center (Florida).
KT: Kilotonne, equal to 1,000 tonnes.
Kuiper belt: A swarm of cometary bodies thought to orbit the Sun beyond Neptune at distances between 30 and 50 AU.

L ...
L-band: A range of microwave radio frequencies in the neighborhood of 1 to 2 GHz.
Lander spacecraft: A spacecraft designed to reach the surface of a planet or moon and survive long enough to telemeter data back to Earth.
Langrangian point: In a system dominated by two attracting bodies (such as Sun and Earth), a point at which a third, much smaller body (such as a satellite) keeps the same position relative to the other two. Theoretically, the Sun-Earth system has 5 Lagrangian points, but only two are important: L1, on the sunward side of Earth, about 4 times the distance of the Moon, and L2 at approximately the same distance on the midnight side. The only two lagrangian stable points, L4 and L5, lie in the orbit of the primary body, leading and trailing it by a 60-degree arc. Jupiter's trojan asteroids can be found orbiting around the Jupiter-Sun L4 and L5 points.
Lanyard: Small rope or cord.
Laser: Light Amplification by the Stimulated Emission of Radiation. A device for producing a coherent monochromatic high-intensity beam of light.
Latitude: Circles in parallel planes to that of the equator defining north-south measurements, also called parallels.
Launch complex: The complex of site, facilities and equipment used to launch a missile or space rocket.
Launch pad: The load-bearing base from which a rocket or spacecraft positioned on its launcher is fired.
Launch window: An interval of time during which a space vehicle can be launched to accomplish a given mission, e.g. a flight to Venus or Mars.
LC: Launch Center.
Leading side: For a satellite that keeps the same face toward the planet, the hemisphere that faces forward, into the direction of motion.
Leap year: Every fourth year, in which a 366th day is added since the Earth's revolution takes 365 days 5 hr 49 min.
LEO: Low Earth Orbit.
LGA: Low-Gain Antenna.
LH2: Liquid Hydrogen.
Liftoff: The start of a rocket's flight from its launch pad. Colloquially, "blast-off".
Light: Electromagnetic radiation that is visible to the eye, in the neighborhood of 1 nanometer wavelength.
Light speed: 299,792,458 meters per second ± 1.2 m/sec (186,282.39 miles/sec). U.S. National Bureau of Standards, 1971.
Light time: The amount of time it takes light or radio signals to travel a certain distance at light speed.
Light year: The distance light travels in one year, approximately 9.46 trillion km (5.88 trillion miles).
LiOH: Lithium Hydroxide.
Liquid hydrogen: A cryogenic rocket fuel which becomes liquid at -423oF.
Liquid oxygen: A cryogenic oxidizer which becomes liquid at -279oF.
Lithosphere: The crust of a planet.
LM: Lunar Module.
LO2: Liquid Oxygen. Also abbreviated LOX.
Local time: Time adjusted for location around the Earth or other planets in time zones.
LOI: Lunar Orbit Insertion.
Longitude: Great circles that pass through both the north and south poles, also called meridians.
Longitude of ascending node: In an orbit, the celestial longitude of the ascending node.
Longitudinal axis: The fore-and-aft line through the center of a space vehicle.
Low Earth orbit: An orbit in the region of space extending from the Earth's surface to an altitude of 2,000 kilometers. Given the rapid orbital decay of objects close to Earth, the commonly accepted definition is between 160-2,000 km above the Earth's surface.
Low-energy charged particle detector: A device designed to characterize the composition, energies, and angular distributions of charged particles in interplanetary space and within planetary systems.
Low-gain antenna: An omni-directional spacecraft antenna that provides relatively low data rates at close range, several AU for example.
LOX: Liquid Oxygen. Also abbreviated LO2.
LRBM: Long Range Ballistic Missile.
LRV: Lunar Roving Vehicle.
LT: Launch Time.
Lunar: Of or pertaining to the Moon.
Lunar module: The craft used by Apollo missions for Moon landings. The lunar module consisted of a descent stage, used to land on the Moon and as a platform for liftoff, and an ascent stage, used as crew quarters and for returning to the orbiting command module.
Lunar roving vehicle: A battery powered wheeled vehicle used by Apollo astronauts to explore the lunar surface.
LV: Launch Vehicle.

M ...
Mach: The ratio of the speed of a vehicle (or of a liquid or gas) to the local speed of sound.
Magnetic field: A region of space near a magnetized body where magnetic forces can be detected.
Magnetic field line: Lines everywhere pointing in the direction of the magnetic force, used as a device to help visualize magnetic fields. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.
Magnetic pole: Two meanings: (1) the points on Earth towards which the compass needle points. (2) A concentrated source of magnetic force, e.g. a bar magnet has two magnetic poles near its end.
Magnetic storm: A disturbance of the Earth's magnetic field initiated by a solar flare or sunspot.
MagnetoHydroDynamics: The study of plasma motion and dynamics in the presence of a magnetic field.
Magnetometer: A device for measuring the strength and direction of the interplanetary and solar magnetic fields.
Magnetopause: The boundary of the magnetosphere, lying inside the bow shock. The location in space where Earth's magnetic field balances the pressure of the solar wind. It is located about 63,000 km from Earth in the direction of the Sun.
Magnetosphere: That region of space surrounding the Earth which is dominated by the magnetic field.
Magnetron: A vacuum tube in which the flow of electrons is subject to the control of an external magnetic field.
Major axis: The maximum diameter of an ellipse.
Manned maneuvering unit: A portable jet-pack device used by astronauts to propel themselves through space independent of a spacecraft.
Mantle: Middle layer of the Earth; between the crust and the core.
Maria: Dark areas on the Moon, actually lava plains, once believed to be seas.
Mars: Fourth planet from the Sun, a terrestrial planet.
Mass: The quantity of matter in a body. It can be determined by measuring the force of gravity (weight) acting on it and dividing this by the gravitational acceleration at that point. Thus, the mass of a given body remains the same everywhere, while its weight changes with the gravitational attraction.
Mass fraction: Mass of a component divided by the total mass of all components in the system.
Mass ratio: Ratio of the total mass of a rocket vehicle to the mass remaining when all the propellant is consumed.
Max Q: Maximum dynamic pressure; the point during launch when the vehicle is subjected to its greatest aerodynamic stress.
Mean: Synonym for average.
Mean solar time: Time based on an average of the variations caused by Earth's non-circular orbit.
Medium Earth Orbit: An orbit in the region of space above low Earth orbit (2,000 kilometers) and below geosynchronous orbit (35,786 kilometers). Sometimes called Intermediate Circular Orbit.
Medium-gain antenna: A spacecraft antenna that provides greater data rates than a low-gain antenna, with wider angles of coverage than a high gain antenna, about 20-30 degrees.
Memory: The faculty of an electronic device to record and store data and/or instructions for future action on a command.
MEO: Medium Earth Orbit.
Mercury: First planet from the Sun, a terrestrial planet.
Meridian: Great circle that passes through both the north and south poles, also called line of longitude.
Mesosphere: A division of the Earth's atmosphere extending from altitudes ranging 18-30 miles to 48-55 miles.
Meteor: The luminous phenomenon seen when a meteoroid enters the atmosphere, commonly known as a shooting star.
Meteorite: A part of a meteoroid that survives through the Earth's atmosphere.
Meteoroid: A solid body, moving in space, that is smaller than an asteroid and at least as large as a speck of dust. Nearly all meteoroids originate from asteroids or comets.
MeV: One million electron volts.
MHz: Megahertz, equal to one million hertz.
Microgravity: An environment of very weak gravitational forces, such as those within an orbiting spacecraft. Microgravity conditions in space stations may allow experiments or manufacturing processes that are not possible on Earth.
Micrometeoroid: Meteoroid less than 1/250th of an inch in diameter.
Micrometeoroid protection: Shielding used to protect spacecraft components from micrometeoroid impacts. Interplanetary spacecraft typically use tough blankets of Kevlar or other strong fabrics to absorb the energy from high-velocity particles.
Microwaves: Radio waves having wavelengths of less than 20 centimeters.
Milky Way: The galaxy which includes the Sun and Earth.
Minor planet: An asteroid.
Missile: An object or a weapon that is fired, thrown, dropped, or otherwise projected at a target; a projectile.
Mixture ratio: Ratio of the masses of the fuel to the oxidizer at any given time.
MMH: Monomethyl Hydrazine, CH3NHNH2. A liquid hypergolic fuel.
MMU: Manned Maneuvering Unit.
Mock-up: A full-size replica or dummy of a vehicle, e.g. a spacecraft, often made of some substitute material such as wood to assess design features.
Modulation: The variation of a property of an electromagnetic wave or signal, such as its amplitude, frequency, or phase.
Module: A self-contained unit of a spacecraft or space station which serves as a building block for the total structure.
Momentum: The product of the mass of a body and its velocity.
Mono-propellant: A rocket propellant consisting of a single substance, especially a liquid containing both fuel and oxidizer, either combined or mixed together.
Moon: A small natural body which orbits a larger one. A natural satellite.
Motor: In spacecraft, a rocket that burns solid propellants.
MRBM: Medium Range Ballistic Missile (range 800-2,400 km).
MSFC: Marshall Space Flight Center (Huntsville, Alabama).
MT: Megatonne, equal to 1 million tonnes.
MT: Moscow Time.
Multiplexer: A mechanical or electrical device for sharing a circuit by two or more coincident signals.
Multistage rocket: A rocket having two or more stages which operate in succession each being discarded as its job is done.

N ...
N2O4: Nitrogen Tetroxide. Also abbreviated NTO.
Nadir: The direction from a spacecraft directly down toward the center of a planet. Opposite the zenith.
NAL: National Aerospace Laboratory of Japan.
NASA: National Aeronautics and Space Administration.
NASDA: National Space Development Agency (Japan).
Nautical mile: The distance spanned by one minute of arc in latitude, defined internationally as 1,852 meters (6,076.1033 feet).
Neptune: Eighth planet from the Sun, a gas giant or Jovian planet.
Neutron: Atomic particles having approximately the same mass as a hydrogen atom; very penetrating.
Newton: That force which gives a mass of 1 kilogram an acceleration of 1 meter per second per second; equal to 100,000 dynes.
NiCd: Nickel Cadmium.
Nitric acid: A liquid oxidizer that reacts spontaneously with hydrazine. Also see IRFNA and IWFNA.
Nitrogen tetroxide: A liquid oxidizer that reacts spontaneously with hydrazine.
Noctilucent clouds: Weakly-luminous clouds, seen at night at heights of about 50 miles (80 km) above the Earth.
Non-coherent: Communications mode wherein a spacecraft generates its downlink frequency independent of any uplink frequency.
NORAD: North American Air Defense Command (USA).
Nose shroud: A cover on the nose of a rocket or spacecraft which jettisons before insertion into orbit.
Nozzle: The projecting aperture at the end of a combustion chamber serving as an outlet for the exhaust gases.
NRC: National Research Council (USA).
NTO: Nitrogen Tetroxide. Also abbreviated N2O4.
Nucleus: The central body of a comet.

O ...
Occultation: The passage of a celestial body across a line between an observer and another celestial object; and the progressive blocking of light, radio waves, or other radiation from a celestial source during such a passage.
OKB: Experimental Construction Bureau.
Omnidirectional: Capable of transmitting or receiving signals in all directions, as an antenna.
OMS: Orbital Maneuvering System.
One-way: Communications mode consisting only of downlink received from a spacecraft.
One-way light time: The elapsed time it takes for light, or a radio signal, to reach a spacecraft or other body from Earth, or vice versa.
Oort cloud: A large swarm of comets theorized to orbit the sun in the neighborhood of 50,000 AU.
Orbit: The path of a body acted upon by the force of gravity. Under the influence of a single attracting body, all orbital paths trace out simple conic sections. Although all ballistic or free-fall trajectories follow an orbital path, the word orbit is more usually associated with the continuous path of a body which does not impact with its primary.
Orbit insertion: The placing of a spacecraft into orbit around a planet or moon.
Orbit trim maneuver: The firing of control rockets to refine a spacecraft's speed and trajectory.
Orbital elements: Six quantities used to mathematically describe an orbit; i.e. semi-major axis, eccentricity, inclination, argument of periapsis, time of periapsis passage and longitude of ascending node.
Orbital mechanics: The study of the motions of artificial satellites and space vehicles moving under the influence of forces such as gravity, drag, and thrust. Also called flight mechanics.
Orbital module: That part of a spacecraft which allows additional volume for crew relaxation and/or experiments. Discarded prior to reentry.
Orbital period: The time taken by an orbiting body to complete one orbit.
Orbital velocity: The velocity necessary to overcome the gravitational attraction of the Earth and so keep a satellite in orbit, about 17,450 mph (28,080 km/hr) close to the Earth.
Orbiter spacecraft: A spacecraft designed to travel to a distant planet or moon and enter orbit. It must carry a substantial propulsive capability to decelerate it at the right moment to achieve orbit insertion.
O-stage: Rocket boosters which operate during part of the burning time of the first stage of a launch vehicle to provide additional thrust.
OTM: Orbit Trim Maneuver.
OTRAG: Orbital Transport- und Raketen- Atktiengesellsschaft.
OWLT: One-Way Light Time.
Oxidizer: An agent that releases oxygen for combination with another substance, creating combustion and gas for propulsion. Alternatively oxidants.

P ...
P & W: Pratt and Whitney (USA).
Parachute: An apparatus used to retard free fall, consisting of a light, usually hemispherical canopy attached by cords and stored folded until deployed in descent.
Parallel: Circle in parallel planes to that of the equator defining north-south measurements, also called line of latitude.
Parking orbit: Orbit in which a space vehicle awaits the next phase of its planned mission.
Parsec: Measure of distance, 1 parsec = approximately 3.26 light years.
Pascal: A unit of pressure equal to one Newton per square meter.
Passive cooling: The use of painting, shading, reflectors and other techniques to cool a spacecraft.
Payload: Revenue-producing or useful cargo carried by a spacecraft; also, anything carried in a rocket or spacecraft that is not part of the structure, propellant, or guidance systems.
PBAN: Polybutadiene acrylic acid acrylonitrile. A polymeric fuel binder.
PDT: Pacific Daylight Time.
Pegasus: A rocket-vehicle concept for transportation of commercial high-priority freight or 172 passengers.
Periapsis: That point in an orbit which is nearest to the primary.
Perigee: That point in a terrestrial orbit which is nearest to the Earth.
Perihelion: That point in a solar orbit which is nearest to the Sun.
Perilune: That point in a lunar orbit which is nearest to the Moon.
Period of revolution: Time of one complete cycle in orbital motion - referred to as a year when applied to Earth.
Period of rotation: Time of one complete cycle - referred to as a day when applied to Earth.
Perturbation: Modifications to simple conic section orbits caused by such disturbances as air drag, non-uniformity of the Earth, and gravitational fields of more distant bodies such as the Moon.
Phase: Two meanings: (1) The particular appearance of a body's state of illumination, such as the full phase of the moon. (2) As applied to electromagnetic waves, phase is the relative measurement of the alignment of two waveforms of similar frequency.
Phase angle: The angle in which waves come to a body.
Photometer: An optical instrument that measures the intensity of light from a source.
Photometry: The measurement of light intensities.
Photon: A quantum of radiant energy.
Photon propulsion: The propulsion of a vehicle by the emission of photons, which possess momentum.
Photosphere: The visible surface of the Sun.
Photovoltaic cells: Crystalline wafers called solar cells which convert sunlight directly into electricity without moving parts.
Pitch: The rotation of a vehicle about its lateral (Y) axis, i.e. movement in elevation.
Planet: A nonluminous celestial body larger than an asteroid or a comet, illuminated by light from a star, such as the sun, around which it revolves. The only known planets are those of the Sun but others have been detected on physical (non-observational) grounds around some of the nearer stars.
Planetoid: An asteroid.
Plasma: A gas-like association of ionized particles that responds collectively to electric and magnetic fields.
Plasma detector: A device for measuring the density, composition, temperature, velocity and three-dimensional distribution of plasmas that exist in interplanetary regions and within planetary magnetospheres.
Plasma engine: A rocket engine in which thrust is obtained from the acceleration of a plasma with crossed electrical and magnetic fields.
Plasma wave: An oscillation or wave in a plasma that falls in the audio range of frequency.
Plasma wave detector: A device for measuring the electrostatic and electromagnetic components of local plasma waves in three dimensions.
Plasmasphere: The region of the atmosphere consisting of cold dense plasma originating in the ionosphere and trapped by the Earth's magnetic field.
PLSS: Portable Life Support System.
Plug nozzle: A doughnut-shaped combustion chamber which discharges engine gases against the surface of a short central cone (the plug). Adapted in the form of an LH2 cooled heat shield, it can be used as a combination rocket/ aerodynamic braking device.
Pluto: Ninth planet from the Sun, considered by many a minor planet.
Plutonium-238: A form of the radioactive element plutonium, characterized by high energy emissions.
PO: Polar Orbit.
Polar orbit: An orbit which passes over the poles.
Polarimeter: An optical instrument that measures the direction and extent of the polarization of light reflected from its targets.
Polymer: A compound used as a binder for solid rocket propellant systems; more generally, a compound consisting of repeating structural units.
Potential energy: The energy of a body due to its position in a field.
Pound: The U.S. customary unit of force defined as the weight of the standard pound at sea level and at the latitude of 45o.
ppm: Parts per million.
Precession: A change in the direction of the axis of spin of a rotating body.
Pressure suit: A suit, with helmet attached, which is inflated to provide body pressure and air, worn by the crew of certain spacecraft and aircraft which fly at great altitudes.
Pressurized: Containing air or other gas at a pressure higher than the pressure outside the chamber.
Primary: The body around which a satellite orbits.
Primitive: Used in a chemical sense, indicating an unmodified material representative of the original composition of the solar nebula.
Probe: An unmanned instrumented vehicle sent into space to gather information.
Prograde: Orbital motion in the same direction as the primary's rotation.
Propellant: A chemical or chemical mixture burned to create the thrust for a rocket or spacecraft.
Propulsion: The process of driving or propelling.
PST: Pacific Standard Time.
PTC: Passive Thermal Control.
Pulsar: Discovered in 1967. Pulsars emit radio signals the pulsations of which are extremely precise. The evidence suggests that pulsars are fast-spinning neutron stars.
Pyrotechnics: The use of electrically initiated explosive devices to operate valves, ignite solid rocket motors, and explode bolts to separate from or jettison hardware, or to deploy appendages.

Q ...
Quasars: Quasi-stellar objects. They are believed to be among the most distant objects in the observable Universe, emitting more energy than some of the most powerful galaxies.

R ...
Radar: System or technique for detecting the position, motion, and nature of a remote object by means of radio waves reflected from its surface.
Radian: Unit of angular measurement equal to the angle at the center of a circle subtended by an arc equal in length to the radius. Equals about 57.296 degrees.
Radiation: Energy in the form of electromagnetic waves or particles.
Radiation belt: The region of high-energy particles trapped in the Earth's magnetic field, also known as the Van Allen belts.
Radio: The least energetic form of electromagnetic radiation, having the lowest frequency and the longest wavelength.
Radio astronomy: The science of astronomy using radio waves instead of light waves.
Radio guidance: A system which is dependent on outside signals for information.
Radioisotopes: Atomic particles which decay by natural radioactivity.
Radioisotope thermoelectric generator: A device that converts the heat produced by the radioactive decay of plutonium-238 into electricity by an array of thermocouples made of silicon-germanium junctions. The Pu-238 is contained within a crash resistant housing.
Radiometry: The detection and measurement of radiant electromagnetic energy, usually in the infrared.
Rankine: A temperature scale, having a degree equal to the Fahrenheit degree but having a zero point at absolute zero. The freezing point of water is at 459.69oR.
RCS: Reaction Control System.
RD: Reaktivnyi Dvigatel. Russian for reaction motor.
RE: Unit of distance equal to the radius of the Earth, 6371.2 km.
Reaction control system: System of thrusters used to control spacecraft attitude.
Reaction wheels: Electrically-powered wheels mounted in three orthogonal axes aboard a spacecraft. To rotate the vehicle in one direction, you spin up the proper wheel in the opposite direction. To rotate the vehicle back, you slow down the wheel.
Readout: The action of a radio transmitter sending data either at the same time as data are acquired or by playback from an electronic memory.
Receiver: An electronic device that receives incoming radio signals and converts them to perceptible forms.
Red dwarf: A small star, on the order of 100 times the mass of Jupiter.
Redundancy: The duplication of certain critical components in a space vehicle.
Reentry: The descent into Earth's atmosphere from space.
Reentry interface: An altitude 400,000 feet; the point at which reentering spacecraft are considered to enter the Earth's atmosphere.
Refraction: The deflection or bending of electromagnetic waves when they pass from one kind of transparent medium into another.
Regenerative cooling: Circulation of a propellant through a jacket around the combustion chamber in order to cool the chamber wall, the propellant subsequently being injected into the combustion chamber.
Relay: An electrical switch employing an armature to open and close circuits.
Rem: Roentgen Equivalent Man. A measure of nuclear radiation causing biological damage.
Remote sensing: Instruments that record characteristics of objects at a distance, sometimes forming an image by gathering, focusing, and recording reflected light from the Sun, or reflected radio waves emitted by the spacecraft.
Rendezvous: A place of meeting at a given time, for example, a spaceship with a space station.
Resolution: Ability to distinguish visual detail, usually expressed in terms of the size (in kilometers) of the smallest features that can be distinguished.
Resonance: A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
Retrograde: Orbital motion in the direction opposite to the primary's rotation.
Retrorocket: A rocket fired to reduce the speed of a spacecraft.
Revolution: Orbital motion about a primary.
RF: Radio Frequency.
RFNA: Red Fuming Nitric Acid; 85% HNO3 + < 5% H2O + 6-15% NOX
Right ascension: With declination, one of the coordinates used to designate the location of an object on the celestial sphere. Right ascension is measured in hours, minutes, and seconds and is similar to longitude on Earth.
Ring current: A very spread-out electric current circling around the Earth, carried by trapped ions and electrons.
RJ-1: A hydrocarbon rocket fuel (a refined kerosene).
RLV: Reusable Launch Vehicle.
Rocket: A missile or vehicle propelled by the combustion of a fuel and a contained oxygen supply. The forward thrust of a rocket results when exhaust products are ejected from the tail.
Rocketdyne: A division of Rockwell International concerned with the design and development of rocket engines (USA).
Roll: The rotational movement of a vehicle about a longitudinal (X) axis.
Rotation: Rotary motion about an axis.
Round-trip light time: The elapsed time it takes for light, or a radio signal, to travel from Earth, be received and immediately transmitted or reflected, and return to the starting point.
RP-1: A hydrocarbon rocket fuel (a refined kerosene).
RTG: Radioisotope Thermoelectric Generator.
RTLT: Round-Trip Light Time.
RV: Reentry Vehicle.

S ...
SAR: Synthetic Aperture Radar.
Satellite: Any body, natural or artificial, in orbit around a planet. The term is used most often to describe moons and spacecraft.
Saturn: Sixth planet from the Sun, a gas giant or Jovian planet.
S-band: A range of microwave radio frequencies in the neighborhood of 2 to 4 GHz, used for communicating with piloted space missions (~2 Ghz).
SCADA: Supervisory Control And Data Acquisition.
Scan platform: An articulated, powered appendage to the spacecraft bus which points in commanded directions, allowing optical observations to be taken independently of the spacecraft's attitude.
Seismometer: A device for measuring movements of the ground.
Semi-major axis: Half the major axis of an ellipse. The mean distance of a planet or satellite from its primary.
Sensor: An electronic device for measuring or indicating a direction or movement.
SEP: Societe Europeene de Propulsion (France).
Sequencer: A mechanical or electrical device which may be set to initiate a series of events and to make events follow a sequence.
Service module: That part of a spacecraft which usually carries a maneuvering engine, thrusters, electrical supply, oxygen and other consumables external to the descent module. Discarded prior to reentry.
SETI: Search for Extra-Terrestrial Intelligence.
Sextant: An instrument that measures angular distances from fixed celestial objects.
Shepherd moon: Moon which gravitationally confines ring particles.
Sidereal time: Time relative to the stars other than the Sun.
Simulator: A device that mimics the operational conditions of equipment or vehicles.
SIS: Satellite Interceptor System.
SL: Sea Level.
SLBM: Submarine Launched Ballistic Missile.
Slug: The U.S. customary unit of mass defined as the mass which receives an acceleration of 1 foot per second per second when a force of 1 pound is applied to it.
SM: Service Module.
Solar: Of or pertaining to the Sun.
Solar array: See solar panel.
Solar cell: A cell that converts sunlight into electrical energy. The light falling on certain substances (e.g. a silicon cell) causes an electric current to flow.
Solar constant: The electromagnetic radiation from the Sun that falls on a unit area of surface normal to the line from the Sun, per unit time, outside the atmosphere, at one astronomical unit.
Solar flare: A sudden brightening in some part of the Sun, followed by the emission of jets of gas and a flood of ultra-violet radiation. The gale of protons which accompanies a flare can be very dangerous to astronauts.
Solar nebula: The large cloud of gas and dust from which the Sun and planets condensed 4.6 billion years ago.
Solar panel: An array of light-sensitive cells attached to a spacecraft and used to generate electrical power for the vehicle in space. Also called solar array.
Solar sensors: Light-sensitive diodes which indicate the direction of the Sun.
Solar wind: A current of charged particles that streams outward from the Sun.
Solid propellant: A rocket propellant in solid form; usually consisting of a mixture of fuel and oxidizer.
Solid rocket booster: A rocket, powered by solid propellants, used to launch spacecraft into orbit.
Sounding rocket: A research rocket used to obtain data from the upper atmosphere.
Space: The universe beyond Earth's atmosphere. The boundary at which the atmosphere ends and space begins is not sharp but starts at approximately 100 miles above Earth's surface.
Space colony: Hypothetical extra-terrestrial habitat, for hundreds, thousands or even millions of people, perhaps established on a moon or planet or as an artificial construction in free space.
Space debris: Man-made objects or parts thereof in space which do not serve any useful purpose.
Spacecraft: A piloted or unpiloted vehicle designed for travel in space.
Spacecraft clock: A counter maintained by the command & data subsystem. It meters the passing of time during the life of the spacecraft, and regulates nearly all activity within the spacecraft systems.
Space platform: A large artificial satellite conceived as a habitable base in space with scientific, exploratory or military applications. A space station.
Space station: An orbiting spacecraft designed to support human activity for an extended time.
Space weather: The popular name for energy-releasing phenomena in the magnetosphere, associated with magnetic storms, substorms and shocks.
SPADATS: Space Detection and Tracking System (USA).
SPASUR: Space Surveillance System (USA).
Specific impulse: Parameter for rating the performance of a rocket engine. Indicates how many pounds or kilograms of thrust are obtained by consumption of a pound or kilogram of propellant in one second.
Spectrometer: An optical instrument that splits the light received from an object into its component wavelengths by means of a diffraction grating; then measuring the amplitudes of the individual wavelengths.
Spectroscopy: The study of the production, measurement and interpretation of electromagnetic spectra.
Spectrum: A particular distribution of wavelengths and frequencies.
Spin stabilization: Spacecraft stabilization accomplished by rotating the spacecraft mass, thus using gyroscopic action as the stabilizing mechanism.
SRB: Solid Rocket Booster.
SRB propellant: Composite propellant used in the Space Shuttle Solid Rocket Boosters. Consists mainly of ammonium perchlorate as the oxidizer, powdered aluminum as the metallic fuel, and PBAN, polybutadiene-acrylic acid-acrylonitrile terpolymer, as the polymeric fuel binder. A small amount of iron oxide is added to increase the burning rate. The final product is a rubbery material not unlike a typewriter eraser.
SRBM: Short Range Ballistic Missile (range <800 km).
SRC: Science Research Council (UK).
SSO: Sun-Synchronous Orbit.
SSPO: Sun-Synchronous Polar Orbit.
Stage: An independently powered section of a rocket or spacecraft, often combined with others to form multistage vehicles.
Star: A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.
Static firing: The firing of a rocket on a special test stand to measure thrust, etc.
Stratosphere: A division of the Earth's atmosphere extending from altitudes ranging 5-10 miles to 18-30 miles.
Subatomic particles: Fundamental components of matter such as electrons or protons.
Subcarrier: Modulation applied to a carrier which is itself modulated with information-carrying variations.
Sublimator: An exposed metal plate, located on the outside of a spacesuit, that functions as a cooling coil to control suit temperatures.
Sub-orbital: Not attaining orbit, i.e. a ballistic space shot.
Sub-satellite: A secondary object released from a parent satellite in orbit, e.g. an electronic "ferret" released by a reconnaissance satellite.
Sunspot cycle: The recurring, eleven-year rise and fall in the number of sunspots.
Sunspots: Dark regions on the Sun which are the centers of large vortices and possess powerful magnetic fields. Maximum sunspot activity occurs in cycles with a period of about 11 years.
Sun synchronous orbit: A walking orbit whose orbital plan precesses with the same period as the planet's solar orbital period. In such an orbit, a satellite crosses periapsis at about the same local time every orbit.
Superior conjunction: Alignment between Earth and a planet on the far side of the Sun.
Superior planets: Planets whose orbits are farther from the Sun than Earth's, i.e. Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Also called outer planets.
Supernova: A large dying star, the final collapse of which is a cataclysmic explosion, hurling its substance into space.
Surface penetrator: A probe designed to penetrate the surface of a body, surviving an impact of hundreds of g's, measuring and telemetering the properties of the penetrated surface.
Surface rover: A semi-autonomous roving vehicle deployed on the surface of a planet or other body, taking images and soil analyses for telemetering back to Earth.
Sustainer engine: An engine that maintains propulsion of a launch vehicle once it has discarded its boosters.
Synthetic aperture radar: A radar imaging instrument which provides a penetrating illumination of radio waves, and is capable of imaging surfaces covered by clouds and haze. SAR images are constructed of a matrix where lines of constant distance or range intersect with lines of constant Doppler shift.

T ...
Tape recorder: A mechanical device for recording digital information on magnetic tape and for playing back the recorded material.
TCM: Trajectory Corrective Maneuver.
TEI: Trans-Earth Injection.
Telecommunication: Any process of communication over considerable distance.
Telemetry: The system for radioing information, including instrument readings and recordings, from a space vehicle to the ground.
Terrestrial: Of or pertaining to the Earth.
Terrestrial planet: Any of the four planets closest to the Sun: Mercury, Venus, Earth, Mars.
Thermal energy: Energy in the form of heat.
Thermal tile: Silica fiber insulation used to protect 70% of the exterior of the Space Shuttle orbiter against reentry temperatures of up to 1430oC. Surface heat dissipates so rapidly that an uncoated tile can be held by its edges with the bare hand while its interior glows red hot.
Thermosphere: The Earth atmosphere between 120 and 250 to 400 km (depending on the solar and geomagnetic activity levels), where temperature has an exponential increase up to a limiting value Texo at the thermopause. The temperature Texo is called the exospheric temperature.
Three-axis stabilization: Stabilization accomplished by nudging a spacecraft back and forth within a deadband of allowed attitude error, using small thrusters or reaction wheels.
Three-way: Coherent communications mode wherein a DSS receives a downlink whose frequency is based upon the frequency of an uplink provided by another DSS.
Throat: That part of a rocket engine between the combustion chamber and nozzle.
Throttle: To decrease the supply of propellant to an engine, reducing thrust. Liquid propellant rocket engines can be throttled; solid rocket motors cannot.
Thrust: The force that propels a rocket or spacecraft measured in pounds, kilograms or Newtons. Thrust is generated by a high-speed jet of gases discharging through a nozzle.
Thrust chamber: The area in a propulsion rocket in which force accumulates before ejection, e.g. the reaction chamber.
Thruster: Rocket engines used for maneuvering spacecraft in space.
Thrust vector control: Control of the thrust vector direction to steer a rocket or spacecraft during powered flight. Thrust vector control is most often achieved by hydraulically gimbaled engines.
Time of periapsis passage: The time in which a planet or satellite moves through its point of periapsis.
TLI: Trans-Lunar Injection.
TNT: Trinitrotoluene, a high explosive.
Tonne: Metric ton, a unit of mass equal to 1,000 kilograms (2,205 pounds).
Torus: Solid geometrical figure with the shape of a doughnut or innertube.
Tracking: The science of monitoring satellite locations by means of radio antennas at ground stations or by using other satellite systems in space.
Tracking station: A station set up to track an object through the atmosphere or space, usually by means of radar or radio.
Trailing side: For a satellite that keeps the same face toward the planet, the hemisphere that faces backwards, away from the direction of motion.
Trajectory: The flight path of a projectile, missile, rocket or satellite.
Transducer: Device for changing one kind of energy into another, typically from heat, position, or pressure into a varying electrical voltage or vice-versa, such as a microphone or speaker.
Trans-Earth injection: The firing of a spacecraft's engines to increase speed and break out of an orbit around the Moon, or another planet, and begin it on a trajectory to Earth.
Trans-Lunar injection: The firing of a spacecraft's engines to increase speed and break out of a parking orbit around Earth and begin it on a trajectory to the Moon.
Trans-Neptunian object: A small body orbiting the Sun beyond Neptune in a region known as the Kuiper belt.
Transmitter: An electronic device that generates and amplifies a carrier wave, modulates it with a meaningful signal, and radiates the resulting signal from an antenna.
Transponder: A device that transmits a response signal automatically when activated by an incoming signal.
Trojan relay system: A method of ensuring uninterrupted radio contact with the surface of any planet in the Solar System at any time first proposed by James Strong in 1967. Two radio satellites, keeping station along the Earth orbit, 60o ahead and 60o behind the Earth, transmit/receive signals from a similar pair of relay satellites at the Trojan equilaterals of another planet. Radio communications via these satellite links, from surface to surface, then becomes possible day and night, despite planetary rotation or orbital displacement. It could be used, for example, in steering a remotely-controlled vehicle on the surface of Mars.
Tropopause: The level separating the troposphere and the stratosphere, occurring at an altitude of 5-10 miles.
Troposphere: A division of the Earth's atmosphere extending from ground level to altitudes ranging 5-10 miles.
True anomaly: The angular distance of a point in an orbit past the point of periapsis, measured in degrees.
TT&C: Tracking, Telemetry and Command.
Turbopump: A pump driven by a gas turbine, generally used to pump propellant into a combustion chamber.
TVC: Thrust Vector Control.
Two-way: Communications mode consisting of downlink received from a spacecraft while uplink is being received at the spacecraft.

U ...
UDMH: Unsymmetrical Dimethyl Hydrazine, (CH3)2NNH2. A liquid hypergolic fuel.
UHF: Ultrahigh Frequency.
Ullage: The amount by which a container, such as a tank, falls short of being full.
Ullage maneuver: To supply positive acceleration to seat propellant in the bottom of its tanks.
Ultrahigh frequency: Short radio waves used for communicating with spacecraft.
Ultraviolet: A band of electromagnetic radiation with a higher frequency and shorter wavelength than visible blue light. Ultraviolet astronomy is generally performed in space, since Earth's atmosphere absorbs most ultraviolet radiation.
Umbilical: A cable conveying power to a rocket or spacecraft before liftoff. Also a tethering or supply line for an astronaut outside a spacecraft.
Universal time: The mean solar time of the meridian of Greenwich, England. Formerly called Greenwich mean time.
Universal time coordinated: The world-wide scientific standard of timekeeping; based upon carefully maintained atomic clocks and accurate to within microseconds. The addition or subtraction of leap seconds, as necessary, keeps it in step with Earth's rotation. Its reference point is Greenwich, England; when it is midnight there, it is midnight UTC.
Universe: All matter and energy, including Earth, the galaxies and all therein, and the contents of intergalactic space, regarded as a whole.
Uplink: The radio signal transmitted to a spacecraft from Earth.
Uranus: Seventh planet from the Sun, a gas giant or Jovian planet.
UT: Universal Time.
UTC: United Technologies Corporation; Chemical Systems Division (USA).
UTC: Universal Time Coordinated.
UV: Ultraviolet.

V ...
Van Allen radiation belts: Two doughnut-shaped zones of radiation about the Earth, concentrated at altitudes of 3,000 and 10,000 miles; named after James A. Van Allen who instrumented the satellite Explorer I. The belts contain charged particles generated by solar flares and trapped by the Earth's magnetic field.
Vector: A quantity that is specified by magnitude, direction and sense.
Velocity trim: See orbit trim maneuver.
Velocity vector: Magnitude of speed plus direction.
Venus: Second planet from the Sun, a terrestrial planet.
Vernier: Rocket engine of small thrust used for fine adjustments in velocity and trajectory.
VfR: Verein fur Raumschiffahrt e. V. (Germany).
VHF: Very High Frequency.
Vidicon: An imaging device consisting of a vacuum tube, in which an electron beam is swept across a phosphor coating on the glass where the image is focused, and its electrical potential varies in proportion to the level of light it encounters. This varying potential becomes the basis of the video signal produced.
VLBI: Very Long Baseline Interferometer.
Volcano: An opening in a planet's crust that allows magma to reach the surface.
Volcanism: Volcanic force or activity.

W ...
Walking orbit: An orbit in which gravitational influences are used to induce a precession in a satellite's orbital plane.
Wavelength: The distance from crest to crest, or trough to trough, of an electromagnetic or other wave. Wavelengths are related to frequency: The longer the wavelength, the lower the frequency.
Weight: The force acting on a body in a gravitational field, equal to the product of its mass and the acceleration of the body produced by the field.
Weightlessness: A state experienced in a ballistic trajectory (i.e. in orbit or free fall) when, because the gravitational attraction is opposed by equal and opposite inertial forces, a body experiences no mechanical stress.
WFNA: White Fuming Nitric Acid; 97.5% HNO3 + 2% H2O + < 0.5% NOX

X ...
X-axis: See roll.
X-band : A range of microwave radio frequencies in the neighborhood of 8 to 12 GHz.
X-rays: A band of electromagnetic radiation intermediate in wavelength between ultraviolet radiation and gamma rays. Because x-rays are absorbed by the atmosphere, x-ray astronomy is performed in space.

Y ...
Yaw: The rotation of a vehicle about its vertical (Z) axis, i.e. movement in azimuth.
Y-axis: See pitch.

Z ...
Z-axis: See yaw.
Zenith: The point on the celestial sphere directly above the observer. Opposite the nadir.
Zero gravity: A condition in which gravity appears to be absent. Zero gravity occurs when gravitational forces are balanced by the acceleration of a body in orbit or free fall.
Zero lift trajectory: A trajectory in which the control system acts to maintain a condition of no aerodynamic lift on the rocket.

Compiled and edited by Robert A. Braeunig, 1996, 1998, 2005, 2006.
Bibliography

 







 http://solargis.info/doc/solar-and-pv-data 







 Glossary of Solar - Terrestrial Physics Terms 







 http://rredc.nrel.gov/solar/glossary/ 







 Glossary of Solar Radiation Resource Terms 







 http://ssd.jpl.nasa.gov/ 







 JPL Solar System Dynamics 







 http://ssd.jpl.nasa.gov/ 







 http://en.wikipedia.org/wiki/Solar_System 







 Solar System 







 

 

 
		

Terms used by meteorologists, forecasters, weather observers, and in weather forecasts
Compiled from several sources

Ablation- The process of being removed. Snow ablation usually refers to removal by melting

Absolute Humidity- The density of water vapor.  It is the mass of the water vapor divided by the volume that it occupies.

Accretion- Growth of precipitation particles by collision of ice crystals with supercooled liquid droplets which freeze on impact

Accessory Clouds- Clouds that are dependent on a larger cloud system for development and continuance. Accessory clouds associated with the thunderstorm include roll, shelf, mammatus, and wall clouds.

Acid Rain- Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (e.g. pH < 5.6).

Additive data- A group of coded remarks in a weather observation that includes pressure tendency, amount of precipitation, and maximum/minimum temperature during specified periods of time.

Adiabatic- changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere.

Adiabatic Process- The change of temperature of air without transferring heat.  In an adiabatic process compression results in warming, and expansion results in cooling.

Advection- The horizontal transport of air, moisture or other atmospheric properties. Commonly used with temperatures, i.e., "warm air advection".

Advection Fog- a type of fog that results from the advection of moist air over a cold surface and the cooling of the air to its dew point that follows; this type of fog is most common in coastal regions.

Advisory- Advisories are issued for weather situations that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life-threatening situations. Advisories are issued for significant events that are occurring, are imminent, or have a very high probability of occurrence.

Aerosol- Particles of matter, solid or liquid, larger than a molecule but small enough to remain suspended in the atmosphere (up to 100? m diameter). Natural origins include salt particles from sea spray and clay particles as a result of weathering of rocks. Aerosols can also originate as a result of man's activities and in this case are often considered pollutants.

Aerovane- Aerovanes are commonly used at many weather stations and airports to measure both wind direction and speed. They are similar to wind vanes and cup anemometers except have three-bladed propellers attached to the end of the vane.

AFOS- the Automation of Field Operations and Services; AFOS is the computer system that links National Weather Service offices and other computer networks, such as the NOAA Weather Wire, to transmit weather information.

AGL- above ground level.

Air- the mixture of gases that make-up the earth's atmosphere.

Air Mass- A large body of air that has similar horizontal temperature and moisture characteristics.

Air-mass Thunderstorm- Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset.

Air Parcel- An imaginary small body of air that is used to explain the behavior of air. A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform throughout.

Air Pollution- The existence in the air of substances in concentrations that are determined unacceptable. Contaminants in the air we breathe come mainly from manufacturing industries, electric power plants, automobiles, buses, and trucks.

Air Pressure- (atmospheric pressure) air pressure is the force exerted on a surface by the weight of the air above it. The internationally recognized unit for measuring this pressure is the kilopascal.

Airstream- A significant body of air flowing in the same general circulation.

Albedo- The percentage of light reflected by an object. Snow covered areas have a high albedo (0.9 or 90%) due to their white color.

Alberta Clipper- A small, fast-moving low-pressure system that forms in western Canada and travels southeastward into the United States. These storms, which generally bring little precipitation, generally precede an Arctic air mass.

Altimeter- An active instrument (see active system) used to measure the altitude of an object above a fixed level.

Altimeter setting- That pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea-level of an aircraft on the ground at the location for which the value was determined.

Altitude- Height expressed as the distance above a reference point, which is normally sea level or ground level.

Altocumulus- Mid-altitude clouds with a cumuliform shape.

Altostratus- Mid-altitude clouds with a flat sheet-like shape.

Anabatic- wind flowing up an incline, such as up a hillside; upslope wind.

Anafront- A front at which the warm is ascending the frontal surface up to high altitudes.

Anemometer- An instrument that measures wind speed.

Aneroid barometer- An instrument built around a metal structure that bends with changing air pressure. These changes are recorded on a pointer that moves back and forth across a printed scale.

Angular Momentum- the energy of motion of a spinning body or mass of air or water.

Angular Velocity- the rate at which a spinning body rotates.

Anomaly- The deviation of (usually) temperature or precipitation in a given region over a specified period from the normal value for the same region.

Anticyclone- A large body of air in which the atmospheric pressure is higher than the pressure in the surrounding air.  The winds blow clockwise around an anticyclone in in the Northern Hemisphere.

Anticyclonic- describes the movement of air around a high pressure,  and rotation about the local vertical opposite the earth's rotation. This is clockwise in the Northern Hemisphere.

Anvil Cloud- The flat, spreading top of a Cb (cumulonimbus), often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind (see back-sheared anvil).

Anvil Crawler - A lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active MCS.

Anvil Dome - A large overshooting top or penetrating top.

Anvil Rollover - A circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil. See cumuliform anvil, knuckles, mushroom.

Anvil Zits - Frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.

Arctic Air- a mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.

Arctic High- a very cold high pressure that originates over the Arctic Ocean.

Arcus- A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds both are types of arcus clouds.

Aridity- A general term used to describe areas suffering from lack of rain or drought. More specifically, a condition in which evaporation exceeds precipitation.

ASOS- Automated Surface Observing System. This system observes sky conditions, temperature and dewpoint, wind direction and speed, and barometric pressure, and precipitation.

Atmosphere- The mass of air surrounding the earth and bound to it more or less permanently by the earth's gravitational attraction.

Atmospheric Pressure- (also called air pressure or barometric pressure) The pressure asserted by the mass of the column of air directly above any specific point.

Atmospheric Stability- An indication of how easily a parcel of air is lifted.  If the air is very stable it is difficult to make the parcel rise.  If the air is very unstable the parcel may rise on its own once started.

Aurora Borealis- Also known as the northern lights - The luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centered around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.

Automated Weather Station- An unmanned station with various sensors that measure weather elements such as temperature/wind/pressure and transmit these readings for use by meteorologists.

VHRR- Advanced Very High Resolution Radiometer. Main sensor on U.S. polar orbiting satellites.

Avalanche- a large mass of rapidly moving snow down a steep mountain slope.

AVN- Aviation Model generated every 12 hours by NCEP.

AWIPS- Advanced Weather Information Processing System. New NWS computer system integrating graphics, satellite and radar imagery. The successor to AFOS.

Back Door Cold Front- A front that moves east to west in direction rather than the normal west to east movement. For instance, one that enters Southern New England from the Gulf of Maine.

Back-building Thunderstorm- A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.

Back-sheared Anvil- A thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential.

Backing Wind- Wind which shifts in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). Backing winds with height are indicative of cold air advection (CAA).   The opposite of veering winds.

Ball lightning- A relatively rarely seen form of lightning, generally consisting of an orange or reddish ball of the order of a few cm to 30 cm in diameter and of moderate luminosity, which may move up to 1 m/s horizontally with a lifetime of a second or two.

Barber Pole- A thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.

Baroclinic Zone- A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems.

Barogram- The graphic record of pressure produced by a barograph.

Barograph- An instrument that provides a continuous record of atmospheric pressure.

Barometer- An instrument for measuring atmospheric pressure.

Barometric pressure- The actual pressure value indicated by a pressure sensor.

Barometric Tendency- The amount and direction of change in barometer readings over a three-hour period.

Barotropic System- A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development.

Bear's Cage- A region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm. The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.

Beaufort Scale- A scale that indicates the wind speed using the effect wind has on certain familiar objects.

Beaver('s) Tail- A particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest.

Black Ice- thin, new ice that forms on fresh water or dew covered surfaces; it is common on roadways during the fall and early winter and appears "black" because of its transparency.

Blizzard- Includes winter storm conditions of sustained winds or frequent gusts of  35 mph or more that cause major blowing and drifting of snow, reducing visibility to less than one-quarter mile for 3 or more hours.  Extremely cold temperatures often are associated with dangerous blizzard conditions.

Blizzard warning- Issued when blizzard condition are expected or are occurring.

Blocking High- A high pressure area (anticyclone), often aloft, that remains nearly stationary or moves slowly compared to west-to-east motion.  It blocks the movement eastward movement of low pressure areas (cyclones) at its latitude..

Blowing Dust- dust that is raised by the wind to moderate heights above the ground to a degree that horizontal visibility decreases to less than seven miles.   Visibilities of 1/8 mile or less over a widespread area are criteria for a Blowing Dust Advisory.

Blowing Sand- Sand particles picked up from the surface of the earth by the wind to moderate heights above the ground, reducing the reported horizontal visibility to less than 7 statute miles.

Blowing Snow- Wind driven snow that reduces visibility to six miles or less causing significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind.

Blowing spray- Water droplets torn by the wind from a body of water, generally from the crests of waves, and carried up into the air in such quantities that they reduce the reported horizontal visibility to less than 7 statute miles.

Blustery- Descriptive term for gusty winds that accompany cold weather.

Bomb Cyclone- An extratropical area of low pressure in which the central pressure drops at least 24 millibars in 24 hours.

Boundary Layer- In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere.

Bow echo- A radar echo which is linear but bent outward in a bow shape.   Damaging straight-line winds often occur near the "crest" or center of a bow
echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.

Box (or Watch Box) - A severe thunderstorm or tornado watch.

Breezy- Wind in the range of 15 mph to 25 mph with mild or warm temperatures.

Brisk- Wind in the range of 15 to 25 mph when the temperature is cold.

Broken Clouds- Clouds which cover between 5/8ths and 7/8ths of the sky.

Buoyancy- That property of an object that enables it to float on the surface of a liquid, or as in the case with air parcels, to ascend and remain freely suspended in the atmosphere.

Bubble High- A mesoscale area of high pressure, typically associated with cooler air from the rainy downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or outflow boundary separates a bubble high from the surrounding air.

Bulk Richardson Number (or BRN)- A non-dimensional number relating vertical stability and vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favorable for supercell development.

Bust- An inaccurate forecast,  usually a situation in which significant weather is expected, but does not occur.

BWER - Bounded Weak Echo Region. (Also known as a vault.) Radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft. This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually.

CAA- Cold Air Advection

Calm- the absence of apparent motion in the air.

Cap (or Capping Inversion)- A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability.

CAPE- Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may exceed 5,000 j/kg. However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent.

Cb- Cumulonimbus cloud

Ceiling- The height of the lowest layer of broken or overcast clouds.

Ceilometer- A device used to evaluate the height of clouds or the vertical visibility into a surface-based obscuration.

Cell- Convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a cumulus or towering cumulus cloud. A typical thunderstorm consists of several cells

Celsius- a temperature scale in which zero is the freezing point of water and one hundred is the boiling point.

Chance- A 30, 40 or 50 percent chance of occurrence of measurable precipitation.

Chinook Wind- A strong downslope wind that causes the air to warm rapidly as a result of compressive heating; called a foehn wind in Europe.

Circulation- The pattern of the movement of air. General circulation is the flow of air of large, semi-permanent weather systems, while secondary circulation is the flow of air of more temporary weather systems.

Cirriform- High altitude ice clouds with a very thin wispy appearance.

Cirrocumulus- Cirrus clouds with vertical development.

Cirrostratus- Cirrus clouds with a flat sheetlike appearance.

Cirrus- High clouds, usually above 18,000 feet, composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands.

Clear- Sky condition of less than 1/10 cloud coverage.

Clear Slot- A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud.

Climate-  The prevalent long term weather conditions in a particular area. Climatic elements include precipitation, temperature, humidity, sunshine and wind
velocity and phenomena such as fog, frost, and hail storms. Climate cannot be considered a satisfactory indicator of actual conditions since it is based upon
a vast number of elements taken as an average.

Climate change- This strictly refers to all forms of climatic inconsistency.  But it  is often used in a more restricted sense to imply a significant change. Within the media, climate change has been used synonymously with global warming. Scientists, however, use the term in a wider sense to include past climate changes also.

Climate Normals- Averages of temperatures, precipitation, snowfall, etc. made over standard 30 year periods.  These normals span across 3 decades and are rederived every 10 years.

Climatology- the scientific study of climate.

Closed Low- A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly.

Cloud- A visible cluster of tiny water and/or ice particles in the atmosphere.

Cloud Base- For a given cloud or cloud layer, it is the lowest level in the atmosphere where cloud particles are visible.

Cloud Condensation Nuclei- small particles in the air on which water vapor condenses and forms cloud droplets.

Cloud Streets- Rows of cumulus or cumulus-type clouds aligned parallel to the low-level flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite photographs.

Cloud Tags - Ragged, detached cloud fragments; fractus or scud.

Cloudburst- A sudden, intense rainfall that is normally of short duration.

Cloudy- the state of the sky when 7/10ths or more of the sky is covered by clouds.

Coastal Flood Warning- Issued when there is widespread coastal flooding expected within 12 hours, more than just typical overwash.

Coastal Flooding- The inundation of land areas along the coast caused by sea water above normal tidal actions.  This is often caused by prolonged strong onshore flow of wind and/or high astronomical tides.

Coastal Forecast- A forecast of wind, wave and weather conditions between the coastline and 25 miles offshore.

Coastal Waters- include the area from a line approximating the mean high water along the mainland or island as far out as 25 miles including the bays, harbors and sounds.

Cold Advection- (CAA) Transport of cold air into a region by horizontal winds.

Cold Air Damming- Cold air damming occurs when a cold dome of high pressure settles over northeastern New England. The clockwise circulation around the high pressure center brings northeasterly winds to the mid Atlantic region. The northeasterly winds bank cold air against the eastern slopes of the Appalachian Mountains.  Warmer air from the west or southwest is lifted above the cold air as it moves instead of warming the surface.

Cold-air Funnel- A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

Cold Front- A narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing.

Cold Low- a low pressure system with cold air mass from near the surface to all vertical levels (also called a cold core low).

Cold Pool- A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

Collar Cloud- Frequently used as a synonym for wall cloud, although it actually is a generally circular ring of cloud surrounding the upper portion of a wall cloud.

Comma Cloud- A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large and intense low-pressure systems.

Condensation- The process by which water vapor becomes a liquid; the opposite of evaporation, which is the conversion of liquid to vapor.

Condensation Nuclei- Small particles in the air around which water vapor condenses.

Conduction- The transfer of heat by molecular action between bodies that are in contact.

Confluence- A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.

Congestus (or Cumulus Congestus)- A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb.

Continental Air Mass- A dry air mass originating over a large land area.

Contrail- A cloud-like stream formed in cold, clear air behind the engines of an airplane.

Convection- The transfer of heat within a the air by its movement.   The term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere.

Convective Outlook- A forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the contiguous United States, issued several times daily by the SPC.

Convective Temperature- The approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding.

Convergence- An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. Divergence is the opposite, where winds cause a horizontal net outflow of air from a specified region.

Coordinated Universal Time (UTC)- The time in the zero degree meridian time zone.

Combined Seas- The combined height of swell and wind waves.

Cooling Degree Day- A form of degree day used to estimate the required energy for cooling. one cooling degree day occurs for each degree the daily mean temperature is above 65 degrees Fahrenheit.

Coriolis Force- An apparent force caused by the rotation of the Earth. In the Northern Hemisphere winds are deflected to the right, and in the Southern Hemisphere to the left. In synoptic scale weather systems (hurricanes and large mid-latitude storms), the Coriolis force causes the air to rotate around a low pressure center in a cyclonic direction. The air flowing around a hurricane spins counter-clockwise in the northern hemisphere

Corona- a disk of light surrounding the sun or moon; this is a result of the diffraction of light by small water droplets.

CRS- Console Replacement System.  This consists of a computer system and computer voice that is used to automate NOAA Weather Radio.

Cumulonimbus Cloud- A vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds.

Cumulus Cloud- A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower.

Cumulus Congestus- A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb.

Cut Off Low- An upper level low pressure system that is no longer in the normal west to east upper air flow. Usually a cut-off low will lie to the South of the established upper air flow.

Cyclogenesis- Development or intensification of a low-pressure center

Cyclone- An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean.

Cyclonic Circulation (or Cyclonic Rotation)- Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above.

Dart Leader- In lightning, the leader which, after the first stroke, initiates each succeeding stroke of a composite flash of lightning.

Debris Cloud- A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado.

Decouple- The tendency for the surface wind to become much lighter than wind above it at night when the surface temperature cools.

Degree Day- a measure of the departure of the daily mean temperature from the normal daily temperature; heating and cooling Degree Days are the departure of the daily mean temperature from sixty-five degrees Fahrenheit.

Dendrite- hexagonal ice crystals with complex and often fernlike branches.

Dense Fog- a fog in which the visibility is less than one-quarter mile.

Dense Fog Advisory- Issued when fog is expected to reduce visibility to 1/4 mile or less over a widespread are for at least 3 hours.

Density Of Air- The mass of air divided by its volume.  The air's density depends on its temperature, its pressure and how much water vapor is in the air.

Density Altitude- The pressure altitude corrected for temperature deviations from the standard atmosphere.  It is used by pilots when setting aircraft performance.

Depression- a region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.

Depth Hoar- Large (one to several millimeters in diameter), cohesionless, coarse, faceted snow crystals which result from the presence of strong temperature gradients within the snowpack

Derechoe- A widespread and usually fast-moving windstorm associated with convection. Derechoes include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.

Dew- Moisture from water vapor in the air that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.

Dew Point- The temperature to which the air must be cooled for water vapor to condense and form fog or clouds.

Diamond Dust- A fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates. (same as ice crystals)

Differential Motion- Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.

Difluence (or Diffluence)- A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence.

Dirty ridge- Most of the time, upper-level ridges bring fairly clear weather as the storms are steered around the ridge. Sometimes, however, strong storms undercut the ridge and create precipitation. Ridges that experience this undercutting by storms are known as dirty ridges because of the unusual precipitation.

Disturbance- a disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather.

Diurnal- Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and falls at night).

Divergence- The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence.

Doldrums- the regions on either side of the equator where air pressure is low and winds are light.

Doppler Radar- A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It determines the intensity of rainfall and uses the Doppler effect to measure the velocity of droplets in the atmosphere.

Downburst- A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado.

Downdraft- A column of generally cool air that rapidly sinks to the ground, usually accompanied by precipitation as in a shower or thunderstorm. .

Downslope wind- Air that descends an elevated plain and consequently warms and dries. Occurs when prevailing wind direction is from the same direction as the elevated terrain and often produces fair weather conditions.

Downstream- In the same direction as a stream or other flow, or toward the direction in which the flow is moving.

Drifting snow- Uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility.

Dry Adiabat- A line of constant potential temperature on a thermodynamic chart.

Dry Line- A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains.

Dry-line Bulge- A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels

Drizzle- Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.

Drought- Abnormally dry weather in a region over an extended period   sufficient to cause a serious hydrological (water cycle) imbalance in the affected area. This can cause such problems as crop damage and water-supply shortage.

Dry Punch- A surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge.

Dry Slot- A zone of dry (and relatively cloud-free) air which wraps east- or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system. A dry slot generally is seen best on satellite photographs.

Dryline- A boundary which separates warm, dry air from warm, moist air. The differences in the two air masses may be significant. The dry line is usually a boundary of instability along which thunderstorms form.

Dust Devil- A small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind.  Dust devils usually develop during hot, sunny days over dry and dusty or sandy areas.

Dust Storm- An area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile.

Dust Plume- A non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front.

Dust Whirl- A rotating column of air rendered visible by dust.

Dynamics- Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.

Easterly Wave- A wavelike disturbance in the tropical easterly winds that usually moves from east to west. Such waves can grow into tropical depressions.

ECMF- European Center for Meteorology Forecast model. 

Eddy- A small volume of air that behaves differently from the predominant flow of the layer in which it exists, seemingly having a life of its own. An example of such would be a tornado, which has its own distinct rotation, but is different than the large-scale flow of air surrounding the thunderstorm in which the tornado is born.

El Ni?o- A major warming of the equatorial waters in the eastern Pacific Ocean. El Ni?o events usually occur every 3 to 7 years, and are related to shifts in global weather patterns. (Spanish for the "Christ Child", named this because it often begins around Christmas.)

Enhanced greenhouse effect- The natural greenhouse effect has been enhanced by man's emissions of greenhouse gases. Increased concentrations of carbon dioxide, methane and nitrous oxide trap more infra-red radiation, so heating up the atmosphere.

Enhanced Wording- An option used by the SPC in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high.

Entrance Region- The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max. Severe weather potential sometimes increases in this area as a result.

ENSO- El Nino-Southern Oscillation.

Equilibrium Level (or EL)- On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment.

ETA- "Eta" (from Greek) model generated every 12 hours by NCEP

Evaporation- the process of a liquid changing into a vapor or gas.

Excessive Heat Warning- Issued within 12 hours of the onset of the following conditions: heat index of at least 105 degrees Fahrenheit for more than 3 hours per day for 2 consecutive days or heat index more than 115 degrees Fahrenheit for any period of time.

Excessive Heat Watch- Issued for the potential of the following conditions within 12 to 36 hours: heat index of at least 105 degrees Fahrenheit for more than 3 hours per day for 2 consecutive days or heat index more than 115 degrees Fahrenheit for any period of time.

Exit Region- The region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result.

Extended Outlook- a basic forecast of general weather conditions three to five days in the future.

Extratropical cyclone- A storm that forms outside the tropics, sometimes as a tropical storm or hurricane changes. See table below for differences between extratropical and tropical cyclones.

Eye- The low pressure center of a tropical cyclone. Winds are normally calm and sometimes the sky clears.

Eye wall- The ring of thunderstorms that surrounds a storm's eye. The heaviest rain, strongest winds and worst turbulence are normally in the eye wall.

Fahrenheit- the standard scale used to measure temperature in the United States; in which the freezing point of water is thirty-two degrees and the boiling point is two hundred and twelve degrees.

Fair- describes weather in which there is less than 4/10ths of opaque cloud cover, no precipitation, and there is no extreme visibility, wind or temperature conditions.

Fall Wind- a strong, cold, downslope wind.

Feeder Bands- Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually from the east through south (i.e., parallel to the inflow). This term also is used in tropical meteorology to describe spiral-shaped bands of convection surrounding, and moving toward, the center of a tropical cyclone.

Fetch- The area in which ocean waves are generated by the wind. Also refers to the length of the fetch area, measured in the direction of the wind.

Few- A cloud layer that covers between 1/8th and 2/8ths of the sky.

Flanking Line- A line of cumulus connected to and extending outward from the most active portion of a parent cumulonimbus, usually found on the southwest side of the storm. The cloud line has roughly a stair step appearance with the taller clouds adjacent to the parent cumulonimbus. It is most frequently associated with strong or severe thunderstorms.

Flash Flood- A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure or water released from an ice jam.

Flash Flood Warning- Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.

Flash Flood Watch- Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent.

Flood- a condition that occurs when water overflows the natural or artificial confines of a stream or river; the water also may accumulate by drainage over low-lying areas.

Flood Crest- The highest stage or flow occurring in a flood.

Flood Warning- Issued when there is expected inundation of a normally dry area near a stream, other water course; or unusually severe ponding of water.

Flood Stage- The stage at which water overflowing the banks of a river, stream or body of water begins to cause damage.

Flurries- Light snow falling for short durations. No accumulation or just a light dusting is all that is expected.

Foehn- A warm dry wind on the lee side of a mountain range. The heating and drying are due to adiabatic compression as the wind descend downslope.

Fog- Water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than one km (three thousand and three hundred feet).

Fogbow- A rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.

Forecast- A forecast provides a description of the most significant weather conditions expected during the current and following days. The exact content depends upon the intended user, such as the Public or Marine forecast audiences.

Fractus- Ragged, detached cloud fragments

Freeze- Occurs when the surface air temperature is expected to be 32 degrees Fahrenheit or below over a widespread area for a significant period of time.

Freeze Warning- Issued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, regardless if frost develops or not.

Freezing- The change in a substance from a liquid to a solid state.

Freezing Drizzle- Drizzle that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less, possibly producing a thin coating of ice. Even in small amounts, freezing drizzle may cause traveling problems.

Freezing fog- A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0 Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog.

Freezing Level- The altitude in the atmosphere where the temperature drops to 32F.

Freezing Nuclei- Particles suspended in the air around which ice crystals form.

Freezing Rain- Rain that freezes on objects such as trees, cars and roads, forming a coating or glaze of ice. Temperatures at higher levels are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact.

Freshet- the annual spring rise of streams in cold climates as a result of snow melt; freshet also refers to a flood caused by rain or melting snow.

Frog Storm- the first bad weather in spring after a warm period.

Front- The boundary or transition zone between two different air masses. The basic frontal types are cold fronts, warm fronts and occluded fronts.

Frost- The formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the exposed surface falls below 32 degrees Fahrenheit and water vapor is deposited as a solid.

Frost Advisory- Issued during the growing season when widespread frost formation is expected over an extensive area. Surface temperatures are usually in the mid 30s Fahrenheit.

Frost Point- When the temperature to which air must be cooled to in order to be saturated is below freezing.

Frozen Dew- When liquid dew changes into tiny beads of ice. The change occurs after dew formation and then the temperature falls below freezing.

Fujita Scale- System developed by Dr. Theodore Fujita to classify tornadoes based on wind damage. Scale is from F0 for weakest to F5 for strongest tornadoes.

Fujiwhara effect- The Fujiwhara effect describes the rotation of two storms around each other.

Funnel Cloud- A rotating, cone-shaped column of air extending downward from the base of a thunderstorm but not touching the ground. When it reaches the ground it is called a tornado.

Gale- Sustained wind speeds from 34 to 47 knots (39 to 54 mph).

Gale Warning- A marine weather warning for gale force winds from a non tropical system.

Geostationary Satellite- A satellite positioned over the equator that rotates at the same rate as the earth, remaining over the same spot.

Glaciation- The transformation of cloud particles from water droplets to ice crystals. Thus, a cumulonimbus cloud is said to have a "glaciated" upper portion.

Glaze- a layer or coating of ice that is generally smooth and clear, and forms on exposed objects by the freezing of liquid raindrops.

Global warming-A theory that increased concentrations of greenhouse gases are causing an elevation in the Earth's surface temperature.

GOES: Geostationary Operational Environmental Satellite

GOES-8- One of the Geostationary Operational Environmental Satellites. They are owned and run by the National Oceanic and Atmospheric Administration (NOAA), while NASA designs and launches them.

Gradient- The time rate or spatial rate of change of an atmospheric property.

Graupel- Small pellets of ice created when supercooled water droplets coat, or rime, a snowflake. The pellets are cloudy or white, not clear like sleet, and often are mistaken for hail.

Gravity Wave- A wave disturbance in which buoyancy acts as the restoring force on parcels displaced from hydrostatic equilibrium. Waves on the ocean are examples of gravity waves.

Greenhouse Effect- The warming of the atmosphere by the trapping of longwave radiation (heat) being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide.

Ground Fog- Shallow fog (less than twenty feet deep) produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog.

Growing Degree Day- A form of degree day to estimate the approximate dates when a crop will be ready to harvest. one growing degree day occurs when the daily mean temperature is one degree above the minimum temperature required for the growth of that specific crop.

Growing Season- The period of time between the last killing frost of spring and the first killing frost of autumn.

Gust- A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.

Gust Front- The leading edge of the downdraft from a thunderstorm. A gust front may precede the thunderstorm by several minutes and have winds that can easily exceed 80 mph.

Gustnado (or Gustinado)- Gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. .

Hail- Precipitation in the form of balls or irregular lumps of ice produced by liquid precipitation, freezing and being coated by layers of ice as it is lifted and cooled in strong updrafts of thunderstorms..

Halo- A ring or arc that encircles the sun or moon. Halos are caused by the refraction of light through the ice crystals in cirrus clouds.

Hard Freeze- freeze where vegetation is killed and the ground surface is frozen solid.

Harmattan- a hot, dry, and dusty northeasterly or easterly wind that occurs in West Africa north of the equator and is caused by the outflow of air from subtropical high pressure areas.

Haze- Fine dust or salt particles in the air that reduce visibility.

Heat Advisory- Issued within 12 hours of the onset of the following conditions: heat index of at least 105 degrees but less than 115 degrees for less than 3
hours per day. Nighttime lows remain above 80 degrees for 2 consecutive days.

Heat Balance- The equilibrium existing between the radiation received and emitted by a planetary system.

Heat Index- An index that combines air temperature and humidity to give an apparent temperature (how hot it feels). Here is a heat index formula originally from Weatherwise magazine. It gives valid results above 70 deg. F.
(-42.379+2.04901523*t+10.14333127*r-.22475541*t*r-(6.83783e-3)*t^2-(5.481717e-2)*r^2+(1.22874e-3)*t^2*r+(8.5282e-4)*t*r^2-(1.99e-6)*t^2*r^2)
t=temp deg f and r=%rel hum

Heat Island- A dome of elevated temperatures over an urban area caused by the heat absorbed by structures and pavement.

Heat Lightning- Lightning that can be seen, but is too far away for the thunder to be heard.

Heating Degree Day- A form of degree day used to estimate the required energy for heating. One heating degree day occurs for each degree the daily mean temperature is below 65 degrees Fahrenheit.

Heavy snow- Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours.

Heavy Snow Warning- Older terminology replaced by winter storm warning for heavy snow. Issued when 7 or more inches of snow or sleet is expected in the next 24 hours.  A warning is used for winter weather conditions posing a threat to life and property.

Heavy Surf- the result of large waves breaking on or near the shore resulting from swells or produced by a distant storm.

Helicity- A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity).

High- An area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.

High Risk (of severe thunderstorms)- Severe weather is expected to affect more than 10 percent of the area.

High Wind Warning- Issued when sustained winds from 40 to 73 mph are expected for at least 1 hour; or any wind gusts are expected to reach 58 mph or more.

High Wind Watch- Issued when conditions are favorable for the development of high winds over all of or part of the forecast area but the occurrence is still uncertain. The criteria of a high wind watch are listed under the high wind warning and should include the area affected, the reason for the watch and the potential impact of the winds.

Hodograph- A plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height.

Hook Echo- A radar pattern sometimes observed in the southwest quadrant of a tornadic thunderstorm. Appearing like a fishhook turned in toward the east, the hook echo is precipitation aloft around the periphery of a rotating column of air 2-10 miles in diameter.

Horse Latitudes- Subtropical regions where anticyclones produce settled weather.

Hot Spot- Typically large areas of pavement, these "hot spots" are heated much quicker by the sun than surrounding grasses and forests. As a result, air rises upwards from the relatively hot surface of the pavement, reaches its condensation level, condenses, and forms a cloud above the "hot spot".

Humidity- The amount of water vapor in the atmosphere.

Hurricane- A severe tropical cyclone with sustained winds over 74 mph (64 knots).   Normally applied to such storms in the Atlantic Basin and the Pacific Ocean east of the International Date Line.

Hurricane Warning- Warning issued when sustained winds of 74 mph (64 knots) or more are expected within 24 hours.  This implies a dangerous storm surge.

Hydrology- The study of the waters of the earth with relation to the effects of precipitation and evaporation upon the water in streams, rivers, lakes, and its effect on land surfaces.

Hydrologic Cycle- The composite picture of the interchange of water substance between the earth, the atmosphere and the seas which includes the change of state and vertical and horizontal transport.

Hydrosphere- The totality of water encompassing the Earth, comprising all the bodies of water, ice, and water vapor in the atmosphere.

Hygrometer- An instrument used to measure humidity.

Ice age- Periods in the history of the earth characterized by a growth of the ice caps towards the equator and a general lowering of global surface temperatures, especially in temperate mid-latitudes. The most recent ice age ended about 10,000 years ago. Ice advances in this period are known to have altered the whole pattern of global atmospheric circulation.

Ice Crystals- A barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures (around zero degrees F and colder) in a stable atmosphere.

Ice Fog- A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0 Celsius, based at the Earth's surface, which reduces horizontal visibility.  Usually occurs at -20F and below.

Ice Jam- An accumulation of broken river ice caught in a narrow channel that frequently produces local floods during a spring break-up.

Ice pellets- Precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (5
mm), or less. There are two main types.  Hard grains of ice consisting of frozen raindrops and pellets of snow encased in a thin layer of ice.

Ice Storm- Liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage.

Ice storm warning- Older terminology replaced by winter storm warning for severe icing.  Issued when 1/2 inch or more of accretion of freezing rain is expected.  This may lead to dangerous walking or driving conditions and the pulling down of power lines and trees. A warning is used for winter weather conditions posing a threat to life and property.

Indefinite ceiling- The ceiling classification applied when the reported ceiling value represents the vertical visibility upward into surface-based obscuration.

Inflow Bands (or Feeder Bands) - Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm.

Inflow Jets - Local jets of air near the ground flowing inward toward the base of a tornado.

Inflow Notch - A radar signature characterized by an indentation in the reflectivity pattern on the inflow side of the storm. The indentation often is V-shaped, but this term should not be confused with V-notch. Supercell thunderstorms often exhibit inflow notches, usually in the right quadrant of a classic supercell, but sometimes in the eastern part of an HP storm or in the rear part of a storm (rear inflow notch).

Inflow Stinger - A beaver tail cloud with a stinger-like shape.

Infra-Red Radiation- Electromagnetic radiation of lower frequencies and longer wavelengths than visible light (greater than 0.7 microns (? m)). Solar ultra-violet radiation is absorbed by the Earth's surface and re-emitted as infra-red radiation.

Indian Summer- An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

Insolation- Incoming solar radiation. Solar heating; sunshine.

Instability- A state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation.

Intertropical Convergence Zone (ITCZ)- The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.

Inversion- An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere. Temperature inversions trap atmospheric
pollutants in the lower troposphere, resulting in higher concentrations of pollutants at ground levels than would usually be experienced.

Ionosphere- Also known as the thermosphere. A layer in the atmosphere above the mesosphere extending from about 80km above the Earth's surface. It can be considered a distinct layer due to a rise in air temperature with increasing height. Atmospheric densities here are very low.

Iridescence- Brilliant patches of green or pink sometimes seen near the edges of high- or medium-level clouds.

Isentropic Lift- Lifting of air that is traveling along an upward-sloping isentropic surface. Situations involving isentropic lift often are characterized by widespread
stratiform clouds and precipitation.

Isentropic Surface- A two-dimensional surface containing points of equal potential temperature.

Isobar- A line of equal barometric pressure on a weather map.

Isodrosotherm- A line of equal dew point temperature.

Isohyet- A line of equal precipitation amounts.

Isopleth- General term for a line of equal value of some quantity. Isobars, isotherms, etc. all are examples of isopleths.

Isotach - A line of equal wind speed.

Isotherm- A line of equal temperature on a weather map.

January Thaw- A period of mild weather popularly supposed to recur each year in late January.

Jet streak-  A local wind speed maximum within a jet stream.

Jet Stream- Strong winds concentrated within a narrow band in the upper atmosphere.   It normally refers to horizontal, high-altitude winds. The jet stream often "steers" surface features such as front and low pressure systems.

Katabatic- Wind blowing down an incline, such as down a hillside; downslope wind.

Katafront- A front ( usually a cold front) at which the warm air descents the frontal surface.

Kelvin Temperature Scale- A temperature scale in which 0 degrees is the point at which all molecular motion ceases (absolute zero).

Killing Frost- Frost severe enough to end the growing season.  This usually occurs at temperatures below 28F.

Kilopascal - The internationally recognized unit for measuring atmospheric pressure. It is equal to 10 millibars.

Knot- A measure of speed. It is one nautical mile per hour (1.15 mph).  A nautical mile is one minute of one degree of latitude.

Knuckles- Lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm anvil. They usually appear on the upwind side of a back-sheared anvil, and indicate rapid expansion of the anvil due to the presence of a very strong updraft. They are not mammatus clouds.

Lake effect- The effect of a lake (usually a large one) in modifying the weather near the shore and down wind. It is often refers to the enhanced rain or snow that falls downwind from the lake.  This effect can also result in enhanced snowfall along the east coast of New England in winter.

Laminar- Smooth, non-turbulent. Often used to describe cloud formations which appear to be shaped by a smooth flow of air traveling in parallel layers or sheets.

La Nina- A cooling of the equatorial waters in the Pacific Ocean.

Land Breeze- A wind that blows from the land towards a body of water. Also known as an offshore breeze. It occurs when the land is cooler than the water.

Landspout- A tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

Lapse Rate- The change in temperature with altitude in the atmosphere.

Latent Heat- The heat energy that must be absorbed when a substance changes from solid to liquid and liquid to gas, and which is released when a gas condenses and a liquid solidifies.

Layer- An array of clouds and/or obscurations whose bases are at approximately the same level.

Left Front Quadrant (or Left Exit Region)- The area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum.

Left Mover- A thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm.

Leeward- Situated away from the wind; downwind - opposite of windward

Lenticular Clouds- A cloud that generally has the form of a smooth lens. They usually appear in formation as the result of orographic origin. Viewed from the ground, the clouds appear stationary as the air rushes through them.

Lifted Index (or LI)- A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and if thunderstorms develop they are more likely to be stronger.

Lifting- The forcing of air in a vertical direction by an upslope in terrain or by the movement of a denser air mass.

Lifting Condensation Level - The level in the atmosphere where a lifted air parcel reaches its saturation point, and as a result, the water vapor within condenses into water droplets.

Lightning- Any form of visible electrical discharges produced by thunderstorms.

Likely- In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.

Loaded Gun (Sounding)- A sounding characterized by extreme instability but containing a cap, such that explosive thunderstorm development can be expected if the cap can be weakened or the air below it heated sufficiently to overcome it.

Longwave Trough- A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration. Generally, there are no more than about five longwave troughs around the Northern Hemisphere at any given time. Their position and intensity govern general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks, or months.

Low- An area of low pressure, usually accompanied by cyclonic and inward wind flow. Also known as a cyclone.

Low-level Jet- A region of relatively strong winds in the lower part of the atmosphere.

Macroburst- Large downburst with a 2.5 mile or greater outflow diameter and damaging winds lasting 5 to 20 minutes.

Mamma Clouds- Also called mammatus, these clouds appear as hanging, rounded protuberances or pouches on the under-surface of a cloud. With thunderstorms, mammatus are seen on the underside of the anvil. These clouds do not produce tornadoes, funnels, hail, or any other type of severe weather, although they often accompany severe thunderstorms.

Maritime Air Mass- An air mass that forms over water. It is usually humid, and may be cold or warm.

Maximum Temperature- The highest temperature during a specified time period.

Mean Sea Level (MSL)- The average height of the sea surface, based upon hourly observation of the tide height on the open coast or in adjacent waters that have free access to the sea.

Mean Temperature- The average of a series of temperatures taken over a period of time, such as a day or a month.

Medium Range- In forecasting, (generally) three to seven days in advance.

Mercury Barometer- An instrument that measures barometric pressure by measuring the level of mercury in a column.

Meridional flow- A type of atmospheric circulation pattern in which the north and south component of motion is unusually pronounced.  Opposite of zonal flow.  

Mesocyclone- A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm). The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it.

Mesohigh- A mesoscale high pressure area, usually associated with MCSs or their remnants.

Mesolow (or Sub-synoptic Low) - A mesoscale low-pressure center. Severe weather potential often increases in the area near and just ahead of a mesolow.

Mesonet- A regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes.

Mesoscale- Size scale referring to weather systems smaller than synoptic-scale systems but larger than single storm clouds. Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines are an example of mesoscale weather systems.

Mesoscale Convective Complex (MCC)- A large mesoscale convective system, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:

Mesoscale Convective System (MCS)- A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.

Mesosphere- A layer of the atmosphere separated by the ionosphere above and the stratosphere below extending from about 50km-80km above the Earth's surface. The air temperature in mesosphere decreases with height.

META- The mesoscale ETA model.  A mathematical model of the atmosphere run on a computer that makes forecasts out to 30 hours.

Metamorphism- Changes in the structure and texture of snow grains which results from variations in temperature, migration of liquid water and water vapor, and pressure within the snow cover

METAR- A weather observation near ground level. It may include date and time, wind, visibility, weather and obstructions to vision, sky condition, temperature and dew point, sea level pressure, precipitation amount and other data used for aircraft operations.

Meteorologist - A person who studies meteorology. Some examples include research meteorologist, climatologist, operational meteorologist, TV meteorologist.

Meteorology- The study of  the physics, chemistry, and dynamics of the atmosphere and the direct effects of the atmosphere upon the Earth's surface, the oceans, and life in general.

Microburst- A strong localized downdraft from a thunderstorm with peak gusts lasting 2 to 5 minutes.

Microclimate- A local climate that differs from the main climate around it.

Mid-Latitudes- The areas in the northern and southern hemispheres between the tropics and the Arctic and Antarctic circles.

Millibar- A metric unit of atmospheric pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately 1013 millibars.

Minimum Temperature- The lowest temperature during a specified time period.

Mist- Consists of microscopic water droplets suspended in the air which produce a thin grayish veil over the landscape. It reduces visibility to a lesser extent than fog.

Mixing- Air movements (usually vertical) that make the properties of the air with a parcel homogeneous.  It may result in a lapse rate approaching the moist or dry adiabatic rate.

Model- A mathematical representation of a process, system, or object developed to understand its behavior or to make predictions. The representation always involves certain simplifications and assumptions.

Moderate Risk (of severe thunderstorms)- Severe thunderstorms are expected to affect between 5 and 10 percent of the area.

Moisture Advection- Transport of moisture by horizontal winds.

Moisture Convergence- A measure of the degree to which moist air is converging into a given area, taking into account the effect of converging winds and moisture advection. Areas of persistent moisture convergence are favored regions for thunderstorm development, if other factors (e.g., instability) are favorable.

Monsoon- A persistent seasonal wind, often responsible for seasonal precipitation regime. It is most commonly used to describe meteorological changes in
southern and eastern Asia.

Mountain Breeze- System of winds that blow downhill during the night.

Morning Glory - An elongated cloud band, visually similar to a roll cloud, usually appearing in the morning hours, when the atmosphere is relatively stable. Morning glories result from perturbations related to gravitational waves in a stable boundary layer.

MOS- Model Output Statistics.

MRF- Medium Range Forecast model generated every 12 hours by NCEP.

MSL- Mean sea level.

MSLP- Mean sea level pressure.

Muggy- Colloquially descriptive of warm and especially humid weather.

Multicell Cluster Thunderstorm- A thunderstorm consisting of two or more cells, of which most or all are often visible at a given time as distinct domes or towers in various stages of development.

Multivortex Tornado- A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.

Mushroom - A thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.

NCDC: National Climatic Data Center. Located in Asheville, North Carolina, the agency that archives climatic and forecast data from the National Weather Service.

NCEP: National Centers for Environmental Prediction. Central computer and communications facility of the National Weather Service; located in Washington, DC.

Negative Tilt Trough- An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.

NEXRAD: NEXt Generation RADar. A NWS network of about 140 Doppler radars operating nationwide.

NGM: Nested Grid Model generated every 12 hours by NCEP.

NHC: National Hurricane Center. The office of the National Weather Service in Miami that is responsible for tracking and forecasting tropical cyclones.

NOAA- National Oceanic and Atmospheric Administration. A branch of the U.S. Department of Commerce, NOAA is the parent organization of the National Weather Service.

NOAA WEATHER WIRE (NWWS): A computer dissemination network that sends National Weather Service products to the media and public.

NOAA WEATHER RADIO (NWR): Continuous, 24 hour a day VHF broadcasts of weather observations and forecasts directly from National Weather Service offices. A special tone allows certain receivers to alarm when watches or warnings are issued.

Nocturnal Related to nighttime, or occurring at night.

Nor'easter- A low-pressure disturbance forming along the South Atlantic coast and moving northeast along the Middle Atlantic and New England coasts to the Atlantic Provinces of Canada. It usually causes strong northeast winds with rain or snow. Also called a Northeaster or Coastal Storm.

Normal- The long-term average value of a meteorological element for a certain area. For example, "temperatures are normal for this time of year" Usually averaged over 30 years.

Northern Lights- Also known as the aurora borealis.  The luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centered around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.

Nowcast- A short-term weather forecast, generally out to six hours or less.

NSSL- The National Severe Storms Laboratory.

Nucleus- a particle of any nature upon which molecules of water or ice accumulate.

Numerical Forecasting- Forecasting the weather through digital computations carried out by supercomputers.

NWP- Numerical Weather Prediction.

NWS- National Weather Service.

Obscuration- Any phenomenon in the atmosphere, other than precipitation, that reduces the horizontal visibility in the atmosphere.

Occluded Front- A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion.

Offshore Breeze- A wind that blows from the land towards a body of water. Also known as a land breeze.

Offshore Forecast- A marine weather forecast for the waters between 60 and 250 miles off the coast.

Omega- A term used to describe vertical motion in the atmosphere. The "omega equation" used in numerical weather models is composed of two terms, the "differential vorticity advection" term and the "thickness advection" term. Put more simply, omega is determined by the amount of spin (or large scale rotation) and warm (or cold) advection present in the atmosphere. On a weather forecast chart, high values of omega (or a strong omega field) relate to upward vertical motion in the atmosphere. If this upward vertical motion is strong enough and in a sufficiently moist airmass, precipitation results.

Onshore Breeze- A wind that blows from a body of water towards the land. Also known as a seabreeze.

Orographic- Related to, or caused by, physical geography (such as mountains or sloping terrain).

Orographic Lift- The lifting of air as it passes over terrain features such hills or mountains. This can create orographic clouds and/or precipitation.

Orphan Anvil- An anvil from a dissipated thunderstorm, below which no other clouds remain.

Outflow- Air that flows outward from a thunderstorm.

Outflow Boundary- A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature.

Outflow Winds- Winds that blow down fjords and inlets from the land to the sea.

Overcast- Sky condition when greater than 9/10 of the sky is covered by clouds.

Overrunning- A condition that exists when a relatively warm air mass moves up and over a colder and denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.

Overshooting Top (or Penetrating Top)- A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm.

Ozone- A form of oxygen in which the molecule is made of  3 atoms instead of the usual two.  Ozone is usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation.  It is also a primary component of smog.

Ozone Hole- A thinning of the ozone layer over Antarctica, which occurs each spring.

Partly Cloudy- Sky condition when between 3/10 and 7/10 of the sky is covered.   Used more frequently at night.

Partly Sunny- Similar to partly cloudy.  Used to emphasize daytime sunshine.

Patches- Used with fog to denote random occurrence over relatively small areas.

Pendant Echo- Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

Permafrost- A soil layer below the surface of tundra regions that remains frozen permanently.

Polar Air- A mass of very cold, very dry air that forms in polar regions.

Polar front- The semi-permanent, semi-continuous front that encircles the northern hemisphere separating air masses of tropical and polar origin.

Polar Stratospheric Clouds (PSCs)- High altitude clouds that form in the stratosphere above Antarctica during the Southern Hemisphere winter. Their presence seems to initiate the ozone loss experienced during the ensuing Southern Hemisphere spring.

Polar vortex- A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion.

Pollutant- Strictly too much of any substance in the wrong place or at the wrong time is a pollutant. More specifically, atmospheric pollution may be defined as the
presence of substances in the atmosphere, resulting from man-made activities or from natural processes, causing adverse effects to man and the environment.

Polycrystal- A snowflake composed of many individual ice crystals.

POP- Probability of Precipitation. Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period.

Popcorn Convection- Clouds, showers and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.

Positive Area- The area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel.

Positive-tilt Trough- An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Potential Temperature- The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.

Precipitation- Liquid or solid water that falls from the atmosphere and reaches the ground.

Precipitation Shaft- A visible column of rain and/or hail falling from a cloud base. When viewed against a light background, heavy precipitation appears very dark gray, sometimes with a turquoise tinge. This turquoise tinge has been commonly attributed to hail, but its actual cause is unknown.

Pressure- The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

Pressure Change- The net difference between pressure readings at the beginning and ending of a specified interval of time.

Pressure Falling Rapidly- A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.

Pressure Gradient- The rate of decrease of pressure with distance at a fixed level.

Pressure Gradient Force- Force acting on air that causes it to move from areas of higher pressure to areas of lower pressure.

Pressure Rising Rapidly- An increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.

Pressure Tendency. The character and amount of atmospheric pressure change during a specified period of time, usually the 3-hour period preceding an observation.

Pressure Unsteady- A pressure that fluctuates by 0.03 inch of mercury or more from the mean pressure during the period of measurement.

Prevailing Westerlies- Winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east.

Prevailing Wind- The direction from which the wind blows most frequently in any location.

Profiler- An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.

Psychrometer- An instrument used for measuring the water vapor content of the atmosphere.  It consists of two thermometers, one of which is an ordinary glass thermometer, while the other has its bulb covered with a jacket of clean muslin which is saturated with distilled water prior to use.

Pulse Storm- A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.

PVA - Positive Vorticity Advection. Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air. PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential precipitation.

QPF: Quantitative Precipitation Forecast

Quality Of Snow- The amount of ice in a snow sample expressed as a percent of the weight of the sample.

Radar- An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging)

Radiation- Energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending upon the wavelength. Radiation from the Sun has a short wavelength (ultra-violet) while energy re-radiated from the Earth's surface and the atmosphere has a long wavelength (infra-red).

Radiation Fog- Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as ground fog.

Radiational Cooling- Cooling process of the Earth's surface and adjacent air, which occurs when infrared (heat) energy radiates from the surface of the Earth upward through the atmosphere into space. Air near the surface transfers its thermal energy to the nearby ground through conduction, so that radiative cooling lowers the temperature of both the surface and the lowest part of the atmosphere.

Radiosonde- An instrument attached to a weather balloon that transmits pressure, humidity, temperature and winds as it ascends to the upper atmosphere.

Rain- Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle (0.5 mm).

Rain Foot- A horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst.

Rain-Free Base- A horizontal, dark cumulonimbus base that has no visible precipitation beneath it. This structure usually marks the location of the thunderstorm updraft. Tornadoes most commonly develop (1) from wall clouds that are attached to the rain-free base, or (2) from the rain-free base itself. This is particularly true when the rain-free base is observed to the south or southwest of the precipitation shaft.

Rain Gauge- An instrument used to measure rainfall amounts.

Rain Shadow- The region on the lee side of a mountain or mountain range where the precipitation is noticeably less than on the windward side.

Rainbow- Optical phenomena when light is refracted and reflected by moisture in the air into concentric arcs of color. Raindrops act like prisms, breaking the light into the colors of a rainbow, with red on the outer, and blue on the inner edge.

Rankine Temperature Scale- A temperature scale with the degree of the Fahrenheit temperature scale and the zero point of the Kelvin temperature scale.

Rawinsonde- A balloon that is tracked by radar to measure wind speeds and wind directions in the atmosphere.

Reflectivity- Radar term referring to the ability of a radar target to return energy; used to estimate precipitation intensity and rainfall rates.

Refraction- The bending of light as it passes through areas of different density, such as from air through ice crystals.

Relative Humidity- The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage).

Retrogression (or Retrograde Motion)- Movement of a weather system in a direction opposite to that of the basic flow in which it is embedded, usually referring to a closed low or a longwave trough which moves westward.

Return Flow- South winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).

RFC- River Forecast Center. The Northeast  River Forecast Center is located in Taunton, MA..

Ridge- An elongated area of high pressure in the atmosphere. Opposite of a trough.

Right Entrance Region (or Right Rear Quadrant)- The area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum.

Right Mover- A thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers.)

Rime- Tiny balls of ice that form when tiny drops of water (usually not precipitation) freeze on contact with the surface.

River Flood Warning- Issued when main stem rivers (such as the Merrimack, Charles, Connecticut, etc) are expected to reach a level above flood stage.

Roll Cloud- A relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes.

Rope (or Rope Funnel) - A narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado.

Rope Cloud - In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary.

Rope Stage - The dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel). Damage still is possible during this stage.

Rossby Waves- Long waves that form in air or water that flows almost parallel to the equator, which results form the effect of the earth's rotation.

Rotor Cloud. A turbulent cloud formation found in the lee of some large mountain barriers. The air in the cloud rotates around an axis parallel to the mountain range.

RUC-  Rapid Update Cycle, a numerical model run at NCEP that focuses on short-term (up to 12 h) forecasts and small-scale (mesoscale) weather features. Forecasts are prepared every 3 hours for the contiguous United States.

Runway Visual Range (RVR)- An instrumentally-derived value, based on standard calibrations, that represents the horizontal distance a pilot may see down the runway from the approach end.

Saffir-Simpson Hurricane Damage Potential Scale- A scale that measures hurricane intensity, developed by Herbert Saffir and Robert Simpson.

St. Elmo's Fire- A luminous, and often audible, electric discharge that is intermediate in nature between a spark discharge  and a point discharge (with its diffuse, quiescent, and non-luminous character). It occurs from objects, especially pointed ones, when the electric field strength near their surfaces attains a value near 100,000 volts per m. Aircraft flying through active electrical storms often develop corona discharge streamers from antennas and propellers, and even from the entire fuselage and wing structure. It is seen also, during stormy weather, emanating from the yards and masts of ships at sea.

Sandstorm- Particles of sand carried aloft by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.

Santa Ana Winds- Relatively warm, dry winds that blow into Southern California coastal areas from an anticyclone located over the high deserts of California or Nevada. The warmth and dryness are due to compressional heating.

Satellite Photo- A photograph of the earth taken by weather satellites that shows areas of cloud.

Saturation- A condition of the atmosphere in which a certain volume of air holds the maximum water vapor it can hold at a specific temperature.

Saturation Vapor Pressure (water)- The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water. This is the maximum water vapor the air can hold for any given combination of temperature and pressure

Scattered- A cloud layer that covers between 3/8ths and 1/2 of the sky.

Scud Clouds- Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

Secondary Cold Front- A front that follows a primary cold front and ushers in even colder air.

Sea Breeze- A wind that blows from a sea or ocean towards a land mass. Also known as an onshore breeze. It occurs when the land is warmer than the water.

Sea-level Pressure- The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.

Sensible Heat- The excess radiative energy that has passed from the Earth's surface to the atmosphere through advection, conduction, and convection processes.

Severe Thunderstorm- A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4" or more. A thunderstorm with winds greater than 39 mph and/or hail greater than ? inch is defined as approaching severe.

Severe Thunderstorm Warning- Issued when thunderstorms are expected to have wind gusts to 58 mph or above or hail 3/4 inch or more in diameter.

Severe Thunderstorm Watch- Issued when conditions are favorable for the development of severe thunderstorms in and close to a defined area.

Shallow fog- Fog in which the visibility at 6 feet above ground level is 5/8ths of a mile or more.

Shear (Wind Shear)- Variation in wind speed and/or direction over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Shelf Cloud- A low-level horizontal accessory cloud that appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front. The leading edge of the shelf is often smooth and at times layered or terraced. It is most often seen along the leading edge of an approaching line of thunderstorms, accompanied by gusty straight winds as it passes overhead and followed by precipitation. The underside is concave upward, turbulent, boiling, or wind-torn. Tornadoes rarely occur with the shelf cloud.

Short-Fuse Warning- A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

Shortwave (Shortwave Trough)- A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.

Shortwave Radiation- The radiation received from the sun and emitted in the spectral wavelengths less than 4 microns. It is also called 'solar radiation'.

Shower- Precipitation that is intermittent, both in time, space or intensity.

Sky Condition- The state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types.

Sky Cover- The amount of the sky which is covered by clouds or obscurations in contact with the surface.

Sleet- Rain drops that freeze into ice pellets before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air near the surface and refreezes.

Slight Risk (of severe thunderstorms)- Severe thunderstorms are expected to affect between 2 and 5 percent of the area. A slight risk generally implies that severe weather events are expected to be isolated.

Sling Psychrometer- A psychrometer in which the wet and dry bulb thermometers are mounted upon a frame connected to a handle.  The psychrometer may be whirled by hand in order to provided the necessary ventilation.

Slight Chance- In probability of precipitation statements, usually equivalent to a 20 percent chance.

Small Craft Advisory- A marine advisory for winds 25 to 33 knots (29  to 38 mph) or seas of 5 feet or more, that may cause hazardous conditions for operators of small vessels.

Smog- Pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health.

Smoke- A suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled great distances (25 to 100 statute miles or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.

Snow- Frozen precipitation composed of ice particles in complex hexagonal patterns.   Snow forms in cold clouds by the direct transfer of water vapor to ice.

Snow Advisory- Older terminology replaced by winter weather advisory. An advisory issued when 4, 5, or 6 inches of snow or sleet is expected in 24 hours.   It is expected to create hazardous or restricted travel conditions, but not as severe as expected with a winter storm.

Snow Depth- The vertical height of frozen precipitation on the ground. For this purpose, frozen precipitation includes ice pellets, glaze, hail, any combination of these, and sheet ice formed directly or indirectly from precipitation.

Snow Flurries- Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation.

Snow Grains- Precipitation of very small, white, opaque grains of ice.

Snow Pellets- Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm).

Snow Shower- Snow falling at varying intensities for brief periods of time. Some accumulation is possible.

Snow Squalls- Intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning.

Snowburst- Very intense shower of snow, often of short duration, that greatly restricts visibility and produces periods of rapid snow accumulation.

Snowfall- The depth of new snow that has accumulated since the previous day or since the previous observation.

Snowflake- White ice crystals that have combined in a complex branched hexagonal form.

Solar Energy- The energy produced by the sun.

Sounder- A special kind of radiometer that measures changes in atmospheric temperature with height, as well as the content of various chemical species in the atmosphere at various levels. The High Resolution Infrared Radiation Sounder (HIRS), found on NOAA polar-orbiting satellites, is a passive instrument. See passive system.

Sounding- A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location ( example). Soundings are used extensively in weather forecasting, e.g., to determine instability, locate temperature inversions etc.

Southern Oscillation- A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events.

Special Marine Warning- Issued for brief or sudden occurrence of sustained wind or frequent gusts of 34 knots or more.  This is usually associated with severe thunderstorms or waterspouts.

SPC- Storm Prediction Center. Located in Norman, OK. This office is responsible for monitoring and forecasting severe convective weather in the continental U.S. This includes the issuance of Tornado and Severe Thunderstorm Watches.

Speed Shear- The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

Spin-up- A small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.

Spray- An ensemble of water droplets torn by the wind from an extensive body of water, generally from the crests of waves, and carried up into the air in such quantities that it reduces the horizontal visibility.

Squall- A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute.

Squall Line- Any non-frontal line or narrow band of active thunderstorms. The term is usually used to describe solid or broken lines of strong or severe thunderstorms.

Stability- An indication of how easily a parcel of air is lifted.   If the air is very stable it is difficult to make the parcel rise.  If the air is very unstable the parcel may rise on its own once started.

Stable Air- Air with little or no tendency to rise, that is usually accompanied by clear dry weather.

Standard Atmosphere- A hypothetical vertical distribution of the atmospheric temperature, pressure, and density, which by international agreement is considered to be representative of the atmosphere for pressure-altimeter calibrations and other purposes (29.92INS or 1013hPa).

Standing Lenticular Cloud- A, more or less, isolated cloud with sharp outlines that is generally in the form of a smooth lens or almond. These clouds often form on the lee side of and generally parallel to mountain ranges. Depending on their height above the surface, they may be reported as stratocumulus standing lenticular cloud (SCSL); altocumulus standing lenticular cloud (ACSL); or cirrocumulus standing lenticular cloud (CCSL).

Statement- Provides the public with information concerning the status of existing warnings.

Station Identifier- A group of four alphabetic characters used to identify a location that makes weather observations.

Station Pressure- The pressure that is read from a barometer but is not adjusted to sea level.

Stationary Front- The boundary between cool and warm air masses in that are not moving.

Stationary wave- Wave (flow pattern with periodicity in time and/or space) that is fixed relative to Earth.

Steam fog- Fog that is formed when water vapor is added to air which is much colder than the vapor's source.  This is most common when very cold air drifts across relatively warm water.

Steering Winds (Steering Currents) - A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Storm- In marine usage, winds 48 knots (55 mph) or greater.

Storm Surge- A rise of the sea level alone the shore that builds up as a storm (usually a hurricane) moves over water. It is a result of the winds of the storm and low atmospheric pressures.

Storm Track- the path that a low pressure area follows.

Storm Warning- A marine wind warning for sustained winds greater of 48 knots (55 mph) or more from a non-tropical system.

Storm-relative- Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

Storm-scale- Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale, mesoscale.

Straight Line Winds- Thunderstorm winds most often found with the gust front. They originate from downdrafts and can cause damage which occurs in a "straight line", as opposed to tornadic wind damage which has circular characteristics.

Stratiform- Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development.

Stratocumulus- Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves.

Stratosphere-- The layer of atmosphere above the troposphere and below the mesosphere (between 10 km and 50 km) generally characterized by an increase in
temperature with height.

Stratus- A flat, low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds.

Striations- Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud.

Sublimation- The change from ice directly to water vapor or from water vapor to ice with out going through the liquid water phase.

Subsidence- Downward moving (sinking) air over a broad area that is associated with warming air and little cloud formation.

Subtropical Jet- The branch of the jet stream that is found in the lower latitudes.

Subtropical storm- A low pressure system that develops in subtropical waters (north of 20 north degrees latitude) and initially has non-tropical features (see table below for a list of tropical features) but does have some element of a tropical cyclone's cloud structure (located close to the center rather than away from the center of circulation).

Suction Vortex (sometimes Suction Spot) - A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.

Supercell Thunderstorm- A severe thunderstorm whose updrafts and downdrafts are in near balance allowing the storm to maintain itself for several hours. Supercells often produce large hail and tornadoes.

Supercooled Water- Water that stays in liquid form if undisturbed even though it has been cooled to a temperature below its normal freezing point.

Supersaturation- The condition which occurs in the atmosphere when the relative humidity is greater than 100 percent.

Surface Hoar- The deposition (sublimation) of ice crystals on a surface which occurs when the temperature of the surface is colder than the air above and colder than the frost point of that air.

Surface Pressure- The pressure that is read from a barometer but is not adjusted to sea level.

Sustained Winds- The wind speed obtained by averaging the observed values over a one minute period.

SWEAT Index- Severe Weather ThrEAT index. A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds.

Synoptic Chart- Chart showing meteorological conditions over a region at a given time; weather map.

Synoptic Scale (Large Scale)- Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale.

TAF- A weather forecast for aircraft operations at an airport.

Tail Cloud- A low tail-shaped cloud extending outward from the northern quadrant of a wall cloud. Motions in the tail cloud are toward the wall cloud with rapid updraft at the junction of tail and wall cloud. This horizontal cloud is not a funnel or tornado.

Tail-end Charlie- The thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms.

Teleconnection - A strong statistical relationship between weather in different parts of the globe. For example, there appears to be a teleconnection between the tropics and North America during El Ni?o.

Temperate Zone- The area of the globe between the tropics and the polar regions.

Temperature- a measure of the warmth or coldness of an object or substance with reference to a standard value.

Terrestrial Radiation- The total infrared radiation emitted by the Earth.

Thermal- A small rising parcel of warm air produced when the earth's surface is unevenly heated.

Thermodynamics- In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Thermometer- An instrument for measuring temperature.

Theta-e (or Equivalent Potential Temperature) - The temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars.

Theta-e Ridge- An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.

Thunder- The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand.

Thunderstorm- A storm with lightning and thunder, produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail.

Tilted Storm or Tilted Updraft- A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

Topography- Generally, the lay-out of the major natural and man-made physical features of the earth's surface. Bridges, highways, trees, rivers and fields are all components that make up this topography.

Tornadic Activity- The occurrence or disappearance of tornadoes, funnel clouds, or waterspouts.

Tornado- A violent rotating column of air, in contact with the ground, pendant from a cumulonimbus cloud. A tornado does not require the visible presence of a funnel cloud.   It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours.

Tornado Alley- The area of the United States in which tornadoes are most frequent. It encompasses the great lowland areas of the Mississippi, the Ohio, and lower Missouri River Valleys. Although no state is entirely free of tornadoes, they are most frequent in the Plains area between the Rocky Mountains and Appalachians

Tornado Family - A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

Tornado Warning- Issued when there is likelihood of a tornado within the given area based on radar or actual sighting. It is usually accompanied by conditions indicated for Severe Thunderstorm Warning.

Total-Totals Index- A stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb.

Towering Cumulus- A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)

Trade Winds- Persistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low.  They blow northeasterly in the Northern Hemisphere.

Transverse Bands- Bands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence.

Transverse Rolls- Elongated low-level clouds, arranged in parallel bands and aligned parallel to the low-level winds but perpendicular to the mid-level flow.

Triple Point- The intersection point between two boundaries (dry line, outflow boundary, cold front, warm front etc.), often a focus for thunderstorm development.

Tropical Air- An air mass that has warm temperatures and high humidities and develops over tropical or sub-tropical areas.

Tropical Depression- Tropical mass of thunderstorms with a cyclonic wind circulation and winds near the surface between 23 mph and 39 mph.

Tropical Disturbance- An organized mass of thunderstorms in the tropics than lasts for more than 24 hours, has a slight cyclonic circulation, and winds less than 23 mph.

Tropical Storm- An organized low pressure system in the tropics with wind speeds between 38 and 74 mph.

Tropical Storm Warning- A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours.

Tropical wave- A kink or bend in the normally straight flow of surface air in the tropics which forms a low pressure trough, or pressure boundary, and showers and thunderstorms. Can develop into a tropical cyclone.

Tropics- The area of the globe from latitudes 23.5 degrees north to 23.5 degrees south.

Tropopause- The boundary between troposphere and the stratosphere.   It is usually characterized by an abrupt change in temperature with height from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).

Troposphere - The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height. It's the layer of the atmosphere where most of the weather occurs.

Trough- An elongated area of relatively low atmospheric pressure surface or aloft. Usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

Turbulence- Disrupted flow in the atmosphere that produces gusts and eddies.   At times this can be violent and can cause the up and down movement of a plane.

Turkey Tower- A narrow, individual cloud tower that develops and falls apart rapidly.

TVS- Tornadic Vortex Signature. Doppler radar signature in the radial velocity field indicating intense, concentrated rotation - more so than a mesocyclone.

Twister- A colloquial term for a tornado.

Typhoon- A hurricane that forms in the Western Pacific Ocean.

UKMET: United Kingdom forecast model.

Ultraviolet radiation- The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5 percent of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.

Unstable Air- Air that rises easily and can form clouds and rain.

Updraft- A small-scale current of rising air. This is often associated with cumulus and cumulonimbus clouds.

Upper Level System- A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.

Upslope Flow- Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upstream- Toward the source of the flow, or located in the area from which the flow is coming.

UTC- Coordinated Universal Time. The time in the zero degree meridian time zone.

UVI- Ultraviolet Index

UVV- Upward Vertical Velocity.

Valley Breeze- System of winds that blow uphill during the day.

Vapor Pressure- The pressure exerted by water vapor molecules in a given volume of air

Variable Ceiling- A ceiling of less than 3,000 feet which rapidly increases or decreases in height by established criteria during the period of observation.

Veering Wind- Wind which changes in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). Veering winds with height are indicative of warm air advection (WAA).

Vertical Shear- The rate of change of wind speed or direction, with a given change in height.

Vertically-stacked System- A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere.

Vicinity- A proximity qualifier used to indicate weather phenomena observed between 5 and 10 statute miles of the usual point of observation but not at the
station.

VIL- Vertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D units that takes into account the three-dimensional reflectivity of an echo. The maximum VIL of a storm is useful in determining its potential severity, especially in terms of maximum hail size.

Visibility- The greatest horizontal distance an observer can see and identify a prominent object.

Virtual temperature- The temperature a parcel of air would have if the moisture in it were removed and its specific heat was added to the parcel.

Virga- Precipitation falling from the base of a cloud and evaporating before it reaches the ground.

Volcanic Ash- Fine particles of rock powder that originate from a volcano and that may remain suspended in the atmosphere for long periods.

Vort Max- (Short for vorticity maximum), a center, or maximum, in the vorticity field of an airmass.

Vorticity- A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.

Vortex- An atmospheric feature that tends to rotate.  It has vorticity and usually has closed streamlines.

WAA: Warm Air Advection

Walker cell- A zonal circulation of the atmosphere confined to equatorial regions and driven principally by the oceanic temperature gradient. In the Pacific, air flows westward from the colder, eastern area to the warm, western ocean, where it acquires warmth and moisture and subsequently rises. A return flow aloft and subsidence over the eastern ocean complete the cell.

Wall Cloud- A local and often abrupt lowering of a rain-free cumulonimbus base into a low-hanging accessory cloud, from 1 to 4 miles in diameter. The wall cloud is usually situated in the southwest portion of the storm below an intense updraft marked by the main cumulonimbus cloud and associated with a very strong or severe thunderstorm. When seen from several miles away, many wall clouds exhibit rapid upward motion and rotation in the same sense as a tornado, except with considerably lower speed. A rotating wall cloud usually develops before tornadoes or funnel clouds by a time which can range from a few minutes up to possibly an hour.

Warm Advection- Transport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning.

Warm Front- A narrow transitions zone separating advancing warmer air from retreating cooler air. The air behind a warm front is warmer and typically more humid than the air it is replacing.

Warning- Forecast issued when a particular weather or flood hazard is "imminent" or already occurring (e.g., tornado warning, flash flood warning).  A warning is used for conditions posing a threat to life or property.

Warning Stage- The level of a river or stream which may cause minor flooding, and at which concerned interests should take action.

Watch- Forecast issued well in advance to alert the public of the possibility of a particular weather related hazard (e.g. tornado watch, flash flood watch). The occurrence, location and timing may still be uncertain.

Watch Box (or Box) - A severe thunderstorm or tornado watch.

Water- a transparent, odorless, tasteless liquid; composed of hydrogen and oxygen.

Water Equivalent- The liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow.

Water Vapor- Water substance in a gaseous state that comprises one of the most important of all the constituents of the atmosphere.

Waterspout-  A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that reaches the surface of the water, (i.e. a tornado over water).

Wave- In meteorology any pattern identifiable on a weather map that has a cyclic pattern, or, a small cyclonic circulation in the early stages of development that moves along a cold front.

Wave Crest- The highest point in a wave.

Wave Trough- The lowest point in a wave.

Wavelength- Physical distance of one period (wave repeat).

Weather- State of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness. Also, weather is the meteorological day-to-day variations of the atmosphere and their effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.

Weather Balloon- Large balloon filled with helium or hydrogen that carries a radiosonde (weather instrument) aloft to measure temperature pressure and humidity as the balloon rises through the air. It is attached to a small parachute so that when the balloon inevitably breaks, the radiosonde doesn't hurtle back to earth dangerously quickly.

Weather Synopsis- a description of weather patterns affecting a large area.

Wedge (or Wedge Tornado) - A large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base.

WFO: Weather Forecast Office.  The Southern New England WFO is located in Taunton, MA. Other WFOs for the Northeast area located in Albany, NY, Gray, ME, and Upton, NY.

Whiteout- A condition caused by falling and/or blowing snow that reduces visibility to nothing or zero miles; typically only a few feet. Whiteouts can occur rapidly often blinding motorists and creating chain-reaction crashes involving multiple vehicles. Whiteouts are most frequent during blizzards.

Wind- Air in motion relative to the surface of the earth.

Wind Advisory- Issued for sustained winds 31 to 39 mph for at least 1 hour or any gusts 46 to 57 mph.  However, winds of this magnitude occurring over an area that frequently experiences such winds would not require the issuance a wind advisory.

Wind Aloft- The wind speeds and wind directions at various levels in the atmosphere above the area of surface.

Wind Chill-  The additional cooling effect resulting from wind blowing on bare skin. The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold.  The (equivalent) wind chill temperature is the temperature the body "feels" for a certain combination of wind and air temperature

Wind Chill Factor- The apparent temperature which describes the cooling effect on exposed skin by the combination of temperature and wind, expressed as the loss of body heat. Increased wind speed will accelerate the loss of body heat.  The formula to calculate wind chill is: WC=.0817(3.71 V^.5 + 5.81 - .25 v)(T-91.4)+91.4 where V=wind speed in MPH and T=temperature F.

Wind Chill Advisory- Issued when the wind chill index is expected to be between -25F and -39F for at least 3 hours.  This is using the wind chill of the sustained wind, not gusts.

Wind Chill Warning- Issued when life-threatening wind chills of -40F or colder are expected for at least 3 hours.   This is using the wind chill of the sustained wind, not gusts.

Wind Direction- The direction from which the wind is blowing.

Wind Shear- Variation in wind speed and/or direction over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Wind Speed- The rate at which air is moving horizontally past a given point.  It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, or gust).

Wind Vane- An instrument that determines the direction from which a wind is blowing.

Wind Wave- A wave that is caused by the action of wind on the surface of water.

Windward- Upwind, or the direction from which the wind is blowing; the opposite of leeward.

Winter Storm- A heavy snow event. A snow accumulation of  more than 6 inches in 12 hrs or more than 12 inches in 24 hrs. 

Winter Storm Watch- A significant winter storm may affect your area, but its occurrence, location and timing are still uncertain. A winter storm watch is issued to provide 12 to 36 hours notice of the possibility of severe winter weather. A watch will often be issued when neither the path of a developing winter storm nor the consequences of the weather
event are as yet well defined. Ideally, the winter storm watch will eventually be upgraded to a warning when the nature and location of the developing weather event becomes more apparent. A winter storm watch is intended to provide enough lead time so those who need to set plans in motion can do so.

Winter Storm Warning- Issued when 7 or more inches of snow or sleet is expected in the next 24 hours, or 1/2 inch or more of accretion of freezing rain is expected.  A warning is used for winter weather conditions posing a threat to life and property.

Winter Weather Advisory- Issued when 4, 5, or 6 inches of snow or sleet is expected in 24 hours; or any accretion of freezing rain or freezing drizzle is expected on road surfaces; or when blowing or drifting snow is expected to occasionally reduce visibility to 1/4 mile or less.

Wiresonde- an atmospheric sounding instrument that is used to obtain temperature and humidity information between ground level and height of a few thousand feet; this instrument is supported by a captive balloon while traveling from the ground level.

World Meteorological Organization (WMO)- A specialized UN agency responsible for the establishment of meteorological stations and networks, and the monitoring of meteorological observations.

Wrapping Gust Front- A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.

WSR-88D- Weather Surveillance Radar - 1988 Doppler; NEXRAD unit.

Yellow Wind- a strong, cold, dry west wind of eastern Asia that blows across the plains during winter and carries a yellow dust from the desert.

Youg- a hot wind during unsettled summer weather in the Mediterranean.

Young Ice- newly formed flat, sea or lake ice generally between two and eight inches thick.

Zigzag Lightning- ordinary lightning of a cloud-to-ground discharge that appears to have a single lightning channel.

Zodiac- the position of the sun throughout a year as it appears to move through successive star groups or constellations.

Zonal Flow (Zonal Wind)- Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant.

Zone Of Maximum Precipitation- The belt of elevation at which the annual precipitation is greatest in a mountain region.

Zulu time- Same as UTC, Universal Coordinated Time.  Is is called Zulu because Z is often appended to the time to distinguish it from local time. 







 http://w1.weather.gov/glossary/ 







 National Weather Service Glossary 







 http://w1.weather.gov/glossary/ 







 Glossary of Meteorological Terms
[A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S] [T] [U] [V] [W] [X] [Y] [Z]


- A -

Acid Rain- rain that is especially acidic as the result of water vapor condensing onto particles of sulfate or nitrogen oxide.

Adiabatic- changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere.

Advection- the horizontal transport of air, moisture, vorticity or other atmospheric properties; commonly used in describing the transport of moisture and temperature.

Advection Fog- a type of fog that results from the advection of moist air over a cold surface and the cooling of the air to its dew point that follows; this type of fog is most common in coastal regions.

AFOS- the Automation of Field Operations and Services; AFOS is the computer system that links National Weather Service offices and other computer networks, such as the NOAA weather wire, to transmit weather information.

AGL- above ground level.

Air- the mixture of gases that make-up the earth's atmosphere.

Air Mass- a large body of air that has similar temperature and moisture characteristics.

Albedo- the reflectiveness of a surface given as either a percentage or a decimal.

Anabatic- wind flowing up an incline, such as up a hillside; upslope wind.

Anemometer- an instrument used to measure wind speed.

Angular Momentum- the energy of motion of a spinning body or mass of air or water.

Angular Velocity- the rate at which a spinning body rotates.

Anticyclone- a body of air in which the atmospheric pressure is higher than the pressure in the surrounding air; a high or high pressure area.

Anticyclonic- describes the movement of air around a high pressure; and rotation about the local vertical oppostie the earth's rotation.

Arctic Air- a mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.

Arctic High- a very cold high pressure that originates over the Arctic Ocean.

ASOS- the Automated Surface Observation System; this system observes sky condition, weather, temperature, dew point, wind direction and speed, pressure and precipitation.

Atmosphere- the mass of air surrounding the earth and bound to it more or less permanently by the earth's gravitational attraction.

Avalanche- a large mass of rapidly moving snow down a steep mountain slope.

AWIPS- the Advanced Weather Information Processing System; this new computer system integrates computer graphics, satellite and radar imagery. (This system is scheduled to be put in all National Weather Service offices by the end of the 1990s).



- B -

Backing- relates to time trend and wind direction; backing winds trend counter-clockwise and are indicative of cold air advection.

Bar- an obstacle formed at the shallow entrance at the mouth of a river or bay that empties into the ocean; when a swift ebb current and heavy seas rolling in from the Pacific meet, large breakers develop over the bar.

Barometer- an instrument used to measure atmospheric pressure.

Beaufort Scale- a scale that indicates the wind speed using the effect wind has on certain familiar objects.

Black Ice- thin, new ice that forms on fresh water or dew covered surfaces; it is common on roadways during the fall and early winter and appears "black" because of its transparency.

Blizzard- includes winter storm conditions of sustained winds greater than thirty-five mph that cause major blowing and drifting of snow, reducing visibility to less than one-quarter mile.

Blowing Dust- dust that is raised by the wind to moderate heights above the ground to a degree that horizontal visibility decreases to less than seven miles.

Blowing Snow- wind blown snow that reduces horizontal visibility.

Breaker- a large sea surface wave that has become too steep and unstable; as the wave breaks a whitecap will form.

Broken Clouds- opaque clouds that cover 6/10ths to 9/10ths of the sky.



- C -

Calm- the absence of apparent motion in the air.

Ceiling- the height of the lowest layer of broken and overcast clouds.

Celsius- a temperature scale in which zero is the freezing point of water and one hundred is the boiling point.

Chinook- a strong downslope wind that causes the air to warm rapidly as a result of compressive heating; called a Foehn wind in Europe.

Clear- the appearance of the sky when it is without clouds, or less than 1/10th of the sky is covered by clouds.

Climatology- the scientific study of climate.

Cloud Condensation Nuclei- small particles in the air on which water vapor condenses and forms cloud droplets.

Cloudy- the state of the sky when 7/10ths or more of the sky is covered by clouds.

Coastal Flooding- the submersion of land areas along the ocean coast and other inland waters caused by sea water over and above normal tide action.

Coastal Waters- include the area from a line approximating the mean high water along the mainland or island as far out as sixty nautical miles including the bays, harbors and sounds.

Cold Front- the leading edge of a cold air mass as it moves toward warmer air; its movement is characterized by a drop in temperature and humidity after the front passes.

Cold Low- a low pressure system with cold air mass from near the surface to all vertical levels (also called a cold core low).

Combined Seas- the interaction of wind waves and swell; the combined seas height is equal to the square root of the sum of the squares of the wind waves and swell.

Condensation- the occurrence of vapor transforming into a liquid.

Conduction- the transfer of heat between bodies that are in contact.

Convection- the transfer of heat within a gas or liquid by their movement.

Corona- a disk of light surrounding the sun or moon; this is a result of the diffraction of light by small water droplets.

Cyclone- a body of air in which the pressure is lower than that of the surrounding air; a low or low pressure area.



- D -

Degree Day- a measure of the departure of the daily mean temperature from the normal daily temperature; heating and cooling Degree Days are the departure of the daily mean temperature from sixty-five degrees Fahrenheit.

Dense Fog- a fog in which the visibility is less than one-quarter mile.

Depression- a region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.

Dew- water drops on an object at or near the ground formed from condensation of water vapor from the surrounding air.

Dew Point- the temperature to which a certain amount of air must be cooled in order for saturation to occur (a cloud or fog to form); the drier the air the lower the dew point and the more air is needed to be cooled for a cloud or fog to form.

Disturbance- a disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather.

Divergence- refers to the spreading out of winds.

Doldrums- the regions on either side of the equator where air pressure is low and winds are light.

Doppler Radar- radar that determines the intensity of rainfall and velocities of water and air particles; the National Weather Service uses a type of Doppler Radar called the WSR-88D, or NEXRAD; velocities of detected targets are determined by Doppler shift in frequency.

Downburst- a vertical or nearly vertical downward burst of strong winds at ground level, usually associated with a thunderstorm.

Drizzle- a form of precipitation that consists of water droplets less than .5mm.

Dust Devil- a rapidly rotating column of air carrying dust, leaves and other light-weight material; dust devils usually develop during hot, sunny days over dry and dusty or sandy areas.

Dust Storm- an area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile.



- E -

Ebb Current- the movement of a tidal current away form the coast or down an estuary.

El Nino- a great warming of the equatorial waters in the eastern Pacific Ocean; El Nino events occur every three to seven years and are related to shifts in global weather patterns.

Evaporation- the process of a liquid changing into a vapor or gas.

Extended Outlook- a basic forecast of general weather conditions three to five days in the future.



- F -

FAA- the Federal Aviation Administration.

Fahrenheit- the standard scale used to measure temperature in the United States; in which the freezing point of water is thirty-two degrees and the boiling point is two hundred and twelve degrees.

Fair- describes weather in which there is less than 4/10ths of opaque cloud cover and no precipitation; and there is no extreme visibility, wind or temperature conditions.

Fall Wind- a strong, cold, downslope wind.

Fathom- the common unit used to measure depth in the ocean; it is equivalent ot six feet.

Fetch- the area in which waves are formed by a wind with an approximately constant speed and direction.

Foehn Wind- European term for Chinook Wind; or warm, downslope wind.

Flood- a condition that occurs when water overflows the natural or artificial confines of a stream or river; the water also may accumulate by drainage over low-lying areas.

Fog- water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than one km (three thousand and three hundred feet).

Fogbow- a rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.

Freezing Level- the altitude in the atmosphere at which the temperature drops to thirty-two degrees Fahrenheit.

Freezing Rain or Drizzle- is precipitation that falls in liquid form but freezes upon contact with cold objects.

Freshet- the annual spring rise of streams in cold climates as a result of snow melt; freshet also refers to a flood caused by rain or melting snow.

Frog Storm- the first bad weather in spring after a warm period.

Front- the boundary or transition zone of two air masses; if cold air advances and replaces warmer air the front is a cold front, and if warm air advances and replaces cooler air the front is a warm front.

Frost- the formation of thin ice crystals on the ground or other surfaces; frost develops under conditions similar to those of dew, except in temperature of the earth's surface and earthbound objects fall below thirty-two degrees Fahrenheit.

Funnel Cloud- a funnel shaped cloud that extends from a cumulus or cumulonimbus cloud; this cloud is related to a rotating column of air that is not in contact with the ground.



- G -

Gale Warning- a marine weather warning for winds of thirty-four to forty-seven knots (thirty-nine to fifty-four mph).

Geostationary Satellite- a satellite positioned over the equator that revolves around the earth once every twenty-four hours, thus remaining in the same position relative to the earth's surface.

Glaze- a layer or coating of ice that is generally smooth and clear, and forms on exposed objects by the freezing of liquid raindrops.

Gradient- the time rate or spatial rate of change of an atmospheric property.

Greenhouse Effect- the heating of the earth's atmosphere caused by imbalances in the atmosphere's radiation cycle, characterized by so called greenhouse gases such as water vapor and carbon dioxide, that trap the heat from the sun in the atmosphere and inhibit re-radiation of that heat into space from the earth at night. Theoretically, this results in a gradual warming of the atmosphere.

Ground Fog- fog with very little vertical extent (less than twenty feet deep).

Growing Season- the period of time between the last killing frost of spring and the first killing frost of autumn.

Gust Front- the leading edge of a thunderstorm's downdraft air that is most prominent beneath the rain-free base and the leading edge of a thunderstorm; this gust front may precede the thunderstorm by several minutes and have winds that can easily exceed eighty miles an hour.



- H -

Hail- precipitation in the form of balls of ice produced by liquid precipitation, freezing and being coated by layers of ice as it is lifted and cooled in strong updrafts of thunderstorms.

Halos- rings or arcs that seem to encircle the sun or moon and are the result of the refraction of light through the ice ctystals that make up cirrus clouds.

Hard Freeze- freeze where vegetation is killed and the ground surface is frozen solid.

Harmattan- a hot, dry, and dusty northeasterly or easterly wind that occurs in West Africa north of the equator and is caused by the outflow of air from subtropical high pressure areas.

Haze- fine dry or wet dust particles suspended in the that reduce visibility.

Heavy Snow- usually means snowfall of four air inches or more in twelve hours or less, or six inches or more in twenty-four hours or less in lower elevation; in higher elevations it means twelve inches or more in twelve hours or less, or eighteen inches or more in twenty-four hours or less.

Heavy Surf- the result of large waves breaking on or near the shore resulting from swells or produced by a distant storm.

High or High Pressure Area- an area of high pressure that is usually characterized by clear skies and fair weather carried by sinking air; winds rotate clockwise around and outward from the center of the high.

High Wind- the sustained wind of forty mph or greater and/or gust greater than fifty-eight mph.

Horse Latitudes- subtropical regions where anticyclones produce settled weather.

Humidity- the amount of water vapor present in the air.

Hurricane- a tropical cyclone with sustained winds over seventy-four mph.

Hydrologic Cycle- the composite picture of the interchange of water substance between the earth, the atmosphere and the seas which includes the change of state and vertical and horizontal transport.

Hygrometer- an instrument used to measure humidity.



- I -

Ice Jam- an accumulation of broken river ice caught in a narrow channel that frequently produces local floods during a spring break-up.

Ice Storm- liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage.

Inversion- an increase in temperature with height, a reverse of the normal cooling with height.

Iridescence- brilliant patches of green or pink sometimes seen near the edges of high- or medium-level clouds.

Isobar- the line of equal pressure denoted on surface weather maps.

Isotherm- the line of equal temperature denoted on surface weather maps.



- J -

January Thaw- a period of mild weather popularly supposed to recur each year in late January.

Jet Stream- a zone of strong winds concentrated in a narrow band in the upper atmosphere; these winds are often referred to as the storm track since the jet stream often "steers" atmospheric storms.



- K -

Katabatic- wind blowing down an incline, such as down a hillside; downslope wind.

Knot- the unit used to measure wind speed, equal to 1.15 statute miles per hour.



- L -

Land Breeze- a coastal breeze that blows from land out to sea, and is the result of temperature differences when the sea is warmer then the adjacent land.

Latent Heat- the heat energy that must be absorbed when a substance changes from solid to liquid and liquid to gas, and which is released when a gas condenses and a liquid solidifies.

Lightning- any and all forms of visible electrical discharges produced by thunderstorms.

Low or Low Pressure Area- an area of low pressure usually characterized by clouds of precipitation caused by rising air; often related to inclement weather such as winds flowing counter clockwise and into the center of the low.



- M -

Macroburst- a large downburst within a 2.5 mile or larger outflow diameter and damaging winds lasting five to twenty-five minutes.

Meteorology- the study of phenomena of the atmosphere and all the processes that take place in the atmosphere and their relationships with processes at the surface of the earth.

Microclimate- a local climate that differs from the main climate around it.

Microwave Radiation- electromagnetic radiation which comprises the highest frequency radio energy.

Millibar- the scientific unit used in measuring and reporting atmospheric pressure.

Mist- very fine water droplets at ground level that occur in air with a halo.

Monsoon- a seasonal change in wind direction bringing dry air or heavy rain.

MSL- Mean sea level.



- N -

NOAA- Natural Oceanic and Atmospheric Administration; parent agency of the National Weather Service in the Department of Commerce.

NOAA Weather Wire- a computer network that transmits weather information.



- O -

Overcast- when more than 9/10ths of the sky is covered with clouds.

Ozone- a form of oxygen in which the molecule is made of three atoms instead of the usual two. Ozone is a primary component of photochemical smog.



- P -

Permafrost- a soil layer below the surface of tundra regions that remains frozen permanently.

Partly Cloudy- the appearance of the sky when 3/10ths to 6/10ths of the sky is covered with clouds.

Precipitation- all forms of water that fall from the sky and reach the ground.

Pressure- a measure of the weight of the air, that is usually measured with a barometer in meteorology.



- Q -

Quality Of Snow- the amount of ice in a snow sample expressed as a percent of the weight of the sample.



- R -

Radar- a method of detecting the distance, size, and movement of objects by their reflection of radio waves.

Radiosonde- a balloon carrying instruments for measuring conditions in the upper atmosphere.

Rain- a type of precipitation that is in the form of water droplets larger than 0.5mm.

Rawinsonde- a balloon that is tracked by radar to measure wind speeds and wind directions in the atmosphere.

Relative Humidity- the amount of water vapor in the air as compared to the percent of the amount of water vapor it would take to saturate the air.

Ridge- an elongated high pressure area in the atmosphere that lies between two low pressure areas.

Rossby Wabes- long waves that form in air or water that flows almost parallel to the equator, which results form the effect of the earth's rotation.



- S -

Sea Breeze- a coastal breeze that blows from sea to land during daytime in summer, that is caused by warm air rising over land and being replaced by cooler air from over the sea.

Shower- a type of precipitation that has a stopping-and-starting nature or rapid changes in intensity; and occurs when the atmosphere is unstable.

Sleet- a mixture of rain and snow that fall when the temperature is around the freezing level.

Small Craft Advisory- a marine advisory issued for sustained winds and/or frequent gusts greater than twenty knots.

Smog- a mixture of smoke and fog; a natural fog contaminated by industrial pollutants.

Stable Air- air with little or no tendency to rise, that is usually accompanied by clear dry weather.

Statement- provides the public with information concerning the status of existing severe weather conditions; and is used to cancel a watch or warning or delete part of a watch or warning that refers to areas that are no longer being threathened by a certain weather hazard.

Stationary Front- the boundary between cool and warm air in which neither air mass is advancing.

Storm Surge- a rise above the usual water level along the shore that is the result of strong onshore winds and/or reduced atmospheric pressure; the actual surge height is the difference of the observed water level minus the predicted tide.

Storm Track- the path that a low pressure area follows.

Storm Warning- a marine wind warning for winds greater than forty-eight knots (fifty-five mph).

Supercooled Water- water that stays in liquid form if undisturbed even though it has been cooled to a temperature below its normal freezing point.



- T -

Temperature- a measure of the warmth or coldness of an object or substance with reference to a standard value.

Tidal Piling- happens when unusually high water levels occur as the result of an accumulation of successive incoming tides that do not completely drain due to opposing strong winds and/or waves.

Tornado- a violent, rotating column of air extending from the ground to a thunderstorm.

Trade Winds- easterly-blowing winds that are found on either side of the equator and blow northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere.

Tropical Air- an air mass that has warm temperatures and high humidities and develops over tropical or sub-tropical areas.

Trough- an elongated relatively low pressure area between two high pressure areas.

Typhoon- a hurricane that occurs in eastern Asia.



- U -

Unstable Air- air that rises easily and can form clouds and rain.



- V -

Virga- rain falling from a cloud that evaporates before reaching the ground.

Vorticity- the tendency of a moving gas or liquid to rotate around an axis, forming a vortex.



- W -

Warning- is used when severe weather conditions are occurring, imminent or are expected within the following twelve to twenty-four hours.

Watch- informs the public that conditions exist that are favorable for the occurence of severe weather conditions within twelve to thirty-six hours of the given announcement.

Water- a transparent, odorless, tasteless liquid; composed of hydrogen and oxygen.

Waterspout- a column of rapidly spiraling air that develops over warm, usually shallow, water in seas or large lakes.

Water Vapor- water substance in a gaseous state that comprises one of the most important of all the constituents of the atmosphere.

Weather Synopsis- a description of weather patterns affecting a large area.

Wind- air in motion relative to the surface of the earth.

Wind Aloft- the wind speeds and wind directions at various levels in the atmosphere above the area of surface weather observations, usually above about two thousand feet AGL.

Wind Chill- the additional cooling effect resulting from wind blowing on bare skin.

Wind Direction- the direction from which the wind is blowing.

Wind Shear- the change in wind direction; vertical wind shear is the change in wind speed with height.

Wind Wave- a wave that is caused by the action of wind on the surface of water.

Wiresonde- an atmospheric sounding instrument that is used to obtain temperature and humidity information between ground level and height of a few thousand feet; this instrument is supported by a captive balloon while traveling from the ground level.



- X -

X-Band- a frequency band of microwave radiation in which radars operate.



- Y-

Yellow Wind- a strong, cold, dry west wind of eastern Asia that blows across the plains during winter and carries a yellow dust from the desert.

Youg- a hot wind during unsettled summer weather in the Mediterranean.

Young Ice- newly formed flat, sea or lake ice generally between two and eight inches thick.



- Z -

Zigzag Lightning- ordinary lightning of a cloud-to-ground discharge that appears to have a single lightning channel.

Zodiac- the position of the sun throughout a year as it appears to move through successive star groups or constellations.

Zonal Wind- the wind or wind componant along the local parallel of latitude.

Zone Of Maximum Precipitation- the belt of elevation at which the annual 







 Solar Energy Glossary 







 

The solar glossary contains definitions for technical terms related to solar power and photovoltaic (PV) technologies, including terms having to do with electricity, power generation, and concentrating solar power (CSP).

A B C D E F G H I J K L M
N O P Q R S T U V W Z

III-V cell — A high-efficiency solar cell made from materials including Group III and Group V elements from the periodic table .
A

absorber — In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).

AC — See alternating current. 

acceptor — A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.

activated shelf life — The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.

activation voltage(s) — The voltage(s) at which a charge controller will take action to protect the batteries.

adjustable set point — A feature allowing the user to adjust the voltage levels at which a charge controller will become active.

acceptor — A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.

AIC — See amperage interrupt capability.

air mass (sometimes called air mass ratio) — Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).

alternating current (AC) — A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

ambient temperature — The temperature of the surrounding area.

amorphous semiconductor — A non-crystalline semiconductor material that has no long-range order.

amorphous silicon — A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.

amperage interrupt capability (AIC) — direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current.

ampere (amp) — A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

ampere-hour (Ah/AH) — A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

ampere hour meter — An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).

ancillary services — Services that assist the grid operator in maintaining system balance. These include regulation and the contingency reserves: spinning, non-spinning, and in some regions, supplemental operating reserve.

angle of incidence — The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.

annual solar savings — The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.

anode — The positive electrode in an electrochemical cell (battery). Also, the earth or ground in a cathodic protection system. Also, the positive terminal of a diode.

antireflection coating — A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.

array — See photovoltaic (PV) array.

array current — The electrical current produced by a photovoltaic array when it is exposed to sunlight.

array operating voltage — The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load.

autonomous system — See stand-alone system.

availability — The quality or condition of a photovoltaic system being available to provide power to a load. Usually measured in hours per year. One minus availability equals downtime.

azimuth angle — The angle between true south and the point on the horizon directly below the sun.

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B

balance of system — Represents all components and costs other than the photovoltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.

balancing area — A metered segment of the power system, maintained by a balancing area authority, that ensures the total of all electrical generation equals the total of all system loads.

band gap — In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.

band gap energy (Eg) — The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.

barrier energy — The energy given up by an electron in penetrating the cell barrier; a measure of the electrostatic potential of the barrier.

base load — The average amount of electric power that a utility must supply in any period.

base load generating plants — Typically coal or nuclear generating units that are committed and dispatched at constant or near-constant levels with minimum cycling. They are often the sources of lowest-cost of energy when run at very high capacity factors.

battery — Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

battery available capacity — The total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state of charge, age, and cut-off voltage.

battery capacity — The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

battery cell — The simplest operating unit in a storage battery. It consists of one or more positive electrodes or plates, an electrolyte that permits ionic conduction, one or more negative electrodes or plates, separators between plates of opposite polarity, and a container for all the above.

battery cycle life — The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.

battery energy capacity — The total energy available, expressed in watt-hours (kilowatt-hours), which can be withdrawn from a fully charged cell or battery. The energy capacity of a given cell varies with temperature, rate, age, and cut-off voltage. This term is more common to system designers than it is to the battery industry where capacity usually refers to ampere-hours.

battery energy storage — Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

battery life — The period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.

BIPV — See building integrated photovoltaics.

blocking diode — A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.

boron (B) — The chemical element commonly used as the dopant in photovoltaic device or cell material.

boule — A sausage-shaped, synthetic single-crystal mass grown in a special furnace, pulled and turned at a rate necessary to maintain the single-crystal structure during growth.

British thermal unit (Btu) — The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

building integrated photovoltaics — A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.

bypass diode — A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.

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C

cadmium (Cd) — A chemical element used in making certain types of solar cells and batteries.

cadmium telluride (CdTe) — A polycrystalline thin-film photovoltaic material.

capacity (C) — See battery capacity.

capacity factor — The ratio of the average load on (or power output of) an electricity generating unit or system to the capacity rating of the unit or system over a specified period of time.

captive electrolyte battery — A battery having an immobilized electrolyte (gelled or absorbed in a material).

cathode — The negative pole or electrode of an electrolytic cell, vacuum tube, etc., where electrons enter (current leaves) the system; the opposite of an anode.

cathodic protection — A method of preventing oxidation of the exposed metal in structures by imposing a small electrical voltage between the structure and the ground.

Cd — See cadmium.

CdTe — See cadmium telluride.

cell (battery) — A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably). See also photovoltaic (PV) cell.

cell barrier — A very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell. The barrier inhibits the movement of electrons from one layer to the other, so that higher-energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell. Also called depletion zone or space charge.

cell junction — The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone.

charge — The process of adding electrical energy to a battery.

charge carrier — A free and mobile conduction electron or hole in a semiconductor.

charge controller — A component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status.

charge factor — A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to charge rate.

charge rate — The current applied to a cell or battery to restore its available capacity. This rate is commonly normalized by a charge control device with respect to the rated capacity of the cell or battery.

chemical vapor deposition (CVD) — A method of depositing thin semiconductor films used to make certain types of photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

cleavage of lateral epitaxial films for transfer (CLEFT) — A process for making inexpensive gallium arsenide (GaAs) photovoltaic cells in which a thin film of GaAs is grown atop a thick, single-crystal GaAs (or other suitable material) substrate and then is cleaved from the substrate and incorporated into a cell, allowing the substrate to be reused to grow more thin-film GaAs.

cloud enhancement — The increase in solar intensity caused by reflected irradiance from nearby clouds.

combined collector — A photovoltaic device or module that provides useful heat energy in addition to electricity.

concentrating photovoltaics (CPV) — A solar technology that uses lenses or mirrors to concentrate sunlight onto high-efficiency solar cells.

concentrating solar power (CSP) — A solar technology that use mirrors to reflect and concentrate sunlight onto receivers that convert solar energy to heat. This thermal energy is then used to produce electricity with a steam turbine or heat engine driving a generator.

concentrator — A photovoltaic module, which includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.

conduction band (or conduction level) — An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge.

conductor — The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.

contact resistance — The resistance between metallic contacts and the semiconductor.

contingency reserves — Reserve services that are sufficient to cover the unplanned trip (disconnect) of a large generator or transmission line and maintain system balance. Contingency reserves are generally split between spinning and non-spinning reserves, and are often based on the largest single hazard (generator or transmission capacity).

conversion efficiency — See photovoltaic (conversion) efficiency.

converter — A unit that converts a direct current (dc) voltage to another dc voltage.

copper indium diselenide (CuInSe2, or CIS) — A polycrystalline thin-film photovoltaic material (sometimes incorporating gallium (CIGS) and/or sulfur).

copper zinc tin sulfide/selenide (CZTS) — A polycrystalline thin-film photovoltaic material.

crystalline silicon — A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.

current — See electric current.

current at maximum power (Imp) — The current at which maximum power is available from a module.

current-voltage (I-V) curve — See I-V curve

cutoff voltage — The voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery.

cycle — The discharge and subsequent charge of a battery.

Czochralski process — A method of growing large size, high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.

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D

dangling bonds — A chemical bond associated with an atom on the surface layer of a crystal. The bond does not join with another atom of the crystal, but extends in the direction of exterior of the surface.

days of storage — The number of consecutive days the stand-alone system will meet a defined load without solar energy input. This term is related to system availability.

DC — See direct current.

DC-to-DC converter — Electronic circuit to convert direct current voltages (e.g., photovoltaic module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker.

deep-cycle battery — A battery with large plates that can withstand many discharges to a low state-of-charge.

deep discharge — Discharging a battery to 20% or less of its full charge capacity.

defect — See light-induced defects

demand response — The process of using voluntary load reductions during peak hours.

depth of discharge (DOD) — The ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example, the removal of 25 ampere-hours from a fully charged 100 ampere-hours rated cell results in a 25% depth of discharge. Under certain conditions, such as discharge rates lower than that used to rate the cell, depth of discharge can exceed 100%.

dendrite — A slender threadlike spike of pure crystalline material, such as silicon.

dendritic web technique — A method for making sheets of polycrystalline silicon in which silicon dendrites are slowly withdrawn from a melt of silicon whereupon a web of silicon forms between the dendrites and solidifies as it rises from the melt and cools.

depletion zone — Same as cell barrier. The term derives from the fact that this microscopically thin region is depleted of charge carriers (free electrons and hole).

design month — The month having the combination of insolation and load that requires the maximum energy from the photovoltaic array.

diffuse insolation — Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.

diffuse radiation — Radiation received from the sun after reflection and scattering by the atmosphere and ground.

diffusion furnace — Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material.

diffusion length — The mean distance a free electron or hole moves before recombining with another hole or electron.

diode — An electronic device that allows current to flow in one direction only. See also blocking diode and bypass diode.

direct beam radiation — Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.

direct current (DC) — A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.

direct insolation — Sunlight falling directly upon a collector. Opposite of diffuse insolation.

discharge — The withdrawal of electrical energy from a battery.

discharge factor — A number equivalent to the time in hours during which a battery is discharged at constant current usually expressed as a percentage of the total battery capacity, i.e., C/5 indicates a discharge factor of 5 hours. Related to discharge rate.

discharge rate — The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

disconnect — Switch gear used to connect or disconnect components in a photovoltaic system.

dispatching (economic dispatch) — A method by which system operators decide how much output should be scheduled from plants.

distributed energy resources (DER) — A variety of small, modular power-generating technologies that can be combined with energy management and storage systems and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid.

distributed generation — A popular term for localized or on-site power generation.

distributed power — Generic term for any power supply located near the point where the power is used. Opposite of central power. See also stand-alone systems.

distributed systems — Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.

donor — In a photovoltaic device, an n-type dopant, such as phosphorus, that puts an additional electron into an energy level very near the conduction band; this electron is easily exited into the conduction band where it increases the electrical conductivity over than of an undoped semiconductor.

donor level — The level that donates conduction electrons to the system.

dopant — A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).

doping — The addition of dopants to a semiconductor.

downtime — Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.

dry cell — A cell (battery) with a captive electrolyte. A primary battery that cannot be recharged.

duty cycle — The ratio of active time to total time. Used to describe the operating regime of appliances or loads in photovoltaic systems.

duty rating — The amount of time an inverter (power conditioning unit) can produce at full rated power.

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E

edge-defined film-fed growth (EFG) — A method for making sheets of polycrystalline silicon for photovoltaic devices in which molten silicon is drawn upward by capillary action through a mold.

electric circuit — The path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.

electric current — The flow of electrical energy (electricity) in a conductor, measured in amperes.

electrical grid — An integrated system of electricity distribution, usually covering a large area.

electricity — Energy resulting from the flow of charge particles, such as electrons or ions.

electrochemical cell — A device containing two conducting electrodes, one positive and the other negative, made of dissimilar materials (usually metals) that are immersed in a chemical solution (electrolyte) that transmits positive ions from the negative to the positive electrode and thus forms an electrical charge. One or more cells constitute a battery.

electrode — A conductor that is brought in conducting contact with a ground.

electrodeposition — Electrolytic process in which a metal is deposited at the cathode from a solution of its ions.

electrolyte — A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell.

electron — An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom. The movement of electrons in an electrical conductor constitutes an electric current.

electron hole pair — The result of light of sufficient energy dislodging an electron from its bond in a crystal, which creates a hole. The free electron (negative charge) and the hole (positive charge) are a pair. These pairs are the constituents of electricity.

electron volt (eV) — The amount of kinetic energy gained by an electron when accelerated through an electric potential difference of 1 Volt; equivalent to 1.603 x 10^-19; a unit of energy or work.

energy — The capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.

energy audit — A survey that shows how much energy used in a home, which helps find ways to use less energy.

energy contribution potential — Recombination occurring in the emitter region of a photovoltaic cell.

energy density — The ratio of available energy per pound; usually used to compare storage batteries.

energy imbalance service — A market service that provides for the management of unscheduled deviations in individual generator output or load consumption.

energy levels — The energy represented by an electron in the band model of a substance.

epitaxial growth — The growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal.

equalization — The process of restoring all cells in a battery to an equal state-of-charge. Some battery types may require a complete discharge as a part of the equalization process.

equalization charge — The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time.

equalizing charge — A continuation of normal battery charging, at a voltage level slightly higher than the normal end-of-charge voltage, in order to provide cell equalization within a battery.

equinox — The two times of the year when the sun crosses the equator and night and day are of equal length; occurring around March 20 or 21 (spring equinox) and September 22 or 23 (fall equinox).

exciton — A quasi-particle created in a semiconductor that is composed of an electron hole pair in a bound state. An exciton can be generated by and converted back into a photon.

external quantum efficiency (external QE or EQE) — Quantum efficiency that includes the effect of optical losses, such as transmission through the cell and reflection of light away from the cell.

extrinsic semiconductor — The product of doping a pure semiconductor.

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F

Fermi level — Energy level at which the probability of finding an electron is one-half. In a metal, the Fermi level is very near the top of the filled levels in the partially filled valence band. In a semiconductor, the Fermi level is in the band gap.

fill factor — The ratio of a photovoltaic cell's actual power to its power if both current and voltage were at their maxima. A key characteristic in evaluating cell performance.

fixed tilt array — A photovoltaic array set in at a fixed angle with respect to horizontal.

flat-plate array — A photovoltaic (PV) array that consists of non-concentrating PV modules.

flat-plate module — An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.

flat-plate photovoltaics (PV) — A PV array or module that consists of nonconcentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array.

float charge — The voltage required to counteract the self-discharge of the battery at a certain temperature.

float life — The number of years that a battery can keep its stated capacity when it is kept at float charge.

float service — A battery operation in which the battery is normally connected to an external current source; for instance, a battery charger which supplies the battery load< under normal conditions, while also providing enough energy input to the battery to make up for its internal quiescent losses, thus keeping the battery always up to full power and ready for service.

float-zone process — In reference to solar photovoltaic cell manufacture, a method of growing a large-size, high-quality crystal whereby coils heat a polycrystalline ingot placed atop a single-crystal seed. As the coils are slowly raised the molten interface beneath the coils becomes single crystal.

frequency — The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

frequency regulation — This indicates the variability in the output frequency. Some loads will switch off or not operate properly if frequency variations exceed 1%.

Fresnel lens — An optical device that focuses light like a magnifying glass; concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point.

full sun — The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).

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G

Ga — See gallium.

GaAs — See gallium arsenide.

gallium (Ga) — A chemical element, metallic in nature, used in making certain kinds of solar cells and semiconductor devices.

gallium arsenide (GaAs) — A crystalline, high-efficiency compound used to make certain types of solar cells and semiconductor material.

gassing — The evolution of gas from one or more of the electrodes in the cells of a battery. Gassing commonly results from local action self-discharge or from the electrolysis of water in the electrolyte during charging.

gassing current — The portion of charge current that goes into electrolytical production of hydrogen and oxygen from the electrolytic liquid. This current increases with increasing voltage and temperature.

gel-type battery — Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

gigawatt (GW) — A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.

grid — See electrical grid.

grid-connected system — A solar electric or photovoltaic (PV) system in which the PV array acts like a central generating plant, supplying power to the grid.

grid-interactive system — Same as grid-connected system.

grid lines — Metallic contacts fused to the surface of the solar cell to provide a low resistance path for electrons to flow out to the cell interconnect wires.

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H

harmonic content — The number of frequencies in the output waveform in addition to the primary frequency (50 or 60 Hz.). Energy in these harmonic frequencies is lost and may cause excessive heating of the load.

heterojunction — A region of electrical contact between two different materials.

high voltage disconnect — The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging.

high voltage disconnect hysteresis — The voltage difference between the high voltag disconnect set point and the voltage at which the full photovoltaic array current will be reapplied.

hole — The vacancy where an electron would normally exist in a solid; behaves like a positively charged particle.

homojunction — The region between an n-layer and a p-layer in a single material, photovoltaic cell.

hybrid system — A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.

hydrogenated amorphous silicon — Amorphous silicon with a small amount of incorporated hydrogen. The hydrogen neutralizes dangling bonds in the amorphous silicon, allowing charge carriers to flow more freely.

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I

incident light — Light that shines onto the face of a solar cell or module.

independent system operator (ISO) — The entity responsible for maintaining system balance, reliability, and electricity market operation.

indium oxide — A wide band gap semiconductor that can be heavily doped with tin to make a highly conductive, transparent thin film. Often used as a front contact or one component of a heterojunction solar cell.

infrared radiation — Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.

ingot — A casting of material, usually crystalline silicon, from which slices or wafers can be cut for use in a solar cell.

input voltage — This is determined by the total power required by the alternating current loads and the voltage of any direct current loads. Generally, the larger the load, the higher the inverter input voltage. This keeps the current at levels where switches and other components are readily available.

insolation — The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour. See also diffuse insolation and direct insolation.

interconnect — A conductor within a module or other means of connection that provides an electrical interconnection between the solar cells.

internal quantum efficiency (internal QE or IQE) — A type of quantum efficiency. Refers to the efficiency with which light not transmitted through or reflected away from the cell can generate charge carriers that can generate current.

intrinsic layer — A layer of semiconductor material, used in a photovoltaic device, whose properties are essentially those of the pure, undoped, material.

intrinsic semiconductor — An undoped semiconductor.

inverted metamorphic multijunction (IMM) cell — A photovoltaic cell that is a multijunction device whose layers of semiconductors are grown upside down. This special manufacturing process yields an ultra-light and flexible cell that also converts solar energy with high efficiency.

inverter — A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.

ion — An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.

irradiance — The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.

ISPRA guidelines — Guidelines for the assessment of photovoltaic power plants, published by the Joint Research Centre of the Commission of the European Communities, Ispra, Italy.

i-type semiconductor — Semiconductor material that is left intrinsic, or undoped so that the concentration of charge carriers is characteristic of the material itself rather than of added impurities.

I-V curve — A graphical presentation of the current versus the voltage from a photovoltaic device as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterizes cell performance.

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J

joule — A metric unit of energy or work; 1 joule per second equals 1 watt or 0.737 foot-pounds; 1 Btu equals 1,055 joules.

junction — A region of transition between semiconductor layers, such as a p/n junction, which goes from a region that has a high concentration of acceptors (p-type) to one that has a high concentration of donors (n-type).

junction box — A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

junction diode — A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes.

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K

kerf — The width of a cut used to create wafers from silicon ingots, often resulting in the loss of semiconductor material.

kilowatt (kW) — A standard unit of electrical power equal to 1000 watts, or to the energy consumption at a rate of 1000 joules per second.

kilowatt-hour (kWh) — 1,000 thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.

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L

langley (L) — Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2.

lattice — The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material.

lead-acid battery — A general category that includes batteries with plates made of pure lead, lead-antimony, or lead-calcium immersed in an acid electrolyte.

levelized cost of energy (LCOE) — The cost of energy of a solar system that is based on the system's installed price, its total lifetime cost, and its lifetime electricity production.

life — The period during which a system is capable of operating above a specified performance level.

life-cycle cost — The estimated cost of owning and operating a photovoltaic system for the period of its useful life.

light-induced defects — Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

light trapping — The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers.

line-commutated inverter — An inverter that is tied to a power grid or line. The commutation of power (conversion from direct current to alternating current) is controlled by the power line, so that, if there is a failure in the power grid, the photovoltaic system cannot feed power into the line.

liquid electrolyte battery — A battery containing a liquid solution of acid and water. Distilled water may be added to these batteries to replenish the electrolyte as necessary. Also called a flooded battery because the plates are covered with the electrolyte.

load — The demand on an energy producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amperes or watts in reference to electricity.

load circuit — The wire, switches, fuses, etc. that connect the load to the power source.

load current (A) — The current required by the electrical device.

load forecast — Predictions of future demand. For normal operations, daily and weekly forecasts of the hour-by-hour demand are used to help develop generation schedules to ensure that sufficient quantities and types of generation are available when needed.

load resistance — The resistance presented by the load. See also resistance.

locational marginal price (LMP) — The price of a unit of energy at a particular electrical location at a given time. LMPs are influenced by the nearby generation, load level, and transmission constraints and losses.

low voltage cutoff (LVC) — The voltage level at which a charge controller will disconnect the load from the battery.

low voltage disconnect — The voltage at which a charge controller will disconnect the load from the batteries to prevent over-discharging.

low voltage disconnect hysteresis — The voltage difference between the low voltage disconnect set point and the voltage at which the load will be reconnected.

low voltage warning — A warning buzzer or light that indicates the low battery voltage set point has been reached.

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M

maintenance-free battery — A sealed battery to which water cannot be added to maintain electrolyte level.

majority carrier — Current carriers (either free electrons or holes) that are in excess in a specific layer of a semiconductor material (electrons in the n-layer, holes in the p-layer) of a cell.

maximum power point (MPP) — The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 volts.

maximum power point tracker (MPPT) — Means of a power conditioning unit that automatically operates the photovoltaic generator at its maximum power point under all conditions.

maximum power tracking — Operating a photovoltaic array at the peak power point of the array's I-V curve where maximum power is obtained. Also called peak power tracking.

measurement and characterization — A field of research that involves assessing the characteristics of photovoltaic materials and devices.

megawatt (MW) — 1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

megawatt-hour — 1,000 kilowatt-hours or 1 million watt-hours.

metrology — The science of measurement.

microgroove — A small groove scribed into the surface of a solar cell, which is filled with metal for contacts.

micrometer (micron) — One millionth of a meter.

minority carrier — A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.

minority carrier lifetime — The average time a minority carrier exists before recombination.

modified sine wave — A waveform that has at least three states (i.e., positive, off, and negative). Has less harmonic content than a square wave.

modularity — The use of multiple inverters connected in parallel to service different loads.

module — See photovoltaic (PV) module.

module derate factor — A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.

monolithic — Fabricated as a single structure.

movistor — Short for metal oxide varistor. Used to protect electronic circuits from surge currents such as those produced by lightning.

multicrystalline — A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.

multijunction device — A high-efficiency photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum.

multi-stage controller — A charging controller unit that allows different charging currents as the battery nears full state_of_charge.

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N

nanometer — One billionth of a meter.

National Electrical Code (NEC) — Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, "Solar Photovoltaic Systems" which should be followed when installing a PV system.

National Electrical Manufacturers Association (NEMA) — This organization sets standards for some non-electronic products like junction boxes.

NEC — See National Electrical Code.

NEMA — See National Electrical Manufacturers Association.

nickel cadmium battery — A battery containing nickel and cadmium plates and an alkaline electrolyte.

nominal voltage — A reference voltage used to describe batteries, modules, or systems (i.e., a 12-volt or 24-volt battery, module, or system).

normal operating cell temperature (NOCT) — The estimated temperature of a photovoltaic module when operating under 800 w/m2 irradiance, 20°C ambient temperature and wind speed of 1 meter per second. NOCT is used to estimate the nominal operating temperature of a module in its working environment.

n-type — Negative semiconductor material in which there are more electrons than holes; current is carried through it by the flow of electrons.

n-type semiconductor — A semiconductor produced by doping an intrinsic semiconductor with an electron-donor impurity (e.g., phosphorus in silicon).

n-type silicon — Silicon material that has been doped with a material that has more electrons in its atomic structure than does silicon.

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O

ohm — A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.

one-axis tracking — A system capable of rotating about one axis.

open-circuit voltage (Voc) — The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.

operating point — The current and voltage that a photovoltaic module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

orientation — Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.

outgas — See gassing.

overcharge — Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.

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P

packing factor — The ratio of array area to actual land area or building envelope area for a system; or, the ratio of total solar cell area to the total module area, for a module.

panel — See photovoltaic (PV) panel.

parallel connection — A way of joining solar cells or photovoltaic modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage.

passivation — A chemical reaction that eliminates the detrimental effect of electrically reactive atoms on a solar cell's surface.

peak demand/load — The maximum energy demand or load in a specified time period.

peak power current — Amperes produced by a photovoltaic module or array operating at the voltage of the I-V curve that will produce maximum power from the module.

peak power point — Operating point of the I-V (current-voltage) curve for a solar cell or photovoltaic module where the product of the current value times the voltage value is a maximum.

peak power tracking — See maximum power tracking.

peak sun hours — The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 w/m2.

peak watt — A unit used to rate the performance of solar cells, modules, or arrays; the maximum nominal output of a photovoltaic device, in watts (Wp) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified.

phosphorous (P) — A chemical element used as a dopant in making n-type semiconductor layers.

photocurrent — An electric current induced by radiant energy.

photoelectric cell — A device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers.

photoelectrochemical cell — A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use.

photon — A particle of light that acts as an individual unit of energy.

photovoltaic(s) (PV) — Pertaining to the direct conversion of light into electricity.

photovoltaic (PV) array — An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.

photovoltaic (PV) cell — The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.

photovoltaic (PV) conversion efficiency — The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

photovoltaic (PV) device — A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device. Solar photovoltaic devices are made of various semiconductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multicrystalline, or amorphous forms.

photovoltaic (PV) effect — The phenomenon that occurs when photons, the "particles" in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available.

photovoltaic (PV) generator — The total of all PV strings of a PV power supply system, which are electrically interconnected.

photovoltaic (PV) module — The smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, (and protective devices such as diodes) intended to generate direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).

photovoltaic (PV) panel — often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current).

photovoltaic (PV) system — A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.

photovoltaic-thermal (PV/T) system — A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system or solar thermal system.

physical vapor deposition — A method of depositing thin semiconductor photovoltaic films. With this method, physical processes, such as thermal evaporation or bombardment of ions, are used to deposit elemental semiconductor material on a substrate.

P-I-N — A semiconductor photovoltaic (PV) device structure that layers an intrinsic semiconductor between a p-type semiconductor and an n-type semiconductor; this structure is most often used with amorphous silicon PV devices.

plates — A metal plate, usually lead or lead compound, immersed in the electrolyte in a battery.

plug-and-play PV system — A commercial, off-the-shelf photovoltaic system that is fully inclusive with little need for individual customization. The system can be installed without special training and using few tools. The homeowner plugs the system into a PV-ready circuit and an automatic PV discovery process initiates communication between the system and the utility. The system and grid are automatically configured for optimal operation.

P/N — A semiconductor photovoltaic device structure in which the junction is formed between a p-type layer and an n-type layer.

pocket plate — A plate for a battery in which active materials are held in a perforated metal pocket.

point-contact cell — A high efficiency silicon photovoltaic concentrator cell that employs light trapping techniques and point-diffused contacts on the rear surface for current collection.

polycrystalline — See multicrystalline.

polycrystalline silicon — A material used to make photovoltaic cells, which consist of many crystals unlike single-crystal silicon.

polycrystalline thin film — A thin film made of multicrystalline material.

power — The amount of electrical energy available for doing work, measured in horsepower, Watts, or Btu per hour.

power conditioning — The process of modifying the characteristics of electrical power (for e.g., inverting direct current to alternating current).

power conditioning equipment — Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use. A collective term for inverter, converter, battery charge regulator, and blocking diode.

power conversion efficiency — The ratio of output power to input power of the inverter.

power density — The ratio of the power available from a battery to its mass (W/kg) or volume (W/l).

power factor (PF) — The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.

primary battery — A battery whose initial capacity cannot be restored by charging.

projected area — The net south-facing glazing area projected on a vertical plane.

p-type semiconductor — A semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity (e.g., boron in silicon).

pulse-width-modulated (PWM) wave inverter — A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.

PV — See photovoltaic(s).

pyranometer — An instrument used for measuring global solar irradiance.

pyrheliometer — An instrument used for measuring direct beam solar irradiance. Uses an aperture of 5.7° to transcribe the solar disc.

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Q

quad — One quadrillion Btu (1,000,000,000,000,000 Btu).

qualification test — A procedure applied to a selected set of photovoltaic modules involving the application of defined electrical, mechanical, or thermal stress in a prescribed manner and amount. Test results are subject to a list of defined requirements.

quantum efficiency (QE) — The ratio of the number of charge carriers collected by a photovoltaic cell to the number of photons of a given energy shining on the cell. Quantum efficiency relates to the response of a solar cell to the different wavelengths in the spectrum of light shining on the cell. QE is given as a function of either wavelength or energy. Optimally, a solar cell should generate considerable electrical current for wavelengths that are most abundant in sunlight.

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R

ramp — A change in generation output.

ramp rate — The ability of a generating unit to change its output over some unit of time, often measured in MW/min.

Rankine cycle — A thermodynamic cycle used in steam turbines to convert heat energy into work. Concentrating solar power plants often rely on the Rankine cycle. In CSP systems, mirrors focus sunlight on a heat-transfer fluid. This is used to creates steam, which spins a turbine to generate electricity.

rated battery capacity — The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature. See also battery capacity.

rated module current (A) — The current output of a photovoltaic module measured at standard test conditions of 1,000 w/m2 and 25°C cell temperature.

rated power — Rated power of the inverter. However, some units can not produce rated power continuously. See also duty rating.

reactive power — The sine of the phase angle between the current and voltage waveforms in an alternating current system. See also power factor.

recombination — The action of a free electron falling back into a hole. Recombination processes are either radiative, where the energy of recombination results in the emission of a photon, or nonradiative, where the energy of recombination is given to a second electron which then relaxes back to its original energy by emitting phonons. Recombination can take place in the bulk of the semiconductor, at the surfaces, in the junction region, at defects, or between interfaces.

rectifier — A device that converts alternating current to direct current. See also inverter.

regulator — Prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.

remote systems — See stand-alone systems.

reserve capacity — The amount of generating capacity a central power system must maintain to meet peak loads.

resistance (R) — The property of a conductor, which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.

resistive voltage drop — The voltage developed across a cell by the current flow through the resistance of the cell.

reverse current protection — Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See also blocking diode.

ribbon (photovoltaic) cells — A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.

RMS — See root mean square.

root mean square (RMS) — The square root of the average square of the instantaneous values of an ac output. For a sine wave the RMS value is 0.707 times the peak value. The equivalent value of alternating current, I, that will produce the same heating in a conductor with resistance, R, as a dc current of value I.

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S

sacrificial anode — A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.

satellite power system (SPS) — Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous Earth orbit. A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.

scheduling — The general practice of ensuring that a generator is committed and available when needed. It also can refer to scheduling of imports or exports of energy into or out of a balancing area.

Schottky barrier — A cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal.

scribing — The cutting of a grid pattern of grooves in a semiconductor material, generally for the purpose of making interconnections.

sealed battery — A battery with a captive electrolyte and a resealing vent cap, also called a valve-regulated battery. Electrolyte cannot be added.

seasonal depth of discharge — An adjustment factor used in some system sizing procedures which "allows" the battery to be gradually discharged over a 30-90 day period of poor solar insolation. This factor results in a slightly smaller photovoltaic array.

secondary battery — A battery that can be recharged.

self-discharge — The rate at which a battery, without a load, will lose its charge.

semiconductor — Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

semicrystalline — See multicrystalline.

series connection — A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.

series controller — A charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery.

series regulator — Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array.

series resistance — Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.

shallow-cycle battery — A battery with small plates that cannot withstand many discharges to a low state-of-charge.

shelf life of batteries — The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

short-circuit current (Isc) — The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.

shunt controller — A charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less.

shunt regulator — Type of a battery charge regulator where the charging current is controlled by a switch connected in parallel with the photovoltaic (PV) generator. Shorting the PV generator prevents overcharging of the battery.

Siemens process — A commercial method of making purified silicon.

silicon (Si) — A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It's commonly found in sand and quartz (as the oxide).

sine wave — A waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle.

sine wave inverter — An inverter that produces utility-quality, sine wave power forms.

single-crystal material — A material that is composed of a single crystal or a few large crystals.

single-crystal silicon — Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon.

single-stage controller — A charge controller that redirects all charging current as the battery nears full state-of-charge.

smart grid — An intelligent electric power system that regulates the two-way flow of electricity and information between power plants and consumers to control grid activity.

soft costs — Non-hardware costs related to PV systems, such as financing, permitting, installation, interconnection, and inspection.

solar cell — See photovoltaic (PV) cell.

solar constant — The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 watts per square meter or 492 Btu per square foot.

solar cooling — The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners.

solar energy — Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

solar-grade silicon — Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.

solar insolation — See insolation.

solar irradiance — See irradiance.

solar noon — The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky.

solar panel — See photovoltaic (PV) panel.

solar resource — The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.

solar spectrum — The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation.

solar thermal electric systems — Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.

space charge — See cell barrier.

specific gravity — The ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. Used as an indicator of battery state-of-charge.

spinning reserve — Electric power plant or utility capacity on-line and running at low power in excess of actual load.

split-spectrum cell — A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See also mulitjunction device.

sputtering — A process used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate.

square wave — A waveform that has only two states, (i.e., positive or negative). A square wave contains a large number of harmonics.

square wave inverter — A type of inverter that produces square wave output. It consists of a direct current source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency.

Staebler-Wronski effect — The tendency of the sunlight to electricity conversion efficiency of amorphous silicon photovoltaic devices to degrade (drop) upon initial exposure to light.

stand-alone system — An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage.

standard reporting conditions (SRC) — A fixed set of conditions (including meteorological) to which the electrical performance data of a photovoltaic module are translated from the set of actual test conditions.

standard test conditions (STC) — Conditions under which a module is typically tested in a laboratory.

standby current — This is the amount of current (power) used by the inverter when no load is active (lost power). The efficiency of the inverter is lowest when the load demand is low.

stand-off mounting — Technique for mounting a photovoltaic array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

starved electrolyte cell — A battery containing little or no free fluid electrolyte.

state-of-charge (SOC) — The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

storage battery — A device capable of transforming energy from electric to chemical form and vice versa. The reactions are almost completely reversible. During discharge, chemical energy is converted to electric energy and is consumed in an external circuit or apparatus.

stratification — A condition that occurs when the acid concentration varies from top to bottom in the battery electrolyte. Periodic, controlled charging at voltages that produce gassing will mix the electrolyte. See also equalization.

string — A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.

sub-hourly energy markets — Electricity markets that operate on time steps of 5 minutes. Approximately 60% of all electricity in the United States is currently traded in sub-hourly markets, running at 5-minute intervals so that maximum flexibility can be obtained from the generation fleet.

substrate — The physical material upon which a photovoltaic cell is applied.

subsystem — Any one of several components in a photovoltaic system (i.e., array, controller, batteries, inverter, load).

sulfation — A condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.

superconducting magnetic energy storage (SMES) — SMES technology uses the superconducting characteristics of low-temperature materials to produce intense magnetic fields to store energy. It has been proposed as a storage option to support large-scale use of photovoltaics as a means to smooth out fluctuations in power generation.

superconductivity — The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.

superstrate — The covering on the sunny side of a photovoltaic (PV) module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum.

surge capacity — The maximum power, usually 3-5 times the rated power, that can be provided over a short time.

system availability — The percentage of time (usually expressed in hours per year) when a photovoltaic system will be able to fully meet the load demand.

system operating voltage — The photovoltaic array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.

system storage — See battery capacity.

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T

tare loss — Loss caused by a charge controller. One minus tare loss, expressed as a percentage, is equal to the controller efficiency.

temperature compensation — A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than ±5°C from ambient temperature.

temperature factors — It is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures.

thermophotovoltaic cell (TPV) — A device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.

thick-crystalline materials — Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.

thin film — A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make photovoltaic cells.

thin film photovoltaic module — A photovoltaic module constructed with sequential layers of thin film semiconductor materials. See also amorphous silicon.

tilt angle — The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

tin oxide — A wide band-gap semiconductor similar to indium oxide; used in heterojunction solar cells or to make a transparent conductive film, called NESA glass when deposited on glass.

total AC load demand — The sum of the alternating current loads. This value is important when selecting an inverter.

total harmonic distortion — The measure of closeness in shape between a waveform and it's fundamental component.

total internal reflection — The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.

tracking array — A photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.

transformer — An electromagnetic device that changes the voltage of alternating current electricity.

transparent conducting oxide (TCO) — A doped metal oxide used to coat and improve the performance of optoelectronic devices such as photovoltaics and flat panel displays. Most TCO films are fabricated with polycrystalline or amorphous microstructures and are deposited on glass. The current industry-standard TCO is indium tin oxide. Indium is relatively rare and expensive, so research is ongoing to develop improved TCOs based on alternative materials.

tray cable (TC) - may be used for interconnecting balance-of-systems.

trickle charge — A charge at a low rate, balancing through self-discharge losses, to maintain a

tunneling — Quantum mechanical concept whereby an electron is found on the opposite side of an insulating barrier without having passed through or around the barrier.

cell or battery in a fully charged condition.

two-axis tracking — A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).

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U

ultraviolet — Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

underground feeder (UF) — May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures.

underground service entrance (USE) — May be used within battery enclosures and for interconnecting balance-of-systems.

uninterruptible power supply (UPS) — The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

utility-interactive inverter — An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system's output is fully synchronized with the utility power.

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V

vacuum evaporation - The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

vacuum zero — The energy of an electron at rest in empty space; used as a reference level in energy band diagrams.

valence band — The highest energy band in a semiconductor that can be filled with electrons.

valence level energy/valence state — Energy content of an electron in orbit about an atomic nucleus. Also called bound state.

varistor — A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

vented cell — A battery designed with a vent mechanism to expel gases generated during charging.

vertical multijunction (VMJ) cell — A compound cell made of different semiconductor materials in layers, one above the other. Sunlight entering the top passes through successive cell barriers, each of which converts a separate portion of the spectrum into electricity, thus achieving greater total conversion efficiency of the incident light. Also called a multiple junction cell. See also multijunction device and split-spectrum cell.

volt (V) — A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

voltage — The amount of electromotive force, measured in volts, that exists between two points.

voltage at maximum power (Vmp) — The voltage at which maximum power is available from a photovoltaic module.

voltage protection — Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

voltage regulation — This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.

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W

wafer — A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.

watt — The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

waveform — The shape of the phase power at a certain frequency and amplitude.

wet shelf life — The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

window — A wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.

wire types — See Article 300 of National Electric Code for more information.

work function — The energy difference between the Fermi level and vacuum zero. The minimum amount of energy it takes to remove an electron from a substance into the vacuum.

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Z

zenith angle — the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).

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Solar Energy 101 Solar energy terms & technology

Cell
A solar cell is the basic component of solar modules. Cells are manufactured from wafers. Learn how a solar cell works

CO2
An abbreviation for carbon dioxide. CO2 is well-known as a “green-house gas.” When people talk about “Carbon Footprint,” they are usually referring to CO2 emissions.

Conversion efficiency
This measure gauges the percentage of solar (light) power reaching a module that is converted into electrical power. Conventional cells now range in the high percentage teens. Theoretical and laboratory conversion rates typically are much higher than rates from mass production.

Crystal
A crystal is a solid with molecular building blocks, such as atoms or ions, that have arranged themselves in an identically repeating pattern along all three spatial dimensions.

Energy Payback
The amount of time required for a solar panel to generate the amount of energy it took to manufacture it. Modern PV panels have an energy payback of 1 to 3 years depending on where they’re installed; over a 30+ year life, a PV system will return 10 to 30 times the energy that went into making them.

Grid Connected
A grid-connected solar electric system generates its own electricity and feeds its excess power into the utility grid for later use. Grid-connected solar electric systems are eligible for many incentives and rebates.

Inverter
The inverter electronically converts Direct Current (DC) power into Alternating Current (AC) power and is required for grid-connected solar power systems.

    Micro inverters can be installed on each solar module. The cost is typically higher, but they provide certain advantages for very small or geometrically complex PV systems.
    String inverters are the most common type of inverter for residential and small commercial PV systems.
    Central inverters are used for very large PV systems.

Insolation
Sunlight varies by region. Insolation is a measure of the solar radiation energy in a given region. It is usually expressed as the intensity of light energy per unit of land area. As a whole, the U.S., especially the sunny Southwest and Southeast, offers strong insolation. By comparison, Germany, the PV industry’s leader, gets little better sun than Alaska.

kWh
The abbreviation for kilowatt-hours, a unit of energy. One kWh represents a 1,000 watt load operated over a period of one hour—that's the amount of energy an average plasma TV uses. Electricity rates are most commonly expressed in cents per kilowatt hour.

Micron
In cell production, manufacturing steps occur in dimensions measured in microns. In the metric system, a micron is one millionth of a meter, or one thousandth of a millimeter. For reference, a human hair measures about 100 microns across.

Million Solar Roofs Initiative
This is a state of California initiative to facilitate the installation of solar energy systems on one million US buildings by 2010. This initiative is largely responsible for the rebates and incentives available in California and serves as a model for many other state solar programs.

Module
Commonly called a “solar panel,” a PV module is composed of multiple solar cells that are electrically connected to increase the total power output and are encapsulated in tempered glass for weather protection and ease of handling.

Monocrystalline
Silicon is a metal and, therefore, its atoms are organized into a crystalline structure. Monocrystalline refers to silicon that is pulled as a single, completely homogenous crystal, as SolarWorld does in the USA. Monocrystalline PV cells are generally the most efficient and reliable available.

MW
The abbreviation for megawatt, a unit of power or capacity of a generator. 1 MW = 1,000 kW = 1,000,000 W.

Net-Metering
Net-metering allows the solar electric system to send excess electricity back through the electric meter to the utility. The meter actually runs backwards! Your utility will bill you for the net (kWh used – kWh generated). While most utilities will not pay you cash if you generate more than you use, many will carry forward a credit toward your next bill.

Photon
A photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. Solar radiation arrives on the surface of photovoltaic cells in the form of photons, providing the main energy that activates cells to produce electricity.

Photovoltaics (PV)
Photo = light; Volt = electrical potential — Literally: electricity from light. Einstein won the Nobel Prize for discovering that when light shines on certain materials (like silicon), an electrical current is generated. Learn how a PV module (solar panel) works.

Polycrystalline
Officially called “multicrystalline,” this refers to cast silicon where several crystals form in a mold. SolarWorld’s German factories use this casting process as it is less expensive than pulling single crystals, but the resulting cells generally have lower efficiency due to internal resistance along the grain boundaries. Polycrystalline cells can be recognized by a visible grain, a “metal flake effect.”

Silicon
The basic material used to make solar cells. It is the second most abundant element in the earth’s crust, after oxygen.

Silicon carbide
Silicon carbide (SiC), silicon bonded with carbon, is another member of the silicon family of materials used in the PV industry. It is a common abrasive in many industries, used in everything from grinding to sandblasting. At SolarWorld, crushed SiC is mixed into a water slurry and applied to saw wires. Silicon carbide, not wire, actually does the cutting.

Stand-alone system
Energy generated by these systems is stored in batteries and then subsequently used. Also known as “off-grid,” these systems are not connected to the utility grid.

Wafer
A sawn silicon disc, used as the starting point for manufacturing a solar cell.
 







 Glossary of Solar Energy Terms

Alternating Current (AC) - The flow of electricity that constantly changes direction between positive and negative sides. Almost all power produced by electric utilities in the United States moves in current that shifts direction at a rate of 60 times per second.

Ambient Temperature - The temperature of the surrounding area.

Amorphous Silicon - A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.

Ampere (Amp) - The unit of measure that indicates how much electricity flows through a conductor. It is like using cubic feet per second to measure the flow of water. For example, a 1,200-watt, 120-volt hair dryer pulls 10 amperes of electric current (amps = watts/volts).

Ampere-Hour (Ah/AH) - A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

Annual Solar Savings - The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.

Average Demand - The energy demand for a given location over a period of time. For example, the number of kilowatt-hours used in a 24-hour period, divided by 24 hours, tells the average demand for that location in that time.

Avoided Cost - The amount of money an electric utility would need to spend for the next increment of electric generation to produce or purchase.

Azimuth Angle - The angle between true south and the point on the horizon directly below the sun.

Battery - Batteries are often sold with a solar electric system. The primary purpose is to store the electricity not immediately used, which could be used at some later time.

BIPV (Building-Integrated Photovoltaics) - A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.

Btu (British Thermal Unit) - The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

Capacity Factor - The ratio of the average load on (or power output of) an electricity generating unit or system to the capacity rating of the unit or system over a specified period of time.

Circuit - One or more conductors through which electricity flows.

Concentrator - A photovoltaic module, which includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.

Converter - An electrical apparatus that changes the quantity or quality of electrical energy.

Crystalline Silicon - A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.

Customer Load - The amount of power your site uses. Load may be expressed in kilowatts (capacity) or kilowatt-hours (energy). A site's peak kilowatts generally refers to when electric demand requirements are highest.

Demand - The level at which electricity is delivered to end-users at a given point in time. Electric demand in measured in kilowatts.

Direct Current (DC)- The flow of electricity that flows continuously in one direction. Frequency - The number of cycles through which an alternating current moves in each second. Standard electric utility frequency in the United States is 60 cycles per second, or 60 Hertz (Hz).

Electrical Grid - The electricity transmission and distribution system that links power plants to customers through high-power transmission line service.

Energy - The capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.

Energy Audit - A survey that shows how much energy used in a home, which helps find ways to use less energy.

Fixed Tilt Array - A photovoltaic array set in at a fixed angle with respect to horizontal.

Grid-Connected System - A solar electric or photovoltaic (PV) system in which the PV array acts like a central generating plant, supplying power to the grid.

Hertz - The unit of electromagnetic frequency that is equal to one cycle per second.

Interconnection - The linkage of transmission lines between two utilities, or between a utility and an end-user, enabling power to be moved in either direction.

Insolation - The solar power density incident on a surface of stated area and orientation. It is commonly expressed as average irradiance in watts per square meter (W/m2) or kilowatt-hours per square meter per day (kW·h/(m2·day)) (or hours/day). In the case of photovoltaics it is commonly measured as kWh/(kWp·y) (kilowatt hours per year per kilowatt peak rating)

Inverter - A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.

Irradiance - The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.

Kilowatt (kW) - 1,000 watts. A unit of measure of the amount of electricity needed to operate given equipment. For example, a one kW system is enough power to illuminate 10 light bulbs at 100 watts each. (volts x amps = watts)

Kilowatt-hour (kWh) - The amount of kW produced over a period of time, or one kilowatt of electricity supplied for one hour. For example, a one kW system, if operating at full capacity for 5 hours will produce (or use) 5 kWh of electricity.

Maximum Power Point (MPP) - The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 volts.

Megawatt - One thousand kilowatts or one million watts. One megawatt is enough to power 1,000 average California homes. Meter - A device that measures levels and volumes of customer's electricity use.

Mounting Equipment - Equipment/apparatus used to fasten solar (PV) modules to the roof. Peak Load - The highest electrical demand within a particular period of time.

Multicrystalline - A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.

National Electrical Code (NEC) - Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, "Solar Photovoltaic Systems" which should be followed when installing a PV system.

Net Metering - In the fall of 1998, New York State Governor George Pataki signed the Solar Choice Act, which requires all the investor owned electric utilities in NYS to allow residential customers to interconnect PV (solar electric) systems to the utility's distribution systems. These grid-connected PV systems allow residential customers to run their electric meters backwards, offsetting their normal utility bill. Other than the renewable energy system, no special equipment is needed.

One-Axis Tracking - A system capable of rotating about one axis.

Orientation - Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.

Peak Sun Hours - The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 w/m2.

Photovoltaic Cell or Module or Panel (PV) - A device that produces an electric reaction to light, thereby producing electricity.

Photovoltaic (PV) Array - An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.

Photovoltaic (PV) Conversion Efficiency - The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

Polycrystalline Silicon - A material used to make photovoltaic cells, which consist of many crystals unlike single-crystal silicon.

Solar Energy - Heat and light radiated from the sun.

Solar Panel - Devices that collect energy from the sun (solar energy). This is usually solar photovoltaic (PV) modules that use solar cells to convert light from the sun into electricity, or solar thermal (heat) collectors that use the sun's energy to heat water or another fluid such as oil or antifreeze.

Solar Resource - The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.

Solar Thermal - The process of concentrating sunlight to create high temperatures that are needed to heat fluids, like water (solar hot water) or to vaporize fluid to drive a turbine for electric power generation.

Stand-Alone System - An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage.

Storage - Storage refers to saving surplus electricity produced by a photovoltaic (PV) system. Generally, batteries are used as storage devices.

String - A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.

Tracking Equipment - Structure that houses PV modules and that can automatically follow the sun across the sky throughout the day to maximize output.

Utility Grid - The interconnection of electricity generation plants through the transmission and distribution lines to customers. The grid also refers to the interconnection of utilities through the electric transmission and distribution systems.

Volt (V) - The amount of force required to drive a steady current of one ampere through a resistance of one ohm. Electrical systems of most homes and offices use 120 volts. (volts - watts/amps) (volts = amperes x resistance)

Watt (W) - Electric measurement of power at one point in time, as capacity or demand. For example, light bulbs are classified by wattage. (1,000 watts = 1 kilowatt). 







 Solar Energy Systems Terms and Definitions 







 Solar Energy Systems Terms and Definitions 







 AGM - Absorbed Glass Mat, a sealed battery that uses saturated absorbed glass mats rather than a gelled or liquid electrolyte between the plates. AGM batteries are more expensive than gell cell batteries or flooded types, but they have superior performance.
Alternating Current - Electric current in which the current changes direction (polarity) 120 times per second (in the U.S.) and is commonly referred to as 60 Hertz (cycles per second) AC. Many other countries use 50 Hertz as a standard.
Alternative Energy - A popular term for "non-conventional" energy systems usually on a smaller scale and includes solar electric systems, wind generator systems, and small hydro-electric systems.
Alternator - A generator producing AC (alternating current) rather than DC (direct current). Alternators are more efficient than DC generators and in automobiles the alternator output is converted to 12 volts DC using rectifier diodes built into the alternator.
Amorphous Semiconductor - A non-crystalline semiconductor material for solar cells also called thin film. Easier and cheaper to make than crystalline semiconductors for solar cells, but also less efficient.
Ampere - 1 Ampere (Amp) is the amount of current that flows in a circuit at an electromotive force (voltage) of one Volt and at a resistance of one Ohm.
Amp Hour - A measure current over a period of time. 1 amp being used or generated for one hour equals 1 AmpHour.
Angle of Incidence - For solar energy systems, the angle at which direct sunlight strikes the surface of the solar panel relative to perpendicular. Sunlight with an incident angle of 90 degrees is the most efficient.
Angle of Inclination - For solar energy systems, the angle that a solar array is positioned above horizontal. (90 degrees would be vertical). A general rule of thumb is to set the angle of a solar array to the latitude +/- 15 degrees.
Anode - The positive electrode or connection of an electrolytic cell or semiconductor device.
Anti-reflection Coating - A thin coating of a material applied to a photovoltaic cell that reduces the reflection of light striking its surface.
AEER - (Appliance Energy Efficiency Ratings) Operating efficiency of appliances as set by the U.S. Department of Energy guidelines.
Array (Solar) - Any number of solar photovoltaic modules or panels connected together to provide a single electrical output.
Asynchronous Generator - An electric generator that produces alternating current that matches an existing power source.
Autonomous System - A stand-alone power system that has no Utility connection. Most systems are designed for 3 to 5 days of power from batteries without recharging from a solar source or standby generator.
AWG - The abbreviation for American Wire Gauge, a standard system for designating the size of electrical wires.
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Backup Energy System - A back-up electric power system using batteries and an inverter, an AC generator, or both.
Battery - A device containing electrolytic cells to store electrical energy in chemical form.
Blocking Diode - A semiconductor device used to stop the flow of current from one direction, but allow it in the opposite direction.
British Thermal Unit (BTU) - The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.
Bus (electrical) - An electrical conductor that serves as a common connection point for multiple connections.
Bypass Diode - A diode connected across a solar cell in a photovoltaic module such that the diode will conduct if the cell becomes reverse biased due to shading or the failure of other cells.
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Cathode - The negative electrode or connection of an electrolytic cell or semiconductor device.
Cell (battery) - The basic unit of an electrochemical battery. A lead acid cell produces about 2.12 volts and a 12 volt battery uses 6 of these cells and fully charged measures about 12.72 volts.
Cell (solar) - The basic unit of a photovoltaic solar panel. A 12 volt solar panel typically has 36 individual cells, a 24 volt solar panel uses 72 cells.
Charge Controller - An electronic device that regulates the voltage from the solar panel array to ensure maximum transfer of energy and prevent overcharging the battery bank.
Circuit - An arrangement of individual electronic components or devices that use the flow of electrical current thru them to perform useful work or functions.
Circuit Breaker - A safety device used to stop the flow of electricity in an electric device or circuit to prevent damage or fire when an overload condition occurs.
Combiner Box - A solar array junction box where multiple solar modules are electrically connected together and fusing devices may be located.
Conductor - Any material through which electricity can flow. Gold, silver, copper, and aluminum (in that order) make excellent conductors.
Conventional Power - Power generation from sources such as hydro, petroleum, natural gas, coal, or nuclear power plants.
Conversion Efficiency (solar panel) - The ratio of the energy produced by a photovoltaic device to the energy received or consumed expressed as a percentage. Solar panels are typically 9% to 14% efficient.
Converter (DC) - Typically, an electronic device for changing 120 volts AC to lower voltage DC.
Crystalline Silicon Photovoltaic Cell - A type of photovoltaic cell made from a single crystal or a polycrystalline slice of silicon. Individual cells are then joined together to form a solar module.
Current (Electrical) - The flow of electrons in an electrical circuit, measured in amperes (amps).
Cycle (AC) - In alternating current (AC), the current goes from zero to maximum in one direction then zero to maximum in the other direction then repeats. In the United States a complete positive/negative cycle occurs 60 times each second and is known as 60 cycle AC.
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Deep Discharge - Discharging a battery down to 20 percent or less of its full charge condition.
Diffuse Solar Radiation - Unfocused sunlight scattered by atmospheric particles and arriving at the earth's surface from all directions.
Dimmer (switch/control) - An electronic device that allows light levels to be adjusted from dim to full brightness.
Diode - A semiconductor device that allows current to flow in one direction only.
Direct Current (DC) - Electric current in which the flow of electrons is in one direction only. Opposite of Alternating Current (AC).
Discharge Rate - The rate that energy is removed from a battery, usually expressed in AmpHours.
DOD (Depth of Discharge) - The percentage that a battery is discharged from a fully charged condition.
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Earth Sheltered Home - A home with large earthen berms around exterior walls to provide wind protection and insulation.
Electrical Grid - An large integrated system of electricity distribution from centralized locations to individual homes and businesses.
Electric Circuit - See 'circuit'.
Electric Current - See 'current'.
Electrolyte - A liquid or gel type conductor of electricity that carries current by the movement of ions (instead of electrons) between the plates in batteries.
Electromagnetic Energy - Energy generated from an electromagnetic field produced by a magnet or an electric current flowing through a conductor.
Electromagnetic Field (EMF) - The electrical and magnetic fields produced by any device operating with electricity.
Electron - An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton. Electrons surround the positively charged protons in the nucleus of an atom and the number of electrons determine the atomic element. Elements that loose electrons easily make good conductors and elements whose electrons are tightly bound together make good insulators. It is this movement of electrons which is the electric current in circuits.
Electronic Ballast - A high voltage transformer that excites the ions in fluorescent lamps.
Emissions - Substances or by products (usually polluting) resulting from energy production.
Energy Efficiency Ratio (EER) - A measure of the energy efficiency of room air conditioners based on the U.S. Department of Energy guidelines.
Equinox - The two times of the year when the sun crosses the equator and night and day are of equal length, occurring on March 21st (spring equinox) and September 23 (fall equinox) in the northern hemisphere.
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Filament (tungsten) - A coil of tungsten wire suspended in a vacuum or inert gas-filled bulb. When electricity flows thru the tungsten "filament" it radiates energy in the form of heat and light. This is the basic operation of inefficient incandescent lamps.
Float Charge - The float charging voltage is the voltage required to maintain a battery in a fully charged condition after it has been charged.
Fluorescent Light - An electric lamp using a phosphor coated glass tube that glows when ions in the tube are excited with high voltage electricity. Much more efficient than incandescent type lamps.
Foot Candle - A unit of illuminance equal to one lumen per square foot.
Foot Pound - The amount of work done by lifting one pound one foot.
Fossil Fuels - Fuels such as oil, natural gas, and coal are fossil fuels believed to have been formed in the ground from the decayed remains of plants and animals over millions of years.
Frequency - The number of cycles through which an alternating current changes direction twice each second. In the U.S. the standard frequency for electricity is 60 cycles per second (60 Hertz).
Fuel Cell - An electrochemical device that converts chemical energy directly into electricity.
Full Sun - The amount of energy in sunlight striking the earth's surface at noon on a clear day (about 1,000 Watts per Square Meter).
Fuse - A safety device which stops the flow of electricity to prevent damage and fire under overload conditions. See 'ciruit breaker'
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Gel Battery - A lead-acid battery in which the electrolyte between the plates is composed of a gelled substance instead of water which is used in automotive batteries.
Generator (AC) - An electric generator driven by a internal cumbustion engine to produce electricity for stand-alone systems and for back-up electrical power. The fuel used may be gasoline, diesel, or propane (LPG).
Gigawatt (GW) - 1,000,000,000 watts, 1 million kilowatts, or 1,000 megawatts.
Green Power - A popular term for energy produced from clean, renewable energy resources such as wind, solar, or hydro electric systems.
Grid - A common term referring to an electricity transmission and distribution system run by large utility companies.
Grid Tie System - An independent electrical power system that is connected to the utility grid so that power can be supplied by the grid when needed and fed back into the grid during excess power production.
Ground - The electrical potential where voltage is zero or at a minimum. Connecting one side of the electric system and metallic surfaces of electric systems to ground potential prevents shocks by bleeding the voltage down to a safe level.
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Hertz - Short hand for 'cycles per second'. It is the frequency of an AC electric system and is 60 hertz in the U.S.A.
Hybrid System - A renewable or alternative energy system that uses two different sources for power, such as wind generators and solar photovoltaic arrays together to produce electricity.
Hydroelectric Power Plant - A power plant that produces electricity by using water pressure to turn large turbines.
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Incandescent (lamp) - A fairly inefficient type of lamp that uses a wire filament suspended in a vacuum or inert gas-filled bulb that give off heat and light when electricity flows thru the filament.
Infrared Radiation - Electromagnetic radiation below the visible range. Also known as heat radiation.
Inverter (AC) - An electronic device that converts 12, 24, 48, or higher volts DC power from batteries or solar panels to 120/240 volts AC to operate normal appliances.
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Kilowatt (KW) - A standard unit of electrical power equal to 1000 watts.
Kilowatt-Hour (KWH) - 1000 watts produced or consumed for a period of 1 hour.
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Lead Acid Battery - An electrochemical battery that uses lead and lead oxide for electrodes and sulfuric acid for the electrolyte. When the sulfuric acid is mixed with water, it is known as a flooded battery. Batteries whose electrolyte is a gel or absorbed glass mat type are called sealed batteries. Not to be confused with sealed batteries, a maintenance-free battery simply has no way to add water and when low is just discarded.
Line Loss - Energy lost due to inherent inefficiencies in an electrical transmission and distribution system.
Load - The devices and appliances that draw power from an electrical supply system.
Long-Wave Radiation - Infrared or radiant heat energy.
Luminance - The measure of the apparent brightness of an object, measured in lumens.
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Maximum Power Point (MPP) - The point on the current-voltage (I-V) curve of a solar panel, where the product of current times voltage equals maximum wattage.
Maximum Power Point Tracking (MPPT) - A charge controller technique that attempts to supply maximum power to the batteries by tracking the maximum power point (MPP) at all times achieving a 15% to 35% increase over other types of charging techniques.
Megawatt - 1 million watts or 1 thousand kilowatts (1,000,000 watts).
Megawatt-Hour - 1 million watts produced or used for a period of 1 hour.
Module (solar) - A number of individual solar cells connected together in an environmentally protected housing producing a standard output voltage and power. Multiple modules/panels can be assembled into an array for increased power and/or voltage.
Monolithic - Manufactured or assembled as a single structure.
Monocrystalline - A material used in solar cells that uses a complicated crystal growth process. Long silicon rods are produced which are cut into slices of .2 to .4 mm thick discs or wafers which are then processed into individual cells.
Motor Speed Control - An electronic device that manually or automatically varies/controls motor speed as in a multi-speed fan.
Multicrystalline - A material composed of randomly oriented, small individual crystals. (Sometimes referred to as polycrystalline or semicrystalline).
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Name Plate - An identifying tag usually located near the AC cord of an appliance that contains information such as model number, serial number, operating voltage, and power consumption.
National Electrical Code (NEC) - The NEC is a set of regulations and standards that most electrical equipment installations must follow making the electrical system in the United States one of the safest in the world.
Net Metering - Using a single meter to measure usage and generation of electricity by customers with a wind or solar power energy system. The net energy used or produced is either purchased from or sold to the power company.
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Off-Peak (demand)- The times during a 24 hour period of low electricity demand. Opposite of peak-demand.
Ohm - The unit of resistance to the flow of electrons in an electric circuit.
Ohm's Law - The various formulas that define the relationship between resistance(R), voltage(E), and current(I) as in E=I*R, I=E/R, R=E/I, etc.
One-Axis Tracking - A solar tracking system that moves in only 1 direction or axis generally following the sun in its arc across the sky from east to west.
Open-Circuit Voltage (Voc)- The maximum possible voltage across a photovoltaic cell or module in sunlight when no current is flowing.
Ovonic - A device (solar cell) that converts sunlight directly into electricity, invented by Standford Ovshinsky.
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Panel (Solar) - A solar photovoltaic device composed of groups of individual solar cells connected in series, in parallel, or in series-parallel combinations to produce a standard output. See 'module'.
Parallel - A wiring technique where multiple devices are wired together to increase current but voltage remains the same.
Payback Period - The amount of time required before the savings resulting from an energy producing system equals the cost of the system, typically 5 to 10 years. This number has been steadily decreasing as alternative energy systems become more popular and more practical.
Peak Sun Hours - The equivalent number of hours per day when solar radiation averages 1 KiloWatt per square meter. For example, 5 peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the solar radiation for 5 hours been 1 KiloWatt per square meter.
Peak Watt - The maximum nominal output of a PV (solar) device, in Watts (Wp) under standardized test conditions. (usually the most favorable!).
Phantom Load - The small power used by an appliance even when it is turned off. Examples of phantom loads include appliances with electronic clocks or timers and devices with remote controls whose circuits remain active in order to detect the remote control signal to power up.
Photon - A particle of light that acts as an individual unit of energy.
Photovoltaic Array - A group of solar photovoltaic modules connected together to increase voltage and/or power to the level required for a given system.
Photovoltaic Cell - Specially processed semiconductor materials such as silicon, cadmium sulfide, cadmium telluride, and gallium arsenide that convert sunlight directly into electricity. Three common types are mono-crystalline, multi-crystalline, and amorphous/thin film.
Photovoltaic Conversion Efficiency - The ratio of the energy produced by a photovoltaic device to the energy received or consumed expressed as a percentage. Solar panels are typically 9% to 14% efficient.
Photovoltaic Module or Panel - See 'module' or 'solar module'.
Poly-crystalline - A semiconductor (photovoltaic) material composed of randomly oriented, small, individual crystals.
Power - Useful energy that performs work measured in horsepower, Watts, or Btu's. Electrical power is rated in Watts and is equal to the Voltage(E) times the Current(I). As in P=E*I.
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Radiation - The transfer of heat energy through matter or space by means of electromagnetic waves.
Reflective Glass - A window glass that has been coated with a reflective film to reduce unwanted solar heat gain during the summer.
Renewable Energy - Energy from sources that are not easily depleted such as moving water (hydro, tidal and wave power), biomass, geothermal energy, solar energy, wind energy, and energy from solid waste treatment plants.
Resistance (electrical)- The inherent characteristic of a material to inhibit the flow of electrons producing heat in conductors, devices, or components and is measured in Ohms.
Resistor - An electrical device that resists the flow of electrons in electric circuits.
Ribbon Cells (Photovoltaic) - A type of solar photovoltaic device made by pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin continuous sheet of material.
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Safety Disconnect - A switch that disconnects one circuit from another circuit to isolate power generation or storage equipment from each other.
Self Discharge (rate) - The rate at which a battery, without being used will lose its charge over time.
Semiconductor - Any material that has a limited capacity for conducting an electric current. It is neither a good conductor nor a good insulator. This characteristic allows it to perform controlled operations as transistors, diodes, and integrated circuit packages. Semiconductor material can be densely packed and layered thru photographic processes and has given rise to the modern world of electronics. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.
Series - A wiring technique where multiple devices are wired together to increase voltage but current remains the same.
Shallow Cycling - Allowing a battery bank to only loose 20% of its full charge when being used, then recharging back to full charge. With a large bank of batteries, this technique can supply sufficient power between charge cycles and will greatly increase the life and performance of the batteries.
Short Circuit - A generally unwanted condition where maximum current flows freely through an external circuit that has no load or resistance, usually due to the catastrophic failure of a device or component.
Silicon - A chemical element, of atomic number 14, that is semi-metallic, and an excellent semiconductor material because the atoms in the outer shell are neither tightly bound nor loosely bound to the nucleus.
Sine Wave - The wave generated by alternating current generators and sine wave solid-state inverters.
Single-Crystal Material - See 'monocrystalline'.
Sizing - The process of designing a solar electric system to meet the required operating loads based on the total wattage of all appliances in the system.
Solar Array - See 'Photovoltaic Array'.
Solar Cell - The basic unit of a photovoltaic solar panel. A 12 volt solar panel typically has 36 individual cells, a 24 volt solar panel uses 72 cells.
Solar Constant - The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 Watts per square meter or 492 Btu per square foot.
Solar Energy - Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.
Solar Module (Panel) - A number of individual solar cells connected together in an environmentally protected housing producing a standard output voltage and power. Multiple modules/panels can be assembled into an array for increased power and/or voltage.
Solar Noon - The time of the day, at a given location when the sun reaches its highest point in the sky.
Solar Radiation - The various wavelengths of electromagnetic radiation that is emitted by the sun including the visible light we can see.
Solstice - The two times of the year when the sun is apparently farthest north and south of the earth's equator occurring on or around June 21 (summer solstice) and December 21 (winter solstice) in the northern hemisphere.
Square Wave Inverter - A type of inverter that produces square wave output. The square wave inverter is the simplest and the least expensive to purchase, but it produces the lowest quality of power.
Stand-Alone Inverter - An inverter that operates independent of any utility company grid or electric transmission and distribution network.
Stand-Alone System - An system that operates independent of any utility company grid or electric transmission and distribution network.
Stand-By Power - See 'Phantom Power'.
State of Charge (SOC) - The remaining charge available in a battery, expressed as a percentage of the battery when fully charged.
Synchronous Inverter - An inverter that produces alternating current electricity from direct current electricity and uses another alternating current source, such as the electric power grid or a generator to synchronize its output voltage and frequency to the external power source.
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Thin-Film - A layer of semiconductor material a few microns or less in thickness, used to make solar photovoltaic cells.
Tilt Angle (of a Solar Array) - The angle at which a solar array is set to face the sun relative to a horizontal position and is usually adjusted seasonally due to the changing declination of the sun.
Tracking Solar Array - A solar array that follows the path of the sun during the day to maximize the solar radiation it receives. A single axis tracker tracks the sun east to west and a two-axis tracker tracks the daily east to west movement of the sun and the seasonal declination movement of the sun.
Transformer - An electromagnetic device that changes the voltage of AC up or down. It consists of an induction coil with an iron core and a primary and secondary winding, the ratio of which determines output voltage versus input voltage.
Trickle Charge - The small charging voltage required to maintain a battery in a fully charged condition after it has been charged.
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Ultraviolet Radiation- Electromagnetic radiation ranging from 4 to 400 nanometers in wavelength.
Utility Company - Do we have to spell it out for you?
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Visible Radiation - The visible portion of the electromagnetic spectrum ranging from 0.4 to 0.76 microns in wavelength.
Volt - A unit of electrical measurement. One volt will cause a current of one ampere to flow through a resistance of one ohm.
Voltage - The difference in electrical potential that exists between two points in a circuit measured in volts.
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Wafer - A thin section of semiconductor material made by slicing it from a single crystal or rod.
Watt - The unit of electric power in a circuit. One watt equals one ampere under an electrical pressure of one volt.
Watt-Hour - 1 watt produced or consumed for a period of 1 hour.
Wattmeter - A device for measuring power production or usage displayed in watts.
Design your system quickly with our Interactive Design Tools
(Note : These design tools require javascript to be turned on in your browser)
* Check out our easy point & click System Sizing Estimator to quickly & easily calculate the number of solar panels and storage batteries you'll need for a wide range of system sizes.
* Our Battery Bank Design Tool will take the confusion out of wiring up your battery bank. Use 2, 4, 6, or 12 volt batteries to build a system voltage of 12, 24, or 48 volts using series and parallel wiring with just 4 clicks. Battery bank capacities from 300 AmpHours to over 4000 AmpHours are displayed graphically so you can see exactly how to wire the batteries together.
* This Wire Size Calculator will allow you to quickly find the correct wire size in AWG (American Wire Gauge) based on the distance to your solar panel array & the amount of amperage your panels put out. No math required!
New Feature!  Check out our new feature Solar Projects where you can build easy, do-it-yourself projects for your solar power system to save money and have fun doing it!
 







 Solar Power Terminology
	
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These are common terms, descriptions, and definitions you should be aware of when shopping for solar power. Understanding this terminology before you start talking with solar power installers will help you communicate with the contractors. You will likely see these terms in their proposals and marketing material.

    Alternating Current (AC) - The type of electrical power supplied by utilities. This form of electricity has a current that reverses direction at regular intervals. This is the most common type of household electricity. In the USA and Canada, the direction of flow is reversed 120 times a second, resulting in 60 complete cycles per second.
    Angle of Incidence - the angle between the surface of an object and the direction of the sunshine. Solar panels produce the most power when the surface of the solar panel is exactly perpendicular to the direction of sunlight. On some solar power farms, motorized trackers keep each cell pointed in the optimum direction as the sun moves across the sky. More recent experience shows it is better not to rely on mechanical devices to move the solar panels, as they wear out, but invest that money in more solar panels instead.
    Array - A group of solar panels wired together.
    Balance of system (BOS) - all the components and costs other than the solar panels. It includes design costs, site preparation, system installation, support structures, operation and maintenance costs, etc.
    Direct Current (DC) - The type of electricity generated by solar panels. The current flows only in a single direction. The most common form of electricity used in cars, boats and RVs. Small devices such as torches or radios use DC current, usually powered by batteries.
    ESA - the Electrical Safety Authroity, which is the governing body who inspects electrical installations in Ontario, Canada.
    Grid - When used in reference to utility power, it refers to a system of electrical transmission and distribution lines.
    Grid-connected Solar Power - Grid-connected solar power, also known as on-grid or grid-tied solar power, generates electricity for the electric utility grid.
    Inverter - An inverter converts the power generated from solar panels from direct current (DC) to alternating current (AC) for use in the home or for export to the grid. An inverter can also be used to convert DC power from storage batteries to AC power for use in the home or business. Also see Micro Inverter.
    Irradiance - the amount of solar energy that strikes a surface during a specific time period. Measured in kilowatts.
    Kilowatt (kW) - One thousand watts of electrical power. Ten 100-watt light bulbs use one Kilowatt of electrical power.
    Kilowatt-hour (kWh) - One thousand watts of electrical power used or generated in one hour. The kWh is a unit of energy. Most electrical meters measure kWh for billing purposes. A typical Canadian household uses about 10,000 kWh per year.
    Lightning Arrestor - a type of fuse used to protect the solar power system in the event of a lightning strike.
    Micro Inverter - this is a small inverter (see Inverter) that mounts to the back of each solar panel instead of having one large inverter. While it works better when partial shading is a concern, it also introduces multiple points of potential failure, which is a concern for roof mounted solar panels.
    Net metering - A practice used in conjunction with a solar electric system where your electric meter tracks your net power usage, spinning forward when you use electricity from the utility, and spinning backward when your system is generating more electricity than you need.
    Off Grid Solar - An array of solar panels that support a home is not connected to a utility power distribution grid.
    Orientation - A term used to describe the direction that a solar module faces. The two components of orientation are the tilt angle (the angle the panel makes from the horizontal) and the aspect angle (the direction the panel faces such as south or south west).
    Parallel Wiring - A group of electrical devices, such as PV modules, wired together to increase ampacity, while voltage remains constant.
    Passive Solar Home - A home that captures the heat from the sun shining through windows, as opposed to active solar, such as photovoltaic (PV).
    Photovoltaic (PV) - the direct conversion of light into electricity. Photo means "light", voltaic means "electric".
    PV - Abbreviation for photovoltaic.
    Series Wiring - A group of electrical devices, such as PV modules, wired together to increase voltage, while ampacity remains constant.
    Solar Energy - radiant energy emitted by the sun.
    Solar Panel - a panel designed to absorb the sun's rays as a source of energy for generating electricity or heating.
    Solar Panel Efficiency - the percent of electrical energy that a solar panel produces (under optimal conditions) as compared to the total amount of energy from the sun falling on the solar panel. Typical solar panel efficiency is currently between 12% and 18%, although this value is improving over time. See solar panel efficiency for an example and more details
    Solar Power - power obtained by harnessing the energy of the sun's rays.
    Tilt Angle - The angle of inclination of a module measured from the horizontal.
    Watt (W) - A unit of power, or the amount of work done in a period of time. One ampere of current flowing at a potential of one volt produces one watt of power. A typical household light bulb uses 60 watts of power. A person climbing a flight of stairs uses 200 watts of power.
 







 Asteroid

A small solar system object composed mostly of rock. Many of these objects orbit the Sun between Mars and Jupiter. Their sizes range anywhere from 33 feet (10 meters) in diameter to less than 620 miles (1,000 kilometers). The largest known asteroid, Ceres, has a diameter of 579 miles (926 kilometers).

Asteroid Belt

A region of space between Mars and Jupiter where the great majority of asteroids is found.

Atmosphere

The layer of gases surrounding the surface of a planet, moon, or star.

Aurora

A phenomenon produced when the solar wind (made up of energized electrons and protons) disturbs the atoms and molecules in a planet’s upper atmosphere. Some of the energy produced by these disturbances is converted into colorful visible light, which shimmers and dances. Auroras have been seen on several planets in our solar system. On Earth, auroras are also known as the “Northern Lights” (aurora borealis) or “Southern Lights” (aurora australis), depending on in which polar region they appear.

Bolide

Large, brilliant meteors that enter the Earth’s atmosphere. Friction between a fast-moving meteor and Earth’s air molecules generates tremendous heat, which causes the meteor to heat up, glow, and perhaps disintegrate. In some cases, the meteor literally explodes, leaving a visible cloud that dissipates slowly.

Carbonaceous Chondrite

A meteorite with embedded pebble-sized granules that contain significant quantities of organic (complex carbon-rich) matter.

Chromosphere

The middle layer of the solar atmosphere between the photosphere and the corona. The chromosphere is roughly 10,000 kilometers (6,200 miles) thick and is composed primarily of hydrogen. It varies in temperature from below 10,000 Kelvin (18,000° F) to over 100,000 Kelvin (180,000° F).

Comet

A ball of rock and ice, often referred to as a “dirty snowball.” Typically a few kilometers in diameter, comets orbit the Sun in paths that either allow them to pass by the Sun only once or that repeatedly bring them through the solar system (as in the 76-year orbit of Halley's Comet). A comet’s “signature” long, glowing tail is formed when the Sun’s heat warms the coma or nucleus, which releases vapors into space.

Comet Shoemaker-Levy 9 (SL-9)

A comet that became gravitationally bound to Jupiter, colliding with the planet in July 1994. Prior to entering the planet’s atmosphere, the comet broke into several distinct pieces, each with a separate coma and tail.

Convection Zone

The region below a star's surface where energy flows outward by the rising of hot gas known as convection.

Corona

The outermost layer of the atmosphere of a star, including the Sun. The corona is visible during a solar eclipse or when special adapters or filters are attached to a telescope to block the light from the star’s central region. The gaseous corona extends millions of kilometers from the star’s surface and has a temperature in the millions of degrees.

Coronal Hole

Regions in the corona from which the high-speed solar wind is known to originate. Coronal holes, usually found near the Sun's poles, are large regions in the corona that are less dense and cooler than the surrounding region.

Crater

A bowl-shaped depression caused by a comet or meteorite colliding with the surface of a planet, moon, or asteroid. On geologically active moons and planets (like Earth), craters can result from volcanic activity.

Earth

The third planet from the Sun and one of four terrestrial planets in the inner solar system. Earth, the only planet where water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometers (93 million miles) away from the Sun. Earth has one satellite — the Moon.

Erosion

Natural processes that wear or grind away the surface of an object. On Earth, the major agents of erosion are water and wind.

Extraterrestrial

An adjective that means “beyond the Earth.” The phrase “extraterrestrial life” refers to possible life on other planets.

Fault

A geological term that refers to a fracture or a break in a hard surface like the Earth’s crust. This area is a zone of weakness and may be the site of earthquakes or volcanoes. All planets or moons with a hard crust are candidates for faults or breaks on their surfaces.

Flare

A sudden and violent outburst of solar energy that is often observed in the vicinity of a sunspot or solar prominence; also known as a solar flare.

Ganymede

One of Jupiter’s largest moons. Ganymede, the largest satellite in our solar system, is about 5300 kilometers (3300 miles) wide and larger than the planet Mercury.

Gravity Assist

An effect through which an orbiting object, such as a spacecraft or a comet, gains or loses speed by virtue of the gravitational might of a planet or other celestial object that it passes. For example, the Cassini spacecraft in its journey to Saturn used a gravity assist from Earth to increase its velocity by about 36,000 kilometers per hour (22,300 miles per hour).

Great Red Spot

A circulating storm located in Jupiter’s upper atmosphere. The storm, which rotates around the planet in six days, is the width of two to three Earths. Galileo first observed the spot in the 17th century.

Greenhouse Effect

The result of a planet’s atmosphere trapping infrared heat, rather than allowing it to escape into space. This effect increases the planet’s surface temperature, a phenomenon known as global warming.

Habitable Zone

A region around a star where planets with liquid water may be present. A planet on the near edge of the habitable zone would have a surface temperature slightly lower than the boiling point of water. A planet on the distant edge of the habitable zone would have a surface temperature slightly higher than the freezing point of water.

Heliocentric

An adjective meaning “centered on the Sun.”

Impact

When one body strikes another with great force. Some examples include a meteor colliding with the Moon or a comet, such as Shoemaker-Levy 9, slamming into Jupiter.

Impact Event

A collision between two solar system bodies that releases exceptionally large amounts of energy. Some examples are the 1908 Siberian Tunguska impact by a comet or an asteroid and the asteroid that struck Earth 65 million years ago, which may have led to the extinction of the dinosaurs and other species of the Cretaceous-Tertiary era.

Interplanetary Matter

Dust, gas, and other debris found within the solar system.

Interplanetary Space

The region of space surrounding our Sun. Asteroids, comets, Earth, and the solar wind are examples of things occupying interplanetary space.

Io

The innermost of Jupiter’s four large moons. Due to Jupiter's gravitational might, Io is geologically active; its surface is peppered with volcanoes that send sulfurous eruptions into its thin atmosphere. Io appears to have the most active volcanoes in the solar system.

Io Plasma Torus

A bagel-shaped region of trapped sulfur ions around Jupiter that originates from the surface of Io, one of Jupiter’s moons. Gravitational tidal forces between Jupiter, other Galilean moons, and Io cause tidal friction in Io’s interior, producing geysers that spew sulfur at tremendous speeds. Some of the sulfur ions leave Io’s surface and become trapped around Jupiter.

Ionosphere

A region of the Earth’s upper atmosphere where solar radiation ionizes the air molecules. This region affects the transmission of radio waves and extends from 50 to 400 kilometers (30 to 250 miles) above the Earth's surface.

Jovian Atmosphere

The atmosphere surrounding the giant, massive planet Jupiter. The Jovian atmosphere is composed primarily of hydrogen (90 percent) and helium (10 percent). Other minor ingredients include water, hydrogen sulfide, methane, and ammonia.

Jovian Planets

The planets Jupiter, Saturn, Uranus, and Neptune. They are called Jovian planets because of similarities in their composition and location. This group is also known as the “giant planets,” the “gas planets” and, when grouped with the planet Pluto, the “outer planets.”

Jovian Winds

The hurricane-force, high-velocity motion of gas molecules in Jupiter’s atmosphere. The wind speed increases as one travels deeper into Jupiter’s atmosphere. The various patterns of atmospheric winds are easily identified in Jupiter’s upper cloud layer.

Jupiter

The fifth planet from the Sun and the largest planet in our solar system, twice as massive as all the other planets combined. Jupiter is a gaseous planet with a very faint ring system. Four large moons and numerous smaller moons orbit the planet. Jupiter is more than five times the Earth’s distance from the Sun. It completes an orbit around the Sun in about 12 Earth years.

Kuiper Belt

A region in our outer solar system where many "short-period" comets originate. The orbits of short-period comets are less than 200 years. This region begins near Neptune’s orbit at 30 astronomical units (AU) and extends to about 50 AU away from the Sun. An astronomical unit is the average distance between Earth and the Sun. The Kuiper Belt may have as many as 100 million comets.

Lithosphere

The solid part of a planet’s surface, composed of the crust and upper mantle. On Earth, it includes the continents and the sea floor.

Long-Period Comet

A comet having an orbital period greater than 200 years and usually moving in a highly elliptical, eccentric orbit. Comets have orbits that take them great distances from the Sun. Most long-period comets pass through the inner solar system only once. Hale-Bopp is an example of a long-period comet.

Lunar Eclipse

A darkening of the Moon, as viewed from Earth, caused when our planet passes between the Sun and the Moon.

Magnetic-Field Lines

Imaginary lines used to visualize a magnetic field. Magnetic field lines are related to the strength of the magnetic object’s influence and point in the same direction as a compass needle would.

Magnetopshere

A region of space above the Earth’s (or other planet’s) atmosphere where magnetic fields influence the motions of charged particles. The magnetosphere magnetically deflects or traps charged particles from space that would otherwise bombard the planet’s surface.

Mantle

The interior region of a terrestrial (rocky) planet or other solid body that is below the crust and above the core.

Maria

A dark, flat, large region on the surface of the Moon. The term is also applied to the less well-defined areas on Mars. Although maria literally means “seas,” watery regions do not exist on the Moon or Mars. Marias on the Moon may be evidence of past volcanic lava flows.

Mars

The fourth planet in the solar system and the last member of the hard, rocky planets (the inner or terrestrial planets) that orbit close to the Sun. The planet has a thin atmosphere, volcanoes, and numerous valleys. Mars has two moons: Deimos and Phobos.

Mercury

The closest planet to the Sun. The temperature range on Mercury’s surface is the most extreme in the solar system, ranging from about 400° C (750° F) during the day to about –200° C (–300° F) at night. Mercury, which looks like Earth’s moon, has virtually no atmosphere, no moons, and no water.

Meteor

A bright streak of light in the sky caused when a meteoroid enters the Earth’s atmosphere. The streak of light is produced from heat generated by the meteoroid traveling into the Earth’s atmosphere.

Meteorite

The remains of a meteoroid that plunges to the Earth’s surface. A meteorite is a stony or metallic mass of matter that did not completely vaporize when it entered the Earth’s atmosphere.

Meteoroid

A small, solid object moving through space. A meteoroid produces a meteor when it enters the Earth’s atmosphere.

Micrometeoroid

A very small meteoroid with a diameter of less than a millimeter. Micrometeoroids form the bulk of the interplanetary solid matter scattered throughout the solar system.

Moon

A large body orbiting a planet. On Earth’s only moon, scientists have not detected life, water, or oxygen on this heavily cratered body. The Moon orbits our planet in about 28 days.

Nebular Theory

The idea that our solar system originated in a contracting, rotating cloud of gas that flattened to form a disk as it contracted. According to this theory, the Sun formed at the center of the disk and the planets formed in concentric bands of the disk.

Neptune

The eighth planet and the most distant giant gaseous planet in our solar system. The planet is 30 times the Earth’s distance from the Sun, and each orbit takes 165 Earth years. Neptune is the fourth largest planet and has at least eight moons, the largest of which is Triton. Neptune has a ring system, just like all the giant gaseous outer planets.

Oort Cloud

A vast spherical region in the outer reaches of our solar system where a trillion long-period comets (those with orbital periods greater than 200 years) reside. Comets from the Oort Cloud come from all directions, often from as far away as 50,000 astronomical units.

Ozone Layer

A region in the upper atmosphere that has high concentrations of ozone (triatomic oxygen, 03). The ozone layer protects the Earth by absorbing the Sun’s high-energy ultraviolet radiation.

Periodic Comet

A comet in a closed, elliptical orbit within our solar system. These comets typically have orbital periods of less than 200 years. Many comets have orbits that keep them in the inner solar system and allow their trajectories to be calculated with great accuracy and precision. Perhaps the best-known periodic comet is Halley’s comet, whose orbital period is 76 years.

Photosphere

The extremely thin, visible surface layer of the Sun or a star. The average temperature of the Sun’s photosphere is about 5800 Kelvin (about 10,000° F). Although the Sun is completely made up of gas, its gas is so dense that we cannot see through it. When we look at the Sun, we are seeing the photosphere.

Planet

An object that orbits a star. Although smaller than stars, planets are relatively large and shine only by reflected light. Planets are made up mostly of rock or gas, with a small, solid core. In our solar system, the inner planets — Mercury, Venus, Earth, and Mars — are the rocky objects, and most of the outer planets — Jupiter, Saturn, Uranus, and Neptune — are the gaseous ones. Because Pluto is made largely of ice, like a comet, some astronomers do not consider it a true planet.

Planetesimal

A small body of rock and/or ice — under 10 kilometers (6 miles) across — formed during the early stages of the solar system. Planetesimals are the building blocks of planets, but many never combined to form large bodies. Asteroids are one example of planetesimals.

Pluto

A dwarf planet whose small size and composition of ice and rock resembles the comets in the Kuiper Belt, a region beyond Neptune’s orbit where Pluto resides. Pluto was considered the ninth planet until August 2006, when the International Astronomical Union reclassified it as a dwarf planet. Pluto’s orbit is more elliptical than those of the eight solar system planets.

Prominence

An eruption of gas from the chromosphere of a star. Solar prominences are visible as part of the corona during a total solar eclipse. These eruptions occur above the Sun’s surface (photosphere), where gases are suspended in a loop, apparently by magnetic forces that arch upward into the solar corona and then return to the surface.

Protoplanet

A small body that attracts gas and dust as it orbits a young star. Eventually, it may form a planetary body.

Regolith

The layer of loose rock resting on bedrock (sometimes called mantle rock), found on the Earth, the Moon, or a planet. Regolith is made up of soils, sediments, weathered rock, and hard, near-surface crusts. On the surface of the Moon, regolith is a fine rocky layer of fragmentary debris (or dust) produced mainly by meteoroid collisions.

Saturn

The sixth planet in the solar system, noted for its obvious ring structure. Saturn is almost ten times the Earth’s distance from the Sun. The planet completes a circuit around the Sun in about 30 Earth years. Saturn is the second largest and the least dense planet in our solar system. The planet has more than 21 moons, including Titan, the second largest known moon in our solar system.

Secondary Atmosphere

A gas or gases, such as helium, that a planet discharges from its interior after having lost its primary or primordial atmosphere.

Seismic Wave

The transfer of energy throughout a celestial object, such as a planet, resulting from an external impact or an internal event. On Earth, seismic waves are generated primarily by earthquakes.

Short-Period Comet

A comet that orbits mainly in the inner solar system. Short-period comets usually orbit the Sun in less than 200 years. Halley’s comet is an example of a short-period comet.

Solar Constant

The average amount of solar radiation reaching a planet; usually expressed in watts (energy per unit time) per square meter. For Earth, the solar constant equals 1,372 W/m2. Each planet has a unique solar constant depending on its distance from the Sun.

Solar Cycle

The periodic changing of the Sun’s magnetic field, which determines the number of sunspots and the amount of particles emitted in the solar wind. The period of the cycle is about 11 years.

Solar Eclipse

A phenomenon in which the Moon’s disk passes in front of the Sun, blocking sunlight. A total eclipse occurs when the Moon completely obscures the Sun’s disk, leaving only the solar corona visible. A solar eclipse can only occur during a new phase of the Moon.

Solar Maximum

The midpoint in the solar cycle where the amount of sunspot activity and the output of cosmic particles and solar radiation is highest.

Solar Minimum

The beginning and the end of a sunspot cycle when only a few sunspots are usually observed, and the output of particles and radiation is normal.

Solar System

The Sun and its surrounding matter, including asteroids, comets, planets and moons, held together by the Sun’s gravitational influence.

Solar Wind

Streams of charged particles flowing from the Sun at millions of kilometers an hour. The composition of this high-speed solar wind may vary, but it always streams away from the Sun. The solar wind is responsible for the Northern and Southern Lights on Earth and causes the tails of comets to point away from the Sun.

Sprites

Gamma-ray flashes produced in Earth’s atmosphere by severe lightning storms and upper atmospheric events.

Sun

The star at the center of our solar system. An average star in terms of size and mass, the Sun is a yellow dwarf of spectral type G2. It is about 5 billion years old, contains 2 * 1030 kilograms of material, and has a diameter more than 100 times that of Earth.

Sunspot

A region on the Sun’s photosphere that is cooler and darker than the surrounding material. Sunspots often appear in pairs or groups with specific magnetic polarities that indicate electromagnetic origins.

Sunspot Cycle

The change in strength of the Sun’s magnetic field, which determines the number of sunspots and the amount of particles emitted in the solar wind. The period of the cycle is about 11 years.

Terrestrial Planets

The four planets of the inner solar system (Mercury, Venus, Earth, and Mars) are called terrestrial planets because they are made up mostly of rock.

Triton

The largest of Neptune’s satellites. Triton has an atmosphere and is roughly the size of Earth's moon. It has an “ice cap” of frozen nitrogen and methane with “ice volcanoes” that erupt liquid nitrogen, dust, and methane compounds from beneath its frozen surface.

Uranus

The third largest planet in the solar system and the seventh from the Sun. Uranus is 19 times the Earth’s distance from the Sun and completes a circuit around the Sun in about 84 Earth years. This gaseous, giant outer planet has a visible ring system and over 20 moons, the largest of which is Titania. Uranus is tipped on its side, with a rotation axis in nearly the same plane as its orbit.

Van Allen Belt

A region containing charged particles trapped in the Earth’s magnetic force field (magnetosphere). The belt’s lower boundary begins at about 800 kilometers (496 miles) above the Earth’s surface and extends thousands of kilometers into space.

Venus

An inner, terrestrial (rocky) planet that is slightly smaller than Earth. Located between the orbits of Mercury and Earth, Venus has a very thick atmosphere that is covered by a layer of clouds that produce a “greenhouse effect” on the planet. Venus’s surface temperature is roughly 480° C (900° F), making it the hottest planet in the solar system.

Volcano

A break or vent in the crust of a planet or moon that can spew extremely hot ash, scorching gases, and molten rock. The term volcano also refers to the mountain formed by volcanic material. 







 Terms and Definitions 







 A
accretion
    Accumulation of dust and gas into larger bodies. 
albedo
    Reflectivity of an object; ratio of reflected light to incident light. 
albedo feature
    A dark or light marking on the surface of an object that might not be a geological or topographical feature. 
allocthonous
    (1) Material that is formed or introduced from somewhere other than the place it is presently found. (2) Fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event. 
Alpha Centauri
    The closest bright star to our solar system. 
angstrom
    A unit of length = 1.0E-08cm. 
antipodal point
    The point that is directly on the opposite side of the planet; e.g., the Earth's north pole is antipodal to its south pole. 
aphelion
    The point in its orbit where a planet is farthest from the Sun. 
apoapsis
    The point in orbit farthest from the planet. 
apogee
    The point in orbit farthest from the Earth. 
ash
    The fine-grained material produced by a pyroclastic eruption. An ash particle is defined to have a diameter of less than 2 millimeters. 
asteroid number
    Asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N. 
astronomical unit (AU)
    The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116 miles). 
atmosphere
    One atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth. 
aurora
    A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun. 
aurora borealis
    The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis. 
B
bar
    A unit of pressure, equal to the sea-level pressure of Earth's atmosphere; 1 bar = 0.987 atmosphere = 101,300 pascals = 14.5 lbs/square inch = 100,000 Newtons per square meter. 
basalt
    A general term for dark-colored, igneous rocks composed of minerals that are relatively rich in iron and magnesium. 
blackbody temperature
    The temperature of an object if it is reradiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature. 
black hole
    An object whose gravity is so strong that the escape velocity exceeds the speed of light. 
bolide
    An exploding meteorite. 
bow shock
    The outermost part of a planetary magnetosphere; the place where the supersonic flow of the solar wind is slowed to subsonic speed by the planetary magnetic field. 
breccia
    A course-grained rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix. 
butte
    A conspicuous, isolated, flattop hill with steep slopes. 
C
calcium K
    A narrow wavelength of blue light which is emitted and absorbed by ions of the element calcium. 
caldera
    A large, basin-shaped volcanic depression that is more or less circular in form. Most volcanic calderas are produced by collapse of the roof of a magma chamber due to removal of magma by voluminous eruptions or subterranean withdrawal of the magma, although some calderas may be formed by explosive removal of the upper part of a volcano. 
carbonate
    A compound containing carbon and oxygen; an example is calcium carbonate (limestone). 
cataclastic
    A texture found in metamorphic rocks in which brittle minerals have been broken, crushed and flattened during shearing. 
catena
    A chain of craters. 
cavus
    Hollows, irregular depressions. 
central peak
    The exposed core of uplifted rocks in complex meteorite impact craters; the central peak material typically shows evidence of intense fracturing, faulting and shock metamorphism. 
chaos
    A distinctive area of broken terrain. 
chasma
    A canyon. 
chromosphere
    The lower level of the solar atmosphere between the photosphere and the corona. 
cinders
    Loose, vesicular volcanic ejecta 4 to 32 millimeters (.16 to 1.28 inches) in diameter. 
cinder cone
    A conical hill formed by the accumulation of pyroclastic fragments that fall to the ground in an essentially solid condition. 
clast
    A fragment of rock that has been transported, either by volcanic or sedimentary processes. 
colles
    A small hill or knob. 
coma
    The dust and gas surrounding an active comet's nucleus. 
composite volcano
    A volcano composed of interbedded lava and pyroclastic material commonly with steep slopes. 
convection
    Fluid circulation driven by temperature gradients; the transfer of heat by this automatic circulation (see also Educator's Guide to Convection). 
corona
    1) The upper level of the solar atmosphere, characterized by low densities and high temperatures (> 1.0E+06 K); it is not visible from the Earth except during a total eclipse of the sun or by use of special telescopes called coronagraphs. 2) An ovoid-shaped feature. 
coronagraph
    A special telescope which blocks light from the disk of the Sun in order to study the faint solar atmosphere. 
cosmic ray
    Electromagnetic rays of extremely high frequency and energy; cosmic rays usually interact with the atoms of the atmosphere before reaching the surface of the Earth. Some cosmic rays come from outside the solar system while others are emitted from the Sun and pass through holes in the corona. 
crater
    1) A depression formed by the impact of a meteorite. 2) A depression around the orifice of a volcano. 
cratons
    The relatively stable portions of continents composed of shield areas and platform sediments; typically, cratons are bounded by tectonically active regions characterized by uplift, faulting and volcanic activity. 
Cretaceous period
    A geological term denoting the interval of Earth history beginning around 144 million years ago and ending 66 million years ago. [ more ] 
Cretaceous-Tertiary boundary
    A major stratigraphic boundry on Earth marking the end of the Mesozoic Era, best known as the age of the dinosaurs. The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth. 
crystalline
    Rock types made up of crystals or crystal fragments, such as metamorphic rocks that recrystallized in high temperature or pressure environments, or igneous rocks that formed from cooling of a melt. 
D
density
    Measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0, iron is 7.9, and lead is 11.3. 
diaplectic glass
    A natural glass formed by shock pressure from any of several minerals without melting; it is found only in association with meteorite impact craters. 
dielectric constant
    The ratio of electric flux density to electric field. 
disk
    The visible surface of the Sun (or any heavenly body) projected against the sky. 
Doppler effect
    The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. 
dorsum
    A ridge. 
E
e=mc2
    Einstein's famous theory of relativity formula known as the energy-mass relation. The energy e is equal to the mass m multiplied by the speed of light squared c2. A small mass produces an enormous amount of energy. 
eccentric
    Noncircular; elliptical (applied to an orbit). 
eccentricity
    A value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis. 
eclipse
    The cutting off of light from one celestial body by another. 
ecliptic
    The plane of Earth's orbit about the Sun 
effusive eruption
    A relative quiet volcanic eruption which puts out basaltic lava that moves at about the speed one walks. The lava is fluid in nature. The eruptions at the Kilauea volcano on the island of Hawaii are effusive 
ejecta
    Material such as glass and fragmented rock thrown out of an impact crater during its formation. 
ellipse
    A closed curve that is formed from two foci or points in which the sum of the distances from any point on the curve to the two foci is a constant. Johannes Kepler first discovered that the orbits of the planets are ellipses, not circles; he based his discovery on the careful observations of Tycho Brahe. 
en echelon fissures
    Fissures that are parallel in trend to each other, but offset to either the left or right. 
eolian
    Related to wind deposits and associated effects. 
eruption
    The ejection of volcanic materials (lavas, pyroclasts and volcanic gases) onto the surface, either from a central vent, a fissure or a group of fissures. 
explosive eruption
    A dramatic volcanic eruption which throws debris high into the air for hundreds of miles. The lava is low in silicate and can be very dangerous for people near by. An example is Mount St. Helens in 1980. 
F
faculae
    A bright region of the photosphere seen in white light, seldom visible except near the solar limb. 
fault
    A crack or break in the crust of a planet along which slippage or movement can take place. 
filament
    A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun; a filament on the limb of the Sun seen in emission against the dark sky is called a prominence. 
fissure
    A narrow opening or crack of considerable length and depth. 
flare
    A sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted. 
flexus
    A cuspate linear feature 
fluctus
    A flow terrain 
fossa
    A long, narrow, shallow depression. 
G
Gaia Hypothesis
    Named for the Greek Earth goddess Gaea, this hypothesis holds that the Earth should be regarded as a living organism. British biologist James Lovelock first advanced this idea in 1969. 
Galilean moons
    Jupiter's four largest moons: Io, Europa, Ganymede and Callisto; discovered independently by Galileo and Marius. 
graben
    An elongated, relatively depressed crustal unit or block that is bounded by faults on its sides. 
geosynchronous orbit
    A direct, circular, low-inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface. 
granulation
    A pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas. 
greenhouse effect
    An increase in temperature caused when the atmosphere absorbs incoming solar radiation but blocks outgoing thermal radiation; carbon dioxide is the major factor. 
gravity
    A mutual physical force attracting two bodies. 
H
H-alpha
    A narrow wavelength of red light which is emitted and absorbed by the element hydrogen; this wavelength is often used to study the Sun. 
heliocentric
    Sun centered; see Copernicus, Kepler, Galileo. 
heliopause
    The point at which the solar wind meets the interstellar medium or solar wind from other stars. 
heliosphere
    The space within the broundary of the heliopause containing the Sun and solar system. 
hemisphere
    A half of the celestial sphere that is divided into two halves by either the horizon, the celestial equator, or the ecliptic. 
high-pressure mineral phase
    In this phase, mineral forms that are stable only at the extremely high pressures typical of Earth's deep interior but not its surface. Such pressures are generated instantaneously during meteorite impact. Stishovite is the high-pressure polymorph of quartz, a common crustal mineral. 
hot spot
    Center of persistent volcanism, thought to be the surface expression of a rising hot plume in Earth's mantle. 
hummocky
    Uneven, lumpy terrain. 
I
ice
    Planetary scientists use this word to refer to water, methane, and ammonia, which usually occur as solids in the outer solar system. 
igneous
    Rock or mineral that solidified from molten or partly molten material. 
impact melt
    Rocks melted during impact, including small particles dispersed in various impact deposits and ejecta, and larger pools and sheets of melt that coalesce in low areas within the crater. Impact melts are extremely uniform in their composition but highly variable in texture. They are composed predominantly of the target rocks, but can contain a small but measurable amount of the impactor. 
inclination
    The inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic. The inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator. 
inferior planets
    The planets Mercury and Venus are inferior planets because their orbits are closer to the Sun than is Earth's orbit. 
interplanetary magnetic field (IMF)
    The magnetic field carried with the solar wind. 
ion
    An atom or molecular fragment that has a positive electrical charge due to the loss of one or more electrons; the simplest ion is the hydrogen nucleus, a single proton. 
ionosphere
    A region of charged particles in a planet's upper atmosphere; the part of the earth's atmosphere beginning at an altitude of about 400 kilometers (25 miles) and extending outward 400 kilometers (250 miles) or more. 
J
Jovian planet
    Any of the four outer, gaseous planets: Jupiter, Saturn, Uranus, and Neptune. 
K
kelvin (K)
    Zero K is absolute zero; ice melts at 273 K (0° C, 32° F); water boils at 373 K ( 100° C, 212° F). 
kilogram (kg)
    One kilogram is equivalent to 1,000 grams or 2.2 pounds; the mass of a liter of water. 
kilometer (km)
    One kilometer is equivalent to 1,000 meters or 0.62 miles. 
L
labes
    A landslide. 
labyrinthus
    An intersecting valley complex. 
lacus
    A lake. 
Lagrangian point
    One of the solutions to the three-body problem discovered by the eighteenth century French mathematician Lagrange; the two stable Lagrangian points, L-4 and L-5, lie in the orbit of the primary body, leading and trailing it by a 60-degree arc. 
lava
    A general term for molten rock that is extruded onto the surface. 
lava tube
    A tunnel formed underneath the surface of a solidfying lava flow. 
leading hemisphere
    The hemisphere that faces forward, into the direction of motion of a satellite that keeps the same face toward the planet. 
lee
    The side of an object that is sheltered from the wind. 
levee
    An embankment, continuous dike or ridge. 
light
    Electromagnetic radiation that is visible to the eye. 
light year
    The distance light travels in a year, at the rate of 300,000 kilometers per second (671 million miles per hour); 1 light-year is equivalent to 9.46053e12 km, 5,880,000,000,000 miles or 63,240 AU. 
limb
    The outer edge of the apparent disk of a celestial body. 
linea
    An elongate marking. 
lineament
    Linear topographic feature that may depict crustal structure. 
lobate
    Having lobes or resembling a lobe. 
M
macula
    A dark spot. 
magma
    Molten rock within the crust of a planet that is capable of intrusion into adjacent crustal rocks or extrusion onto the surface. Igneous rocks are derived from magma through solidification and related processes or through eruption of the magma at the surface. 
magnetic field
    A region of space near a magnetized body where magnetic forces can be detected. 
magnetograph
    A special telescope which analyzes the color and polarization of sunlight in order to measure the magnetic field of the Sun. 
magnetopause
    The boundary of the magnetosphere, lying inside the bow shock. 
magnetosphere
    The region of space in which a planet's magnetic field dominates that of the solar wind. 
magnetotail
    The portion of a planetary magnetosphere which is pushed in the direction of the solar wind. 
magnitude
    The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude. 
mare
    Latin word for "sea." Galileo thought the dark featureless areas on the Moon were bodies of water, even though the Moon is essentially devoid of liquid water. The term is still applied to the basalt-filled impact basins common on the face of the Moon visible from Earth. 
mensa
    A mesa, flat-topped elevation. 
mesa
    A broad, flattop, erosional hill or mountain, commonly bounded by steep slopes. 
meteor
    The luminous phenomenon seen when a meteoroid enters the atmosphere, commonly known as a shooting star. 
meteorite
    A part of a meteoroid that survives through the Earth's atmosphere. 
meteoroid
    A small rock in space. 
millibar
    This is 1/1000 of a bar; the standard sea-level pressure is about 1,013 millibars. 
minor planets
    Another term used for asteroids. 
mons
    A mountain. 
N
nebula
    A diffuse mass of interstellar dust and gas. 
neutrino
    A fundamental particle supposedly produced in massive numbers by the nuclear reactions in stars; they are very hard to detect because the vast majority of them pass completely through the Earth without interacting. 
nuclear fusion
    A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence E=mc2. This is the source of the Sun's energy and, ultimately, of (almost) all energy on Earth. 
O
oceanus
    An ocean. 
obliquity
    The angle between a body's equatorial plane and orbital plane. 
occultation
    The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star. 
old
    A planetary surface that has been modified little since its formation typically featuring large numbers of impact craters; (compare to young). 
optical depth
    Optical depth is a measure of the transparency of a ring system. When a ring is "optically thick" (i.e., the optical depth is large), the ring is nearly opaque and very little light passes through. When a ring is "optically thin" (i.e., the optical depth is small), very little material is present and most of the light passes through. 
orbit
    The path of an object that is moving around a second object or point. 
ovoid
    Shaped like an egg. 
P
Paleozoic
    A geological term denoting the time in Earth history between 570 and 245 million years ago. 
pahoehoe
    A type of basalt lava flow characterized by a smooth glassy skin, and constructed of innumerable "flow units" called "toes"; pahoehoe flows advance at rates of 1 to 10 meters (3 to 33 feet) hour and are associated with low-effusion-rate eruptions with little to no fountaining. 
palimpsest
    A circular feature on the surface of dark icy moons such as Ganymede and Callisto lacking the relief associated with craters; Pamlimpsests are thought to be impact craters where the topographic relief of the crater has been eliminated by slow adjustment of the icy surface. 
palus
    A swamp. 
patera
    Shallow crater; scalloped, complex edge. 
peak ring
    A central uplift characterized by a ring of peaks rather than a single peak; peak rings are typical of larger terrestrial craters above about 50 kilometers (30 miles) in diameter. 
penumbra
    The outer filamentary region of a sunspot. 
periapsis
    The point in the orbit closest to the planet. 
perigee
    The point in the orbit closest to the Earth. 
perihelion
    The point in its orbit where a planet is closest to the Sun. 
perturb
    To cause a planet or satellite to deviate from a theoretically regular orbital motion. 
photosphere
    The visible surface of the Sun; the upper surface of a convecting layer of gases in the outer portion of the sun whose temperature causes it to radiate light at visible wavelengths; sunspots and faculae are observed in the photosphere. 
phreatic eruption
    A volcanic eruption or explosion of steam, mud or other material that is not incandescent; this form of eruption is caused by the heating and consequent expansion of ground water due to an adjacent igneous heat source. 
plage
    Bright regions seen in the solar chromosphere. 
planar features
    Microscopic features in grains of quartz or feldspar consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure. 
planitia
    Broad plains that occupy lowlands on planetary surfaces. 
planum
    A plateau or high plain. 
plasma
    A low-density gas in which the individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality. 
polarization
    A special property of light; light has three properties, brightness, color and polarization. 
Precambrian
    A geological term denoting the time in Earth history prior to 570 million years ago. 
pressure ridge
    A ridge formed by the uplift of a lava flow crust due to pressure of the flowing lava. 
prominence
    An eruption of hot gases above the photosphere of the Sun. Prominences are most easily visible close to the limb of the Sun, but some are also visible as bright streamers on the photosphere. 
promontorium
    A cape. 
pseudocrater
    A generally circular crater produced by a phreatic eruption resulting from emplacement of a lava flow over wet ground. 
pyroclastic
    Pertaining to clastic (broken and fragmented) rock material formed by volcanic explosion or aerial expulsion from a volcanic vent. 
pumice
    A light vesicular form of volcanic glass with a high silica content; it is usually light in color and will float on water. 
Q
R
radiation
    Energy radiated in the form of waves or particles; photons. 
radiationbelt
    Regions of charged particles in a magnetosphere. 
red giant
    A star that has low surface temperature and a diameter that is large relative to the Sun. 
regio
    Region. 
regolith
    The layer of rocky debris and dust made by metoritic impact that forms the uppermost surface of planets, satellites and asteroids. 
relativity, Theory of
    More accurately describes the motions of bodies in strong gravitational fields or at near the speed of light than newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the photoelectric effect.) 
resolution
    The amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details. 
resonance
    A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5. 
retrograde
    The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic; moving in the opposite sense from the great majority of solar system bodies. 
rhyolite
    Fine-grained extrusive igneous rock, commonly with phenocrysts of quartz and feldspar in a glassy groundmass. 
rift
    A fracture or crack in a planet's surface caused by extension. On some volcanoes, subsurface intrusions are concentrated in certain directions; this causes tension at the surface and also means that there will be more eruptions in these "rift zones." 
rift valley
    An elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike. 
rima
    A fissure. 
Roche limit
    The Roche Limit was first described by Edouard Roche in 1848. It is the closest distance a body can come to a planet without being pulled apart by the planet's tidal (gravity) force. As a result, large moons cannot survive inside the Roche Limit. On July 7, 1992, Comet Shoemaker-Levy 9 broke apart into 21 pieces due to tidal forces when it passed within Jupiter's Roche Limit; on the subsequent pass, each of the comet's pieces collided with Jupiter.

    If a planet and a moon have identical densities, then the Roche Limit is 2.446 times the radius of the planet. The Roche Limits for the ringed planets are:

        Jupiter - 175,000 km (108,000 miles)
        Saturn - 147,000 km ( 92,000 miles)
        Uranus - 62,000 km ( 39,000 miles)
        Neptune - 59,000 km ( 37,000 miles) 

    This limit represents the rough boundary between each planet's ring system and its innermost moons. 
rupes
    The term applied to scarps on planetary surfaces; many scarps are thought to be the surface expression of faults within the crust of the planetary object. 
S
sapping
    A process of erosion where water leaks to the surface through the pores of rocks; as the water flows away, it slowly removes material to form valleys and channel networks. 
satellite
    A body that revolves around a larger body. 
scarp
    A line of cliffs produced by faulting or erosion; a relatively straight, clifflike face or slope of considerable linear extent, breaking the general continuity of the land by separating surfaces lying at different levels. 
scopulus
    A lobate or irregular scarp. 
semimajor axis
    One-half of the longest dimension of an ellipse. 
shatter cone
    Striated conical fracture surfaces produced by meteorite impact into fine-grained, brittle rocks such as limestone. 
shepherd satellite
    A satellite that constrains the extent of a planetary ring through gravitational forces. 
shield
    Any of several extensive regions where ancient Precambrian crystalline rocks are exposed at the Earth's surface. 
shield volcano
    A volcano in the shape of a flattened dome, broad and low, built by flows of very fluid lava. 
shock metamorphism
    The production of irreversible chemical or physical changes in rocks by a shock wave generated by impact, or detonation of high-explosive or nuclear devices. 
siderial
    Of, relating to, or expressed in relation to stars or constellations. 
siderial rotation
    Rotation time measured with respect to the fixed stars rather than the Sun or body orbited. 
siderophile elements
    This phrase literally means iron-loving elements. It includes Iridium, Osmium, Platinum and Plladium, which are found in the metal-rich interiors of chemically segregated asteroids and planets; consequently, these elements are extremely rare on Earth's surface. 
silicate
    A rock or mineral whose structure is dominated by bonds of silicon and oxygen atoms (ie. olivine). 
sinus
    A bay. 
solar cycle
    The approximately 11-year, quasi-periodic variation in the frequency or number of solar active events. 
solar nebula
    The large cloud of gas and dust from which the Sun and planets condensed 4.6 billion years ago. 
solar wind
    A tenuous flow of gas and energetic charged particles, mostly protons and electrons -- plasma -- which stream from the Sun; typical solar wind velocities are almost 350 kilometers (217 miles) per second. 
spatter cone
    A low, steep-sided cone built up from fluid pyroclasts coating the surface around a vent. 
spectroradiometer [SPEC-tro-RAY-dee-om-it-er]
    A device that measures the amount of reflected or radiated energy from a surface in two or more wavelengths. 
spectrum
    The distribution of wavelengths and frequencies. 
speed of light
    Light speed equals 299,792,458 meters/second (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light. 
spicules
    The grass-like patterns of gas seen in the solar atmosphere. 
stratosphere
    The cold region of a planetary atmosphere above the convecting regions (the troposphere), usually without vertical motions but sometimes exhibiting strong horizontal jet streams. 
stishovite
    A dense, high-pressure phase of quartz that has so far been identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters. 
subduction
    The process of one lithospheric plate descending beneath another. 
sublime
    Sublimation occurs when a substance changes directly from a solid to a gas without becoming liquid. 
sulcus
    Subparallel furrows and ridges. 
sulfuric acid
    A heavy, corrosive, oily, dibasic strong acid H2SO4 that is colorless when pure; it is a vigorous oxidizing and dehydrating agent. 
sunspot
    An area seen as a dark spot on the photosphere of the Sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere. 
superior planets
    The planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are superior planets because their orbits are farther from the Sun than Earth's orbit. 
synchronous orbit radius
    The orbital radius at which the satellite's orbital period is equal to the rotational period of the planet. A synchronous satellite with an orbital inclination of zero (same plane as the planet's equator) stays fixed in the sky from the perspective of an observer on the planet's surface. These orbits are commonly used for communications satellites. 
synchronous rotation
    A satellite's rotational period is equal to its orbital period; this causes the same side of a satellite to always face the planet. Synchronous rotation occurs when a planet's gravity produces a tidal bulge in its satellite. The gravitational attraction and bulge acts like a torque, which slows down the satellite until it reaches a synchronous rotation. 
synthetic-aperture radar
    SAR is a side-looking imaging system that uses the Doppler effect to sharpen the effective resolution in the cross-track direction. 
T
target rocks
    The surface rocks that an asteroid or comet impactor smashes into in a meteorite impact event. 
tectonic
    The deformation forces acting on a planet's crust. 
tektites
    Natural, silica-rich, homogeneous glasses produced by complete melting, and dispersed as droplets during terrestrial impact events. Tektites range in color from black or dark brown to gray or green and most are spherical in shape. They have been found in four regional deposits or strewn fields on the Earth's surface: North America, Czechoslovakia, Ivory Coast and Australasia. 
terminator
    The dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk. 
terra
    An extensive land mass. 
tessera
    A tile; polygonal ground. 
tholus
    A small domical mountain or hill. 
tidal forces
    The gravitational pull on planetary objects from nearby planets and moons. When the tidal forces of a planet and several moons are focused on certain moons, particularly if the orbits of the various objects bring them into alignment on a repeated basis, the tidal forces can generate a tremendous amount of energy within the moon. The intense volcanic acivity of Io is the result of the interaction of such tidal forces. 
tidal heating
    The frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites. 
trailing hemisphere
    The hemisphere that faces backwards, away from the direction of motion of a satellite that keeps the same face toward the planet. 
Trojan satellites
    Satellites which orbit at the Lagrangian points, 60° ahead of and 60° behind another satellite. For example, Telesto and Calypso are trojans of Saturn's satellite Tethys. 
troposphere
    The lower regions of a planetary atmosphere where convection keeps the gas mixed and maintains a steady increase of temperature with depth. Most clouds are in the troposphere. 
tuff
    The general term for consolidated pyroclastic debris. 
U
ultraviolet
    Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light. 
umbra
    The dark central region of a sunspot. 
undae
    Dunes. 
V
vallis
    A sinuous valley. 
vastitas
    Widespread lowlands. 
vent
    The opening in the crust through which volcanic material erupts. 
volatile
    Compounds with low melting temperatures, such as hydrogen, helium, water, ammonia, carbon dioxide and methane. 
volcano
    (1) A vent in the planetary surface through which magma and associated gases and ash erupt. (2) The form or structure produced by the erupted materials. 
W
weight
    The gravitational force exerted on a body. 
white dwarf
    A whitish star of high surface temperature and low intrinsic brightness with a mass approximately equal to that of a Sun but with a density many times larger. 
X
x-ray
    Electromagnetic radiation of very short wavelength and very high energy; x-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays. 
Y
young
    When used to describe a planetary surface, "young" means that the visible features are of relatively recent origin, i.e. that older features have been destroyed by erosion or lava flows. Young surfaces exhibit few impact craters and are typically varied and complex; in contrast, an "old" surface is one that has changed relatively little over geologic time. The surfaces of Earth and Io are young; the surfaces of Mercury and Callisto are old. 
Z
zodiac
    Twelve constellations dividing the ecliptic into approximately equal parts. Each month the Sun is in a different constellation of the zodiac. 

 







 Glossary of Astronomy Terms 







 

The universe is a large place full of wondrous and unimaginable things. The amateur astronomy hobby contains words, labels, and technical terms that may seem be a little confusing for both beginners and the more experienced hobbyists.

This glossary of astronomy terms contains definitions for some of the most common words used in astronomy, cosmology, astrophysics, and space exploration. You may click on any yellow highlighted word to jump directly to the definition for that astronomy word. You can also click on any letter of the alphabet below to jump directly to that section in the astronomy glossary listing. 







 A

Absolute Magnitude
A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light-years) away from Earth. On this scale, the Sun has an absolute magnitude of +4.8 while it has an apparent magnitude of -26.7 because it is so close.

Absolute Zero
The temperature at which the motion of all atoms and molecules stops and no heat is given off. Absolute zero is reached at 0 degrees Kelvin or -273.16 degrees Celsius.

Ablation
A process by where the atmosphere melts away and removes the surface material of an incoming meteorite.

Accretion
The process by where dust and gas accumulated into larger bodies such as stars and planets.

Accretion Disk
A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation.

Achondrite
A stone meteorite that contains no chondrules.

Albedo
The reflective property of a non-luminous object. A perfect mirror would have an albedo of 100% while a black hole would have an albedo of 0%.

Albedo Feature
A dark or light marking on the surface of an object that may or may not be a geological or topographical feature.

Altitude
The angular distance of an object above the horizon.

Antimatter
Matter consisting of particles with charges opposite that of ordinary matter.  In antimatter, protons have a negative charge while electrons have a positive charge.

Antipodal Point
A point that is on the direct opposite side of a planet.

Apastron
The point of greatest separation of two stars, such as in a binary star system.

Aperture
The size of the opening through which light passes in an optical instrument such as a camera or telescope. A higher number represents a smaller opening while a lower number represents a larger opening.

Aphelion
The point in the orbit of a planet or other celestial body where it is farthest from the Sun.

Apogee
The point in the orbit of the Moon or other satellite where it is farthest from the Earth.

Apparent Magnitude
The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low apparent magnitude while dim objects will have a higher apparent magnitude.

Asteroid
A small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets.

Astrochemistry
The branch of science that explores the chemical interactions between dust and gas interspersed between the stars.

Astronomical Unit (AU)
A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.

Atmosphere
A layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases.

Aurora
A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun. This phenomenon is known as the Aurora Borealis in the Earth's northern hemisphere and the Aurora Australis in the Earth's Southern Hemisphere.

Aurora Australis
Also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Borealis in the northern hemisphere.

Aurora Borealis
Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Australis in the southern hemisphere.

Axis
Also known as the poles, this is an imaginary line through the center of rotation of an object.

Azimuth
The angular distance of an object around or parallel to the horizon from a predefined zero point.

B

Bar
A unit of measure of atmospheric pressure. One bar is equal to 0.987 atmospheres, 1.02 kg/cm2, 100 kilopascal, and 14.5 lbs/square inch.

Big Bang
The theory that suggests that the universe was formed from a single point in space during a cataclysmic explosion about 13.7 billion years ago. This is the current accepted theory for the origin of the universe and is supported by measurements of background radiation and the observed expansion of space.

Binary
A system of two stars that revolve around a common center of gravity.

Black Hole
The collapsed core of a massive star. Stars that are very massive will collapse under their own gravity when their fuel is exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity. The gravitational pull is so strong that not even light can escape.

Black Moon
A term used to describe an extra new that occurs in a season. It usually refers to the third new moon in a season with four new moons. The term is sometimes used to describe a second new moon in a single month.

Blue Moon
A term used to describe an extra full that occurs in a season. It usually refers to the third full moon in a season with four full moons. Note that a blue moon does not actually appear blue in color. It is merely a coinsidence in timing caused by the fact that the lunar month is slightly shorter than a calendar month. More recently, the term has sometimes been used to describe a second full moon in a single month.

Blueshift
A shift in the lines of an object's spectrum toward the blue end. Blueshift indicates that an object is moving toward the observer. The larger the blueshift, the faster the object is moving.

Bolide
A term used to describe an exceptionally bright meteor. Bolides typically will produce a sonic boom.

C

Caldera
A type of volcanic crater that is extremely large, usually formed by the collapse of a volcanic cone or by a violent volcanic explosion. Crater Lake is one example of a caldera on Earth.

Catena
A series or chain of craters.

Cavus
A hollow, irregular depression.

Celestial Equator
An imaginary line that divides the celestial sphere into a northern and southern hemisphere.

Celestial Poles
The North and South poles of the celestial sphere.

Celestial Sphere
An imaginary sphere around the Earth on which the stars and planets appear to be positioned.

Cepheid Variable
This is a variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter Cepheids will have a longer period.

Chaos
A distinctive area of broken terrain.

Chasma
Another name used to describe a canyon.

Chondrite
A meteorite that contains chondrules.

Chondrule
Small, glassy spheres commonly found in meteorites.

Chromosphere
The part of the Sun's atmosphere just above the surface.

Circumpolar Star
A star that never sets but always stays above the horizon. This depends on the location of the observer.  The further South you go the fewer stars will be circumpolar. Polaris, the North Star, is circumpolar in most of the northern hemisphere.

Circumstellar Disk
A torus or ring-shaped accumulation of gas, dust, or other debris in orbit around a star in different phases of its life cycle.

Coma
An area of dust or gas surrounding the nucleus of a comet.

Comet
A gigantic ball of ice and rock that orbit the Sun in a highly eccentric orbit. Some comets have an orbit that brings them close to the Sun where they form a long tail of gas and dust as they are heated by the Sun's rays.

Conjunction
An event that occurs when two or more celestial objects appear close close together in the sky.

Constellation
A grouping of stars that make an imaginary picture in the sky.

Corona
The outer part of the Sun's atmosphere. The corona is visible from Earth during a total solar eclipse. It is the bright glow seen in most solar eclipse photos.

Cosmic Ray
Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy.

Cosmic String
A tube-like configuration of energy that is believed to have existed in the early universe. A cosmic string would have a thickness smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other.

Cosmogony
The study of celestial systems, including the Solar System, stars, galaxies, and galactic clusters.

Cosmology
A branch of science that deals with studying the origin, structure, and nature of the universe.

Crater
A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.

D

Dark Matter
A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.

Debris Disk
A ring-shaped circumstellar disk of dust and debris in orbit around a star. Debris disks can be created as the next phase in planetary system development following the protoplanetary disk phase. They can also be formed by collisions between planetesimals.

Declination
The angular distance of an object in the sky from the celestial equator.

Density
The amount of matter contained within a given volume. Density is measured in grams per cubic centimeter (or kilograms per liter). The density of water is 1.0, iron is 7.9, and lead is 11.3.

Disk
The surface of the Sun or other celestial body projected against the sky.

Double Asteroid
Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid.

Doppler Effect
The apparent change in wavelength of sound or light emitted by an object in relation to an observer's position. An object approaching the observer will have a shorter wavelength (blue) while an object moving away will have a longer (red) wavelength. The Doppler effect can be used to estimate an object's speed and direction.

Double Star
A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.

Dwarf Planet
A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. It has to have sufficient mass to overcome rigid body forces and achieve hydrostatic equilibrium. Pluto is considered to be a dwarf planet.

E

Eccentricity
The measure of how an object's orbit differs from a perfect circle. Eccentricity defines the shape of an object's orbit.

Eclipse
The total or partial blocking of one celestial body by another.

Eclipsing Binary
A binary system where one object passes in front of the other, cutting off some or all of its light.

Ecliptic
An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.

Ejecta
Material from beneath the surface of a body such as a moon or planet that is ejected by an impact such as a meteor and distributed around the surface. Ejecta usually appear as a lighter color than the surrounding surface.

Electromagnetic Radiation
Another term for light. Light waves created by fluctuations of electric and magnetic fields in space.

Electromagnetic Spectrum
The full range of frequencies, from radio waves to gamma waves, that characterizes light.

Ellipse
An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.

Elliptical Galaxy
A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.

Elongation
The angular distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern elongation is seen at its highest point above the horizon in the evening sky and a planet at greatest western elongation will be seen at its highest point above the horizon in the morning sky.

Ephemeris
A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar system objects.

Equinox
The two points at which the Sun crosses the celestial equator in its yearly path in the sky. The equinoxes occur on or near March 21 and September 22.  The equinoxes signal the start of the Spring and Autumn seasons.

Escape Velocity
The speed required for an object to escape the gravitational pull of a planet or other body.

Event Horizon
The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light.

Evolved Star
A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loose mass in the form of stellar wind.

Extinction
The apparent dimming of star or planet when low on the horizon due to absorption by the Earth's atmosphere.

Extragalactic
A term that means outside of or beyond our own galaxy.

Extraterrestrial
A term used to describe anything that does not originate on Earth.

Eyepiece
The lens at the viewing end of a telescope. The eyepiece is responsible for enlarging the image captured by the instrument.  Eyepieces are available in different powers, yielding differing amounts of magnification.

F

Faculae
Bright patches that are visible on the Sun's surface, or photosphere.

Filament
A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun.

Finder
A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired viewing location.

Fireball
An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even be accompanied by a sonic boom.

Flare Star
A faint red star that appears to change in brightness due to explosions on its surface.

G

Galactic Halo
The name given to the spherical region surrounding the center, or nucleus of a galaxy.

Galactic Nucleus
A tight concentration of stars and gas found at the innermost regions of a galaxy. Astronomers now believe that massive black holes may exist in the center of many galaxies.

Galaxy
A large grouping of stars. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant the their light takes millions of years to reach the Earth.

Galilean Moons
The name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius.

Gamma-ray
The highest energy, shortest wavelength form of electromagnetic radiation.

Geosynchronous Orbit
An orbit in which a satellite's orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless above one position of a planet's surface.

Giant Molecular Cloud (GMC)
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce thousands of stars and are frequently the sites of new star formation.

Globular Cluster
A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually found around the central regions of a galaxy.

Granulation
A pattern of small cells that can be seen on the surface of the Sun. They are caused by the convective motions of the hot gases inside the Sun.

Gravitational Lens
A concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object. Gravitational lensing results in duplicate images of distant objects.

Gravity
A mutual physical force of nature that causes two bodies to attract each other.

Greenhouse Effect
An increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere. Carbon dioxide and water vapor are two of the major gases responsible for this effect.

H

Heliopause
The point in space at which the solar wind meets the interstellar medium or solar wind from other stars.

Heliosphere
The space within the boundary of the heliopause containing the Sun and the Solar System.

Hydrogen
An element consisting of one electron and one proton. Hydrogen is the lightest of the elements and is the building block of the universe. Stars form from massive clouds of hydrogen gas.

Hubble's Law
The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.

Hydrostatic equilibrium
A state that occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. Hydrostatic equilibrium is responsible for keeping stars from imploding and for giving planets their spherical shape.

Hypergalaxy
A system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.

I

Ice
A term used to describe water or a number of gases such as methane or ammonia when in a solid state.

Inclination
A measure of the tilt of a planet's orbital plane in relation to that of the Earth.

Inferior Conjunction
A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.

Inferior Planet
A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.

International Astronomical Union (IAU)
An international organization that unites national astronomical societies from around the world and acts as the internationally recognized authority for assigning designations to celestial bodies and their surface features.

Interplanetary Magnetic Field
The magnetic field carried along with the solar wind.

Interstellar Medium
The gas and dust that exists in open space between the stars.

Ionosphere
A region of charged particles in a planet's upper atmosphere. In Earth's atmosphere, the ionosphere begins at an altitude of about 25 miles and extends outward about 250.

Iron Meteorite
A meteorite that is composed mainly of iron mixed with smaller amounts of nickel.

Irregular Galaxy
A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape.

Irregular Satellite
A satellite that orbits a planet far away with an orbit that is eccentric and inclined. They also tend to have retrograde orbits. Irregular satellites are believed to have been captured by the planet's gravity rather than being formed along with the planet.

J

Jansky
A unit used in radio astronomy to indicate the flux density (the rate of flow of radio waves) of electromagnetic radiation received from outer space. A typical radio source has a spectral flux density of roughly 1 Jy. The jansky was named to honor Karl Gothe Jansky who developed radio astronomy in 1932.

Jet
A narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.

K

Kelvin
A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.

Kepler's First Law
A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler's Second Law
A ray directed from the Sun to a planet sweeps out equal areas in equal times.

Kepler's Third Law
The square of the period of a planet's orbit is proportional to the cube of that planet's semi major axis; the constant of proportionality is the same for all planets.

Kiloparsec
A distance equal to 1000 parsecs.

Kirkwood Gaps
Regions in the main belt of asteroids where few or no asteroids are found. They were named after the scientist who first noticed them.

Kuiper Belt
A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper Belt objects are believed to be remnants of the original material that formed the Solar System. Some astronomers believe Pluto and Charon are Kuiper Belt objects.

L

Lagrange Point
French mathematician and astronomer Joseph Louis Lagrange showed that three bodies could lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable. Such bodies are sometimes referred to as Trojans. The leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5.

Lenticular Galaxy
A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.

Libration
An effect caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time.

Light Year
An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.

Limb
The outer edge or border of a planet or other celestial body.

Local Group
A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.

Luminosity
The amount of light emitted by a star.

Lunar Eclipse
A phenomenon that occurs when the Moon passes into the shadow of the Earth.  A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow.  In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow.

Lunar Month
The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month.

Lunation
The interval of a complete lunar cycle, between one new Moon and the next.  A lunation is equal to 29 days, 12 hours, and 44 minutes.

M

Magellanic Clouds
Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic Clouds are visible in the skies of the southern hemisphere.

Magnetic Field
A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.

Magnetic Pole
Either of two limited regions in a magnet at which the magnet's field is most intense.

Magnetosphere
The area around a planet most affected by its magnetic field. The boundary of this field is set by the solar wind.

Magnitude
The degree of brightness of a star or other object in the sky according to a scale on which the brightest star has a magnitude -1.4 and the faintest visible star has magnitude 6. Sometimes referred to as apparent magnitude. In this scale, each number is 2.5 times the brightness of the previous number. Thus a star with a magnitude of 1 is 100 times brighter than on with a visual magnitude of 6.

Main Belt
The area between Mars and Jupiter where most of the asteroids in our solar system are found.

Major Planet
A name used to describe any planet that is considerably larger and more massive than the Earth, and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of major planets.

Mare
A term used to describe a large, circular plain. The word mare means "sea". On the Moon, the maria are the smooth, dark-colored areas.

Mass
A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.

Matter
A word used to describe anything that contains mass.

Meridian
An imaginary circle drawn through the North and South poles of the celestial equator.

Metal
A term used by astronomers to describe all elements except hydrogen and helium, as in "the universe is composed of hydrogen, helium and traces of metals". This astronomical definition is quite different from the traditional chemistry definition of a metal.

Meteor
A small particle of rock or dust that burns away in the Earth's atmosphere.  Meteors are also referred to as shooting stars.

Meteor Shower
An event where a large number of meteors enter the Earth's atmosphere from the same direction in space at nearly the same time.  Most meteor showers take place when the Earth passes through the debris left behind by a comet.

Meteorite
An object, usually a chunk or metal or rock, that survives entry through the atmosphere to reach the Earth's surface. Meteors become meteorites if they reach the ground.

Meteoroid
A small, rocky object in orbit around the Sun, smaller than an asteroid.

Millibar
A measure of atmospheric pressure equal to 1/1000 of a bar. Standard sea-level pressure on Earth is about 1013 millibars.

Minor Planet
A term used since the 19th century to describe objects, such as asteroids, that are in orbit around the Sun but are not planets or comets. In 2006, the International Astronomical Union reclassified minor planets as either dwarf planets or small solar system bodies.

Molecular Cloud
An interstellar cloud of molecular hydrogen containing trace amounts of other molecules such as carbon monoxide and ammonia.

N

Nadir
A term used to describe a point directly underneath an object or body.

Nebula
A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material the stars are made of.

Neutrino
A fundamental particle produced by the nuclear reactions in stars. Neutrinos are very hard to detect because the vast majority of them pass completely through the Earth without interacting.

Neutron Star
A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy along their axis. These are known as pulsars.

Newton's First Law of Motion
A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

Newton's Second Law of Motion
For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

Newton's Third Law of Motion
In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

Nova
A star that flares up to several times its original brightness for some time before returning to its original state.

Nuclear Fusion
The nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. Nuclear fusion is the reaction that fuels the Sun, where hydrogen nuclei are fused to form helium.

O

Obliquity
The angle between a body's equatorial plane and orbital plane.

Oblateness
A measure of flattening at the poles of a planet or other celestial body.

Occultation
An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon.

Oort Cloud
A theoretical shell of comets that is believed to exist at the outermost regions of our solar system. The Oort cloud was named after the Dutch astronomer who first proposed it.

Open Cluster
A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.

Opposition
The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.

Orbit
The path of a celestial body as it moves through space.

P

Parallax
The apparent change in position of two objects viewed from different locations.

Parsec
A large distance often used in astronomy.  A parsec is equal to 3.26 light-years.

Patera
A shallow crater with a complex, scalloped edge.

Penumbra
The area of partial illumination surrounding the darkest part of a shadow caused by an eclipse.

Perigee
The point in the orbit of the Moon or other satellite at which it is closest to the Earth.

Perihelion
The point in the orbit of a planet or other body where it is closest to the Sun.

Perturb
To cause a planet or satellite to deviate from a theoretically regular orbital motion.

Phase
The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.

Photon
A particle of light composed of a minute quantity of electromagnetic energy.

Photosphere
The bright visible surface of the Sun.

Planemo
A large planet or planetary body that does not orbit a star. Planemos instead wander cold and alone through the cosmos. It is believed that most planemos once orbited their mother star but were ejected from the star system by gravitational interaction with another massive object.

Planet
A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals.

Planetary Nebula
A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colors and shapes.

Planetesimal
A solid object that is believed to exist in protoplanetary disks and in debris disks. Planetesimals are formed from small dust grains that collide and stick together and are the building blocks that eventually form planets in new planetary systems.

Planitia
A low plain.

Planum
A high plain or plateau.

Plasma
A form of ionized gas in which the temperature is too high for atoms to exist in their natural state. Plasma is composed of free electrons and free atomic nuclei.

Precession
The apparent shift of the celestial poles caused by a gradual wobble of the Earth's axis.

Prominence
An explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.

Prograde Orbit
In reference to a satellite, a prograde orbit means that the satellite orbits the planet in the same direction as the planet's rotation. A planet is said to have a prograde orbit if the direction of its orbit is the same as that of the majority of other planets in the system.

Proper Motion
The apparent angular motion across the sky of an object relative to the Solar System.

Protoplanetary Disk
A rotating circumstellar disk of dense gas surrounding a young newly formed star. It is thought that planets are eventually formed from the gas and dust within the protoplanetary disk.

Protostar
Dense regions of molecular clouds where stars are forming.

Pulsar
A spinning neutron star that emits energy along its gravitational axis. This energy is received as pulses as the star rotates.

Q

Quadrature
A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.

Quasar
An unusually bright object found in the remote areas of the universe. Quasars release incredible amounts of energy and are among the oldest and farthest objects in the known universe. They may be the nuclei of ancient, active galaxies.

Quasi-Stellar Object
Sometimes also called quasi-stellar source, this is a star-like object with a large redshift that gives off a strong source of radio waves. They are highly luminous and presumed to be extragalactic.

R

Radial Velocity
The movement of an object either towards or away from a stationary observer.

Radiant
A point in the sky from which meteors in a meteor shower seem to originate.

Radiation
Energy radiated from an object in the form of waves or particles.

Radiation Belt
Regions of charged particles in a magnetosphere.

Radio Galaxy
A galaxy that gives off large amounts of energy in the form of radio waves.

Red Giant
A stage in the evolution of a star when the fuel begins to exhaust and the star expands to about fifty times its normal size. The temperature cools, which gives the star a reddish appearance.

Redshift
A shift in the lines of an object's spectrum toward the red end. Redshift indicates that an object is moving away from the observer. The larger the redshift, the faster the object is moving.

Regular Satellite
A satellite that orbits close to a planet in a nearly circular, equatorial orbit. Regular satellites are believed to have been formed at the same time as the planet, unlike irregular satellites which are believed to have been captured by the planet's gravity.

Resonance
A state in which an orbiting object is subject to periodic gravitational perturbations by another.

Retrograde Motion
The phenomenon where a celestial body appears to slow down, stop, them move in the opposite direction. This motion is caused when the Earth overtakes the body in its orbit.

Retrograde Orbit
The orbit of a satellite where the satellite travels in a direction opposite to that direction of the planet's rotation.

Right Ascension
The amount of time that passes between the rising of Aries and another celestial object. Right ascension is one unit of measure for locating an object in the sky.

Ring Galaxy
A galaxy that has a ring-like appearance. The ring usually contains luminous blue stars. Ring galaxies are believed to have been formed by collisions with other galaxies.

Roche Limit
The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At a lesser distance the tidal forces of the primary would break up the secondary.

Rotation
The spin of a body about its axis.

S

Saros Series
Also known as a saros cycle, a period of 223 synodic months that can be used to predict solar and lunar eclipses. The saros cycle is equal to 6,585.3 days (18 years 11 days 8 hours).

Satellite
A natural or artificial body in orbit around a planet.

Scarp
A line of cliffs produced erosion or by the action of faults.

Seyfert Galaxy
A main-sequence star that rotates rapidly, causing a loss of matter to an ever-expanding shell.

Shell Star
A type of star which is believed to be surrounded by a thin envelope of gas, which is often indicated by bright emission lines in its spectrum.

Shepherd Satellite
A satellite that constrains the extent of a planetary ring through gravitational forces. Also known as a shepherd moon.

Sidereal
Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.

Sidereal Month
The average period of revolution of the Moon around the Earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.

Sidereal Period
The period of revolution of a planet around the Sun or a satellite around its primary.

Singularity
The center of a black hole, where the curvature of space time is maximal. At the singularity, the gravitational tides diverge. Theoretically, no solid object can survive hitting the singularity.

Small Solar System Body
A term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets or dwarf planets. These include most of the asteroids, comets, and other small bodies in the Solar System.

Solar Cycle
The approximately 11-year quasi-periodic variation in frequency or number of solar active events.

Solar Eclipse
A phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar eclipse occurs when the Moon is close enough to completely block the Sun's light. An annular solar eclipse occurs when the Moon is farther away and is not able to completely block the light. This results in a ring of light around the Moon.

Solar Flare
A bright eruption of hot gas in the Sun's photosphere.  Solar prominences are usually only detectable by specialized instruments but can be visible during a total solar eclipse.

Solar Nebula
The cloud of dust and gas out of which the Solar System was believed to have formed about 5 billion years ago.

Solar Wind
A flow of charged particles that travels from the Sun out into the Solar System.

Solstice
The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of the Summer and Winter seasons.

Spectrometer
The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.

Spectroscopy
The technique of observing the spectra of visible light from an object to determine its composition, temperature, density, and speed.

Spectrum
The range of colors that make up visible white light. A spectrum is produced when visible light passes through a prism.

Spicules
Grass-like patterns of gas seen in the atmosphere of the Sun.

Spiral Galaxy
A galaxy that contains a prominent central bulge and luminous arms of gas, dust, and young stars that wind out from the central nucleus in a spiral formation. Our galaxy, the Milky Way, is a spiral galaxy.

Star
A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.

Star Cluster
A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.

Steady State Theory
The theory that suggests the universe is expanding but exists in a constant, unchanging state in the large scale. The theory states that new matter is being continually being created to fill the gaps left by expansion. This theory has been abandoned by most astronomers in favor of the big bang theory.

Stellar Wind
The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind. A star's stellar wind is strongest near the end of its life when it has consumed most of its fuel.

Stone Meteorite
A meteorite that resembles a terrestrial rock and is composed of similar materials.

Stony Iron
A meteorite that contains regions resembling both a stone meteorite and an iron meteorite.

Sunspot
Areas of the Sun's surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the Sun. Sunspots are usually associated disturbances in the Sun's electromagnetic field.

Supergiant
The stage in a star's evolution where the core contracts and the star swells to about five hundreds times its original size. The star's temperature drops, giving it a red color.

Supermoon
A term used to describe a full moon that occurs during the Moon's closest approach to the Earth. During a supermoon, the Moon may appear slightly larger and brighter than normal.

Superior Conjunction
A conjunction that occurs when a planet passes behind the Sun and is on the opposite side of the Sun from the Earth.

Superior Planet
A planet that exists outside the orbit of the Earth. All of the planets in our solar system are superior except for Mercury and Venus. These two planets are inferior planets.

Supernova
A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life.  Supernovae are the most powerful forces in the universe. All of the heavy elements were created in supernova explosions.

Supernova Remnant
An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many resemble "bubbles" in space.

Synchronous Rotation
A period of rotation of a satellite about its axis that is the same as the period of its orbit around its primary. This causes the satellite to always keep the same face to the primary. Our Moon is in synchronous rotation around the Earth.

Synodic Month
The period of time it takes the Moon to make one complete revolution around the Earth. A Synodic month is equal to 29.53 days and is measured as the time between a lunar phase and the return of that same phase.

Synodic Period
The interval between points of opposition of a superior planet.

T

Tektite
A small, glassy material formed by the impact of a large body, usually a meteor or asteroid. Tektites are commonly found at the sites of meteor craters.

Telescope
An instrument that uses lenses and sometimes mirrors to collect large amounts of light from distant objects and enable direct observation and photography. A Telescope can also include any instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves.

Terminator
The boundary between the light side and the dark side of a planet or other body.

Terrestrial
A term used to describe anything originating on the planet Earth.

Terrestrial Planet
A name given to a planet composed mainly of rock and iron, similar to that of Earth.

Tidal Force
The differential gravitational pull exerted on any extended body within the gravitational field of another body.

Tidal Heating
Frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and/or other neighboring satellites.

Transit
The passage of a celestial body across an observer's meridian; also the passage of a celestial body across the disk of a larger one.

Trans-Neptunian Object (TNO)
Any one of a number of celestial objects that orbit the Sun at a distance beyond the orbit of the planet Neptune.

Trojan
An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter's Lagrange points. Saturn's moons Helene, Calypso and Telesto are also sometimes called Trojans.

U

Ultraviolet
Electromagnetic radiation at wavelengths shorter than the violet end of visible light. The atmosphere of the Earth effectively blocks the transmission of most ultraviolet light, which can be deadly to many forms of life.

Umbra
The area of total darkness in the shadow caused by an eclipse.

Universal Time (UT)
Also known as Greenwich Mean Time, this is local time on the Greenwich meridian. Universal time is used by astronomers as a standard measure of time.

V

Van Allen Belts
Radiation zones of charged particles that surround the Earth. The shape of the Van Allen belts is determined by the Earth's magnetic field.

Variable Star
A star that fluctuates in brightness. These include eclipsing binaries.

Visible Light
Wavelengths of electromagnetic radiation that are visible to the human eye.

Virgo Cluster
A gigantic cluster of over 2000 galaxies that is located mainly within the constellation of Virgo. This cluster is located about 60 million light-years from Earth.

Visual Magnitude
A scale used by astronomers to measure the brightness of a star or other celestial object. Visual magnitude measures only the visible light from the object. On this scale, bright objects have a lower number than dim objects.

W

Wavelength
The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation.

White Dwarf
A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a planetary nebula, with the white dwarf star at its center.

X

X-ray
Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays.

X-ray Astronomy
The field of astronomy that studies celestial objects by the x-rays they emit.

X-ray Star
A bright celestial object that gives off x-rays as a major portion of its radiation.

Y

Yellow Dwarf
An ordinary star such as the Sun at a stable point in its evolution.

Z

Zenith
A point directly overhead from an observer.

Zodiac
An imaginary belt across the sky in which the Sun, moon, and all of the planets can always be found.

Zodiacal Light
A faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise.  Zodiacal light is caused by sunlight reflecting off small particles of material in the plane of the Solar System.

 







 Glossary

colorbar

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

colorbar

Absolute magnitude- the brightness a star would be as seen from a distance of 10 parsecs

Absolute Zero- the lowest possible temperature, at which substances contain no heat energy, and atomic movement has stopped

Accretion- accumulation of dust and gas into larger bodies such as stars, planets, and moons

Accretion disk- a disk of hot, glowing matter spiraling into a black hole

Active galaxy- a galaxy under-going a violent outburst in its central regions

Adaptive optics- the technology that allows, based on a laser beam aimed through the atmosphere, a computer to make very slight modifications to a telescope's mirror, which will correct for atmospheric distortions

Albedo- the reflecting power of a non-luminous body; a perfect reflector would have an albedo of 100 per cent

Albedo feature- a dark or light marking on the surface of an object that may not be a geological or topographical feature

Altazimuth mounting- a telescope mounting that swings from side to side parallel to the horizon, and up and down

Angstrom unit- the hundred-millionth part of a centimeter (10-10 m)

Annihilation- total destruction of matter in a burst of energy

Anthropic principle- we see the universe the way it is because if it were different we would not be here to observe it

Antigravity- a gravitational field that repels, rather than attracts, matter and light rays

Antimatter- the exact opposite of matter; when matter meets the tiny amount of antimatter in the universe, the two annihilate each other (see antiparticle)

Antiparticle- an atomic particle that has exactly the opposite properties of its counter-part (e.g. a positron and an electron)

Antipodal point- the point that is directly on the opposite side of the planet
Roman Numeral 	Description
I 	Perfect seeing, without a quiver
II 	Slight undulations, with some moments of calm lasting several seconds
III 	Moderate seeing, with larger air tremors
IV 	Poor seeing, with constant troublesome undulations
V 	Very poor seeing, scarcely allowing the making of a rough sketch

Antoniadi Scale- a roman numeral indicates the quality of seeing according to the following scale:

Aperture- the diameter of an opening through which light passes in an optical instrument

Aphelion- the position of a planet (or other body) when it is at its furthest from the sun

Apparent magnitude- the brightness of an object as seen from Earth

Arcuate- having the form of a bow; curved; arc-shaped

Asteroid- a small rocky object orbiting the sun, less than 1,000 kilometers in diameter

Astronomical Unit (A.U.)- the average distance between the Earth and the sun, roughly 150 million km (93 million miles)

Atmosphere- the layer of gases enveloping a celestial object

Atom- the smallest part of an element that can take part in a chemical reaction; most of the mass of the an atom is concentrated in its nucleus, which is about .000000000001 meters (.01 angstroms) across

Aurora- curtains and arks of light in the sky visible over middle and high latitudes; they are caused by particles from the sun hitting the Earth's atmosphere and causing some of its gases to glow

Axis- the imaginary line through the center of a planet, star, or galaxy around which it rotates; also, a similar line through a telescope mounting

Bar- equals 0.987 atmosphere = 1.02 kg/cm2= 100 kilopascal = 14.5 lbs/inch2

Baryon- baryons are composed of three quarks; they include protons and neutrons

Big Bang- the violent event that gave birth to our universe

Big Crunch- the ultimate collapse of the universe that may take place in the future if the universe starts to contract

Binary system- system of two stars that orbit around each other

Black dwarf- a dead star with a maximum possible mass of 1.4 solar masses that has cooled to a point where it no longer glows with residual heat

Black hole- a collapsed object with such strong gravity that nothing can escape it; as a result, the object is black, and it is a hole because nothing can escape from it

Blazar- a type of active galaxy that is angled in such a way to us that we look almost directly at its accretion disk and jet

Blue shift- a shift in the wave-length of radiation emitted by an object when it is approaching us; the Doppler shift makes the wave fronts bunch closer together, causing the light to appear of a shorter wavelength, and hence bluer

Bolide- a fireball that produces a sonic boom

Brown dwarf- a "failed star" in the sense that when it was finished forming, it did not have enough mass to begin fusion; it does not shine as a star does, but can generate heat through very slow gravitational contraction -- this works because when a gas is compressed, it gains temperature

Caldera- a volcanic crater

Carbonate- a compound containing carbon and oxygen (e.g. calcium carbonate AKA limestone)

Casimir effect- the attractive pressure between two flat, parallel metal plates placed very near to each other in a vacuum; the pressure is due to a reduction in the usual number of virtual particles in the space between the plates

Catadioptric telescope- a telescope that uses both mirrors and lenses to form and image

Catena- a chain of craters

Cavus- a hollow, irregular depression

Charge-coupled device (CCD)- a computer-controlled electronic detector that can record an image

Celestial equator- the imaginary line encircling the sky midway between the two celestial poles

Celestial poles- the imaginary points on the sky where Earth's rotation axis, extended infinitely, would touch the imaginary celestial sphere

Celestial sphere- the imaginary sphere enveloping the Earth upon which the stars, galaxies, and other celestial objects all appear to lie

Cepheid variable- a variable star of short period; the fluctuations are regular and are linked with its real luminosity; the longer the period, the more luminous the star

Chandrasekhar limit- the maximum possible mass of a stable cold star, above which it must collapse into a black hole

Chaos- a distinctive area of broken terrain

Chasma- a canyon

Chromosphere- that part of the sun's atmosphere that lies just above its visible surface, or photosphere

Circumpolar stars- stars that never set when seen from a given location

Colles- small hills or knobs

Collimation- the procedure of aligning a telescope's optics

Coma- the dust and gas surrounding an active comet's nucleus

Comet- a small body composed of ices and dust which orbits the sun on an elongated path

Conjunction- the moment when two celestial objects lie closest together in the sky

Conservation of angular momentum- the law of science that states that momentum must be conserved within a system

Conservation of energy- the law of science that states that energy (or its equivalent in mass) can neither be created nor destroyed

Constellation- one of the eighty-eight official patterns of stars into which the night sky is divided

Convection- fluid circulation driven by large temperature gradients; the transfer of heat by this automatic circulation

Corona- the high-temperature outermost atmosphere of the sun, visible from Earth only during a total solar eclipse

Coronagraph- a special telescope which blocks light from the disk of the Sun in order to study the faint solar atmosphere

Cosmic censor- mythical being who dictates that singularities must be surrounded by an event horizon

Cosmic ray- an extremely energetic (relativistic) charged particle

Cosmological Constant- a mathematical device used by Einstein to give space-time an inbuilt tendency to expand

Cosmology- the study of the universe as a whole

Core- in a star, the central region that is undergoing nuclear fusion; in a galaxy, the innermost few light-years

Crater- bowl-shaped depression formed by the impact of a meteoroid; depression around the orifice of a volcano

Culmination- the maximum altitude of a celestial body from the celestial equator

Dark adaptation- the process by which the human eye increases sensitivity under conditions of low, or none, illumination

Dark matter- invisible matter that is believed to make up 99% of the mass of the universe

Declination- the angular distance of a celestial object north or south of the celestial equator; it corresponds to latitude on the Earth

Density- degree of "solidity" of a body: its mass divided by its volume

Direct motion- the movement of a celestial body from west to east - that is, in the same direction as that of the Earth around the sun

Disk- the visible surface of the Sun (or any heavenly body) projected against the sky

Doppler effect- change in the observed frequency of sound or radiation that takes place when the observer and the source are moving relative to each other

Dorsum- a ridge

Double-star system- a system of two stars in orbit around each other

Duality- a correspondence between apparently different theories that lead to the same physical results

Dust- microscopic grains in space that absorb starlight; the grains are "soot" left by dying stars, and they will sometimes clump together in huge dark clouds

Dwarf star- a star, such as the sun, that lies on the Main Sequence

Eccentricity- a measure of how closely a planet's orbit approximates to a perfect circle

Eclipse- when one celestial body passes in front of another, dimming or obscuring its light (e.g. solar, lunar, and eclipsing binaries)

Ecliptic- the apparent yearly path of the sun against the stars

Einstein-Rosen bridge- the "throat" of a black hole in one universe connecting up with one in a different universe; in theory, it is a bridge from one universe to another

Electromagnetic radiation- radiation made up of magnetic and electrical fields that move at the speed of light

Electron- tiny particle with a negative charge, often in orbit around the nucleus of an atom

Electroweak unification energy- the energy (around 100 GeV) above which the distinction between the electromagnetic force and the weak force disappears

Elementary particle- a particle that, it is believed, cannot be subdivided

Ellipse- the oval, closed path followed by a celestial object moving under gravity (e.g. a planet around the sun)

Elongation- the angular distance of a planet from the sun or a satellite from its primary planet

Equatorial mounting- a telescope mounting which has one axis parallel to the Earth's rotational axis, so the motion of the heavens can be followed with a single movement

Equinox- the two points at which the sun crosses the celestial equator; the spring equinox is about March 21, and the autumnal equinox is about September 22

Ergosphere- region surrounding a spinning black hole, between the static limit and the outer event horizon, in which it is impossible to be at rest

Escape velocity- speed a body needs to travel in order to escape the surface gravity of a star or planet

Event- a point in space-time, specified by its time and place

Event horizon- the "edge" of a black hole: and imaginary surface where the escape velocity reaches the speed of light

Exclusion Principle- the idea that two identical spin-1/2 particles cannot have (within the limits set by the uncertainty principle) both the same position and the same velocity

Extinction- the apparent reduction in the brightness of a star or planet when low over the horizon because more of its light is absorbed by the Earth's atmosphere

Eyepiece- a set of lenses used to magnify the image produced by a telescope's objective

Faculae- the bright patches on the sun's photosphere

Farrum- a pancake-like structure (like those on Venus)

Field- something that exists throughout space and time, as opposed to a particle that exists at only one point at a time

Filament- a strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun; a filament on the limb of the Sun seen in emission against the dark sky is called a prominence

Finder- a small, low-power telescope attached to and aligned with a larger one; its wider field of view makes it useful for locating celestial objects

Fireball- any meteor brighter than Venus, about magnitude -4

Flare, solar- brilliant outbreaks in the solar atmosphere, normally detectable only by spectroscopic methods

Flare star- a faint red star that has short-lived explosions on its surface; these explosions cause the star to appear temporarily brighter

Fluctus- flow terrain

Fossa- long, narrow, shallow depression

Fraunhofer Lines- the dark lines in the spectrum of the sun

Frequency- for a wave, the number of complete cycles per second

Gaia Hypothesis- named for the Greek Earth goddess Gaea, holds that the Earth as a whole should be regarded as a living organism and that biological processes stabilize the environment. First advanced by British biologist James Lovelock in 1969

Galactic cluster- (see open cluster)

Galaxy- a huge gathering of stars, gas, and dust, bound by gravity and having a mass ranging from 100,000 to 10 trillion times that of the sun; there are spiral, elliptical, and irregular types of galaxies

Gamma rays- the highest energy, shortest wavelength electromagnetic radiation of all

Gegenschein- a round or elongated spot of light in the sky at a point 180 degrees from the sun; also called counter glow

General relativity- the theory of relativity that describes how matter behaves in the presence of strong gravitational fields

Geodesic- the shortest (or longest) path between two points

Geosynchronous orbit- a direct, circular, low inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface

Globular cluster- a spherical cluster that may contain up to a million stars

Grand unification energy- the energy above which, it is believed, the electromagnetic force, weak force, and strong force become indistinguishable from each other

Grand Unified Theory (GUT)- a theory that unifies the electromagnetic, strong, and weak energy forces

Granulation- a pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas

Gravitational lens- distortion of an image - or the production of many images - by a powerful gravitational field

Gravitational waves- ripples in space that travel at the speed of light, produced by the movement of very massive bodies

Gravity- force of attraction that is felt between two masses, such as the pull between the Earth and the Moon

Hadron- particles made of quarks that are influenced by the Strong Nuclear Force; includes mesons and bosons, but excludes leptons

Heliocentric- sun-centered solar system theories

Heliopause- the point at which the solar wind meets the interstellar medium or solar wind from other stars

Heliosphere- the space within the boundary of the heliopause containing the Sun and solar system

Hertzsprung-Russel (HR) Diagram- a graph whose horizontal axis plots star color (or temperature) against a vertical axis plotting stellar luminosity (or absolute magnitude)

Ice- used by planetary scientists to refer to water, methane, and ammonia which usually occur as solids in the outer solar system

Imaginary time- time measured using imaginary numbers (i.e. the square root of negative one)

Inclination- measure of the tilt of a planet's orbital plane, in relation to that of the Earth

Inferior planet- either of the two planets, Mercury and Venus, that orbit between the sun and the Earth

Inferometry- if one combines two or more telescope's light, the resulting image would have the resolution of a telescope the size of the distance separating them

Infrared- heat radiation, inter../mediate in wavelength between light and radio waves

Interplanetary Magnetic Field (IMF)- the magnetic field carried with the solar wind

Kelvin (K)- 0 Kelvin is absolute zero; water melts at 273 K; water boils at 373 K; developed by William Thomson

Lacus- lake

Lagrange points- Lagrange showed that three bodies can lie at the apexes of an equilateral triangle which rotates in its plane; if one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable (such bodies are sometimes referred to as Trojans); the leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5

Last stable orbit- the closest an object can circle a black hole without being pulled in

Lepton- one of the fundamental types of particles, with quarks being the other; common examples are electrons and neutrinos

Libration- an effect caused by the apparent slight "wobbling" of the Moon from side to side, as seen from Earth; as a result, a total of fifty-nine per cent of the Moon can be observed from Earth, though no more than fifty per cent at one time

Lidar- an instrument similar to radar that operates at visible wavelengths

Light-year- distance covered by a ray of light traveling at 300,000 kps (186,000 mps) in a year; it is about 9,460,528,404,880 km (5,878,499,814,140 miles)

Light cone- a surface in space-time that marks out the possible directions for light rays passing through a given event

Limb- an edge or border, as of the sun, Moon, or any planet

Local Group- a gathering of roughly thirty galaxies to which the Milky Way belongs

Luminosity- the total intrinsic brightness of a star or galaxy

Lunar month- The average time between successive new or full moons, equal to 29 days 12 hours 44 minutes

Lunation- the interval between one new moon and the next: that is, 29 days, 12 hours, and 44 minutes

MACHO- an acronym for "Massive Compact Halo Object" - supposed dark, massive objects surrounding our galaxy

Magnetosphere- the are around a planet in which its magnetic field is dominant

Magnetotail- the portion of a planetary magnetosphere which is pushed away from the sun by the solar wind

Magnetic field- the field responsible for magnetic forces, now incorporated along with the electric field, into the electromagnetic field

Magnitude- a logarithmic unit used to measure the optical brightness of celestial objects; numerically lower magnitudes are brighter than numerically larger ones; a five-magnitude difference represents a 100-fold change in brightness

Main Sequence- the band on the HR Diagram where stars lie for much of their life

Mare- literally "sea" (a very bad misnomer, still in use for historical reasons); really a large circular plain

Mass- amount of matter making up a body

Matter- what everything that we know of is made of; the opposite of antimatter

Meridian- an imaginary line on the sky that runs due north and south and passes through your zenith

Meson- matter that is composed of two quarks

Metal- used by astrophysicists to refer to all elements except hydrogen and helium, as in: "The universe is composed of hydrogen, helium and traces of metals;" (note: this is quite different from the usual chemistry definition)

Meteor- the bright, transient streak of light produced by a piece of space debris burning up as it enters the atmosphere at high speed

Meteorite- any piece of space debris that reaches the Earth's surface intact

Milky Way- a soft, glowing band of light encircling the sky, it is the disk of the spiral galaxy in which the sun lies, seen from the inside

Microwave background radiation- the radiation from the glowing of the hot early universe, now so greatly red-shifted that it appears not as light but as microwaves (radio waves with a wavelength of a few centimeters)

Mini black hole- one of many tiny black holes with the mass of a mountain but the size of an atom that are believed to have been created in the Big Bang

Minor planets- the official term used for asteroids

Missing mass- several seperate experiments show that there is more matter in the universe than we can see; more than that, this so-called "dark matter" accounts for about 25% of the universe's total mass-energy, but there is a "dark energy" which appears to be moving everything apart from everything else (acting only at very large scales) that seems to account for 70% of the universe's mass-energy content

Mons- mountain (plural is montes)

Naked singularity- a singularity that is not surrounded by an event horizon

Nebula- a cloud of dust and gas in space, from which new stars are created

Neutrino- miniscule particle with little or no mass and no charge that travels at the speed of light

Neutron- electrically neutral particle that makes up part of the nucleus of an atom

Neutron star- collapsed star composed mainly of neutrons; pulsars are young, fast-spinning neutron stars

No boundary condition- the idea that the universe is finite but has no boundary (in imaginary time)

Nova- a white dwarf star in a binary system that brightens suddenly by several magnitudes as gas pulled away from its companion star explodes in a thermonuclear reaction

Nuclear fusion- nuclear reaction in which one kind of atom, under extreme heat and pressure, is combined with another and forms a different one

Nucleus:

    the central part of an atom, consisting only of protons and neutrons, held together by the strong force
    the central part of a comet, whose existence is not a function of its distance from the sun -- the only "permenant" feature of a comet

Objective- the main light gathering optical element in a telescope; it may be a lens or a mirror

Oblateness- the degree of flattening at the poles of a celestial body

Occulation- the covering up of one celestial object by another, such as the Moon passing in front of a star or planet as seen from Earth

Open cluster- a group of some few hundred stars bound by gravity and moving through space together

Opposition- the position of a superior planet when exactly opposite the sun in the sky, as seen from Earth; the planet is then best placed for observation

Orbit- the path followed by any celestial object moving under the control of another's gravity

Orbital family- a group of asteroids that follows the same relative orbital path, velocity, and is usually seen close together; they are thought to have once been one asteroid that was broken apart due to a collision in the past

Parallax- the apparent change in position of a nearby star due to Earth's orbital motion around the sun

Parsec- a unit of distance equal to 3.26 light-years; it is the distance at which a star would have a parallax of one second of arc

Particle accelerator- a machine that, using electromagnets, can accelerate moving charged particles, giving them more energy

Patera- a shallow crater with a scalloped, complex edge

Penumbra- the outer part of an eclipse shadow; also, the lighter area surrounding the center of a sunspot

Perihelion- a planet or comet's closest approach to the sun

Periodic time- (see sidereal period)

Perturb- to cause a planet or satellite to deviate from a theoretically regular orbital motion

Phase- the varying illuminated part of the Moon and planets caused by the relative locations of the object, the Earth, and the sun

Photon- a quantum of light

Photosphere- the visible surface of the sun or a star

Plage- bright regions seen in the solar chromosphere

Planetary nebula- a shell of gas puffed off by a star late in its life; their often round appearance led to the name

Planitia- plateau or high plain

Plank's Quantum Principle- the idea that light (or any other classical waves) can be emitted or absorbed only in discrete quanta, whose energy is proportional to their wavelength

Positron- the positively charged antiparticle of the electron

Precession- a slow periodic wobble in the Earth's axis caused by the gravitational pull of the sun and Moon

Prominence, solar- a loop of cooler gas seen above the sun's surface, which sometimes erupts outwards into space

Proton- a positively charged particle that forms part of the nucleus of an atom

Protoplanet- a stage in the formation of a planet which implies the body is nearly full-size

Protostar- a stage in the formation of a star which implies the body is nearly full-size; the star is still within its parent nebula, and does not yet produce energy through nuclear fusion

Pulsar- collapsed star composed mainly of neutrons; pulsars are young, fast-spinning neutron stars

Quantum- the indivisible unit in which waves may be emitted or absorbed

Quantum chronodynamics (QCD)- the theory that describes the interactions of quarks and gluons

Quantum mechanics- the theory developed from Plank's Quantum Principle and Heisenberg's Uncertainty Principle

Quasar- the brilliant core of a distant young active galaxy with outer regions that are often too faint to be visible

Quark- a (charged) elementary particle that feels the strong force; protons and neutrons are composed of three quarks each

Radar- a system using pulsed radio waves to detect the position of objects by measuring the time it takes a single pulse to reach the object and be reflected back

Radial velocity- the movement of a celestial body toward or away from an observer

Radiant- the point on the sky from where a shower of meteors appears to come

Radiation- made up of magnetic and electrical fields that move at the speed of light

Radio galaxy- active galaxy that gives out as much energy in radio waves as it does in light

Radio telescope- a telescope that picks up radio waves from objects in space

Radioactivity- the spontaneous breakdown of one type of atomic nucleus into another

Red giant- an old star whose outer layers have billowed out and cooled down

Red shift- shift in the light of a retreating object toward red wavelengths, caused by the Doppler effect

Reflector- a telescope that forms an image with mirrors

Refractor- a telescope that forms an image with a lens

Relativity- (see general relativity and special relativity)

Resolving power- the ability of a telescope to image two closely spaced objects as separate

Resonance- a state in which one orbiting object is subject to periodic gravitational perturbations by another

Retrograde:

    an apparent westward motion of a planet, asteroid, or comet relative to the stars
    the motion of a moon that orbits contrarty to the spin of its planet -- a strong indication that the moon was captured and did not form in the vicinity of the planet

Rift valley- an elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike

Right ascension- the celestial coordinate analogous to longitude on Earth

Roche Limit- the closest one celestial object can get to another before the weaker object is broken apart by tidal effects

Satellite- any small object orbiting a larger one, although the term is most often used for rocky or man-made objects orbiting a planet

Scarp- line of cliffs produced by faulting or erosion

Schwarzchild Radius- radius of the event horizon around a black hole

Seeing- a measure of the steadiness of the atmosphere; good seeing is essential to using high magnification

Semimajor axis- the semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru the foci of the ellipse with endpoints on the ellipse itself; the semimajor axis of a planetary orbit is also the average distance from the planet to its primary

Seyfert galaxy- a galaxy that has a small, bright nucleus and faint spiral arms; it is often a strong radio source

Shepherd moon- a satellite which constrains the extent of a planetary ring through gravitational forces

Sidereal period- the time, relative to the stars, needed for a planet or moon to make one rotation or revolution around its primary body

Singularity- the center of a black hole; a point (or ring) of infinite density that occupies zero space

Singularity theorem- a theorem that shows that a singularity must exist under certain circumstances - in particular, that the universe must have started with a singularity

Solar cycle- the approximately 11-year quasi-periodic variation in frequency or number of solar active events

Solar filter- a filter that reduces the sun's light to a level where you can view it with a telescope; only those filters which fit over a telescope's objective are safe to use

Solar mass- the mass of the sun; it is used as a standard weight against which other celestial objects can be compared

Solar nebula- the cloud of gas and dust that began to collapse about 5 billion years ago to form the Solar System

Solar System- our sun with everything that orbits it: nine planets (plus their satellites), thousands of asteroids, and countless comets, meteors, and other debris

Solar wind- charged particles from the sun that travel into the Solar System at about 1.5 million kph (932,000 mph)

Solstice- the time when the sun reaches its greatest northern or southern declination

Space-Time- the four-dimensional description of the universe in which length, breadth, and height make up the first three spatial dimensions, while time makes up the fourth dimension

Spaghettification- gravitational stretching of a body falling into a black hole

Spatial dimension- any of the three dimensions that are space-like - that is, any except the time dimension (i.e. length, width, depth)

Special relativity- a branch of relativity dealing with the behavior of objects traveling close to the speed of light

Spectrograph- an instrument that breaks the light from a celestial object into its component colors for analysis

Spectroscopic binary- a very close double that is recognizable only by the periodic splitting of lines in the combined spectrum of the two stars, owing to the opposite Doppler effects resulting from their motions

Spectrum- the range of color produced when light is split up by a prism of diffraction grating

Spicules- grass-like patterns of gas seen in the solar atmosphere

Static limit- a limit close to a black hole inside of which is impossible to remain at rest

Stationary state- one that is not changing with time; a sphere spinning at a constant rate is stationary because it looks identical at any given instant

Stellar mass black hole- a black hole produced by the explosion of a massive star as a supernova; most weigh about ten solar masses

String Theory- a theory of physics in which particles are described as waves on strings; strings have length but no other dimension

Strong force- the strongest of the four fundamental forces, with the shortest range of all; it holds the quarks together within protons and neutrons, and holds the protons and neutrons together to form atoms

Sunspot- a highly magnetized dark spot on the sun's surface, cooler than surrounding area

Supergiant- the stage in the evolution of a massive star when its core contracts, its surface expands to about 500 solar radii, and its temperature drops, giving the star its red color

Superior planet- any planet beyond the orbit of the Earth in the Solar System

Supermassive black hole- a black hole located at the center of a galaxy; these holes, formed by material falling onto the galaxy's core, may weight billions of solar masses

Supernova- an explosion of a massive star at the end of its life; the star may briefly equal an entire galaxy in brightness

Supernova remnant- the gaseous debris, rich in heavy elements, thrown off by a supernova

Synchronous orbit radius- the orbital radius at which the satellite's orbital period is equal to the rotational period of the planet; a synchronous satellite with an orbital inclination of zero (same plane as the planet's equator) stays fixed in the sky from the perspective of an observer on the planet's surface (such orbits are commonly used for communications satellites; it is also seen in the cases of Pluto and Charon)

Synchronous rotation- said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary; this implies that the satellite always keeps the same hemisphere facing its primary (e.g. the moon); it also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward; most of the satellites in the solar system rotate synchronously

Synodic month- see lunar month

Synodic period- the interval between the successive oppositions, conjunctions, etc., of a celestial body

Telrad- a sighting device for telescopes which projects a bull's-eye on the sky

Terminator- the boundary between the illuminated and dark portions of a planet or satellite

Terra- extensive land mass

Terrestrial- refers to either Mercury, Venus, Earth, or Mars, due to them all having a solid surface, and they are within the inner solar system

Tholus- small dome-shaped mountain or hill

Tidal heating- frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites (e.g. Jupiter's moon Io)

Tidally locked- two bodies are tidally locked when their rotations are such that they always present the same side to the other -- their day is as long as their year; examples include Earth's moon, and the Pluto-Charon system

Transit:

    the instant when a celestial object crosses the meridian
    when an object passes in front of another object

Ultraviolet (UV)- the portion of the spectrum with wavelengths just shorter than the bluest light visible

Umbra- the dark inner part of an eclipse shadow; also, the dark central part of a sunspot

Uncertainty Principle- the principle, formulated by Heisenberg, that one can never be exactly sure of both the position and the velocity of a particle; the more accurately one knows the one, the less accurately one can know the other

Undae- dunes (literally 'waves')

Vallis- sinuous valley (plural: valles)

Van Allen Belts- radiation zones of charged particles surrounding the Earth

Variable star- any star, the brightness of which appears to change, with periods ranging from minutes to years

Virtual particle- in quantum mechanics, a particle that can never be directly detected, but whose existence does have measurable effects

Visible spectrum- the wavelengths of light to which the human eye is sensitive

Vastitas- widespread lowlands

Volatile- As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes Hydrogen (H), Helium (He), water (H20), ammonia (NH3), carbon dioxide (CO2), and methane (CH4)

Wave/particle duality- the concept in quantum mechanics that there is no distinction between waves and particles; particles may sometimes behave like waves, and waves like particles

Wavelength- the distance between eave crests on any train of electromagnetic radiation; short ones are more energetic than long ones

Weak force- the second weakest of the four fundamental forces, with a very short range; it affects all matter particles, but not force-carrying particles

Weight- the force exerted on a body by a gravitational field; it is proportional to, but not the same as, its mass

White dwarf- a collapsed core of a normal star such as the sun after it has lost its outer layers

White hole- the exact opposite of a black hole; an object that spews out matter and energy

Wormhole- an object with two mouths in different parts of our universe connected by a tunnel that allows two-way traffic; they may be safe shortcuts through space

X-ray source- a region of extremely hot gas; matter torn away from a normal star by a black hole or a neutron star becomes violently heated and emits x rays

Yellow dwarf- an ordinary star such as the sun at a comparatively stable and long-lived stage of evolution

Zenith- the point on the celestial sphere directly overhead

Zodiac- the twelve constellations straddling the ecliptic through which the sun, Moon, and planets appear to move during the year

Zodiacal light- a faint cone of light rising from the horizon after sunset or before sunrise; it is caused by sunlight reflected from thinly spread interplanetary material lying in the main plane of the Solar System 







 
Glossary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z     Links

A
accretion
    Accumulation of dust and gas into larger bodies such as stars, planets and moons. 
Adams, John Couch 1819-1892
    English astronomer and mathematician who, at the age of 24, was the first person to predict the position of a planetary mass beyond Uranus. But, unfortunately, Adams did not publish his prediction. Galle confirmed the existence of Neptune based on independent calculations done by Le Verrier. (4k jpg) 
albedo
    the ratio of the amount of light reflected by an object and the amount of incident light; a measure of the reflectivity or intrinsic brightness of an object (a white, perfectly reflecting surface would have an albedo of 1.0; a black perfectly absorbing surface would have an albedo of 0.0). 
albedo feature
    A dark or light marking on the surface of an object that may not be a geological or topographical feature. 
antipodal point
    the point that is directly on the opposite side of the planet 
aphelion
    the point in its orbit where a planet is farthest from the Sun; when refering to objects orbiting the Earth the term apogee is used; the term apoapsis is used for orbits around other bodies. (opposite of perihelion) 
arcuate
    having the form of a bow; curved; arc-shaped 
Arago, Dominique François Jean 1786 - 1853
    French astronomer and physicist and Director of the Paris Observatory, who discovered the phenomenon of the production of magnetism by rotation 
d'Arrest, Heinrich Louis
    Danish astronomer who assisted Galle with the first observations of Neptune. After receiving its predicted position from Le Verrier, Galle and d'Arrest began searching. With Galle at the eyepiece and d'Arrest reading the chart, they scanned the sky and checked that each star seen was actually on the chart. Just a few minutes after their search began, d'Arrest cried out, "That star is not on the map!" and earned his place in the history books. (more) 
asteroid
    (also "planetoid") a medium-sized rocky object orbiting the Sun; smaller than a planet, larger than a meteoroid 
asteroid number
    asteroids are assigned a serial number when they are discovered. It has no particular meaning except that asteroid N+1 was discovered after asteroid N. (see appendix 5) 
astronomical unit (AU)
    = 149,597,870.691 km; the average distance from the Earth to the Sun. 1 AU is a long way -- at 100 miles per hour (160 kph) it would take over 100 years to go 1 AU. 
atmosphere
    = 1.013 bars = 1.03 kg/cm^2 = 14.7 pounds per square inch, standard atmospheric pressure at sea level on Earth. 
aurora
    (Latin for "dawn") a glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun 
aurora borealis
    the "Northern Lights"; caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere. A similar effect happens in the southern hemisphere where it is known as the aurora australis. 
B
bar
    = 0.987 atmosphere = 1.02 kg/cm^2 = 100 kilopascal = 14.5 lbs/square inch. 
Barnard, Edward Emerson 1857-1923
    American astronomer; discovered Jupiter's satellite Amalthea and Barnard's star, the second-nearest star system to the Sun. 
Barsoom
    The local name for Mars in Edgar Rice Burroughs' SF books. 
billion
    I use the American version of "billion" which means 1,000,000,000 (1e9); not the British version which means 1e12. 
Bode, Johann Elert 1747-1826
    German astronomer, known for the bogus "Bode's Law" which attempts to explain the sizes of the planetary orbits. 
bolide
    a fireball that produces a sonic boom 
Bond, William Cranch 1789-1859
    One of the earliest American astronomers of note; rose from poverty and overcame a lack of formal education to become the first director of the Harvard College Observatory where he studied Saturn and (with Lassell) discovered its moon Hyperion. 
Brahe, Tycho 1546-1601
    (a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical observations formed the basis for Johannes Kepler's laws of planetary motion. (141k jpg; 38k jpg; more; The Noble Dane: Images of Tycho Brahe, from the Museum of the History of Science in Oxford) 
brown dwarf
    An object between 0.013 and 0.080 solar masses (13 to 80 Jupiter-masses): too small for normal nuclear fusion but big enough to fuse deuterium. Brown dwarfs are larger than planets but smaller than stars. 
C
caldera
    crater formed by an explosion or collapse of a volcanic vent. 
carbonate
    a compound containing carbon and oxygen (i.e. calcium carbonate a.k.a. limestone). 
Cassini, Giovanni Domenico 1625-1712
    (a.k.a. Jean Dominique) Italian-born French astronomer and first director of the Royal Observatory in Paris; discoverer of four of Saturn's moons (Tethys, Dione, Rhea and Iapetus) and the major division in its rings. (13k jpg; more) 
catena
    chain of craters. 
cavus
    Hollow, irregular depression. 
chaos
    distinctive area of broken terrain. 
chasma
    canyon. 
Christy, James W.
    discoverer of Pluto's moon Charon (331k jpg (Jim is the seated figure at left)) 
chromosphere
    the lower level of the solar atmosphere between the photosphere and the corona 
colles
    small hills or knobs. 
coma
    the dust and gas surrounding an active comet's nucleus 
comet
    a medium-sized icy object orbiting the Sun; smaller than a planet 
conjunction
    An inferior planet is said to be "in inferior conjunction" when it is directly between the Earth and the Sun. It is "in superior conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet is "in conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet obviously cannot have an inferior conjunction. When the Earth is at inferior conjunction with respect to an observer on a superior planet we say that planet is "in opposition" from Earth's perspective. . (nice diagram) 
Congress
    the legislative branch of the US Government; has proven to be a much more hostile environment for scientific spacecraft than the vastness of space. 
convection
    fluid circulation driven by temperature gradients in the presence of gravity; the transfer of heat by this mechanism. 
Copernicus, Nicolaus 1473-1543
    Polish astronomer who advanced the heliocentric theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time as the Ptolemaic view of the universe, which was the prevailing theory for over 1000 years, was deeply ingrained in the prevailing philosophy and religion. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century BC, a fact known to Copernicus but long ignored.) ( 12k gif; 129k jpg; more) 
corona
    ovoid-shaped feature. 
corona
    the uppermost level of the solar atmosphere, characterized by low densities and high temperatures (> 1.0E+06 K). 
coronagraph
    a special instrument which blocks light from the disk of the Sun in order to study the faint solar atmosphere. 
cosmic ray
    an extremely energetic (relativistic) charged particle. 
crater
    bowl-shaped depression formed by the impact of a meteoroid; depression around the orifice of a volcano. 
D
density
    measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3. 
disaster
    literally "bad stars"; particularly apt in reference to a major asteroid impact. 
disk
    the visible surface of the Sun (or any heavenly body) projected against the sky. 
doppler effect
    (Christian Doppler 1803-1853) the apparent change in wavelength of sound or light caused by the motion of the source, observer or both. (see also) 
dinosaurs
    large animals that lived in the Mesozoic Era from 230 to 65 million years ago; most probably wiped out by the impact of a large asteroid or comet. 
direct
    rotation or orbital motion in a counterclockwise direction when viewed looking down from above the north pole of the primary (i.e. in the same sense to most satellites); the opposite of retrograde. The north pole is the one on the same side of the ecliptic as the Earth's north pole. (The word "prograde" is sometimes used to mean "direct" in this sense.) 
dorsum
    ridge. 
dwarf planet
    A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. Currently there are three objects: Pluto, Ceres and Eris officially designated as dwarf planets. (See planet.) 
E
eccentricity
    the eccentricity of an ellipse (planetary orbit) is the ratio of the distance between the foci and the major axis. Equivalently the eccentricity is (ra-rp)/(ra+rp) where ra is the apoapsis distance and rp is the periapsis distance. 
effusive eruption
    a relative quiet volcanic eruption which puts out basaltic lava that moves at about the speed one walks; the lava is fluid in nature; the eruptions at the Kilauea volcano on the island of Hawaii are effusive 
Einstein, Albert 1879-1955
    German-American physicist; developed the Special and General Theories of Relativity which along with Quantum Mechanics is the foundation of modern physics. (See fusion, speed of light) (96k gif) 
ellipse
    oval. That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Kepler based on the careful observations by Tycho Brahe. 
erg/sec
    = 1e-10 kilowatts. 
explosive eruption
    a dramatic volcanic eruption which throws debris high into the air for hundreds of miles; lava is low in silicate; can be very dangerous for people near by; an example is Mount St. Helens in 1980 
exponential notation
    "1.23e4" means "1.23 times 10 to the fourth power" or 12,300; "5.67e-8" means "5.67 divided by 10 to the eighth power" or 0.0000000567. 
F
facula
    bright spot. 
farrum
    pancake-like structure 
filament
    a strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun; a filament on the limb of the Sun seen in emission against the dark sky is called a prominence. 
fireball
    a meteor brighter than magnitude -3 
fissure
    a narrow opening or crack of considerable length and depth. 
flare
    a sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted. 
flexus
    cuspate (pointed) linear feature. 
fluctus
    flow terrain. 
fossa
    long, narrow, shallow depression. 
Franklin, Benjamin 1706-1790
    American public official, writer, and scientist. Played a major part in the American Revolution and helped draft the Constitution. His numerous scientific and practical innovations include the lightning rod, bifocal spectacles, and a stove. 
G
Gaia Hypothesis
    named for the Greek Earth goddess Gaea, holds that the Earth as a whole should be regarded as a living organism and that biological processes stabilize the environment. First advanced by British biologist James Lovelock in 1969. 
Galle, Johann Gottfried 1812-1910
    German astronomer who, with Heinrich Louis d'Arrest, made the first observation of Neptune based on calculations by Le Verrier. Though Galle was the first to observe Neptune, its discovery is usually credited to Adams (who made an earlier calculation) and Le Verrier. 
Galilean Moons
    Jupiter's four largest moons: Io, Europa, Ganymede and Callisto; discovered independently by Galileo and Marius. (Galileo proposed that they be named the Medicean stars, in honor of his patron Cosimo II de Medici; the present names are due to Marius) 
Galileo Galilei 1564-1642
    Italian astronomer and physicist. The first to use a telescope to study the stars. Discoverer of the first moons of an extraterrestrial body (see above). Galileo was an outspoken supporter of Copernicus's heliocentric theory. In reaction to Galileo, the Church declared it heresy to teach that the Earth moved and silenced him. The Church clung to this position for 350 years; Galileo was not formally exonerated until 1992. (16k gif; 136k jpg) (See also the Galileo exhibit at Institute and Museum of History of Science, Florence ITALY; The Galileo Project from Rice and APOD 980913) 
gegenschein
    a round or elongated spot of light in the sky at a point 180 degrees from the Sun. Also called counterglow. 
George III 1738-1820
    King of Great Britain and Ireland (1760-1820). His government's policies fed American colonial discontent, leading to revolution in 1776. 
geosynchronous orbit
    a direct, circular, low inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface. 
granulation
    a pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas. 
greenhouse effect
    increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere (carbon dioxide and water vapor are the major factors). Very important on Venus and Earth but very weak on Mars. 
H
Hale, George Ellery 1868-1938
    American astronomer who founded the Yerkes, Mt. Wilson and Palomar observatories. (72k gif) 
Hall, Asaph 1829-1907
    American astronomer who discovered the two moons of Mars, Deimos and Phobos. 
Halley, Edmond 1656-1742
    English astronomer who applied Newton's laws of motion to historical comet data and predicted correctly the reappearance of the comet which now bears his name. (12k jpg; more) 
heliocentric
    Sun-centered; see Copernicus, Kepler, Galileo. 
heliopause
    the point at which the solar wind meets the interstellar medium or solar wind from other stars. 
heliosphere
    the space within the boundary of the heliopause containing the Sun and solar system. 
Herschel, Sir William 1738-1822
    British astronomer who discovered Uranus and cataloged more than 800 double stars and 2,500 nebulae. (365k html/gif) 
Hubble, Edwin Powell 1889-1953
    American astronomer whose observations proved that galaxies are "island universes", not nebulae inside our own galaxy. His greatest discovery was the linear relationship between a galaxy's distance and the speed with which it is moving. The Hubble Space Telescope is named in his honor. (133k html/gif; 60k gif; bio material) 
Huygens, Christiaan 1629-1695
    Dutch physicist and astronomer who first described the nature of Saturn's rings (1655) and discovered its moon Titan; also pioneered the use of the pendulum in clocks. (7k jpg; more) 
I
ice
    used by planetary scientists to refer to water, methane, and ammonia which usually occur as solids in the outer solar system. 
inclination
    the inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator. 
Inquisition, The
    A Renaissance Catholic court instituted to seek out and prosecute heretics. 
inferior planets
    the planets Mercury and Venus are called inferior planets because their orbits are closer to the Sun than is Earth's orbit. (The other planets are called "superior" planets.) 
interplanetary magnetic field (IMF)
    the magnetic field carried with the solar wind. 
ionosphere
    a region of charged particles in a planet's upper atmosphere; the part of the Earth's atmosphere beginning at an altitude of about 25 miles and extending outward 250 miles or more. 
J
K
Keeler, James E. 1857-1900
    American astrophysicist, probable discoverer of the dark narrow gap in the outer part of the A ring of Saturn, and the second director of Lick Observatory. Keeler was (probably accidentally) cheated of his rightful fame when the A ring gap became known as "Encke's Division". Encke had earlier seen a broad, poor contrast feature in the A-ring (now called the "Encke Minimum" by some amateurs) which is quite different from the sharp, distinct gap that Keeler recorded on the very first night of observing with the Lick 36-inch refractor. Images from Voyager and Cassini show a much smaller gap near the very outer edge of the A ring which has been named the Keeler Gap. Of course, Keeler never saw it. On the other hand, the "Encke" gap which was seen by Keeler may have been seen even earlier by Francesco De Vico, William Lassell and/or the Rev. William R. Dawes. 
Kelvin (K)
    0 Kelvin is absolute zero; H2O melts at 273 K (= 0° C = 32° F); H2O boils at 373 K (= 100° C = 212° F). (developed by William Thomson). 
Kepler, Johannes 1571-1630
    German astronomer and mathematician. Considered a founder of modern astronomy. Using the positional data carefully amassed by Tycho Brahe, Kepler formulated the famous three laws of planetary motion. They comprise a quantitative formulation of Copernicus's theory that the planets revolve around the Sun. (16k jpg; 86k jpg; more; yet more) 
kilogram (kg)
    = 1000 grams = 2.2 pounds, the mass of a liter of water. (see also) 
kilometer (km)
    = 1000 meters = 0.62 miles. 
Kowal, Charles T. 1940-
    American astronomer; discovered Leda and the comet-like object 2060 Chiron (aka 95 P/Chiron). 
Kuiper, Gerard 1905-1973
    Dutch-born American astronomer best known for his study of the surface of the Moon; discovered Miranda and Nereid, found an atmosphere on Titan. (Dr.Kuiper was solidly Americanized; his name is pronounced to rhyme with "viper.") (a short bio) 
L
labes
    landslide. 
labyrinthus
    intersecting valley complex. 
lacus
    lake. 
Lagrange, Joseph Louis 1736-1813
    French (originally Italian, Giuseppe Luigi Lagrangia; born in Turin, moved to Paris and became a French citizen) mathematician and astronomer; made a number of contributions to the study of celestial mechanics. (5k gif) trojans 
Lagrange points
    Lagrange showed that three bodies can lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable. Bodies at such points are sometimes referred to as Trojans. The leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5. Collinear with the two large bodies are the L1, L2 and L3 unstable equilibrium points which can sometimes be useful places for spacecraft, eg SOHO. (more and more) 
Lassell, William 1799-1880
    British astronomer, discovered Neptune's largest satellite, Triton and (with Bond) discovered Saturn's moon Hyperion. A successful brewer before turning to astronomy. (22k jpg; more) 
Le Verrier, Urbain Jean Joseph 1811-1877
    French mathematician whose prediction of the position of an undiscovered planet (Neptune) that caused perturbations in the orbit of Uranus was the first to be confirmed (by Galle) though Adams had made a similar but unpublished prediction some months earlier. 
lidar
    an instrument similar to radar that operates at visible wavelengths. 
limb
    the outer edge of the apparent disk of a celestial body 
light-year
    = 9.46053e12 km (= 5,880,000,000,000 miles = 63,239 AU); the distance traveled by light in a year. 
linea
    elongate marking. 
liter
    = 1000 cm3 = 1.06 US quarts 
Lowell, Percival 1855-1916.
    American astronomer. He founded the Lowell Observatory in Arizona (1894), where his studies of Mars led him to believe that the linear markings (first noted by Schiaparelli) on the surface were "canals" and therefore that the planet was inhabited by intelligent beings. His successors later discovered Pluto. (26k gif) 
lunar month
    The average time between successive new or full moons, equal to 29 days 12 hours 44 minutes. Also called synodic month. 
M
macula
    dark spot. 
magnetosphere
    the region of space in which a planet's magnetic field dominates that of the solar wind. 
magnetotail
    the portion of a planetary magnetosphere which is pushed away from the Sun by the solar wind. 
magnitude
    The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512. Also called apparent magnitude. 
mare
    literally "sea" (a very bad misnomer, still in use for historical reasons); really a large circular plain (plural: "maria") 
Marius, Simon 1573-1624
    (a.k.a. Mayr) German astronomer who gave Jupiter's "Galilean" moons their names. He and Galileo both claimed to have discovered them in 1610 and likely did so independently. Marius was also the first to observe the Andromeda Nebula with a telescope and one of the first to observe sunspots. (more) 
mensa
    mesa, flat-topped elevation. 
metal
    used by astrophysicists to refer to all elements except hydrogen and helium, as in: "the universe is composed of hydrogen, helium and traces of metals". (Note: this is quite different from the usual chemistry definition.) 
meteor
    (also "shooting star" or "falling star") a bright streak of light in the sky caused by the entry into Earth's atmosphere of a meteoroid or a small icy particle. Very large, bright ones are called fireballs and bolides 
meteorite
    a rock of extra-terrestrial origin found on Earth 
meteoroid
    a small rocky object orbiting the Sun; smaller than an asteroid 
millibar
    1/1000 of a bar. Standard sea-level pressure is about 1013 millibars. 
minor planets
    the official term used for asteroids. 
mons
    mountain (plural: montes) 
N
Neujmin, Grigoriy N.
    Ukrainian astronomer; discovered the asteroid 951 Gaspra. 
neutrino
    a fundamental particle supposedly produced in massive numbers by the nuclear reactions in stars. They are very hard to detect since the vast majority of them pass completely through the Earth without interacting. 
Newton, Isaac 1642-1727
    English cleric and scientist; discovered the classical laws of motion and gravity; the bit with the apple is probably apocryphal. (10k jpg) 
Nicholson, Seth Barnes 1891-1963
    American astronomer; discovered Lysithea, Ananke, Carme and Sinope; also did important work on sunspots. 
nuclear fusion
    a nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence E=mc2. This is the source of the Sun's energy therefore ultimately of (almost) all energy on Earth. 
O
oceanus
    literally "ocean"; really a large circular plain 
old
    a planetary surface that has been modified little since its formation typically featuring large numbers of impact craters (compare young). 
Oort, Jan Hendrik 1900-1992
    Dutch astronomer made major contributions to knowledge of the structure and rotation of our galaxy. More or less as a sideline, Oort studied comets as well. The result of this work was a theory, now widely accepted, that the Sun is surrounded by a distant cloud of comet-stuff, now called the Oort cloud, bits of which are occasionally hurled into the solar system as comets. ( more) 
opposition
    A superior planet is said to be "in opposition" when it is directly on the opposite side of the Earth from the Sun. This is generally the closest it comes to the Earth and the time at which it is most easily visible. (nice diagram) 
ovoid
    shaped like an egg 
P
palus
    literally "swamp"; really a small plain 
parsec
    = 206265 AU = 3.26 light year 
patera
    shallow crater; scalloped, complex edge. 
penumbra
    literally, "dim light"; the outer filamentary region of a sunspot. 
perihelion
    the point in its orbit where a planet is closest to the Sun. when referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion) 
Perrine, Charles Dillon 1867-1951
    Argentine-American astronomer who discovered Himalia and Elara. 
perturb
    to cause a planet or satellite to deviate from a theoretically regular orbital motion . 
photosphere
    the visible surface of the Sun; sunspots and faculae are observed in the photosphere. 
plage
    bright regions seen in the solar chromosphere. 
Piazzi, Giuseppe 1746-1826
    Astronomer, born in Ponte di Valtellina, Italy. He became a Theatine monk, professor of theology in Rome (1779), and professor of mathematics at the Academy of Palermo (1780). He set up an observatory at Palermo in 1789, published a star catalog (1803, 1814) and discovered and named the first minor planet, Ceres. (more) 
Pickering, William Henry 1858-1938
    American astronomer. His photographs of Mars, among the earliest obtained, provided a basis for his opposition to Lowell's observations of supposed canals on Mars. Discovered Phoebe. 
planet
    The recently adopted IAU resolution states that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:

        A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
        A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
        All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies". 

    So by this official definition there are exactly eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto, Ceres and Eris (aka 2003 UB313) are "dwarf planets" with a potentially large number of additional objects falling into this category in the near future. 
planitia
    low plain. 
planum
    plateau or high plain. 
Pope, Alexander 1688-1744
    English writer best remembered for his satirical mock-epic poems The Rape of the Lock and The Dunciad. 
prominence
    a strand of relatively cool gas in the solar corona which appears bright when seen at the edge of the Sun against the blackness of space. 
promontorium
    cape; headland 
Ptolemy 87-150
    (aka Claudius Ptolemaeus) Alexandrian astronomer, mathematician, and geographer who based his astronomy on the belief that all heavenly bodies revolve around the Earth. (10k gif; more) 
Q
R
red giant
    a star that has low surface temperature and a diameter that is large relative to the Sun. 
regio
    region. 
Relativity, Theory of
    more accurately describes the motions of bodies in strong gravitational fields or near the speed of light than newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not specifically for Relativity but rather for his 1905 work on the photoelectric effect and "services to Theoretical Physics".) (see Spacetime Wrinkles, an excellent WWW site from NCSA) 
resolution
    the amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details 
resonance
    A state in which one orbiting object is subject to periodic gravitational perturbations by another. 
reticulum
    reticular (net-like) pattern 
retrograde
    rotation or orbital motion in a clockwise direction when viewed from above the north pole of the primary (i.e. in the opposite sense to most satellites); the opposite of direct. The north pole is the one on the same side of the ecliptic as the Earth's north pole. 
rift valley
    an elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike. 
rima
    fissure. 
Roche limit
    the closest a fluid body can orbit to its primary without being pulled apart by tidal forces. A solid body may survive within the Roche limit if the tidal forces do not exceed its structural strength. The Roche limit is calculated with the equation

    RL = 2.456*R*(p'/p)^(1/3)

    where p' is the density of the planet, p is the density of the moon, and R is the radius of the planet. (here's the math) 
rupes
    scarp. 
S
scarp
    line of cliffs produced by faulting or erosion. 
Schiaparelli, Giovanni Virginio 1835-1910
    Italian astronomer who in 1877 first observed the "canals" on Mars. He believed that the features he observed included straight lines that joined in a complicated pattern. He called these lines 'canali', which means 'channels'. However, the Italian word was mistranslated into the English word 'canals'. That, combined with the suspicious straightness of the lines, bespoke of artificial structures, and this created a furor. Speculations concerning the possibility of intelligent life on Mars sprang up in the popular press. Even astronomers felt the pull of that dramatic possibility. Foremost among these was Percival Lowell, who carried matters far beyond Schiaparelli. 
scopulus
    lobate or irregular scarp. 
semimajor axis
    the semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by rp = a(1-e) and ra = a(1+e). 
Shakespeare, William 1564-1616
    English playwright and poet; wrote some good skits. 
shepherd satellite
    (or 'shepherd moon') a satellite which constrains the extent of a planetary ring through gravitational forces. (See Pandora for a nice image.) 
sidereal
    of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite. 
sidereal month
    The average period of revolution of the moon around the earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time. 
silicate
    a compound containing silicon and oxygen (e.g. olivine) 
sinus
    literally "bay"; really a small plain 
solar cycle
    the approximately 11-year quasi-periodic variation in frequency or number of solar active events. 
solar nebula
    the cloud of gas and dust that began to collapse about 5 billion years ago to form the solar system. 
solar wind
    a tenuous flow of gas and energetic charged particles, mostly protons and electrons -- plasma -- which stream from the Sun; typical solar wind velocities are near 350 kilometers per second. 
speed of light
    = 299,792,458 meters/second (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light; Scotty and Geordi know better. 
spicules
    grass-like patterns of gas seen in the solar atmosphere. 
stellar classification
    Stars given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2. 
sublime (or sublimate)
    to change directly from a solid to a gas without becoming liquid 
sulcus
    subparallel furrows and ridges. 
sunspot
    an area seen as a dark spot on the photosphere of the Sun; sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups; they appear dark because they are cooler than the surrounding photosphere. 
superior planets
    the planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are called superior planets because their orbits are farther from the Sun than Earth's orbit. (Mercury and Venus are called "inferior" planets.) 
synchronous orbit radius
    the orbital radius at which the satellite's orbital period is equal to the rotational period of the planet. A synchronous satellite with an orbital inclination of zero (same plane as the planet's equator) stays fixed in the sky from the perspective of an observer on the planet's surface (such orbits are commonly used for communications satellites). 
synchronous rotation
    said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward. Most of the satellites in the solar system rotate synchronously. 
T
tectonic
    deformation forces acting on a planet's crust. 
terminator
    the dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk. 
terra
    extensive land mass. 
tessera
    tile; terrain formed of polygonal pattern 
tholus
    small domical mountain or hill. 
Thomson, William 1824-1907
    aka Lord Kelvin, British physicist who developed the Kelvin scale of temperature. Also supervised the laying of a trans-Atlantic cable. (10k gif) 
tidal heating
    frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites. 
Tombaugh, Clyde 1906-1997
    American astronomer; discovered Pluto. (more, more, 4k gif) 
Trekkie
    (also "Trekker") a devotee of the science fiction program Star Trek. 
Trojan
    an object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter's Lagrange points: 588 Achilles, 624 Hektor, and 911 Agamemnon. Saturn's satellites Helene, Calypso and Telesto are also sometimes called Trojans. 
U
umbra
    the dark central region of a sunspot. 
undae
    dunes (literally 'waves'). 
V
vallis
    sinuous valley (plural: valles) 
Van Allen, James A.
    American physicist who discovered the Earth's radiation belts (that now bear his name) with an instrument aboard the first successful American satellite, Explorer 1. 
vastitas
    widespread lowlands. 
Verne, Jules 1828-1905
    French writer who is considered the founder of modern science fiction. His novels include "Journey to the Center of the Earth" and "From the Earth to the Moon". 
volatile
    As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes hydrogen, helium, water, ammonia, carbon dioxide and methane. 
W
white dwarf
    a whitish star of high surface temperature and low intrinsic brightness with a mass approximately equal to that of a Sun but with a density many times larger. 
X
Y
young
    When used to describe a planetary surface "young" means that the visible features are of relatively recent origin, i.e. that older features have been destroyed (e.g. by erosion or lava flows). Young surfaces exhibit few impact craters and are typically varied and complex. In contrast an "old" surface is one that has changed relatively little over geologic time. The surfaces of Earth, Titan and Io are young; the surfaces of Mercury, Callisto and most other solid bodies in our solar system are old. 
Z
zodiacal light
    a faint glow from light scattered off of interplanetary dust along the plane of the ecliptic. 







 
Other Glossaries

General
  

  
AstroGlossary from StarDate Online
  
Astronomy Unbound, an electronic text has an excellent glossary
  
 Terms from LANL
  
Astronomy Picture of the Day Glossary
  
Planetary Science Glossary from Planetary Science Research Discoveries
  
Glossary from the Munich Astronomical Archive
  
Galileo SSI Glossary
  
Astronomiae Historia / History of Astronomy
  
Solar terminology
  
 Laws, rules, principles, effects, paradoxes, limits,  constants, experiments, & thought-experiments in physics
Astronomy Topics

  
 Solar-Terrestrial Terms
  
Solar Glossary
  
Solar System Dynamics Glossary from JPL
  
 Planetary Ring Terms
  
 Comet Definitions
  
 Glossary of Astronomical and Comet Terms from ICQ/CBAT/MPC
  
Short Glossary of Lunar Geology
  
 Comet and Meteor Shower Glossary
  
Glossary on Meteor Astronomy
  
Relativity Term from NCSA
  
History Topics: Mathematical Astronomy
Scientists

  
 People from LANL
  
 Biographies of Mathematicians, including some who were also astronomers and/or physicists (includes many portraits).
  
Institute and Museum of History of Science, Florence ITALY (Biographies in the  Galileo exhibit)
  
Astronomical People (with images)
  
People from the University of Michigan's wonderful "Windows to the Universe", includes many pictures
Mythology

  
Encyclopedia Mythica, an encyclopedia of mythology and more
  
Bulfinch's Mythology
  
The Gods and Goddesses of Ancient Mythology
Spaceflight

  
 Glossary of Spaceflight Terminology from JPL
  
NASA Thesaurus
  
NASA NGST Glossary
  
Dictionary Of Technical Terms For Aerospace Use from NASA LeRC
Place names
      

  
Gazetteer of Planetary Nomenclature from USGS
Acronyms

  
 Commonly Used Acronyms in Astronomy
  
Frequently Seen Space/Astronomy Acronyms

 







 A
ACCREDITED
An educational institution which has been recognized as maintaining standards that qualify graduates for admission to higher, or more specialized, institutions or professional practice.

ASTEROID
A rocky space object which can be from a few hundred feet to several hundred km wide. Most asteroids in our solar system orbit the Sun in a belt between Mars and Jupiter.

ASTRONOMER
An expert in the study of the Sun, Moon, stars, planets, and other space bodies.

ASTRONOMICAL UNIT
The approximate distance from the Sun to the Earth which is equal to 150,000,000 kilometers.

ASTROPHYSICS
The branch of astronomy that deals with the physics of stellar phenomena.

ATMOSPHERE
The layers of gases which surround a star, like our Sun, or a planet, like our Earth.

AXIS
An imaginary straight line around which an object rotates.

B
BACTERIA
Living things that have only one cell and are so small they can only be seen with a microscope.

BASALT
A dark, fine-grained volcanic rock.

BIG BANG THEORY
A theory which states that the Universe began to expand after a super powerful explosion of concentrated matter and energy.

BLACK HOLE
The leftover core of a super massive star after a supernova. Black holes exert a tremendous gravitational pull.

C
CELSIUS
A scale on a thermometer where the interval between the boiling point and the freezing point of water is divided into 100 degrees. Freezing point is represented by 0 degrees and boiling point is represented by 100 degrees. The same scale is also utilized in Centigrade.

CENTIGRADE
See CELSIUS.

CHARGED PARTICLES
electrons, protons, ions.

COMA
A hazy cloud which surrounds the nucleus of a comet.

COMET
Frozen masses of gas and dust which have a definite orbit through the solar system.

CORONA
The very hot outermost layer of a star's atmosphere. Our Sun's corona can only be seen during a total solar eclipse.

COSMONAUT
An astronaut from the former Soviet Union or the current republic of Russia.

COSMOLOGICAL
Having to do with the study of the history, structure, and changes in the universe.

D
DECAYING ORBIT
A path around an object which decreases in size with time. For example, when a satellite enters a decaying orbit above Earth, its orbit size decreases to the point that it enters Earth's atmosphere where it burns up.

DENSITY
The mass per unit of volume of a substance.

DOPPLER SHIFT
A shift in an object's spectrum due to a change in the wavelength of light that occurs when an object is moving toward or away from Earth.

E
ELECTROMAGNETIC SPECTRUM
The entire range of the different types of electromagnetic radiation, or waves. It goes from the very long wave, low frequency, radio waves through infrared waves and visible light waves to the very high frequency and short waves of the gamma-rays and X-rays. Those wavelengths in the visible light range have a specific color associated with them when they pass through a prism. The lower frequency, longer wavelengths produce a red while those with higher frequency, shorter wavelengths produce a violet. Those wavelengths which fall somewhere in between these two points produce the orange, yellow, green, and blue also found in a spectrum.

ELECTROMAGNETIC WAVE
A wave of electric and magnetic energy that is generated when an electric charge is accelerated.

ELLIPTICAL
Shaped like an elongated closed curve.

ENERGY
Usable heat or power; in physics, it is the capacity of a physical system to perform work.

F
FAHRENHEIT
A scale on a thermometer where the freezing point of water is represented by 32 degrees and the boiling point is represented by 212 degrees.

FUSION
A nuclear reaction in which an element with small atoms fuses to form an element with larger atoms, releasing large amounts of energy.

G
GALAXY
A cluster of stars, dust, and gas held together by gravity.

GAMMA-RAYS
Penetrating short wave electromagnetic radiation of very high frequency.

GEOSYNCHRONOUS
An orbit in which a satellite's rate of revolution matches the Earth's rate of rotation. This allows the satellite to stay over the same site on the Earth's surface at all times.

GRAVITATIONAL FIELD
The volume over which an object exerts a gravitational pull.

GRAVITATIONAL FORCE
See Gravity

GRAVITATIONAL PULL
See Gravity

GRAVITY
The force of attraction between two objects which is influenced by the mass of the two objects and the distance between the two objects.

GYROSCOPE
A heavy wheel or disk mounted so that its axis can turn freely in one or more directions. A spinning gyroscope tends to resist change in the direction of its axis.

H
HABITAT
The place in which an organism lives and obtains the materials it needs in order to survive.

HELIOCENTRIC
Having the Sun as a center, such as a heliocentric solar system.

I
IMPACT CRATERS
Craters which are the result of a collision between a large body, such as a planet or satellite, and a smaller body such as an asteroid or meteorite.

INFRARED WAVES
Electromagnetic radiation with long wavelengths which is found in the invisible part of the spectrum. Human beings experience infrared waves as heat.

ION
An electrically charged particle. Ions may be negatively or positively charged.

J
K
KELVIN
A scale for measuring temperature where 0 Kelvin is equal to -273.16 degrees Celsius. Zero Kelvin is referred to as absolute zero, the point at which all motion within molecules comes to a stop.

KILOGRAM
1000grams. A kilogram equals 2.2 pounds.

KILOMETER
1000 meters. A kilometer equals 0.6214 miles.

KILOPARSEC
1000 parsecs. A parsec equals 3.26 light years.

L
LIGHT YEAR
The distance light can travel in one year, which is 9,500,000,000,000 kilometers.

M
MAGNETIC FIELD
The area in which an attractive or repelling force exists between two magnets or in association with the element iron. The Earth's magnetic field is thought to be due to the liquid iron-nickel which is in its core. This magnetic field protects Earth from constant bombardment by high-energy charged particles.

MANTLE
The middle layer of a planet located between the crust, or surface, and the core.

MASS
The measure of the amount of matter in an object.

MATTER
Anything which has mass and occupies space.

METEOR
Meteoroids which burn up in the atmosphere of a space body, such as the Earth, prior to impacting on the surface.

METEORITE
Fragments of material that fall from space and impact on other larger space bodies.

METEOROID
Fragments of material which vaporize when they have a close encounter with a space body which has an atmosphere.

METRIC TON
1000 kilograms. A metric ton equals 2,204 pounds.

MICROMETEOROID
Very small pieces of matter which are encountered in space.

MICROWAVE
Electromagnetic radiation which has a long wavelength (between 1 mm and 30 cm). Microwaves can be used to study the universe, communicate with satellites in orbit around Earth, and cook popcorn.

MYLAR
A tough polyester material used as an insulator.

N
NASA
The National Aeronautics and Space Administration which oversees the space program in the United States.

NEBULA
A low density cloud of gas and dust in which a star is born.

O
OORT CLOUD
A huge cloud which is thought to surround our solar system and reach over halfway to the nearest star. Comets originate in the Oort Cloud.

ORBIT
A specific path followed by a planet, satellite, etc.

P
PARSEC
One parsec equals 3.26 light-years.

PAYLOAD BAY
The main body of the Space Shuttle where the payload, or cargo, is stored.

PHOTOSYNTHESIS
The process by which plants use carbon dioxide, nutrients, and sunlight to produce food.

PHYSICIST
A person who studies physics.

PHYSICS
The science of matter and energy, and of interactions between the two. A person who studies physics is called a physicist.

PLAINS
Vast, flat areas with low elevation.

PROBES
Unmanned spacecraft which are launched into space in order to collect data about the solar system and beyond. Space probes are not necessarily designed to return to Earth.

Q
QUANTUM MECHANICS
A theory in physics which is based on 2 ideas: (1) light can be emitted or absorbed only in discrete quantities called quanta, whose energy is proportional to their wavelength; and (2) you can never be exactly sure of the position and velocity of a particle, the more accurately you know the one, the less accurately you can know the other.

QUASAR
A distant energy source which gives off vast amounts of radiation, including radio waves and X-rays.

R
RADIO WAVES
A type of electromagnetic radiation which has the lowest frequency, the longest wavelength, and is produced by charged particles moving back and forth. Radio waves are not blocked by clouds in the Earth's atmosphere.

RETROGRADE
Having a direction which is opposite that of similar bodies.

REVOLUTION
The circling of a smaller object around a larger object.

ROTATION
The spinning of an object on its axis.

S
SATELLITE
An object that revolves around a larger primary body. Satellites may be naturally occurring, such as the Moon, or they may be man-made, such as the Hubble Space Telescope and the Compton Gamma-Ray Observatory.

SILICON
A non-metallic chemical element.

SOLAR ECLIPSE
A shadow which falls on an area of Earth when the Moon moves between the Sun and Earth.

SOLAR FLARES
A magnetic storm on the Sun's surface which shows up as a sudden increase in brightness.

SOLAR PROMINENCES
Gases trapped at the edge of the Sun which appear to shoot outward from the Sun's surface.

SOLAR SYSTEM
The Sun and all of the planets, comets, etc. which revolve around it.

SOLAR WIND
A continuous stream of charged particles which are released from the Sun and hurled outward into space at speeds up to 800 kilometers per second. Solar winds are very prominent after solar flare activity.

SOLAR WINGS
A panel of solar cells which converts sunlight into electrical energy.

SPECTROGRAPH
The image of the electromagnetic spectrum produced by a spectroscope.

SPECTROSCOPE
An instrument which separates visible light into its various wavelengths. Each wavelength corresponds to a specific color in the spectrum.

SPECTRUM
A band of colors which forms when visible light passes through a prism. The band ranges in color from violet (shorter wavelength) to red (longer wavelength).

SUNSPOT
A magnetic storm on the the Sun's surface which appears as a dark area. A sunspot is approximately 1500 degrees Celsius cooler than it's surrounding material. The number of sunspots we see on the Sun at any given time appears to cycle every 11 years.

T
TECTONIC ACTIVITY
A shifting of an object's surface due to changes in the material underlying the surface.

TELESCOPE
Any of various devices, sometimes made with an arrangement of lenses, mirrors, or both, used to detect and observe distant objects by their emission, transmission, reflection, or other interaction with invisible radiation.

THERMOMETER
An instrument for measuring temperature.

U
ULTRAVIOLET RAYS
Invisible electromagnetic radiation which is comprised of very short wavelengths. Human beings get a sunburn from the ultraviolet rays emitted by the Sun.

UNIVERSE
The vast expanse of space which contains all of the matter and energy in existence.

V
VOLCANISM
Volcanic activity.

W
X
X-RAYS
Penetrating electromagnetic radiation which has an extremely short wavelength.

Y
Z

Shuttle

 







 
Space Terms

If you ever wonder the meaning of an astronomical word, search no further and browse below to find the definition of the space term. The following are terms from A-Z related to space & astronomy:

-A-

Absolute magnitude – also known as absolute visual magnitude, relates to measuring a heavenly object’s brightness when viewed from 10 parsec or 32+ light years.

Absolute zero – The international community agreed to define absolute zero as equivalent to -273.15°C on the Celsius scale or-459.67°F on the Fahrenheit scale. It is the theoretical temperature entropy reaches its minimum value.

Absorption lines are a dark feature in the spectrum of a star formed by cooler gases in a star’s outer layer.

Accretion disks arise when material, usually gases, are transferred from one celestial object to another. There are two places astronomers find accretion disks, binary star systems and galactic nuclei.

Achromatic lens is a combination of lenses made of different glass. These bring two wavelengths into focus (normally red & blue) on the same plane. Achromatic lenses are used to take chromatic aberrations away from images.

Active galactic nuclei is a region in the center of a galaxy that has a higher than normal brightness. It is a class of galaxies that emit a large amount of energy from their center more than ordinary galaxies.

Active optics – Technology developed in the 80’s for reflecting telescopes. The construction enables telescopes to move 8 meter primary mirrors. As the name suggests, it works by “actively” adjusting the telescope mirrors.

Adaptive optics – technology used to improve performance of optical systems through the reduction of rapidly changing optical distortion. It is used to remove atmospheric distortion through the use of astronomical telescopes and laser communication.

Airy disk – Named after George Airy, it is the central spot in a diffraction pattern of a stars image in focus in a telescope.

Albedo is the ratio that light is reflected by a planet or satellite to that received by it. It is the ratio of total-reflected light.

Altazimuth mount is a two-axis mount used to support and rotate an instrument in two common perpendicular axes, vertical (altitude) and horizontal (azimuth).

Altitude is the height of anything above given a planetary reference plane. In astronomy the angular distance of a heavenly body above the horizon.

Anaglyph is a composite picture printed in two colors to produce a 3D image viewed through eye glasses having lenses of the same colors.

Andromeda galaxy is a spiral galaxy that is nearly two and a half million light years away in the constellation Andromeda.

Angular size is the angle between two lines of sight to its two opposite sides. It is a measure of how large an object actually appears to be.

Anisotropy is the state of being directionally dependent. The property of being anisotropic and having a different value when measured in different directions.

Annular eclipse, a solar eclipse in which the moon covers all but the bright ring around the circumference of the sun. When Sun and Moon are exactly in line, because the Moon is smaller, the Sun appears as a bright ring (annulus) surrounding the moon.

Antimatter is matter composed of anti-particles: antiprotons, antineutrons, and positrons. Hypothetically a type of matter identical to physical matter except that the atoms are made of: anti-electrons, anti-protons, and anti-neutrons.

Aperture is a hole, gap, or slit and any other small opening. Diameter of the objective of a telescope.

Aphelion is the point of orbit of a planet or comet which is farthest away from the sun.

Panchromatic – sensitive to light of all colors in the visible spectrum.

Apogee is the point in the orbit of an object (moon, satellite, etc…) orbiting the earth that is at the greatest distance from the center of the earth.

Apparent magnitude is the measure of brightness of a celestial body as seen from Earth as seen without atmosphere.

Apparition is the appearance or time when a comet is visible such as Halley’s Comet.

Archeoastronomy, the study of how people of the past “understand phenomena in the sky and how those phenomena affect their cultures.” Branch of archaeology that deals with use by prehistoric civilizations of astronomical techniques to establish seasons or cycle of the year, as evidenced in megaliths and other ritual structures.

ArcMinute is a unit of angular distance equal to a 60th of a degree.

ArcSecond a 60th part of a minute of an arcminute.

Asterism is a group of starts. Also a pattern of stars seen from earth which is not part of an established constellation.

Asteroids are any of thousands of smaller bodies or planetoids that orbit around the Sun. they range in size from 1.6 miles to 480 miles.

Asteroid belt is the region in space between the orbits of Mars and Jupiter where most asteroids are located.

Astrometry is the branch of astronomy dealing with the measurement of the positions and motions of heavenly bodies.

Astronomical unit (AU) a unit of length which is equal to the mean distance of the earth from the Sun.

Astronomy is the science that deals with the material universe beyond the earth’s atmosphere. Natural science engaged with the study of celestial objects.

Astrophotography a specialized branch of photography that captures images of astronomical objects and large portions of the night sky.

Aurora is a radiant emission from the upper atmosphere that occurs intermittently over the middle and high altitudes of both hemispheres. They appear in the form of luminous bands, streamers, or the like. This is caused by the constant bombardment of the atmosphere by charged particles attracted by earth’s magnetic lines.

Aurora Australis or Southern Lights are dynamic displays of light that appear in the Antarctic Skies in winter. They are nature’s light show. It is the name given to light emitted by atoms, molecules, and ions that have been excited by energetic charged particles. Common colors are pale green and pink in spiral curtains, arcs and streamers.

Aurora Borealis also know as Northern Polar lights are natural occurring light display in the heavens in the Northern hemisphere. They are nature’s light show. It is the name given to light emitted by atoms, molecules, and ions that have been excited by energetic charged particles. Common colors are pale green and pink in spiral curtains, arcs and streamers.

Auto guider is a tool used in astrophotography to track celestial objects that are photographed from drifting away from the field of view.

Autumnal equinox is the time that signals the end of the summer months and the beginning of winter. It is when the Sun passes the equator.

Averted vision – a technique to view faint objects using peripheral vision. You do not look at the object directly, but just off to the side, you do this while concentrating on the object.

Axis is the line which an object rotates. A straight line about which a body or geometric object rotates or may be conceived to rotate.

Azimuth is the arc of the horizon measured clockwise from the south point, in astronomy, or from the north point, in navigation, to the point where a vertical circle through a given heavenly body intersects the horizon.

-B-

Barlow lens named after its creator Peter Barlow, is a removable lens that can be attached to the eyepiece of a telescope and improves magnification.

Barnard’s Star is a red dwarf star that is six light years away from earth. It is also known as “Barnard’s Runaway Star”.

Barred spiral galaxy is a spiral galaxy that has a centric bar-shaped configuration made-up of stars.

Baryon is a proton, neutron, or any elementary particle that decays into a set of particles that includes a proton.

Baseline is a line that serves as a basis for measurement, calculation, or location. A line between two points or telescopes of an interferometer.

Big Bang is a theory offered by cosmologists related to the early development of the universe.

Binary star is a star system composed of two stars that orbit a common center. The primary star is brightest; the secondary is referred to as the companion star.

Binoculars are optical devices providing good depth effect for both eyes. This consists of two small telescopes fitted side by side.

Black hole theoretically a massive object formed at the beginning of the universe or by a gravitational collapse of a star exploding as a supernova. The gravitational field is intense that no electromagnetic radiation can escape.

Blazar is a compact quasar. An active galaxy with very active and highly variable radio, electromagnetic, and optical emissions.

Blink comparator is used by astronomers, it is an optical instrument used to detect differences in two photographs of the same object by rapidly switching between the two, one picture at a time.

Blueshift is a shift toward shorter wavelengths on the spectral lines of a celestial object. This is caused by the movement of the object toward the object.

Bok globule is small interstellar clouds of very cold gas and dust that are thick. Because of the thickness, they are totally opaque to visible light; however, they can be studied using infrared and radio techniques.

Bolide is a fireball. A large brilliant meteor that explodes.

Bose-Einstein condensate also known as superatom. A phase of matter in which all bosons in a given physical system have been cooled to a temperature near absolute zero and enter the same quantum state.

Brown dwarf is a cold and dark star too small to initiate nuclear reactions that generate heat and light.

Buckyball is a natural occurring type of carbon recognized as C60. The molecular structure looks like the geodesic domes designed by Buckminster Fuller.

Bulge The generally spherical and central region of a spiral galaxy.

-C-

Cannibal coronal mass ejections are fast moving solar eruptions that overtake and often absorb their slower moving kin.

Carbon star is a cool, red giant having a spectrum with strong bands of carbon compounds.

Carbonaceous chondrites are recognized as a group of chondritic meteorites composed of at least 7 recognized groups.

Cassegrain telescope is a reflecting telescope in which the light, passing through a central opening in the primary mirror, is brought into focus a short distance behind it by a secondary mirror.

Cataclysmic variable are stars that invariably increase in brightness and decrease to a nearly dormant state.

Catadioptric telescope is a telescope that uses a combination of mirrors and lenses to increase the focal length of the telescope while allowing it to be folded into a more convenient and compact size.

Charge-coupled device (CCD) is a silicon chip used to detect light. A more efficient device at collecting light than regular film.

Celestial pole are two points in which the extended axis of the earth cuts the celestial sphere and about which the stars seem to revolve.

Celestial sphere is an imaginary spherical shell formed by the sky represented as an infinite sphere. The observer’s position is the given center of the sphere.

Cepheid variable is a variable star in which changes in brightness are due to alternate contractions and expansions in volume.

Chandrasekhar limit: named after Indian astrophysicist Subrahmanyan Chandrasekhar, is the mass limit above which a star has too much mass to become a white dwarf after gravitational collapse.

Charles Messier: French astronomer recognized for publishing the astronomical catalogue that consist of nebulae, star clusters that later become known as “103 Messier objects.”

Chondrite is a stony meteorite containing chodrules. Unchanged meteorites due to melting.

Chromosphere is a layer of the sun’s atmosphere. A gaseous envelope that surrounds the sun outside the photosphere from which large quantities of hydrogen and other gases erupt from.

Circumpolar: Circumpolar stars are permanently above the horizon from a given observing point on Earth; that is to say, they never set. At Earth’s Geographical North Pole (90° north latitude), all stars in the sky are circumpolar. On Earth’s equator, no stars are circumpolar.

Clock drive is a mechanism that causes an equatorial telescope to revolve about its polar axis so that it keeps the same star in its field of view.

Coated optics are optical elements that have refracting and reflecting surfaces coated with one or more coatings of dielectric or metallic material.

Collapsar is a gravitationally collapsed star.

Collimation: Perfectly aligning a telescope’s optics.

Coma: The shroud of gas surrounding a comet’s nucleus.

Coma Berenices is a constellation in the northern sky near Boötes and Leo that contains a prominent cluster of galaxies and the north pole of the Milky Way.

Comet is a celestial body moving about the sun consisting of a central mass surrounded by an envelope of dust and gas that may form a tail that streams away from the sun.

Comet nucleus: The solid, central part of a comet, also known as a “dirty snowball.” It is made of rock, dust, and frozen gases.

Conjunction: A moment when two or more objects appear close together in the sky.

Constellation is any of various groups of stars to which definite names have been given, as Ursa Major, Ursa Minor, Boötes, Cancer, Orion.

Convection: The transfer of heat by the circulation or movement of heated parts of a liquid or gas.

Core: The central region of a planet, star, and galaxy.

Corona: A faintly colored luminous ring appearing to surround a celestial body visible through a haze or thin cloud, especially such a ring around the moon or sun, caused by diffraction of light from suspended matter in the intervening medium.

Coronagraph is an instrument for observing and photographing the sun’s corona, consisting of a telescope fitted with lenses, filters, and diaphragms that simulate an eclipse.

Coronagraph mask is a circular shaped instrument designed to block light from a star’s disk. This allows the area close to the target to be studied.

Coronal mass ejection (CME) is a large-scale solar event involving an ejection of hot plasma that may accelerate charged particles and travel as far as the Earth’s orbit, preceded by a shock front that may create a magnetic storm on Earth

Cosmic microwave background: Microwave radiation that permeates the universe and represents the still cooling heat generated in the universe.

Cosmic ray: A radiation of high penetrating power that originates in outer space and consists partly of high-energy atomic nuclei.

Cosmological constant is a term introduced by Einstein into his field equations of general relativity to permit a stationary, nonexpanding universe: it has since been abandoned in most models of the universe

Cosmology is a branch of astronomy that deals with the general structure and evolution of the universe.

Cosmos: The world or universe seen as an orderly, harmonious system.

Crescent: A lunar or planetary phase wherein less than half the surface is illuminated.

Critical density: The density of a pure element or compound at a critical point. Density of the universe that provide enough gravity to bring the expansion to halt.

Crust is the outermost geological thin layer of an asteroid, moon, or planet.

Cryovolcanism is an icy volcano. When water and other liquids or vapor-phase volatiles, together with gas-driven solid fragments, onto the surface of a planet or moon due to internal heating.

-D-

Damocloid – An elliptical shaped asteroid with a comet-like orbit. It is rare and named after asteroid 5335 Damocles, the first of its kind discovered.

Dark adaptation – The ability of the human eye to adjust seeing dim objects in the dark.

Dark energy – Negative gravity that plays a role in the acceleration in the expansion of the universe.

Dark matter – a term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.

Dark nebula – Dust grains that appear as clouds and is thick enough to shade light from stars in the background.

Declination – Angular distance of an object in the sky, above or below the celestial equator.

Deep-sky objects – Objects that are located beyond the solar system, usually consisting of galaxies, nebulae, stars, and star clusters.

Degree angular Scale interferometer (DASI) – Used to measure temperature and polarization in the Cosmic Microwave background. This is located at NSF Amundsen-Scott South Pole station.

Denison Olmsted – American physicist and astronomer born in Hartford, Connecticut. He is attributed for founding meteor science. He demonstrated that meteors are cosmic in origin and not an atmospheric phenomenon.

Density – Amount of matter contained in a given volume. Usually measured in grams per cubic centimeter.

Deuterium – An isotope of hydrogen with one proton and one neutron in the nucleus having an atomic weight of 2.014.

Diffraction – Spreading out of light as it passes the edge of an obstacle.

Dobsonian telescope – A telescope with a stable altazimuth mount that rotates easily.

Doppler effect – The change in wavelength of sound or light emitted by an object in relation to an observer’s position. An object approaching the observer will have a shorter wavelength (blue) while an object moving away will have a longer (red) wavelength. The Doppler effect is used to estimate an object’s speed and direction.

Double Star – Grouping of two stars. The grouping may look distinct, where the stars appear close together, or physical, such as a binary system.

Double asteroid – Two asteroids that orbit around each other and linked by the gravity between them.

Dust – Minute particles floating in space.

Dwarf galaxy – Small galaxy that contain a few million stars, it is the most common kind of galaxy in the universe.

Dwarf star – A Smaller star. Any star of average to low brightness, mass, and size.

-E-

Eccentric – Deviation from a circle, applied when describing the shape of an orbit.

Eclipse – the total or partial blocking of one celestial body by another.

Eclipsing binary – binary star with an orbital plane oriented so that one star passes in front of the other, thus completely or partially blocking the light from the other star during each orbital period.

Ecliptic – the great circle formed by the intersection of the plane of the earth’s orbit with the celestial sphere; the apparent annual path of the sun in the heavens.

Edwin Hubble – American astronomer who pioneered the understanding of the universe. He showed that other galaxies existed, specifically the Milky Way. Born in Marshfield, Missouri then later moved to Chicago at the age of 9. Young Edwin Hubble had always been fascinated with science. He attended Oxford University on a Rhodes scholarship and studied law. He later realized that his true passion was astronomy; in 1917 Hubble received his doctorate in astronomy from the University of Chicago.

Ejecta – material from beneath the surface of a body such as a moon or planet that is ejected by an impact from a meteor and distributed on the surface. Ejecta usually appear lighter in color than the surrounding surface.

Electromagnetic radiation – Radiation that travels through space at the speed of light, and increases the interplay of oscillating and magnetic fields. The radiation has a wavelength and frequency.

Electromagnetic Spectrum – The range of all kinds of wavelengths of electromagnetic radiation. These include short to long wavelength gamma rays, x-rays, ultra-violet, optical, infrared and radio waves.

Electron – Negatively charged elementary particle found outside, but is attached to, the nucleus of an atom.

Electron Flux – Rate of flow of electrons through a reference surface.

Electron volt – A unit of energy equal to the energy gained by an electron that falls through a potential difference of one volt.

Element – Fundamental unit of matter consisting of fixed number of protons. Number of neutrons and electrons may vary.

Ellipse – An oval shape. Johannes Kepler discovered the orbits of planets are elliptical in shape and not circular.

Elliptical galaxy – A galaxy whose structure is shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.
Elongation – The angular separation of an object from the sun.

Emission – Discharge of electromagnetic radiation from an object.

Emission nebula – Cloud of hot gas being illumined from within by the radiation of energetic, young stars.

Ephemeris – Table that identifies the positions of astronomical objects at certain intervals.

Equatorial mount – a telescope mount in which one axis lies parallel to Earth’s rotational axis; the motion of the telescope about this axis compensates for Earth’s rotation.

Equinox – Two points in which the sun crosses the celestial equator in its yearly path in the sky. Equinoxes signal the start of spring and autumn seasons that occur on or near March 21 and September 22, respectively.

Escape velocity – Speed required for something or an object, to be free of the gravitational pull of a planet or other body.

Evening star – Venus, when it appears in the evening sky.

Event horizon – An invisible boundary around a black hole from which nothing can escape the gravitational pull, not even light.

Exit pupil – Image of the objective lens or primary mirror of a telescope formed on the eye side of the eyepiece.

Exobiologist – a person who studies the origin, development, and distribution of ‘living’ systems that may exist outside of Earth.

Extragalactic – Beyond the Milky Way galaxy.

Extrasolar – beyond the sun.

Extraterrestrial – beyond earth.

Eye relief – the distance between the eyeball and the lens nearest the eye of an eyepiece at which an observer can clearly see the entire field of view

Eyepiece – a magnifying lens used to view the image produced by a telescope’s primary lens or mirror.

-F-

Far ultraviolet – Ultraviolet radiation with the shortest wavelengths.

Field of view – The area of the sky visible through a telescope or binoculars.

Filter – A device that transmits light of only certain wavelengths. Used by astronomers to observe view specific wavelengths and to minimize the light of exceptionally bright objects.

Finder scope – a small, low-powered telescope attached to a larger telescope that helps the observer locate objects in the sky.

Fireball – A very bright meteor.

First quarter – Phase of the moon a quarter of the way around its orbit from new moon. Eastern portion is visibly bright during this phase.

Flare – The sudden, violent outburst of energy from a star’s surface.

Focus – Point at which rays of light passing through a lens meet.

Focal length – Distance from a lens or mirror to the point it draws light to a focus.

Focal ratio – The ratio of the focal length of a lens or mirror to its diameter.

Focuser – the device on a telescope that holds an eyepiece and moves to allow an observer to bring light to a sharp focus.

Fork mount – an equatorial mount in which the telescope swings in declination between the two prongs of a fork.

Frequency – the number of wave crests or troughs that pass a particular point in a given interval of time (usually one second); usually expressed in hertz (cycles per second)

Full moon – Phase of the moon when it is halfway around its orbit from new moon and opposite the sun in the sky; the full disk is illuminated.

-G-

Galactic disk – Disk of a spiral galaxy.

Galactic nucleus – central region of a galaxy. Contains a high density of stars and gas and a super massive black hole.

Galactic plane – Projection of the Milky Way’s disk on the sky.

Galaxy – an enormous gravitationally bound assemblage of millions or billions of stars.

Galaxy cluster – Gravitationally bound assemblage of dozens to thousands of galaxies.

Galilean moons/satellites – Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto as discovered by Galileo in 1610.

Gamma rays – the highest energy, shortest wavelength form of electromagnetic radiation.

Gamma-ray burst – Short and intense burst of high energy radiation emanating from the distant universe.

Gas giant – Planets made primarily of gas, these include Jupiter, Saturn, Uranus, and Neptune.

General relativity – Theory of relativity governing accelerated motion that describes gravity as a curvature of space-time.

German equatorial mount – Mount in which the declination axis sits on top of the polar axis, with the telescope on one end of the declination axis and a counterweight on the other.

Giant molecular cloud – Interstellar clouds of cold gas and dust that contain tens or hundreds of thousands of solar masses.

Gibbous – The phase of the moon between first quarter and last quarter, when the moon appears more than half illuminated.

Globular cluster – A roughly spherical congregation of hundreds of thousands of stars; most globular clusters consist of old stars and exist in a galaxy’s halo.

Granulation – A pattern of small cells that can be seen on the surface of the Sun. They are caused by the convective motions of the hot gases inside the Sun.

Gravitational lens – A concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object. Gravitational lensing results in duplicate images of distant objects.

Gravity – the attractive force that all objects exert on one another; the greater an object’s mass, the stronger its gravitational pull.

Gravity or Gravitational waves – Weak, wavelike disturbances which represent the radiation related to the gravitational force; produced when massive bodies are accelerated or otherwise disturbed.

Greenhouse Effect – An increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere. Carbon dioxide and water vapor are two of the major gases responsible for this effect.

-H-

Habitable zone – Zone around a star in which a planet can maintain liquid on its surface.

Halo – Outer region of a galaxy, contains globular clusters, a few stray stars, and dark matter.

Heliacal rising – the period of time when an object, such as a star, is briefly seen in the eastern sky before dawn and is no longer hidden from the glare of the sun.

Heliopause – The point at which the solar wind meets the interstellar medium or solar wind from other stars.

Heliosphere – a vast region around the sun dominated by the solar wind.

Helium – Second lightest element, consists of two protons, two neutrons and two electrons. Eight percent of the atoms in the universe are helium.

Hertz – A unit of frequency equal to one cycle per second.

Hertzsprung-Russell diagram – a diagram that plots luminosity against temperature for a group of stars.

HII region – an area filled with clouds of ionized hydrogen; the ionization is usually caused by radiation from newborn stars.

Hubble law – the principle that a distant galaxy’s recessional velocity is proportional to its distance from Earth

Hubble space telescope (HST) – The Hubble Space Telescope makes its observations from above Earth’s atmosphere. The telescope orbits 600 kilometers (375 miles) above Earth, working around the clock. It was originally designed in the 1970s and launched in 1990. The telescope is named for astronomer Edwin Hubble.

Hydrazine – Colorless liquid which burns quickly and used as rocket and missile fuel.

Hydrogen – the simplest and lightest element; usually consists of just a single proton and electron; about 90 percent of the atoms in the universe are hydrogen.

Hypered film – Film that has been treated, usually with gas, to enhance its response to low light levels.

Hypergalaxy – A system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.

-I-

Igneous rock – Rock formed by the solidification of magma.

Inclination – Angle between a planet’s orbit and the ecliptic place; Angle between a satellite’s orbit and its host planet’s rotational plane.

Inferior conjunction – The configuration of an inferior planet when it lies between the sun and Earth.

Inferior planet – A planet that orbits the sun inside earth’s orbit, these would be Mercury and Venus.

Inflation – a brief and extraordinarily rapid period of expansion a fraction of a second after the Big Bang.

Infrared – a form of light with slightly lower energy than visible light but with greater energy than radio waves.

Interacting galaxies – galaxies caught in each other’s gravitational embrace, often results in galactic mergers or extreme star formation.

Interference or interferometric fringes – a wave-like pattern resulting from the successful combination of two beams of light which amplifies the light.

Interferometer – A system of two or more widely separated telescopes that achieves the resolving power of a much larger telescope.

Interferometry – The technique of using two or more widely separated telescopes to achieve the resolving power of a much larger telescope.

Intergalactic – Space between the galaxies.

International Space Station – A global cooperative program between the United States, Russia, Canada, Japan, and Europe, for the joint development, operation, and utilization of a permanently habitat in space close to low-Earth orbit.

Interplanetary – Space between the planets.

Interstellar – Space between the stars of a galaxy.

Interstellar medium – Gas and dust located between the stars.

Ion – an electrically charged atom due to the loss or gain of one or more electrons.

Ionization – Process an atom gains or loses electrons.

Ionized gas – Gas heated to a state where it contains ions and free-floating electrons. Also referred to as plasma.

Ionosphere – An atmospheric layer with a high concentration of ions and free electrons.

Irregular galaxy – Galaxy without a clearly defined spiral or elliptical shape.

Isotope – Forms of an element wherein all atoms have the same number of protons but different numbers of neutrons.

-J-

Jet – a narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.

JPL (Jet Propulsion Laboratory) – The lead U.S. center for robotic exploration of the solar system located in Pasadena, California; JPL spacecraft have visited recognized planets with the exception of Pluto.

Jet stream – a high-speed, wandering wind current in the upper troposphere that blows from west to east and affects weather

Jovian planet – A planet with the same attributes of Jupiter (gas giant).

-K-

Kelvin – a unit of temperature equal to one degree on the Celsius scale and 1.8 degrees on the Fahrenheit scale; also the absolute temperature scale defined so that 0 kelvin is absolute zero.

Kuiper Belt – a region in the outer solar system beyond Neptune’s orbit that contains billions of small, icy bodies; Pluto is the largest known Kuiper Belt Object.

-L-

L chondrite – a chondrite (a stony meteorite containing small, round, silicate granules called chondrules) that has a low amount of iron.

Lagrange point – One of five locations in space relative to two bodies where less massive body can maintain a stable orbit around a common center of mass.

Large magellanic cloud – Irregular galaxy that orbits the Milky Way Galaxy.

Last quarter – Phase of the moon three quarters of the way around its orbit from the new moon, the western side is lit.

Latitude – the angular distance north or south from the equator to a point on Earth’s surface, measured on the meridian of the point.

Lens – Curved piece of glass that brings light to a focus.

Lenticular galaxy – a galaxy possessing a large bulge and small disk.

Libration – the small oscillations in the moon’s motion that allow Earth-based observers to see slightly more than half the moon’s surface.

Light pollution – Light, typically from artificial sources, that reaches the night sky, obscuring the view of faint astronomical objects.

Light-gathering power – the ability of a telescope to collect light; the larger a telescope’s aperture, the greater its light-gathering power.

Light-year – the distance light travels in one year, equivalent to approximately 5.9 trillion miles (9.5 trillion km).

Limb – Edge of a celestial object.

Limiting magnitude – the apparent magnitude of the faintest objects that can be seen given the local observing conditions and any telescope, film, or other detector you may be using.

LINER galaxy – A low-ionization nuclear emission-line region galaxy belongs to a common class of otherwise normal galaxies that display low-ionization line emissions near their central regions.

Local Group – the galaxy cluster containing 35 galaxies to which the Milky Way Galaxy belongs.

Local supercluster – the galaxy supercluster to which the Local Group belongs; it spreads over 100 million light-years and boasts the Virgo Cluster as its dominant member.

Long-period comet – Comets that have orbital periods greater than 200 years.

Longitude – the angular distance of a particular place on Earth as measured east or west from the prime meridian running through Greenwich, England.

Luminosity – the total amount of light that an object radiates.

Lunar eclipse – a phenomenon caused by the Earth passing between the sun and moon.

Lunar month – the period of one complete revolution of the moon around Earth, 29.5 days.

Lunation – the time between two successive new moons; approximately 29.5 days.

-M-

Magnetograph – A recording magnetometer used for recording variations in the earth’s magnetic field.

Magnetometer – An instrument that measures the intensity of earth’s magnetic field.

Magnetopause – The boundary space between the earth’s magnetosphere and interplanetary space (40,000 miles / 65,000 km) above the earth, marked by an abrupt decrease in the earth’s magnetic induction.

Magnetosphere – The dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind.

Magnitude – The measurement of an object’s brightness; the lower the number, the brighter the object.

Main sequence – The band of stars on a Hertzsprung-Russell diagram stretching from the upper left to the lower right; stars spend most of their lives in the main sequence phase, in which they are fusing hydrogen into helium in their cores.

Maksutov telescope – A catadioptric telescope that uses a deeply curved meniscus lens as the correcting plate. (See Catadrioptric Telescope)

Mantle – The portion of a planet’s interior above the core but below the crust.

Mare – Dark and smooth area on the surface of the moon or on a planet.

Mass – A measure of the total amount of matter within an object.

Mass loss – The loss of mass by a star during its evolution; some of the causes of mass loss include stellar winds, bipolar outflows, and the ejection of material in a planetary nebula or supernova.

Megaparsec – One million parsecs, equivalent to 3.26 million light-years.

Meridian – Imaginary circle on the celestial sphere that connects the zenith to the north, or south, celestial pole.

Messier Catalog – A catalog of 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s.

Meteor – A flash of light that occurs when a meteoroid burns up in earth’s atmosphere, also known as shooting star.

Meteor showers – Period of meteor activity that occurs when Earth collides with many meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point.

Meteor storm – Rare events that occur when Earth encounters dense regions within a meteor stream. Such encounters can increase normal meteor rates by more than 1,000 meteors per minute.

Meteorite – Rock from space that survives as it passes through the earth’s atmosphere and falls to the ground.

Meteoroid – Small rock that orbits the sun.

Microgravity – A condition the force of gravity is very low, producing a near-weightless environment.

Microlensing – Effect of gravity from a small astronomical body focusing light rays, similar to lenses.

Micron – One millionth of a meter.

Microwaves – Most energetic form of radio waves.

Milky Way – Spiral galaxy containing our solar system. It can be observed by the naked eye as a faint luminous band stretching across the heavens, containing approximately a trillion stars, most of which are too distant to be seen individually.

Millisecond pulsar – Neutron star rotates hundreds of times per second, which typically accretes matter from a stellar companion.

Minor planet – Rocky body that orbits the sun; also recognized as an asteroid.

Mirror – Piece of glass coated with a highly reflective material.

Molecule – Combination of two or more atoms that represent the smallest part of a compound that has the chemical properties of that compound.

Moon – Smaller body orbiting a larger body; often refers to earth’s moon.

Morning star – Venus, when it appears in the morning sky.

Multicultural astronomy – the variety of ways cultures of the past and present have observed, recorded, interpreted, and made use of astronomy to structure their lives, and in some cases satisfy their curiosity about the universe.

Multiple star system – Gravity bound system in which two or more stars orbit a common center of mass.

MUSES-C – The MUSES-C Mission will investigate an asteroid known as an Earth-approaching type. Through this mission, the Institute of Space and Astronautical Science (ISAS) in Japan intends to establish the technology to bring back samples of an asteroid’s surface to Earth.

MUSES stands for a series of missions performed launched by the MU rocket and C means the third mission of this series.

-N-

Naked eye – something visible without the aid of binoculars or a telescope.

Near-infrared – light from the part of the infrared band of the electromagnetic spectrum closest to the visible range.

Nebula – a cloud of interstellar gas and dust; some nebulae represent stellar nurseries, others represent stellar graveyards.

Neutrino – a subatomic particle produced in nuclear reactions and in supernovae that very rarely interacts with matter; neutrinos have no electrical charge and travel at or very close to the speed of light.

Neutron – a subatomic particle with no electric charge that resides in an atomic nucleus; it has about the same mass as a proton.

Neutron star – the collapsed, extraordinarily dense, city-sized remnant of a high-mass star.

New moon – the phase in which the moon is in the same direction as the sun in Earth’s sky, so it is unilluminated and invisible.

Newtonian telescope – a reflecting telescope in which a flat secondary mirror (called the diagonal) in the center of the tube reflects light to a focus outside the tube.

NGC – New General Catalogue, a 19th-century compendium of deep-sky objects such as galaxies, globular clusters, and nebulae.

NGC Objects – deep-sky objects such as galaxies, globular clusters, and nebulae included in the New General Catalogue.

North Celestial Pole – the point in the sky to which Earth’s Geographical North Pole points.

Nova – An explosion on the surface of a white dwarf that is accreting matter from a companion star, which causes the system to temporarily brighten by a factor of several hundred to several thousand.

Nuclear fusion – The process by which two atomic nuclei combine to form a heavier atomic nucleus; this is the energy source that causes most stars to shine.

Nucleosynthesis – the creation of heavy elements from lighter ones by nuclear fusion.

Nucleus – the central region of an atom, comet, or galaxy.

-O-

O-type star – A hot, massive blue star that emits strongly at ultraviolet wavelengths and has a surface temperature of roughly between 28,000 to 40,000 Kelvin’s.

OB Association – Loose grouping of O and B stars, which are the most luminous, most massive, and shortest-lived stars.

Objective – Telescopes primary lens or mirror that gathers light and brings it to a focus.

Obliquity – the angle between the plane of the earth’s orbit and that of the earth’s equator, equal to 23°27'; the inclination of the earth’s equator.

Occultation – The passage of one object in front of a smaller one, temporarily obscuring all or part of the background object from view.

Omega – 1. The ratio of the density of the universe to the critical density 2. The 24th letter of the Greek alphabet.

Omega centauri – Massive globular cluster in the southern constellation Centaurus located about 17,000 light-years from Earth; also known as NGC 5139.

Omega nebula – Also known as the Swan Nebula, M17, NGC 6618, the Horseshoe Nebula, and the Lobster Nebula. One of the Milky Way’s numerous stellar nurseries; the Omega Nebula is about 5,000 light-years from Earth and can be seen in the constellation of Sagittarius the Archer.

Oort cloud – Cloud of cometary nuclei that surrounds the sun at a distance of many thousands of astronomical units.

Open cluster – System containing a few dozen to a few thousand stars that formed from the same stellar nursery.

Opposition – Best time to observe a planet. The moment a planet far from the sun than Earth appears opposite the sun in the sky.

Optical double – Two stars at different distances that lie along nearly the same line of sight and thus appear close together.

Optics – Study of light and its properties; Lenses or mirrors.

Orbit – Curved path, usually elliptical in shape, an object follows around a bigger object or a common center of mass.

Orbital period – The length of time it takes one body to orbit another.

Outgassing – Release of gas from rocky body.

-P-

PAHs – Polycyclic aromatic hydrocarbons (PAHs). A class of stable organic molecules. Flat molecules made of carbon and hydrogen atoms. These are common and highly carcinogenic. It is one of the by-products of combustion from automobiles and airplanes.

Parallax – Shift of a nearby object against a fixed background due to the movement of the observer. Astronomers observe the parallax of stars to measure the distances of these same stars.

Parsec – Distance an object would have to be from earth so that its parallax when seen from two points separated by 1 AU is equivalent to one arcsecond, equivalent to 3.26 light years.

Patera – A shallow crater with a scalloped and complex edge; saucer shaped volcanic structure.

Penumbra – Outer filament region of a sunspot. Lighter region of a sunspot surrounding the umbra (dark center).

Penumbral eclipse – When the moon passes into the outer ring of earth’s shadow, causing a slight shading in the moon’s appearance.

Periastron – Location in an objects orbit where it is closest to the star it orbits.

Perigree – Position of a satellite’s orbit when it is closest to earth.

Perihelion – Position of an object, or body, when it is closest to the sun.

Period – Measured interval a regular event takes place.

Periodic comet – Comet that has been observed to circle, orbit, the sun more than once.

Phase – Cycle of changes in the appearance of a moon or a planet.

Photometer – An instrument that measures light emitted by an object.

Photometry – Degree and measurement of light intensities.

Photons – Single waves of light.

Photosphere – Visible surface of the sun.

Photovoltaic – When light energy or emissions are converted into electricity.

Pixel – Short name for “picture element.” Individual light detectors on a CCD chip.

Planck scale – A unit of measurement scientists utilize to describe the universe. One unit (length) of Planck is 10^-33 centimeters.

Planet – A gaseous, rocky body that orbits a star.

Planetary nebula – Gas ejected by dying, low mass stars that appear as glowing shells.

Planetesimals – Asteroid sized bodies in a new planetary system that collide and form larger bodies.

Planisphere – Map of the sky in two-dimensions with an adjustable overlay and shows a part of the sky that is visible anytime of the night or year.

Plasma – Gas heated to a state wherein it contains ions and free floating electrons.

Plasmasphere – Area of cold and high density plasma above the ionosphere.

Plate tectonics – Theory describing the possibility on how earth’s crust is broken into plates, suggesting that those plates move thru and across earth’s surface.

Polar cap – Icy region of a planet, specifically the north and south pole.

Polarization – When the direction of electric or magnetic fields in an electromagnetic wave changes in a regular pattern.

Position angle – Direction in the heavens one celestial object from another, measured eastward from due north.

Power – Capability of a telescope or binoculars to increase the size of an object that is far away.

Poynting-Robertson effect – Interplanetary particles that are dragged and is caused by its interaction with solar radiation. This causes particles to lose momentum in their orbit and is drawn towards the sun.

Precession – Periodic change in the direction of an objects axis caused by the gravitational influence from another body.

Primary lens – Main lens of a telescope that gathers light bringing the object into focus.

Primary mirror – Main mirror of a telescope that gathers and reflects light to bring the object in focus.

Prime meridian – The line of longitude that runs through Greenwich, England.

Prism – A piece of glass that breaks white light into it’s basic colors, it is wedge shaped.

Prograde – Same direction a planet rotates. An object that move, or looks like it moves in the same direction of solar system bodies or moons.

Prominence – A massive eruption of gas streaming off the surface of the sun towards the corona.

Proper motion – Annual movement of a star across the sky.

Proton – Subatomic particle that is found in an atom’s nucleus and possesses a positive electric charge.

Protoplanet – Gas, dust, and rocks that gradually becomes a whole planet.

Protoplanetary disk – Disk of gas and dust surrounding a new planet; planets that form through the collision of particles inside the disk.

Protostar – Cloud of hot, dense gas and dust that gravitationally collapses to form a star.

Proxima Centauri – Nearest star to the sun at 4.2 light years away.

Pulsar – A rotating neutron star that showers earth with regular pulses of electromagnetic radiation.

-Q-

Quadrillion – a number represented in the U.S. with a 1 followed by 15 zeros, in the U.K., 1 followed by 24 zeros.

Quantum mechanics – Law in physics describing the behavior of matter at the atomic and subatomic level.

Quasar – Highly energetic core in a young galaxy believed to be powered by a big black hole; Short for quasi-stellar object.

-R-

Radial velocity – Acceleration of an object going away from or headed towards an observer.

Radiant – Location in the sky where meteors belonging to a meteor shower appear to come from. 2. Very bright and shining.

Radiation – Electromagnetic waves as it relates to astronomy.

Radiation pressure – Amount of pressure applied on a surface by electromagnetic radiation or light.

Radio galaxy – Galaxy that emanates a large amount of radio waves.

Radio telescope – Designed to observe radio waves coming from space.

Radio waves – Type of light with the longest wavelength with the least energy.

Radiometer – Instrument to measure total energy or power from an object in the form of radiation, especially infrared radiation.

Red dwarf – Smaller star with a low mass, cooler, and less luminous than the sun.

Red giant – Cool star nearing the end of its cycle. These have expanded up a hundred times the diameter of the sun.

Red supergiant – Cool star nearing the end of its cycle. These have expanded from a hundred to a thousand times the diameter of the sun.

Redshift – Multiplication of wavelength of light coming from an object due to its motion away from earth; expansion of the universe; strong gravitational field.

Reflection nebula – Gas and dust clouds made visible due to the dust reflection from the light of nearby star.

Reflector – Telescope using curved mirrors to gather light.

Refractor – Telescope using a glass lens to gather light.

Regolith – Soil from the moon produced meteorites hitting the surface.

Relativity – Theory in physics developed by Albert Eistein. Describes measurement made by two observers who are in relative motion.

Resolution (Resolving power) – A camera or a telescopes ability to capture fine details of a subject.

Reticle – Using two fine wires as part of a grid attached to part of the focal plane or a telescope eyepiece. This is used to locate the position and size of a celestial object.

Retrograde – Viewing objects that move or appear be moving in the opposite direction of a solar system bodies.

Reusable launch vehicle (RLV) – A spacecraft that may be reused on successive missions. A single stage to orbit spacecraft.

Revolution – Orbital motion of a body around a common center of mass or another body.

Ribonucleic acid – Nucleic acid containing genetic information.

Rich clusters – Galaxy clusters with high population densities.

Rich-field telescope – Designed to show a larger field of view at low magnification.

Right ascension – Angular Distance of a celestial object located east of the vernal equinox; outer space sphere equivalent to longitude.

Rotation – Spin of an asteroid, planet, star, moon, or galaxy on its central axis.

Rotation period – Measurable interval an asteroid, planet, star, moon, or galaxy completes one rotation.

-S-

Satellite – Small body or object that goes (orbit) around a planet or asteroid.

Scarp – Cliffs created by erosion and fault movement.

Schmidt camera – Catadioptric telescope used as a camera to photograph wide-angle pictures of the sky.

Schmidt-Cassegrain telescope – Small telescope wherein light passes through a correcting lens located at the front of the telescope; it then reflects off a primary mirror back to a secondary mirror, which then directs the light through a hole in the primary and out the back of the scope; this is a popular telescope for backyard observers.

Secondary mirror – A small mirror used in a telescope that redirects light gathered by a primary mirror.

Seeing – State of observing phenomena created by earth’s atmosphere that blurs images of astronomical objects.

Semimajor axis – Average distance an orbiting body has from its main body.

SETI – “search for extra-terrestrial intelligence”

Seyfert galaxy – Galaxy with a bright nucleus coupled with spectral emission lines, first discovered by Carl Seyfert in 1943.

Shock wave – Powerful wave emanating from a sudden change in density, temperature, or pressure traveling through a medium faster than sound travels on that same medium.

Short-period comet – A comet that orbits less than 200 years.

Sidereal – Relating to or measured in association with the stars.

Sidereal year – Amount of time a body revolves around another with respect to the stars.

Siderostat – A movable flat mirror that reflects light from a celestial object to a given location.

Singularity – An area wherein space and time are infinitely distorted.

Small Magellanic Cloud – An irregular and small galaxy orbiting the Milky Way galaxy.

Solar eclipse – When the moon passes between the earth and the sun.

Solar filter – A safety precaution, a filter used to block almost all the suns light when being viewed.

Solar irradiance – Radiant energy given by the sun over all wavelengths that falls each moment on one square meter of earth’s atmosphere.

Solar mass – Amount of mass contained in the sun, equivalent to 330,000 times to that of earth.

Solar system – System that includes the sun and the smaller bodies (planets, moons, etc…) that orbit the sun.

Solar wind – Stream of charged minute particles coming from the sun.

Solstice – Two points on the celestial sphere wherein the sun is farthest north or south of the equator.

South Celestial Pole – Point in the sky earth’s South Pole points.

Space weathering – Process of changing the surface of an object in space by impacts from small meteors, cosmic rays, and even the solar wind.

Space-time – When the three dimensions of space come together with one dimension of time wherein the events can be exactly calculated.

Special relativity – Theory of relativity applied concerning uniform motion. It proposes that the equivalence of mass and energy and differs from Newtonian physics only when speeds approach that of light.

Spectra – Plural of spectrum. Radiant source energy.

Spectral class – Classification of stars based on its spectrum as dictated by the surface temperature.

Spectral line – Specific wavelength of light that corresponds to the energy exchange of an atom or molecule.

Spectrograph/Spectrometer – Instrument coupled to a telescope that records the spectrum of an astronomical object.

Spectroheliograph – Instrument to photograph the sun on a single wavelength of light.

Spectroscope – An apparatus to explore spectra.

Spectroscopy – In astronomy, it is the study of astronomical subjects.

Spectrum – 1. Whole range of electromagnetic radiation, also known as light. 2. Energy created from a radiant source.

Speed of light – Light travels through a vacuum at 186,000 miles per second, or 300,000 km per second. Distance light travels in a unit of time through a specific substance.

Spicules – Supersonic jet about 300 miles(500 km) in diameter found in the chromosphere of the Sun.

Spiral arm – Concentration of young stars, gas and dust that are finds its way out of the nucleus of a spiral galaxy.

Spiral galaxy – Spiral shaped system composed of stars, gas clouds, and dust, numbering in the billions.

Standard candle – In astronomy, refers to an object known for its brightness and is sometimes used to determine distances.

Star – Sphere of hot gas held together by gravity and emanates brightness by itself; common stars utilize nuclear fusion from its core to generate energy.

Star atlas – Collection of maps using a coordinate system to mark positions of astronomical objects, stars, galaxies, and nebula.

Star hopping – Techniques using familiar patterns of stars to hop from one part of the sky to another; this is done through the use of a telescope and the naked eye.

Star party – Gathering of friends and other like-minded people to observe the night sky.

Starburst galaxy – Galaxy going through a high rate of star formation.

Stellar evolution – Process, that include changes a star goes through during its existence.

Stellar wind – Torrent of charged atomic particles emanating from stars. Release of gas from a star’s surface.

Sterocomparator – Device that allows astronomers to view two separate images of the same region in the sky at the same time.

Stone Meteorite – Meteorite resembling a terrestrial rock made of similar materials.

Sublimate – Transition of solid substance that is evaporated into a gas without reaching the liquid phase.

Summer – Season in the northern hemisphere that commences around June 21.

Sunspot – Dark, temporary cool spot found on the surface of the sun.

Sunspot cycle – Cycle that averages eleven years at which the number of sunspots decreases and increases.

Supercluster – Huge congregation of galaxy clusters that span hundreds and millions of light years away.

Superfluid – State of matter exhibiting frictionless flow. Liquid helium is the one element that produces this when cooled to absolute zero.

Superior conjunction – Constitution of an inferior planet when it lies on the far side of the sun.

Superior planet – Planets that are farther from the sun than earth: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Superluminal motion – Movement that looks to be faster than the speed of light.

Supermassive black hole – Black hole located at the center of a galaxy containing millions or billions of solar masses.

Supernova – Destructive explosion of a star.

Supernova remnant – Growing cloud of gas that is the outer layers of star that just exploded.

Synchronous rotation – Identical rate of rotation of a satellite or moon to the main and bigger object it orbits.

Synchrotron emission – Electromagnetic field from high-energy electrons that are moving in a given magnetic field.

Synodic Period – Interval between points of opposition in a superior planet.

-T-

Telescope – Instrument used to brighten and magnify the view of astronomical objects.

Tera (trillion) – American use, one followed by 12 zeros

Terminator – Boundary of a planet or moon separating the lighted from the unlighted sides.

Terrestrial – Related to the earth.

Terrestrial planet – Small and rocky planet which includes Mercury, Venus, Earth, and Venus.

Thermal radiation – Electromagnetic radiation coming from an object that is not at absolute zero.

Tidal force – disparity in gravitational force between two points on an object caused by the gravity of another object; this leads to a deformation of an object.

Tides – Distortion of a body caused by the gravitational influence on another body.

Trans-Neptunion Object – Object in our solar system lying beyond the orbit of Neptune.

Transit – Passage of a smaller body in front of a larger body. Passage of a celestial body across an observer’s meridian.

Transparency – Clarity of the sky.

Trapezium – Open cluster of young stars, protostars, gas, and dust in the Orion Nebula that feature four stars forming a the trapezium.

Tremolite – Common mineral in metamorphic rocks, made up mainly of calcium and magnesium.

Trojan – Asteroid lying in or near the Lagrange points 60 degrees for or aft Jupiter along the planet’s orbit.

Tropical year – Time earth revolves around the sun in relation to the vernal equinox.

True field of view – Angle of sky viewed through an eyepiece attached to a telescope.

Type la supernova – the explosion of a white dwarf that occurs when it accretes enough mass from a companion star to go above the Chandrasekhar limit.

Type II quasars – a quasar enshrouded in gas and dust that emits very little visible light, however, is easily seen in the infrared and x-ray region of the electromagnetic spectrum

Type II supernova – the explosion of a massive star that occurs when its core runs out of nuclear fuel; these explosions leave behind a neutron star or a black hole

-U-

Ultraviolet light or radiation – Radiation with a higher amount of energy than visible light, not as much as x-rays.

Umbra – Perfect and whole shadow of an opaque body, like a planet, wherein direct light from the source of brightness is totally reduced. 2) Area of complete darkness on the shadow made by an eclipse.

UIB – unidentified infrared bands – Unknown objects in space that produce unidentifiable infrared emission patterns.

Universal time – Also recognized as Greenwich Mean Time, forming the basis in all civil time keeping. Local time centered in Greenwich, England.

Universe – All that exists

UT – short for Universal time

UV (ultraviolet) – short for Ultra Violet.

-V-

Van Allen Belts – Dual belts of charged particles from a solar wind trapped in earth’s magnetic field above the atmosphere. Radiation zone of charged particles surrounding Earth. Shape of Van Allen belts is determined by Earth’s magnetic field.

Variable star – Star with varying luminosity.

Vernal equinox – Time of the year when the sun moves across the celestial equator towards the north, usually around March 21.

Vignetting – Decreased illumination over an image plane in a camera or in some cases a telescope, this causes a distortion close to the edge of an image.

Virgo Cluster – 2,500 known galaxies near the north galactic pole of the constellation Virgo that is 60 million light years from earth.

Visible light – Wavelengths in the electromagnetic spectrum that the human eye can see.

Voids – Big regions of empty space found amidst galaxy clusters and superclusters.

Volatiles – Chemical compounds that become gaseous at very low temperatures.

-W-

Waning/Waxing – Interval between full and new moon

Wavelength – Distance between two wave crests.

Weight – Force applied on an object because of gravity.

White dwarf – Dense remains of an intermediate mass star like the sun that has collapsed and is the same size as earth.

Winter – Season in the Northern Hemisphere that begins December 21.

Wolf-Rayet star – Luminous and hot star having temperatures reaching 90,000 kelvins.

-X-

X-rays – Type of electromagnetic radiation that is like light but has a shorter wavelength capable of penetrating solid objects and ionizing gases.

X-class flares – Most energetic kind of solar flares and the brightest.

X ray star – Bright object emitting x rays as a primary component of its radiation.

-Y-

Yellow dwarf – Ordinary star, like the sun and is at its stable point in its transformation.

-Z-

Zenith – Point on the celestial sphere directly above an observer.

Zenith hourly rate – Meteorites expected to be viewed per hour during a meteor shower, where the meteor showers radiance is at an observer’s zenith.

Zodiac – Imaginary belt across the sky wherein the solar system can always be found.

Zodiacal light – Cone of light that can be observed above the horizon before sunrise or after sunset. This is caused by small particles of reflected sunlight.
 







 NOAA Technical Memorandum NWS SR-145
A COMPREHENSIVE GLOSSARY OF
WEATHER TERMS FOR STORM SPOTTERS
NOAA/NWS/WFO Norman
Contents

    Introduction
    Glossary (A-B)
    Glossary (C-H)
    Glossary (I-R)
    Glossary (S-Z)
    Acknowledgements
    Bibliography
    Figures

Glossary

Quick Alpha Access: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

 
-S-

*Scud (or Fractus) - Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

SELS - SEvere Local Storms Unit, former name of the Operations Branch of the Storm Prediction Center (SPC) in Norman, OK (formerly in Kansas City, MO).

*Severe Thunderstorm - A thunderstorm which produces tornadoes, hail 0.75 inches or more in diameter, or winds of 50 knots (58 mph) or more. Structural wind damage may imply the occurrence of a severe thunderstorm. See approaching (severe).

Shear - Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

*Shelf Cloud - A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

Short-Fuse Warning - A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

Shortwave (or Shortwave Trough) - A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.

Slight Risk (of severe thunderstorms) - Severe thunderstorms are expected to affect between 2 and 5 percent of the area. A slight risk generally implies that severe weather events are expected to be isolated. See high risk, moderate risk, convective outlook.

Sounding - A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location (Fig. 6). Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature, etc.

SPC - Storm Prediction Center. A national forecast center in Norman, Oklahoma, which is part of NCEP. The SPC is responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding) and severe winter weather over the contiguous United States.

Speed Shear - The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

Spin-up - [Slang], a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.

Splitting Storm - A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.

*Squall Line - A solid or nearly solid line or band of active thunderstorms.

Staccato Lightning - A CG lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.

Steering Winds (or Steering Currents) - A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Storm-relative - Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

Storm-scale - Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale, mesoscale.

*Straight-line Winds - Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.

Stratiform - Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).

Stratocumulus - Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.

Stratus - A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus.

Striations - Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. Striations often reveal the presence of rotation, as in the barber pole or "corkscrew" effect often observed with the rotating updraft of an LP storm.

Subsidence - Sinking (downward) motion in the atmosphere, usually over a broad area.

Sub-synoptic Low - Essentially the same as mesolow.

Suction Vortex (sometimes Suction Spot) - A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.

*Supercell - A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm (Fig. 7). Storms exhibiting these characteristics often are called classic supercells; however HP storms (Fig. 3) and LP storms (Fig. 5) also are supercell varieties.

Surface-based Convection - Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.

SWEAT Index - Severe Weather ThrEAT index. A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds as follows:

    SWEAT=(12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where

        Td 850 is the dew point temperature at 850 mb,
        TT is the total-totals index,
        f 850 is the 850-mb wind speed (in knots),
        f 500 is the 500-mb wind speed (in knots), and
        s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer).

    SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers.

    The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed sounding analysis programs.

SWODY1, SWODY2 (sometimes pronounced swoe-dee) - The day-1 and day-2 convective outlooks issued by SELS.

Synoptic Scale (or Large Scale) - Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale, storm-scale.
-T-

*Tail Cloud - A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds. See Fig. 7, supercell.

Compare with beaver tail, which is a form of inflow band that normally attaches to the storm's main updraft (not to the wall cloud) and has a base at about the same level as the updraft base (not the wall cloud).

Tail-end Charlie - [Slang], the thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unimpeded, such a storm often has a higher probability of strengthening to severe levels than the other storms in the line.

Thermodynamic Chart (or Thermodynamic Diagram) - A chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability. (See Fig. 6, sounding.)

Thermodynamics - In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Theta-e (or Equivalent Potential Temperature) - The temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.

Theta-e Ridge - An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.

Tilt Sequence - Radar term indicating that the radar antenna is scanning through a series of antenna elevations in order to obtain a volume scan.

Tilted Storm or Tilted Updraft - A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

*Tornado - A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.

Tornado Family - A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

Total-Totals Index - A stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb. The total-totals index is the arithmetic sum of two other indices: the Vertical Totals Index (temperature at 850 mb minus temperature at 500 mb) and the Cross Totals Index (dew point at 850 mb minus temperature at 500 mb). As with all stability indices there are no magic threshold values, but in general, values of less than 50 or greater than 55 are considered weak and strong indicators, respectively, of potential severe storm development.

Tower - (Short for towering cumulus), a cloud element showing appreciable upward vertical development.

Towering Cumulus - (Same as congestus.) A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)

Transverse Bands - Bands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence. Transverse bands observed at low levels (called transverse rolls or T rolls) often indicate the presence of a temperature inversion (or cap) as well as directional shear in the low- to mid-level winds. These conditions often favor the development of strong to severe thunderstorms.

Transverse Rolls - Elongated low-level clouds, arranged in parallel bands and aligned parallel to the low-level winds but perpendicular to the mid-level flow. Transverse rolls are one type of transverse band, and often indicate an environment favorable for the subsequent development of supercells. Since they are aligned parallel to the low-level inflow, they may point toward the region most likely for later storm development.

T Rolls - [Slang], same as transverse rolls.

Triple Point - The intersection point between two boundaries (dry line, outflow boundary, cold front, etc.), often a focus for thunderstorm development.

Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment).

Tropopause - The upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height). See Fig. 6, sounding.

Troposphere - The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers - see inversion, cap), vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).

Trough - An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

Turkey Tower - [Slang], a narrow, individual cloud tower that develops and falls apart rapidly. The sudden development of turkey towers from small cumulus clouds may signify the breaking of a cap.

TVS - Tornadic Vortex Signature. Doppler radar signature in the radial velocity field indicating intense, concentrated rotation - more so than a mesocyclone. Like the mesocyclone, specific criteria involving strength, vertical depth, and time continuity must be met in order for a signature to become a TVS. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it. A TVS is not a visually observable feature.
-U-

UKMET - A medium-range numerical weather prediction model operated by the United Kingdom METeorological Agency.

Updraft - A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.

Updraft Base - Alternate term for a rain-free base.

Upper Level System - A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term sometimes is used interchangeably with impulse or shortwave.

Upslope Flow - Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upstream - Toward the source of the flow, or located in the area from which the flow is coming.

UVM (or UVV) - Upward Vertical Motion (or Velocity).
-V-

VAD - Velocity Azimuth Display. A radar display on which mean radial velocity is plotted as a function of azimuth. See VWP.

Vault - Same as BWER.

Veering Winds - Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter example is a form of directional shear which is important for tornado formation. Compare with backing winds.

Vertically-stacked System - A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. Such systems typically are weakening and are slow-moving, and are less likely to produce severe weather than tilted systems. However, cold pools aloft associated with vertically-stacked systems may enhance instability enough to produce severe weather.

VIL - Vertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D units that takes into account the three-dimensional reflectivity of an echo. The maximum VIL of a storm is useful in determining its potential severity, especially in terms of maximum hail size.

VIP - Video Integrator and Processor, which contours radar reflectivity (in dBZ) into six VIP levels:

    VIP 1 (Level 1, 18-30 dBZ) - Light precipitation
    VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.
    VIP 3 (Level 3, 38-44 dBZ) - Moderate to heavy rain.
    VIP 4 (Level 4, 44-50 dBZ) - Heavy rain
    VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.
    VIP 6 (Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible.

*Virga - Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst; see dry microburst.

V Notch - A radar reflectivity signature seen as a V-shaped notch in the downwind part of a thunderstorm echo. The V-notch often is seen on supercells, and is thought to be a sign of diverging flow around the main storm updraft (and hence a very strong updraft). This term should not be confused with inflow notch or with enhanced V, although the latter is believed to form by a similar process. See Fig. 7, supercell.

Volume Scan - A radar scanning strategy in which sweeps are made at successive antenna elevations (i.e., a tilt sequence), and then combined to obtain the three-dimensional structure of the echoes. Volume scans are necessary to determine thunderstorm type, and to detect features such as WERs, BWERs, and overhang.

Vorticity - A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.

Vort Max - (Slang; short for vorticity maximum), a center, or maximum, in the vorticity field of a fluid.

VWP - VAD Wind Profile. A radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar. The display is plotted with height as the vertical axis and time as the horizontal axis (a so-called time-height display), which then depicts the change in wind with time at various heights. This display is useful for observing local changes in vertical wind shear, such as backing of low-level winds, increases in speed shear, and development or evolution of nearby jet streams (including low-level jets).

This product often is referred to erroneously as a VAD.
-W-

*Wall Cloud - A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion. See Fig. 7, supercell.

"Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.

Warm Advection - Transport of warm air into an area by horizontal winds.

Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.

Warning - A product issued by NWS local offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning). See short-fuse warning.

Watch - An NWS product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).

Watch Box (or Box) - [Slang], a severe thunderstorm or tornado watch.

Waterspout - In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters.

The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.

Wedge (or Wedge Tornado) - [Slang], a large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base.

The term "wedge" often is used somewhat loosely to describe any large tornado. However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least as wide at the ground as it is tall, is very rare.

Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many documented wedges have been rated lower. And some violent tornadoes may not appear as wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series of suction vortices without a central condensation funnel). Whether or not a tornado achieves "wedge" status depends on several factors other than intensity - in particular, the height of the environmental cloud base and the availability of moisture below cloud base. Therefore, spotters should not estimate wind speeds or F-scale ratings based on visual appearance alone. However, it generally is safe to assume that most (if not all) wedges have the potential to produce strong (F2/F3) or violent (F4/F5) damage.

WER - Weak Echo Region. Radar term for a region of relatively weak (reflectivity at low levels on the inflow side of a thunderstorm echo, topped by stronger reflectivity in the form of an echo overhang directly above it (see Fig. 2). The WER is a sign of a strong updraft on the inflow side of a storm, within which precipitation is held aloft. When the area of low reflectivity extends upward into, and is surrounded by, the higher reflectivity aloft, it becomes a BWER.

Wet Microburst - A microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst. See dry microburst.

Wind Shear - See shear.

Wrapping Gust Front - A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.

WSR-57, WSR-74 - NWS Weather Surveillance Radar units, replaced by WSR-88D units.

WSR-88D - Weather Surveillance Radar - 1988 Doppler; NEXRAD unit.
-Z-

Zonal Flow - Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal. Compare with meridional flow.

Acknowledgements

The author expresses his most sincere thanks to the many individuals who provided input to the glossary. The following individuals were instrumental in contributing, through helpful comments and suggestions: David Andra, Dave Beusterien, Dr. Harold Brooks, Bill Bunting, Don Burgess, E. Brian Curran, Dr. Charles A. Doswell III, Mike Emlaw, Mike Foster, Dave Gold, Paul Janish, Tim Marshall, Alan Moller, Mike Morgan, Steve Parker, Steve Piltz, Robert Prentice, Jim Purpura, Gene Rhoden, Lans Rothfusz, Dan Smith, Greg Stumpf, Steve Vasiloff. Steve Nelson and Doug Speheger both were instrumental in setting up the glossary on the WSFO Norman home page.

Bibliography

    American Meteorology Society, 1990: Glossary of Meteorology. American Meteorological Society Press, Boston.
    Caracena, Fernando, Ronald L. Holle, and Charles A. Doswell III, 1989: Microbursts - A Handbook for Visual Identification. NOAA, Environmental Research Laboratories, National Severe Storms Laboratory.
    Doswell, Charles A. III, 1982: The Operational Meteorology of Convective Weather. Volume I: Operational Mesoanalysis. NOAA Technical Memorandum NWS NSSFC-5.
    Doswell, Charles A. III, 1985: The Operational Meteorology of Convective Weather. Volume II: Storm Scale Analysis. NOAA Technical Memorandum ERL ESG-15.
    Fujita, T. T., 1985: The Downburst - Microburst and Macroburst. SMRP Research Paper No. 210, University of Chicago, 122 pp.
    Marshall, Tim, 19--: Storm Chase Manual. Published annually in association with Storm Track. Contact: 1336 Brazos Blvd, Lewisville TX 75067.
    Marshall, Tim (Editor): Storm Track. Published bi-monthly by Master Graphics, Lewisville TX. Contact the editor, 1336 Brazos Blvd, Lewisville TX 75067, for subscription information.
    National Weather Service, 1982: Spotter's Guide for Identifying and Reporting Severe Local Storms. Available from most National Weather Service offices, or from the U. S. Dept. of Commerce, NOAA, Rockville MD 20852.

Figures

    Figure 1, Bow Echo
    Figure 2, Weak Echo Region, Bounded Weak Echo Region
    Figure 3, High-Precipitation Supercell Thunderstorm
    Figure 4, Line Echo Wave Pattern
    Figure 5, Low-Precipitation Supercell Thunderstorm
    Figure 6, Upper Air Sounding
    Figure 7, Classic Supercell Thunderstorm
 







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STORMFAX®  Weather Words

A | B | C | D | E | F | G | H | I | J | L | M | N | O | P | R | S | T | V | W


ACID RAIN: Cloud droplets combining with gaseous pollutants, such as oxides of sulfur and nitrogen, to make falling rain or snow acidic.

AIR MASS: A widespread body of air with consistent temperature and moisture characteristics.

ALBERTA CLIPPER: A storm system that develops near the Province of Alberta in Canada and moves rapidly east-southeast into the Great Lakes and on into the Northeast.

ALTOCUMULUS: A principal cloud type, white and/or gray in color, present in the mid-altitudes with a cumuliform-like shape.

ALTOSTRATUS: A principal cloud type, gray or bluish in color, present in the mid-altitudes with a sheet or fibrous appearance.

ANEMOMETER: An instrument which measures wind speed.

ANTICYCLONE: An area of high pressure around which the winds circulate in a clockwise direction in the Northern Hemisphere.  It is usually responsible for fair, dry weather.

BACKDOOR COLD FRONT: A cold front that approaches from the north-northeast instead of from the usual west-northwest direction.

BAROMETER: An instrument for measuring atmospheric pressure.

BEAUFORT WIND SCALE: A system of estimating and reporting wind speed devised by British Rear-Admiral, Sir Francis Beaufort in 1805, based on observations of the effects of the wind.

BERMUDA HIGH: An area of high pressure centered over the western Atlantic Ocean.  This weather system produces a southerly wind which often moves warm, humid air into the Northeast.

BIOMETEOROLOGY: The portion of the science of meteorology that deals with the effects of weather and climate on health and the human body.

BLIZZARD: Severe weather characterized by snow with winds in excess of 35 mph and visibility 1/4 mile or less for 3 hours or longer.

BLOWING SNOW: Wind-driven snow that reduces visibility near the ground.  Blowing snow can be either falling snow or snow that has already accumulated but is picked up and moved by strong winds.

CEILING: The height of the lowest layer of clouds reported as broken or overcast and not thin.

CIRRUS: A principal cloud type present at high altitudes above 18,000 feet and composed of ice crystals.

CIRROCUMULUS: A principal cloud type featuring cirrus clouds with vertical development.

CIRROSTRATUS: A principal cloud type featuring cirrus clouds with a flat or sheet-like appearance.

CLEAR: Sky condition with less than 1/10 cloud coverage.

CLIMATE: The statistical collective of weather records during a specified period of time.

COLD FRONT: The front edge of a cold air mass. It often produces precipitation and, frequently, severe weather.

CONDENSATION: The process by which a gas changes into a liquid.

CONVECTION: An atmospheric motion which is predominately vertical.  Warm air rising or cold air sinking or both.

CORIOLIS FORCE: An apparent force on moving particles produced by the rotation of the Earth. In the Northern Hemisphere, the wind is deflected to the right by the coriolis force.

CRYOSEISM: (also called frost quake) A sudden soil expansion caused by water that has frozen quickly after seeping into the ground.  Loud booms often accompany a cryoseism occurrence.

CUMULONIMBUS: A principal cloud type, dense and vertically developed, which produces heavy precipitation. It features an anvil shape on top and a dark base.

CUMULUS: A principal cloud type of vertical elements having a flat base and a bulging upper portion resembling cauliflower.

CYCLONE: An area of low pressure around which the wind circulates in a counterclockwise direction in the Northern Hemisphere. It usually produces wet weather. The term is also used for a hurricane in the Indian Ocean.

DERECHO: A large cluster of thunderstorms that produces widespread, straight-line wind damage.

DEW: Moisture that condenses on objects near the ground.

DEWPOINT: The temperature to which the air must be cooled for water vapor to condense.  The larger the spread of temperature and dewpoint, the drier the air.  This spread is called the dewpoint depression.

DOPPLER RADAR: A type of weather radar which displays atmospheric motion.  It utilizes the Doppler Effect to measure the velocity of particles in the atmosphere.

DOWNBURST: A strong localized downdraft of wind that is experienced beneath a severe thunderstorm.

DRIFTING SNOW: The uneven distribution of snowfall.  Snow depth caused by strong surface winds.

DRIZZLE: Very small, numerous, slowly falling water droplets, with diameters less than 0.5 millimeters.

EL NIŃO: A large scale weakening of the trade winds and warming of the surface layers in the Equatorial eastern and central Pacific Ocean.

FAIR: Less than 4/10 opaque cloud cover, no precipitation, and no extremes in temperature, visibility or wind.

FLASH FLOOD: A flood which is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours.  These conditions are often produced by slow moving thunderstorms or tropical systems which make landfall.

FOG: The visible aggregate of minute water droplets suspended in the atmosphere near the Earth's surface.  A cloud on the ground!

FREEZING RAIN: Rain which falls as liquid then freezes upon impact of a surface, resulting in a coating of ice on exposed objects.

FRONT: The leading edge of an airmass; the transition zone between two distinct airmasses.  Frontal types include cold fronts, warm fronts, occluded fronts and stationary fronts.

FROST: Ice crystals produced from water vapor that has frozen on a surface at or below 32şF/0şC.

FROST QUAKE: (cryoseism) A sudden soil expansion caused by water that has frozen quickly after seeping into the ground.  Loud booms often accompany a cryoseism occurrence.

FROST/FREEZE WARNING: Below-freezing temperatures are expected during the growing season and may cause significant damage to plants and crops.

FROSTBITE: The partial freezing of exposed parts of the body, causing injury to the skin and sometimes to deeper tissues.

FUNNEL CLOUD: A rotating, funnel-shaped cloud extending below the base of a thunderstorm.  When it reaches the ground, it is called a tornado.

GALE: Wind speeds from 39 to 54 mph (34 to 47 knots).

GOES: Geostationary Operational Environmental Satellite.

GUST: A brief, sudden increase in wind speed with a fluctuation greater than 10 knots during a period less than 30 seconds.

GUST-FRONT: The boundary between air flowing into a thunderstorm and the precipitation-cooled air flowing out of the storm.  A shelf cloud may be seen above its surface position.  There is a noticeable wind shift and temperature drop when the gust-front passes (similar to a cold front).

HABOOB: From Arabic, the meteorological term for a dust storm.

HAIL: Frozen precipitation in the form of layered lumps of ice produced by convection within cumulonimbus clouds.

HAZE: Fine dust particles present in a portion of the atmosphere.

HEAT ADVISORY: Issued when the Heat Index is expected to exceed 105 during the day and 80 during the night for at least two consecutive days.

HEAT INDEX: Also known as the Apparent temperature, it is a non-physical value which combines the effect of the air temperature and amount of moisture in the air to illustrate how it "feels."

HEAVY SNOW: In general, snowfall is accumulating at the rate of either 4 inches or more in 12 hours or less, or 6 inches or more in 24 hours or less.

HEAVY SURF: Large waves breaking on or near the shore resulting from swells spawned by a distant storm.

HIGH: An area of high pressure around which the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

HUMIDITY: Generally the measure of the water vapor content of the air.

HURRICANE: An intense tropical cyclone in the North Atlantic, Caribbean Sea, Gulf of Mexico and the Eastern Pacific Ocean with sustained wind speeds in excess of 74 mph (64 knots).

HURRICANE SEASON: The part of the year having a relatively high incidence of hurricanes.  In the Atlantic, Caribbean, Gulf of Mexico, and Central Pacific, the season is from June through November.  The season begins two weeks earlier in the Eastern Pacific.

ICE FOG: (also called frozen fog, pogonip) Fog containing suspended ice crystals. Ice fog can form only in extremely cold areas of the world since water remains a liquid down to -40 °F (-40 °C).

JET STREAM: Relatively strong winds concentrated within a narrow band in the atmosphere.

LA NIŃA: A large scale cooling of the surface layers in the Equatorial eastern and central Pacific Ocean.

LAKE EFFECT: Warm lake water modifies the weather along its shore and for some distance downwind.

LIGHTNING: All of the various forms of electrical discharge produced by thunderstorms.

LOW: A cyclonic storm that often forms along a front, around which the wind blows counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

METEOROLOGY: The study of atmospheric phenomena.

NCDC: National Climatic Data Center. The archive center for climate, observational and forecast data from the National Weather Service (NWS).

NEXRAD: Next Generation Weather Radar.

NHC: National Hurricane Center. The office of the National Weather Service responsible for tracking and forecasting tropical cyclones.

NOAA: National Oceanic and Atmospheric Administration. A branch of the US Department of Commerce, NOAA is the parent organization of the National Weather Service.

NOAA WEATHER RADIO (NWR): The voice of the National Weather Service with 24-hour-a-day broadcasts of weather information, forecasts and warnings.  It is programmed from local National Weather Service offices.

NOR' EASTER: An intense low pressure system that tracks along the east coast of the United States, producing strong northeast winds, large waves, and intense precipitation.

NSSL: National Severe Storms Laboratory. Located in Norman, Oklahoma, the office is responsible for monitoring and forecasting severe convective weather. (Storm Prediction Center)

OCCLUDED FRONT: A composite of two fronts that usually occurs when a cold front overtakes a warm front.

OVERCAST: Sky condition when greater than 9/10 of the sky is covered by clouds.

PARTLY CLOUDY: Sky condition when between 3/10 and 7/10 of the sky is covered by clouds.

PAROEMIEOLOGY: The study of weather folklore.

POGONIP: A term of Native American origin used for ice fogs occurring in mountain valleys of the western United States.

POLAR VORTEX: The cold-core cyclonic circulation, centered in the Earth's polar regions, extending from the middle troposphere to the stratosphere.

PRECIPITATION: Any form of water particles, liquid or frozen, that fall from the atmosphere and reach the ground.

PRESSURE: The force exerted by the weight of air above a given point, usually expressed in millibars (mb) or inches of mercury (in. Hg).

RADAR: A device used to detect precipitation by sending an electromagnetic signal and measuring the intensity of the reflected energy. (RAdio Detection And Ranging)

RADIOSONDE: An instrument connected to a weather balloon that collects meteorological data as it ascends through the atmosphere.

RAIN: Liquid water precipitation generally with a diameter greater than 0.5 mm.

RELATIVE HUMIDITY: The percent of the amount of water vapor in the air compared to the capacity for water vapor in the air.

RIDGE: An elongated area of high pressure in the atmosphere.

SCATTERED CLOUDS: A sky condition when between 1/10 and 5/10 of the sky is covered by clouds.

SEA BREEZE: A local wind that blows from a sea or ocean towards land.  It is caused by the temperature difference between cool air above the water and the warmer land.  The leading edge of the breeze is termed a sea breeze front.

SEVERE THUNDERSTORM: A thunderstorm with wind gusts in excess of 58 mph (50 knots), frequent lightning and hail with a diameter of 3/4" or larger.

SHOWER: Liquid precipitation with frequent changes in intensity or sudden stops or starts.

SLEET: A type of frozen precipitation, consisting of small pellets produced by the freezing of raindrops as they fall.

SNOW: A type of frozen precipitation composed white translucent ice crystals in complex hexagonal forms.

SNOW FLURRY: Also referred to as a snow shower, a very light and brief period of snowfall.

SNOW SQUALL: Intense showers or bands of locally heavy snow, often produced by the lake effect.

SQUALL LINE: A non-frontal band, or line, of thunderstorms.

STATIONARY FRONT: A transition zone between air masses, with neither advancing upon the other.

STORM SURGE: A rise above the normal water level along a shore caused by strong onshore winds and/or reduced atmospheric pressure.  The surge height is the difference of the observed water level minus the predicted tide.

STRATUS: A principal cloud type, gray in color, present at low altitudes with a uniform base.

SUBTROPIC: The climatic region adjacent to the tropics, which are located between 20 and 40 degrees latitude in both hemispheres.

THUNDER: The sound produced by a stroke of lightning as it rapidly heats the air surrounding the bolt.

TORNADO: A violent rotating column of air, extending from the base of a cumulonimbus cloud to the ground, producing damaging winds up to 300 mph.

TROPICAL DEPRESSION: A tropical cyclone in which the maximum 1-minute sustained surface wind is 38 mph or less.  They form from a tropical wave or tropical disturbance.

TROPICAL DISTURBANCE: A discrete system of apparently organized convection originating in the tropics or subtropics, having a non-frontal migratory character and maintaining its identity for 24 hours or more.

TROPICAL STORM: A tropical cyclone in which the 1-minute sustained surface wind ranges 39-73 mph.

TROUGH: An elongated area of low atmospheric pressure.

VIRGA: Falling precipitation that evaporates before reaching the ground.

WARM FRONT: A leading boundary of a warm airmass that is often moving into an area influenced by a cooler airmass.

WARNING: A public notice issued by the National Weather Service when a certain hazard (tornado, severe thunderstorm, flood or winter storm) is imminent.

WATCH: A public notice issued by the National Weather Service when conditions are such that a certain hazard (tornado, severe thunderstorm, flood or winter storm) is possible.

WATERSPOUT: A column of rotating wind over water that has characteristics of a tornado.

WIND CHILL: The accelerated heat loss from exposed skin due to increased wind speed.  A non-physical value which combines the effect of air temperature and the wind speed to illustrate how it "feels." 







 Acid rain
    Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (eg. pH < 5.6).

Afternoon
    Noon to sunset.

Arctic air
    A mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.

Arctic high
    A very cold high pressure that originates over the Arctic Ocean.

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Ball lightning
    A relatively rarely seen form of lightning, generally consisting of an orange or reddish ball of the order of a few cm to 30cm in diameter and of moderate luminosity, which may move up to 1 m/s horizontally with a lifetime of a second or two.

Barber pole
    A thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.

Barometer
    An instrument for measuring atmospheric pressure.

Barometric pressure
    The actual pressure value indicated by a pressure sensor.

Bitterly cold
    In winter, bitterly cold or very cold, refers to more than seven degrees Celsius below normal.

Black ice
    Thin, new ice that forms on fresh water or dew covered surfaces; it is common on roadways during the fall and early winter and appears "black" because of its transparency.

Blizzard
    Includes winter storm conditions of sustained winds or frequent gusts of 35 mph or more that cause major blowing and drifting of snow, reducing visibility to less than one-quarter mile for 3 or more hours. Extremely cold temperatures often are associated with dangerous blizzard conditions. In Canada, a blizzard described as a severe storm that lasts three or more hours, and brings low temperatures, strong winds and poor visibility due to blowing snow. In Australia, it is described as a violent and very cold wind which is loaded with snow, some of which has been raised from snow covered ground.

Blocking high
    A high pressure area (anticyclone), often aloft, that remains nearly stationary or moves slowly compared to west-to-east motion. It blocks the movement eastward movement of low pressure areas (cyclones) at its latitude.

Blowing Dust
    Dust that is raised by the wind to moderate heights above the ground to a degree that horizontal visibility decreases to less than seven miles. Visibilities of 1/8 mile or less over a widespread area are criteria for a Blowing Dust Advisory.

Blowing sand
    Sand particles picked up from the surface of the earth by the wind to moderate heights above the ground, reducing the reported horizontal visibility to less than seven statute miles.

Blowing snow
    Wind driven snow that reduces visibility to six miles or less causing significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind. In Canada, it is described as snow that is lifted by the wind from the earth's surface to a height of two meters or more.

Blowing spray
    Water droplets torn by the wind from a body of water, generally from the crests of waves, and carried up into the air in such quantities that they reduce the reported horizontal visibility to less than seven statute miles.

Blustery
    Descriptive term for gusty winds that accompany cold weather.

Breezy
    Wind in the range of 15 miles per hour to 25 mile per hour with mild or warm temperatures.

Brisk
    Wind in the range of 15 to 25 miles per hour when the temperature is cold.

Broken clouds
    Clouds which cover between 5/8ths and 7/8ths of the sky.

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Calm
    The absence of apparent motion in the air.

Celsius
    A temperature scale in which zero is the freezing point of water and one hundred is the boiling point.

Chinook
    A Chinook is a warm, dry, gusty wind that occasionally occurs to the leeward side of a mountain range, particularly the Rocky Mountains.

Cirrus cloud
    High cloud, delicate, hair-like and feathery looking.

Clear
    Sky condition of less than 1/10 cloud coverage. In the United Kingdom, clear is defined as “No cloud”, and in Australia, it is defined as “Virtually cloud-free”.

Clear slot
    A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud.

Climate
    The prevalent long term weather conditions in a particular area. Climatic elements include precipitation, temperature, humidity, sunshine and wind velocity and phenomena such as fog, frost, and hail storms. Climate cannot be considered a satisfactory indicator of actual conditions since it is based upon a vast number of elements taken as an average.

Cloudburst
    A sudden, intense rainfall that is normally of short duration.

Cloudy
    Cloudy means that clouds cover more than 60 percent of the sky.

Coastal flood warning
    Issued when there is widespread coastal flooding expected within 12 hours, more than just a typical overwash.

Cold
    In the winter, cold refers to four to seven degrees Celsius below normal.

Cold Advection (CAA)
    Transport of cold air into a region by horizontal winds.

Cold front
    A narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing.

Cold low
    A low pressure system with cold air mass from near the surface to all vertical levels (also called a cold core low).

Cold-air-funnel
    A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

Collar cloud
    Frequently used as a synonym for wall cloud, although it actually is a generally circular ring of cloud surrounding the upper portion of a wall cloud.

Combined seas
    The combined height of swell and wind waves.

Condensation
    The process by which water vapor becomes a liquid; the opposite of evaporation, which is the conversion of liquid to vapor. In Australia, it is described as a change from a gas to a liquid.

Confluence
    A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.

Congestus (or Cumulus congestus)
    A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb (Cumulonimbus cloud).

Continental air mass
    A dry air mass originating over a large land area.

Convection
    The transfer of heat within the air by its movement. The term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere.

Convergence
    An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. Divergence is the opposite, where winds cause a horizontal net outflow of air from a specified region.

Cumulonimbus Cloud
    A vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds. In Australia, it is described as a heavy, puffy, heaped, dark cloud of great vertical depth, often bringing rain. Some have a distinctive anvil shaped head. 

Cumulus cloud
    A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower. In Australia, it is described as a cloud with a woolly, heaped appearance that often produces rain. 

Cumulus congestus
    A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb (Cumulonimbus cloud).

Cyclone
    An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean. In Australia, it is described as atmospheric circulations that rotate clockwise in the southern hemisphere, and anti-clockwise in the northern hemisphere. Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.

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Day
    Sunrise to sunset. 

Day of the week (eg. Monday)
    Midnight to midnight.

Debris cloud
    A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado.

Dense fog
    A fog in which the visibility is less than one-quarter mile.

Depression
    A region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.

Dew point
    The temperature to which the air must be cooled for water vapor to condense and form fog or clouds.

Diamond dust
    A fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates.

Difluence
    A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence. 

Disturbance
    A disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather. 

Downburst
    A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. 

Downdraft
    A column of generally cool air that rapidly sinks to the ground, usually accompanied by precipitation as in a shower or thunderstorm. 

Downslope wind
    Air that descends an elevated plain and consequently warms and dries. Occurs when prevailing wind direction is from the same direction as the elevated terrain and often produces fair weather conditions.

Draft
    A draft is a small gusty air current that moves upward or downward abruptly; hence the terms updraft and downdraft.

Drifting snow
    Uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility. In Canada, it is defined as snow blown to a height of less than two meters.

Drizzle
    Small, slowly falling water droplets, with diameters between .2 and .5 millimeters. In Australia, it is defined as fairly uniform precipitation (rain) composed exclusively of very small water droplets (less than 0.5 mm in diameter) very close to one another. 

Drought
    Abnormally dry weather in a region over an extended period sufficient to cause a serious hydrological (water cycle) imbalance in the affected area. This can cause such problems as crop damage and water-supply shortage. In Australia, it is defined as the prolonged absence or marked deficiency of precipitation (rain).

Dry
    Free from rain. Normally used when preceding weather has also been relatively dry and dry weather is expected to continue for a day or so.

Dust storm
    An area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile. In Australia, it is described as a storm which carries large amounts of dust into the atmosphere.

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Early
    Until two hours after sunrise.

El Nińo
    A major warming of the equatorial waters in the eastern Pacific Ocean. El Nińo events usually occur every three to seven years, and are related to shifts in global weather patterns. (Spanish for the "Christ Child” because it often begins around Christmas.)

Evaporation
    The process of a liquid changing into a vapor or gas. 

Evening
    Sunset to midnight.

Extended outlook
    A basic forecast of general weather conditions three to five days in the future.

Extratropical cyclone
    A storm that forms outside the tropics, sometimes as a tropical storm or hurricane changes. See table below for differences between extratropical and tropical cyclones.

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Fahrenheit
    The standard scale used to measure temperature in the United States; in which the freezing point of water is 32 degrees and the boiling point is 212 degrees.

Fair
    Describes weather in which there is less than 4/10ths of opaque cloud cover, no precipitation, and there is no extreme visibility, wind or temperature conditions.

Fall wind
    A strong, cold, downslope wind.

Fine
    No rain or other precipitation (hail, snow etc.). The use of fine is generally avoided in excessively cloudy, windy, foggy or dusty conditions. In particular note that fine means the absence of rain or other precipitation such as hail or snow - not 'good' or 'pleasant' weather.

Flash flood
    A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure or water released from an ice jam. 

Flood
    A condition that occurs when water overflows the natural or artificial confines of a stream or river; the water also may accumulate by drainage over low-lying areas. 

Flood crest
    The highest stage or flow occurring in a flood. 

Flurry
    A flurry or snow shower is a snowfall that suddenly stops and starts and changes rapidly in intensity; the accumulation and extent of the snow are limited.

Fog
    Water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than one km (three thousand and three hundred feet). In Canada, fog is defined as a cloud at ground level, and occurs when air is cooled to its dew point and below, or when atmospheric moisture increases through evaporation from water that is warmer than the air. In the United Kingdom, fog is described to occur when visibility less than one kilometer. In Australia, it is defined as a dense mass of small water droplets or particles in the lower atmosphere.

Fogbow
    A rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside. 

Forecast
    A forecast provides a description of the most significant weather conditions expected during the current and following days. The exact content depends upon the intended user, such as the Public or Marine forecast audiences. 

Freeze
    Occurs when the surface air temperature is expected to be 32 degrees Fahrenheit or below over a widespread area for a significant period of time. In Australia, the term “freeze” is defined as the change from a liquid to a solid.

Freezing
    The change in a substance from a liquid to a solid state. 

Freezing drizzle
    Drizzle that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less, possibly producing a thin coating of ice. Even in small amounts, freezing drizzle may cause traveling problems. 

Freezing fog
    A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below zero degrees Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog. 

Freezing level
    The altitude in the atmosphere where the temperature drops to 32F. 

Freezing rain
    Rain that freezes on objects such as trees, cars and roads, forming a coating or glaze of ice. Temperatures at higher levels are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact. In Canada, freezing rain is described to occur when the air temperature is below zero Celsius near the ground but above zero Celsius higher up.

Front
    The boundary or transition zone between two different air masses. The basic frontal types are cold fronts, warm fronts and occluded fronts.

Frost
    The formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the exposed surface falls below 32 degrees Fahrenheit and water vapor is deposited as a solid. 

Frost point
    When the temperature to which air must be cooled to in order to be saturated is below freezing.

Fujita scale
    System developed by Dr Theodore Fujita to classify tornadoes based on wind damage. Scale is from F0 for weakest to F5 for strongest tornadoes. 

Fujiwhara effect
    The Fujiwhara effect describes the rotation of two storms around each other.

Funnel cloud
    A rotating, cone-shaped column of air extending downward from the base of a thunderstorm but not touching the ground. When it reaches the ground it is called a tornado. 

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Gale warning
    Sustained wind speeds from 34 to 47 knots (39 to 54 mph). 

Glaze
    a layer or coating of ice that is generally smooth and clear, and forms on exposed objects by the freezing of liquid raindrops. 

Global warming
    A theory that increased concentrations of greenhouse gases are causing an elevation in the Earth's surface temperature. 

Good visibility
    More than 10 kilometers. 

Greenhouse effect
    The warming of the atmosphere by the trapping of longwave radiation (heat) being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide. 

Ground fog
    Shallow fog (less than twenty feet deep) produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog. 

Gust
    A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 miles per hour. In Canada, gust is defined as a sudden, brief increase in wind speed that generally lasts less than 20 seconds. In Australia, gust is defined as any sudden increase of wind of short duration, usually a few seconds.

Gustnado
    Gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground.

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Hail
    Precipitation in the form of balls or irregular lumps of ice produced by liquid precipitation, freezing and being coated by layers of ice as it is lifted and cooled in strong updrafts of thunderstorms. 

Haze
    Fine dust or salt particles in the air that reduce visibility. In Canada, haze is defined to consist of fine particles of dust and pollution suspended in the atmosphere, and is distinguished from fog by its bluish or yellowish tinge.

Heat balance
    The equilibrium existing between the radiation received and emitted by a planetary system. 

Heat index
    An index that combines air temperature and humidity to give an apparent temperature (how hot it feels). 

Heat island
    A dome of elevated temperatures over an urban area caused by the heat absorbed by structures and pavement. 

Heat lightning
    Lightning that can be seen, but is too far away for the thunder to be heard. 

Heat wave
    A period of abnormally hot weather lasting several days. 

Heavy snow
    Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours. 

Heavy snow warning
    Older terminology replaced by winter storm warning for heavy snow. Issued when seven or more inches of snow or sleet is expected in the next 24 hours. A warning is used for winter weather conditions posing a threat to life and property. 

Heavy surf
    the result of large waves breaking on or near the shore resulting from swells or produced by a distant storm.

High
    An area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone. In Canada, a high is described as an area of high atmospheric pressure with a closed, clockwise movement of air. 

High cloud
    Cirrus, cirrostratus and cirrocumulus. 

High risk (of severe thunderstorm)
    Severe weather is expected to affect more than 10 percent of the area.

High wind warning
    Issued when sustained winds from 40 to 73 mph are expected for at least one hour; or any wind gusts are expected to reach 58 miles per hour or more. 

High wind watch
    Issued when conditions are favorable for the development of high winds over all of or part of the forecast area but the occurrence is still uncertain. The criteria of a high wind watch are listed under the high wind warning and should include the area affected, the reason for the watch and the potential impact of the winds. 

Hot or very warm
    In summer, hot or very warm means more than seven degrees Celsius above normal.

Hot spot
    Typically large areas of pavement, these "hot spots" are heated much quicker by the sun than surrounding grasses and forests. As a result, air rises upwards from the relatively hot surface of the pavement, reaches its condensation level, condenses, and forms a cloud above the "hot spot". 

Humidity
    The amount of water vapor in the atmosphere. In Canada, it is described as the amount of moisture in the air.

Hurricane
    A severe tropical cyclone with sustained winds over 74 miles per hour (64 knots). Normally applied to such storms in the Atlantic Basin and the Pacific Ocean east of the International Date Line. 

Hygrometer
    An instrument used to measure humidity.

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Ice crystals
    Ice crystals are tiny sprinkles that sparkle in the sunshine like diamond dust and hang in the air. 

Ice fog
    A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below zero degrees Celsius, based at the Earth's surface, which reduces horizontal visibility. Usually occurs at -20 degrees Fahrenheit and below. 

Ice pellets
    Precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (five millimeters), or less. There are two main types. Hard grains of ice consisting of frozen raindrops and pellets of snow encased in a thin layer of ice. 

Ice storm
    Liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage. 

Ice storm warning
    Older terminology replaced by winter storm warning for severe icing. Issued when 1/2 inch or more of accretion of freezing rain is expected. This may lead to dangerous walking or driving conditions and the pulling down of power lines and trees. A warning is used for winter weather conditions posing a threat to life and property. 

Indian summer
    An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather. 

Inflow bands (or feeder bands)
    Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. 

Insolation
    Incoming solar radiation. Solar heating; sunshine. 

Instability
    A state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation. 

Intermittent rain
    Intermittent rain stops and starts repeatedly, although not as abruptly or as frequently as showers. 

Intertropical Convergence Zone (ITCZ)
    The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.

Inversion
    An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere. Temperature inversions trap atmospheric pollutants in the lower troposphere, resulting in higher concentrations of pollutants at ground levels than would usually be experienced.

Iridescence
    Brilliant patches of green or pink sometimes seen near the edges of high- or medium-level clouds.

Isentropic lift
    Lifting of air that is traveling along an upward-sloping isentropic surface. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation.

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Jet stream
    Strong winds concentrated within a narrow band in the upper atmosphere. It normally refers to horizontal, high-altitude winds. The jet stream often "steers" surface features such as front and low pressure systems.

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Katabatic
    Wind blowing down an incline, such as down a hillside; downslope wind.

Katafront
    A front (usually a cold front) at which the warm air descents the frontal surface.

Killing frost
    Frost severe enough to end the growing season. This usually occurs at temperatures below 28 degrees Fahrenheit. In Canada, a killing frost is described as a frost severe enough to destroy annual plants and new growth on trees (in the spring) or to end the growing season (in the fall).

Knot
    A measure of speed. It is one nautical mile per hour (1.15 miles per hour). A nautical mile is one minute of one degree of latitude.

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La Nińa
    A cooling of the equatorial waters in the Pacific Ocean. 

Land breeze
    A wind that blows from the land towards a body of water. Also known as an offshore breeze. It occurs when the land is cooler than the water.

Late
    From sunset (winter), from two hours before sunset (summer).

Leeward
    Situated away from the wind; downwind - opposite of windward.

Left Mover
    A thunderstorm which moves to the left relative to the steering winds and to other nearby thunderstorms; often the northern part of a splitting storm.

Lifting
    The forcing of air in a vertical direction by an upslope in terrain or by the movement of a denser air mass.

Lifting condensation level
    The level in the atmosphere where a lifted air parcel reaches its saturation point, and as a result, the water vapor within condenses into water droplets.

Lightning
    Any form of visible electrical discharges produced by thunderstorms.

Likely
    In probability of precipitation statements, the equivalent of a 60 or 70 percent chance. 

Low
    An area of low pressure, usually accompanied by cyclonic and inward wind flow. Also known as a cyclone.

Low cloud
    Stratus, stratocumulus, cumulus and cumulonimbus.

Low-level jet
    A region of relatively strong winds in the lower part of the atmosphere. 

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Mainly cloudy
    More than half cloud cover but with some breaks in the cloud. It can also be described as cloudy with some sunny periods.

Mainly sunny
    Mainly sunny means sunny with some cloudy periods. 

Mammatus (or mamma clouds)
    These clouds appear as hanging, rounded protuberances or pouches on the under-surface of a cloud. With thunderstorms, mammatus are seen on the underside of the anvil. These clouds do not produce tornadoes, funnels, hail, or any other type of severe weather, although they often accompany severe thunderstorms.

Maritime air mass
    An air mass that forms over water. It is usually humid, and may be cold or warm.

Maximum temperature
    The highest temperature during a specified time period.

Mean temperature
    The average of a series of temperatures taken over a period of time, such as a day or a month.

Medium cloud
    Altostratus, altocumulus and nimbostratus.

Meteorology
    The study of the physics, chemistry, and dynamics of the atmosphere and the direct effects of the atmosphere upon the Earth's surface, the oceans, and life in general.

Mild
    In winter, mild refers to four to seven degrees Celsius above normal.

Minimum temperature
    The lowest temperature during a specified time period.

Mist
    Consists of microscopic water droplets suspended in the air, which produces a thin grayish veil over the landscape. It reduces visibility to a lesser extent than fog. In Australia, it is described as similar to fog, but visibility remains more than a kilometer. 

Moderate risk
    Severe thunderstorms are expected to affect between five and 10 percent of the area.

Moderate visibility
    Five to 10 kilometers’ visibility.

Monsoon
    A persistent seasonal wind, often responsible for seasonal precipitation regime. It is most commonly used to describe meteorological changes in southern and eastern Asia.

Morning
    Sunrise to noon or midnight to noon depending on context.

Mountain breeze
    System of winds that blow downhill during the night.

Muggy
    Colloquially descriptive of warm and especially humid weather.

Multivortex tornado
    A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.

Mushroom
    A thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.

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Night, overnight
    Sunset to sunrise.

No weather data available
    No weather information available for this time slot.

Normal
    The long-term average value of a meteorological element for a certain area. For example, "temperatures are normal for this time of year" Usually averaged over 30 years.

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Offshore breeze
    A wind that blows from the land towards a body of water. Also known as a land breeze.

Offshore forecast
    A marine weather forecast for the waters between 60 and 250 miles off the coast.

Onshore breeze
    A wind that blows from a body of water towards the land. Also known as a sea breeze.

Outflow
    Air that flows outward from a thunderstorm.

Outflow winds
    Winds that blow down fjords and inlets from the land to the sea.

Overcast
    Sky condition when greater than 9/10ths of the sky is covered by clouds. In Canada overcast means grey and dull skies, with extensive cloud cover.

Overrunning
    A condition that exists when a relatively warm air mass moves up and over a colder and denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.

Ozone
    A form of oxygen in which the molecule is made of three atoms instead of the usual two. Ozone is usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation. It is also a primary component of smog.

Ozone hole
    A thinning of the ozone layer over Antarctica, which occurs each spring.

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Partly cloudy
    Sky condition when between 3/10ths and 7/10ths of the sky is covered. Used more frequently at night. In the United Kingdom, it is defined as less than half cloud cover.

Partly sunny
    Similar to partly cloudy. Used to emphasize daytime sunshine.

Permafrost
    A soil layer below the surface of tundra regions that remains frozen permanently.

Polar air
    A mass of very cold, very dry air that forms in polar regions.

Polar front
    The semi-permanent, semi-continuous front that encircles the northern hemisphere separating air masses of tropical and polar origin.

Polar Stratospheric Clouds (PSCs)
    High altitude clouds that form in the stratosphere above Antarctica during the Southern Hemisphere winter. Their presence seems to initiate the ozone loss experienced during the ensuing southern hemisphere spring.

Polar vortex
    A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion.

Poor visibility
    One to five kilometers’ visibility.

POP
    Probability of Precipitation. Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period.

Popcorn convection
    Clouds, showers and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.

Precipitation
    Liquid or solid water that falls from the atmosphere and reaches the ground.

Pressure
    The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

Pressure change
    The net difference between pressure readings at the beginning and ending of a specified interval of time.

Pressure falling rapidly
    A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inches or more.

Pressure rising rapidly
    An increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inches or more.

Pressure tendency
    The character and amount of atmospheric pressure change during a specified period of time, usually the three-hour period preceding an observation.

Prevailing westerlies
    Winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east.

Prevailing wind
    The direction from which the wind blows most frequently in any location.

Pulse storm
    A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.

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Qualitative forecasts
    Forecasts that provide only a categorical value for the predicted variable. Examples of this include “rain/no rain” and “cloudy/partly cloudy”.

Quantitative forecasts
    Forecasts in which the “amount” of the forecast variable is specified. 

Quantitative precipitation forecast (QPF)
    A forecast of rainfall, snowfall or liquid equivalent of snowfall.

Quasi-biennial oscillation (QBO)
    Periodic variation in the direction, either easterly or westerly, of tropical lower stratospheric winds. The direction changes every 26 months.

Quasi-nonhydrostatic (QNH)
    Pilots, air traffic control (ATC) and low frequency weather beacons use this pressure setting to refer to the barometric altimeter setting that causes the altimeter to read altitude above mean sea level within a certain defined region. 

Quasi-stationary front
    A front which is nearly stationary or moves very little since the last synoptic position. Also known as a stationary front.

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Radar
    An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging).

Radiation
    Energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending upon the wavelength. Radiation from the Sun has a short wavelength (ultra-violet) while energy re-radiated from the Earth's surface and the atmosphere has a long wavelength (infra-red).

Radiation fog
    Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as ground fog.

Rain
    Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle (0.5 mm). In Canada the term "rain", used alone, means liquid precipitation of significant duration and extent.

Rain gauge
    An instrument used to measure rainfall amounts.

Rain showers
    Rain showers stop and start suddenly and vary widely in intensity, and are gone in less than an hour.

Rainbow
    Optical phenomena when light is refracted and reflected by moisture in the air into concentric arcs of color. Raindrops act like prisms, breaking the light into the colors of a rainbow, with red on the outer, and blue on the inner edge.

Relative humidity
    The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. It is expressed as a percentage.

Return flow
    South winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).

Ridge
    An elongated area of high pressure in the atmosphere. Opposite of a trough.

River flood warning
    Issued when main stem rivers are expected to reach a level above flood stage.

Roll cloud
    A relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes.

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Sandstorm
    Particles of sand carried aloft by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.

Saturation
    A condition of the atmosphere in which a certain volume of air holds the maximum water vapor it can hold at a specific temperature.

Scattered
    A cloud layer that covers between 3/8ths and 1/2 of the sky.

Scud clouds
    Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

Sea breeze
    A wind that blows from a sea or ocean towards a land mass. Also known as an onshore breeze. It occurs when the land is warmer than the water.

Sea-level pressure
    The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.

Secondary cold front
    A front that follows a primary cold front and ushers in even colder air.

Severe thunderstorm
    A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4 inch or more.

Shallow fog
    Fog in which the visibility at 6 feet above ground level is 5/8ths of a mile or more.

Shelf-cloud
    A low-level horizontal accessory cloud that appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front.

Short-fuse warning
    A warning issued by the National Weather Service for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

Shower
    Precipitation that is intermittent, both in time, space or intensity.

Sky condition
    The state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types.

Sleet
    Rain drops that freeze into ice pellets before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air near the surface and refreezes. In Australia, sleet refers to a mixture of rain and snow or falling snow that is melting into rain.

Slight chance
    In probability of precipitation statements, usually equivalent to a 20 percent chance.

Slight risk (of severe thunderstorms)
    Severe thunderstorms are expected to affect between two and five percent of the area. A slight risk generally implies that severe weather events are expected to be isolated.

Smog
    Pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health.

Smoke
    A suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled great distances (25 to 100 statute miles or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere. 

Snow
    Frozen precipitation composed of ice particles in complex hexagonal patterns. Snow forms in cold clouds by the direct transfer of water vapor to ice.

Snow burst
    Very intense shower of snow, often of short duration, that greatly restricts visibility and produces periods of rapid snow accumulation.

Snow flurries
    Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation.

Snow grains
    Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation. In Canada, Snow grains are described as minute, white and opaque grains of ice. When they hit hard ground, they do not bounce or shatter. They usually fall in very small quantities, and never in the form of a shower.

Snow pellets
    Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm). In Canada, snow pellets are brittle and easily crushed; when they fall on hard ground, they bounce and often break up. They always occur in showers and are often accompanied by snowflakes or rain drops, when the surface temperature is around zero degrees Celsius.

Snow shower
    Snow falling at varying intensities for brief periods of time. Some accumulation is possible.

Snow squalls
    Intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning. In Canada, it is described that a snow squall brings strong winds, flurries and poor visibility.

Snowfall
    The depth of new snow that has accumulated since the previous day or since the previous observation.

Snowflake
    White ice crystals that have combined in a complex branched hexagonal form.

Special marine warning
    Issued for brief or sudden occurrence of sustained wind or frequent gusts of 34 knots or more. This is usually associated with severe thunderstorms or waterspouts.

Spray
    An ensemble of water droplets torn by the wind from an extensive body of water, generally from the crests of waves, and carried up into the air in such quantities that it reduces the horizontal visibility.

Squall
    A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute. In Canada, a squall is defined as a strong, sudden wind which generally lasts a few minutes then quickly decreases in speed. In Australia, it is described as a sudden increase of the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value.

Stability
    An indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started.

Stable air
    Air with little or no tendency to rise, usually accompanied by clear dry weather.

Steam fog
    Fog that is formed when water vapor is added to air which is much colder than the vapor's source. This is most common when very cold air drifts across relatively warm water.

Steering winds (steering currents)
    A prevailing synoptic scale flow which governs the movement of smaller features embedded within it. 

Storm
    In marine usage, winds 48 knots (55 miles per hour) or greater.

Storm surge
    A rise of the sea level alone the shore that builds up as a storm (usually a hurricane) moves over water. It is a result of the winds of the storm and low atmospheric pressures.

Storm warning
    A marine wind warning for sustained winds greater of 48 knots (55 miles per hour) or more from a non-tropical system.

Straight line winds
    Thunderstorm winds most often found with the gust front.

Subtropical storm
    A low pressure system that develops in subtropical waters (north of 20 north degrees latitude) and initially has non-tropical features (see table below for a list of tropical features) but does have some element of a tropical cyclone's cloud structure (located close to the center rather than away from the center of circulation).

Sunny
    Sunny or a few clouds means that less than half the sky has clouds.

Supercell
    A severe thunderstorm whose updrafts and downdrafts are in near balance allowing the storm to maintain itself for several hours. Supercells often produce large hail and tornadoes.

Supersaturation
    The condition which occurs in the atmosphere when the relative humidity is greater than 100 percent.

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Temperature
    A measure of the warmth or coldness of an object or substance with reference to a standard value.

Thunder
    The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand.

Thunderstorm
    A storm with lightning and thunder produced by a Cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail. In Australia, they are described to be usually short-lived and hit on only a small area. 

Tilted storm or tilted updraft
    A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

Today
    Until sunset. 

Tomorrow
    Midnight to midnight.

Tonight
    Sunset to midnight.

Tornado
    A violent rotating column of air, in contact with the ground, pendant from a cumulonimbus cloud. A tornado does not require the visible presence of a funnel cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours.

Trade winds
    Persistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low. They blow northeasterly in the northern hemisphere. 

Tropical storm
    An organized low pressure system in the tropics with wind speeds between 38 and 74 miles per hour. In Australia, it is a term used in the northern hemisphere for a tropical cyclone.

Tropical storm warning
    A warning issued when sustained winds of 39 to 73 miles per hour (34 to 63 knots) are expected within 24 hours. 

Trough
    A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours. In Canada, it is described as an elongated area of relatively low pressure extending from the centre of a region of low pressure. 

Trough
    A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours. In Canada, it is described as an elongated area of relatively low pressure extending from the centre of a region of low pressure. 

Turbulence
    A warning issued when sustained winds of 39 to 73 miles per hour (34 to 63 knots) are expected within 24 hours.

Twister
    A colloquial term for a tornado.

Typhoon
    A hurricane that forms in the Western Pacific Ocean. It is a term used in Australia for a tropical cyclone in the northwestern Pacific with maximum winds above 117 kilometers per hour (63 knots).

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Unstable air
    Air that rises easily and can form clouds and rain.

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Very cool
    Very cool, in the summer, refers to more than seven degrees Celsius below normal.

Very mild
    In the winter, very mild means more than seven degrees Celsius above normal.

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Warm
    In summer, warm refers to four to seven degrees Celsius above normal.

Warm front
    A narrow transitions zone separating advancing warmer air from retreating cooler air. The air behind a warm front is warmer and typically more humid than the air it is replacing. In Canada, it is defined as the trailing edge of a retreating cold air mass and moves in such a way that the warmer air replaces the colder air.

Wave
    In meteorology any pattern identifiable on a weather map that has a cyclic pattern or a small cyclonic circulation in the early stages of development that moves along a cold front. In Canada, A wave, in meteorology, is the intersection of warm and cold fronts.

Weather
    State of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness. Also, weather is the meteorological day-to-day variations of the atmosphere and their effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.

Wind
    Wind is the horizontal movement of air relative to the earth's surface and is caused by variations in temperature and pressure (for instance, air rises as it warms and a cool breeze moves in to take the place of the rising air.) Wind is also known as moving air.

Wind chill
    The additional cooling effect resulting from wind blowing on bare skin. The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. The (equivalent) wind chill temperature is the temperature the body "feels" for a certain combination of wind and air temperature.

Winter storm
    A heavy snow event. A snow accumulation of more than six inches in 12 hours or more than 12 inches in 24 hours.

Winter storm warning
    Issued when seven or more inches of snow or sleet is expected in the next 24 hours, or 1/2 inch or more of accretion of freezing rain is expected. A warning is used for winter weather conditions posing a threat to life and property.

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X-ray
    An electromagnetic wave of very short wavelength, able to pass through many materials opaque to light.

Xenon
    An inert gaseous chemical element, present in trace amounts in the air and used in some kinds of electric light.

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Yellow wind
    A strong, cold, dry west wind of eastern Asia that blows across the plains during winter and carries a yellow dust from the desert.

Youg
    A hot wind during unsettled summer weather in the Mediterranean.

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Zonal flow
    A small amplitude pattern where winds blow mostly west-to-east.

Zonal wind
    Wind blowing east-to-west or west-to-east.

Zulu time
    The mean solar time for the meridian at Greenwich, England, used as a basis for calculating time in communications, military, aviation, maritime and other activities that cross time zones. Zulu time, which is also known as Coordinated Universal Time (UTC), is used internally among people working in the field of weather and meteorology, as well as among weather enthusiasts, for weather reports and forecasts. 







 GLOSSARY OF WEATHER TERMS FOR STORM SPOTTERS/CHASERS
Feel free to print out this glossary for future reference! As of now there's no answering service Dallas to New York to California, or anywhere else really, that offers in-depth information as you'll find in this glossary here. All spotters/chasers should be familiar with the definitions of the terms with asterisks before taking an active role in any spotting/chasing.
A - B- C - D - E - F- G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

-A-

ACCAS (usually pronounced ACK-kis) - AltoCumulus CAStellanus; mid-level clouds (bases generally 8 to 15 thousand feet), of which at least a fraction of their upper parts show cumulus-type development. These clouds often are taller than they are wide, giving them a turret-shaped appearance. ACCAS clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.

*Accessory Cloud - A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.

Advection - Transport of an atmospheric property by the wind.

Air-mass Thunderstorm - Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter. Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable. Therefore the term is somewhat controversial and should be used with discretion.

*Anticyclonic Rotation - Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.

*Anvil - The flat, spreading top of a Cb (cumulonimbus), often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind (back-sheared anvil).

*Anvil Crawler - [Slang], a lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active MCS.

Anvil Dome - A large overshooting top or penetrating top.

Anvil Rollover - [Slang], a circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil.

Anvil Zits - [Slang], frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.

AP - Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions.

Approaching (severe levels) - A thunderstorm which contains winds of 35 to 49 knots (40 to 57 mph), or hail 1/2 inch or larger but less than 3/4 inch in diameter.

Arcus - A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds both are types of arcus clouds.

AVN - AViatioN model; one of the operational forecast models run at NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800 GMT. As of fall 1996, forecast output was available operationally out to 72 hours only from the 0000 and 1200 runs. At 0600 and 1800, the model is run only out to 54 hours.

-B- 

Back-building Thunderstorm - A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.

Backing Winds - Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds. In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.

*Back-sheared Anvil - [Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential.

*Barber Pole - [Slang], a thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.

Baroclinic Zone - A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.

Barotropic System - A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone.

Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.

As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.

*Bear's Cage - [Slang], a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm. The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.

*Beaver('s) Tail - [Slang], a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes.

Spotters should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud typically is attached to the wall cloud and has a cloud base at about the same level as the wall cloud itself. A beaver tail, on the other hand, is not attached to the wall cloud and has a cloud base at about the same height as the updraft base (which by definition is higher than the wall cloud). Unlike the beaver tail, the tail cloud forms from air that is flowing from the storm's main precipitation cascade region (or outflow region). Thus, it can be oriented at a large angle to the pseudo-warm front.

Blue Watch (or Blue Box) - [Slang], a severe thunderstorm watch.

Boundary Layer - In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly. There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a part of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly 10 meters thick, but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.

Bow Echo - A radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.

Box (or Watch Box) - [Slang], a severe thunderstorm or tornado watch.

Bust - [Slang], an inaccurate forecast or an unsuccessful storm chase; usually a situation in which thunderstorms or severe weather are expected, but do not occur.

BWER - Bounded Weak Echo Region. (Also known as a vault.) Radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft. This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually.

-C- 

CA - Cloud-to-Air lightning.

Cap (or Capping Inversion) - A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However if the cap is removed or weakened, then explosive thunderstorm development can occur. The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.

CAPE - Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may exceed 5,000 j/kg. However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on a sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.)

*Cb - Cumulonimbus cloud, characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil. Also known colloquially as a "thunderhead."

*CC - Cloud-to-Cloud lightning.

*Cell - Convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a cumulus or towering cumulus cloud. A typical thunderstorm consists of several cells (see multi-cellular thunderstorm). The term "cell" also is used to describe the radar echo returned by an individual shower or thunderstorm. Such usage, although common, is technically incorrect.

*CG - Cloud-to-Ground lightning flash.

Chaser Convergence - Any unplanned gathering of storm chasers, often near a storm of interest. These are seen regularly during the spring convective season.

CIN - Convective INhibition. A measure of the amount of energy needed in order to initiate convection. Values of CIN typically reflect the strength of the cap. They are obtained on a sounding by computing the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is cooler than the former. (This area sometimes is called negative area.)

*Cirrus - High-level clouds (16,000 feet or more), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.

*Classic Supercell - See supercell.

*Clear Slot - A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.

Closed Low - A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see cutoff low).

*Cloud Tags - Ragged, detached cloud fragments; fractus or scud.

*Cold-air Funnel - A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

Cold Pool - A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

*Collar Cloud - A generally circular ring of cloud that may be observed on rare occasions surrounding the upper part of a wall cloud. This term sometimes is used (incorrectly) as a synonym for wall cloud.

Comma Echo - A thunderstorm radar echo which has a comma-like shape. It often appears during latter stages in the life cycle of a bow echo.

*Condensation Funnel - A funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc...).

Convection - Generally, transport of heat and moisture by the movement of a fluid. In meteorology, the term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds, and ACCAS clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

Convective Outlook - A forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the contiguous United States, issued several times daily by the SPC. The terms approaching, slight risk, moderate risk, and high risk are used to describe severe thunderstorm potential. Local versions sometimes are prepared by local NWS offices.

Convective Temperature - The approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding. Calculation of the convective temperature involves many assumptions, such that thunderstorms sometimes develop well before or well after the convective temperature is reached (or may not develop at all). However, in some cases the convective temperature is a useful parameter for forecasting the onset of convection.

Convergence - A contraction of a vector field; the opposite of divergence. Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," vertical motion may result: upward forcing if convergence is at low levels, or downward forcing (subsidence) if convergence is at high levels. Upward forcing from low-level convergence increases the potential for thunderstorm development (when other factors, such as instability, are favorable).

*Core Punch - [Slang], a penetration by a vehicle into the heavy precipitation core of a thunderstorm. Core punching is not a recommended procedure for storm spotting or chasing.

*Cumuliform Anvil - A thunderstorm anvil with visual characteristics resembling cumulus-type clouds (rather than the more typical fibrous appearance associated with cirrus). A cumuliform anvil arises from rapid spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil rollover, knuckles, mushroom.

*Cumulus - Detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers. Tops normally are rounded while bases are more horizontal.

Cutoff Low - A closed low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression).

"Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies. Therefore, the recommended usage of the terms is to reserve the use of "cutoff low" only to those closed lows which clearly are detached completely from the westerlies.

*Cyclic Storm - A thunderstorm that undergoes cycles of intensification and weakening (pulses) while maintaining its individuality. Cyclic supercells are capable of producing multiple tornadoes (i.e., a tornado family) and/or several bursts of severe weather. A storm which undergoes only one cycle (pulse), and then dissipates, is known as a pulse storm.

Cyclogenesis - Development or intensification of a low-pressure center (cyclone).

*Cyclonic Circulation (or Cyclonic Rotation) - Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. Nearly all mesocyclones and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, such as gustnadoes, occasionally rotate anticyclonically (clockwise).

-D- 

dBZ - Nondimensional "unit" of radar reflectivity which represents a logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity factor, Z. The value of Z is a function of the amount of radar beam energy that is backscattered by a target and detected as a signal (or echo). Higher values of Z (and dBZ) thus indicate more energy being backscattered by a target. The amount of backscattered energy generally is related to precipitation intensity, such that higher values of dBZ that are detected from precipitation areas generally indicate higher precipitation rates.

However, other factors can affect reflectivity, such as width of the radar beam, precipitation type, drop size, or the presence of ground clutter or AP. WSR-88D radars can detect reflectivities as low as -32 dBZ near the radar site, but significant (measurable) precipitation generally is indicated by reflectivities of around 15 dBZ or more. Values of 50 dBZ or more normally are associated with heavy thunderstorms, perhaps with hail, but as with most other quantities, there are no reliable threshold values to confirm the presence of hail or severe weather in a given situation.

*Debris Cloud - A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado. This term is similar to dust whirl, although the latter typically refers to a circulation which contains dust but not necessarily any debris. A dust plume, on the other hand, does not rotate. Note that a debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel.

Delta T - A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development.

Derecho - (Pronounced day-RAY-cho), a widespread and usually fast-moving windstorm associated with convection. Derechos include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.

Dew Point (or Dew-point Temperature) - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).

Differential Motion - Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.

Directional Shear - The component of wind shear which is due to a change in wind direction with height, e.g., southeasterly winds at the surface and southwesterly winds aloft. A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development.

Diurnal - Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and diurnal falls at night).

Divergence - The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence. Divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm development (if other factors also are favorable).

Doppler Radar - Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna )

*Downburst - A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.

*Downdraft - A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft.

Downstream - In the same direction as a stream or other flow, or toward the direction in which the flow is moving.

Dry Adiabat - A line of constant potential temperature on a thermodynamic chart.

Dry Line - A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west). The dry line typically advances eastward during the afternoon and retreats westward at night.

However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day. A typical dry line passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime; see dry punch). These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward. See LP storm.

Dry-line Bulge - A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels (i.e., a surface dry punch). Severe weather potential is increased near and ahead of a dry line bulge.

Dry-line Storm - Generally, any thunderstorm that develops on or near a dry line. The term often is used synonymously with LP storm, since the latter almost always occurs near the dry line.

*Dry Microburst - A microburst with little or no precipitation reaching the ground; most common in semi-arid regions. They may or may not produce lightning. Dry microbursts may develop in an otherwise fair-weather pattern; visible signs may include a cumulus cloud or small Cb with a high base and high-level virga, or perhaps only an orphan anvil from a dying rain shower. At the ground, the only visible sign might be a dust plume or a ring of blowing dust beneath a local area of virga. Compare with wet microburst.

Dry Punch - [Slang], a surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge. A dry punch aloft above an area of moist air at low levels often increases the potential for severe weather.

*Dust Devil - A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl). Dust devils form in response to surface heating during fair, hot weather; they are most frequent in arid or semi-arid regions.

*Dust Plume - A non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front. If rotation is observed, then the term dust whirl or debris cloud should be used.

*Dust Whirl - A rotating column of air rendered visible by dust. Similar to debris cloud; see also dust devil, gustnado, tornado.

Dynamics - Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.

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ECMWF - European Center for Medium-Range Weather Forecasting. Operational references in forecast discussions typically refer to the ECMWF's medium-range forecast model.

*Elevated Convection - Convection occurring within an elevated layer, i.e., a layer in which the lowest portion is based above the earth's surface. Elevated convection often occurs when air near the ground is relatively cool and stable, e.g., during periods of isentropic lift, when an unstable layer of air is present aloft. In cases of elevated convection, stability indices based on near-surface measurements (such as the lifted index) typically will underestimate the amount of instability present. Severe weather is possible from elevated convection, but is less likely than it is with surface-based convection.

Energy Helicity Index (or EHI) - An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms. It is related directly to storm-relative helicity in the lowest 2 km (SRH, in m2/s2) and CAPE (in j/kg) as follows: EHI = (CAPE x SRH)/160,000. Thus, higher values indicate unstable conditions and/or strong vertical shear. Since both parameters are important for severe weather development, higher values generally indicate a greater potential for severe weather. Values of 1 or more are said to indicate a heightened threat of tornadoes; values of 5 or more are rarely observed, and are said to indicate potential for violent tornadoes. However, there are no magic numbers or critical threshold values to confirm or predict the occurrence of tornadoes of a particular intensity.

Enhanced V - A pattern seen on satellite infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a V-shaped region of colder cloud tops extending downwind from the thunderstorm core. The enhanced V indicates a very strong updraft, and therefore a higher potential for severe weather.

Enhanced V should not be confused with V notch, which is a radar signature

Enhanced Wording - An option used by the SPC in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high. The statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF VERY DAMAGING TORNADOES" appears in tornado watches with enhanced wording. Severe thunderstorm watches may include the statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF EXTREMELY DAMAGING WINDS," usually when a derecho event is occurring or forecast to occur. See PDS watch.

Entrance Region - The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also exit region, left exit region.

Equilibrium Level (or EL) - On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment.

The height of the EL is the height at which thunderstorm updrafts no longer accelerate upward. Thus, to a close approximation, it represents the height of expected (or ongoing) thunderstorm tops. However, strong updrafts will continue to rise past the EL before stopping, resulting in storm tops that are higher than the EL. This process sometimes can be seen visually as an overshooting top or anvil dome.

Eta Model - One of the operational numerical forecast models run at NCEP. The Eta is run twice daily, with forecast output out to 48 hours.

Exit Region - The region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also entrance region, right entrance region.

-F- 

F scale - See Fujita Scale.

*Feeder Bands - Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually from the east through south (i.e., parallel to the inflow). Same as inflow bands. This term also is used in tropical meteorology to describe spiral-shaped bands of convection surrounding, and moving toward, the center of a tropical cyclone or hurricane.

*Flanking Line - A line of cumulus or towering cumulus clouds connected to and extending outward from the most active part of a supercell, normally on the southwest side. The line normally has a stair-step appearance, with the tallest clouds closest to the main storm, and generally coincides with the pseudo-cold front.

*Forward Flank Downdraft - The main region of downdraft in the forward, or leading, part of a supercell, where most of the heavy precipitation is. Compare with rear flank downdraft. See pseudo-warm front

Front - A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

Fractus - Ragged, detached cloud fragments; same as scud.

Fujita Scale (or F Scale) - A scale of wind damage intensity in which wind speeds are inferred from an analysis of wind damage:

F0 (weak): 40- 72 mph, light damage.

F1 (weak): 73-112 mph, moderate damage.

F2 (strong): 113-157 mph, considerable damage.

F3 (strong): 158-206 mph, severe damage.

F4 (violent): 207-260 mph, devastating damage.

F5 (violent): 261-318 mph, (rare) incredible damage.

All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.

*Funnel Cloud - A condensation funnel extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is NOT in contact with the ground (and hence different from a tornado). A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.

-G- 

Ground Clutter - A pattern of radar echoes from fixed ground targets (buildings, hills, etc...) near the radar. Ground clutter may hide or confuse precipitation echoes near the radar antenna.

*Grunge - [Slang], anything in the atmosphere that restricts visibility for storm spotting, such as fog, haze, precipitation (steady rain or drizzle), widespread low clouds (stratus), etc...

*Gust Front - The leading edge of gusty surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud. See also downburst, gustnado, outflow boundary.

*Gustnado (or Gustinado) - [Slang], gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud.

-H- 

Helicity - A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2.

High Risk (of severe thunderstorms) - Severe weather is expected to affect more than 10 percent of the area. A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major severe weather outbreak. (See slight risk, moderate risk, convective outlook.)

Hodograph - A plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height. Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc...).

Hook (or Hook Echo) - A radar reflectivity pattern characterized by a hook-shaped extension of a thunderstorm echo, usually in the right-rear part of the storm (relative to its direction of motion). A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.

*HP Storm or HP Supercell - High-Precipitation storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone. Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events.

Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous. See bear's cage.

Humidity - Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

-I- 

Impulse - See upper level system.

*Inflow Bands (or Feeder Bands) - Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone.

*Inflow Jets - Local jets of air near the ground flowing inward toward the base of a tornado.

Inflow Notch - A radar signature characterized by an indentation in the reflectivity pattern on the inflow side of the storm. The indentation often is V-shaped, but this term should not be confused with V-notch. Supercell thunderstorms often exhibit inflow notches, usually in the right quadrant of a classic supercell, but sometimes in the eastern part of an HP storm or in the rear part of a storm (rear inflow notch).

*Inflow Stinger - A beaver tail cloud with a stinger-like shape.

Insolation - Incoming solar radiation. Solar heating; sunshine.

Instability - The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms. See lifted index

Inversion - Generally, a departure from the usual increase or decrease in an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs. An inversion is present in the lower part of a cap.

Isentropic Lift - Lifting of air that is traveling along an upward-sloping isentropic surface.

Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms.

Isentropic Surface - A two-dimensional surface containing points of equal potential temperature.

Isobar - A line connecting points of equal pressure.

Isodrosotherm - A line connecting points of equal dew point temperature.

Isohyet - A line connecting points of equal precipitation amounts.

Isopleth - General term for a line connecting points of equal value of some quantity. Isobars, isotherms, etc... all are examples of isopleths.

Isotach - A line connecting points of equal wind speed.

Isotherm - A line connecting points of equal temperature

-J- 

Jet Max (or Speed Max, Jet Streak) - a point or area of relative maximum wind speeds within a jet stream.

Jet Streak - A local wind speed maximum within a jet stream.

Jet Stream - Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low-level jet.

-K- 

*Knuckles - [Slang], lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm anvil. They usually appear on the upwind side of a back-sheared anvil, and indicate rapid expansion of the anvil due to the presence of a very strong updraft. They are not mammatus clouds. See also cumuliform anvil, anvil rollover.

-L- 

*Laminar - Smooth, non-turbulent. Often used to describe cloud formations which appear to be shaped by a smooth flow of air traveling in parallel layers or sheets.

*Landspout - [Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

Lapse Rate - The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.

Left Front Quadrant (or Left Exit Region) - The area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also entrance region, right rear quadrant.

*Left Mover - A thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm. See also right mover.

LEWP - Line Echo Wave Pattern. A bulge in a thunderstorm line producing a wave-shaped "kink" in the line. The potential for strong outflow and damaging straight-line winds increases near the bulge, which often resembles a bow echo. Severe weather potential also is increased with storms near the crest of a LEWP.

Lifted Index (or LI) - A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms.

However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.

Loaded Gun (Sounding) - [Slang], a sounding characterized by extreme instability but containing a cap, such that explosive thunderstorm development can be expected if the cap can be weakened or the air below it heated sufficiently to overcome it.

Longwave Trough - A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration. Generally, there are no more than about five longwave troughs around the Northern Hemisphere at any given time. Their position and intensity govern general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks, or months. Smaller disturbances (e.g., shortwave troughs) typically move more rapidly through the broader flow of a longwave trough, producing weather changes over shorter time periods (a day or less).

Low-level Jet (abbrev. LLJ) - A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).

The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.

*LP Storm (or LP Supercell) - Low-Precipitation storm (or Low-Precipitation supercell). A supercell thunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic supercell, except without the heavy precipitation core. LP storms often exhibit a striking visual appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation. They are capable of producing tornadoes and very large hail. Radar identification often is difficult relative to other types of supercells, so visual reports are very important. LP storms almost always occur on or near the dry line, and thus are sometimes referred to as dry line storms.

LSR - Local Storm Report. A product issued by local NWS offices to inform users of reports of severe and/or significant weather-related events.

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*Mammatus Clouds - Rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil). Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well.

*MCC - Mesoscale Convective Complex. A large MCS, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs: Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more. Duration: Size criteria must be met for at least 6 hours. Eccentricity: Minor/major axis at least 0.7. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.

*MCS - Mesoscale Convective System. A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.

Medium Range - In forecasting, (generally) three to seven days in advance.

Meridional Flow - Large-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.

*Mesocyclone - A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm). The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it. Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration. Therefore, a mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone).

Mesohigh - A mesoscale high pressure area, usually associated with MCSs or their remnants.

Mesolow (or Sub-synoptic Low) - A mesoscale low-pressure center. Severe weather potential often increases in the area near and just ahead of a mesolow.

Mesolow should not be confused with mesocyclone, which is a storm-scale phenomenon.

Mesonet - A regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes. Also see mobile mesonet.

Mesoscale - Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines, MCCs, and MCSs are examples of mesoscale weather systems.

*Microburst - A small, concentrated downburst affecting an area less than 4 kilometers (about 2.5 miles) across. Most microbursts are rather short-lived (5 minutes or so), but on rare occasions they have been known to last up to 6 times that long.

Mid-level Cooling - Local cooling of the air in middle levels of the atmosphere (roughly 8 to 25 thousand feet), which can lead to destabilization of the entire atmosphere if all other factors are equal. Mid-level cooling can occur, for example, with the approach of a mid-level cold pool.

Mobile Mesonet - A group of vehicles equipped with observation stations, usually used for research. Also see mesonet.

Moderate Risk (of severe thunderstorms) - Severe thunderstorms are expected to affect between 5 and 10 percent of the area. A moderate risk indicates the possibility of a significant severe weather episode. See , high risk, slight risk, convective outlook.

Moisture Advection - Transport of moisture by horizontal winds.

Moisture Convergence - A measure of the degree to which moist air is converging into a given area, taking into account the effect of converging winds and moisture advection. Areas of persistent moisture convergence are favored regions for thunderstorm development, if other factors (e.g., instability) are favorable.

Morning Glory - An elongated cloud band, visually similar to a roll cloud, usually appearing in the morning hours, when the atmosphere is relatively stable. Morning glories result from perturbations related to gravitational waves in a stable boundary layer. They are similar to ripples on a water surface; several parallel morning glories often can be seen propagating in the same direction.

MRF - Medium-Range Forecast model; one of the operational forecast models run at NCEP. The MRF is run once daily, with forecast output out to 240 hours (10 days).

*Multi-cell(ular) Thunderstorm - A thunderstorm consisting of two or more cells, of which most or all are often visible at a given time as distinct domes or towers in various stages of development. Nearly all thunderstorms (including supercells) are multi-cellular, but the term often is used to describe a storm which does not fit the definition of a supercell.

*Multiple-vortex (or Multi-vortex) Tornado - a tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging. See suction vortex.

*Mushroom - [Slang], a thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.

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NCEP - National Centers for Environmental Prediction

Negative-tilt Trough - An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.

NEXRAD - NEXt-Generation Weather RADar. Technologically-advanced weather radar being deployed to replace WSR-57 and WSR-74 units. NEXRAD is a high-resolution Doppler radar with increased emphasis on automation, including use of algorithms and automated volume scans. NEXRAD units are known as WSR-88D.

NGM - Nested Grid Model; one of the operational forecast models run at NCEP. The NGM is run twice daily, with forecast output out to 48 hours.

NMC - National Meteorological Center, with headquarters near Washington D.C.; now known as NCEP.

NOAA - National Oceanographic and Atmospheric Administration.

Nocturnal - Related to nighttime, or occurring at night.

Nowcast - A short-term weather forecast, generally out to six hours or less.

NSSFC - National Severe Storms Forecast Center, in Kansas City MO; now known as SPC and located in Norman, OK.

NSSL - National Severe Storms Laboratory, in Norman OK.

NWP - Numerical Weather Prediction.

NWS - National Weather Service

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Occluded Mesocyclone - A mesocyclone in which air from the rear-flank downdraft has completely enveloped the circulation at low levels, cutting off the inflow of warm unstable low-level air.

Orographic - Related to, or caused by, physical geography (such as mountains or sloping terrain).

Orographic Lift - Lifting of air caused by its passage up and over mountains or other sloping terrain.

*Orphan Anvil - [Slang], an anvil from a dissipated thunderstorm, below which no other clouds remain.

*Outflow Boundary - A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc...; see triple point).

Overhang - Radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. (The latter area is known as a weak-echo region, or WER.) The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side).

*Overshooting Top (or Penetrating Top) - A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell. A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm or a cyclic storm.

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PDS Watch - [Slang], a tornado watch with enhanced wording (Particularly Dangerous Situation).

Pendant Echo - Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

Penetrating Top - Same as overshooting top.

Popcorn Convection - [Slang], Showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms: they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

Positive CG - A CG flash that delivers positive charge to the ground, as opposed to the more common negative charge. Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not considered possible to distinguish between a positive CG and a negative CG. (Some claim to have observed a relationship between staccato lightning and positive CGs, but this relationship is as yet unproven.)

Positive-tilt Trough - An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Potential Temperature - The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.

PPINE - Plan Position Indicates No Echoes, referring to the fact that a radar detects no precipitation within its range.

Profiler - An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.

Pseudo-Cold Front - A boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the mesocyclone center, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded mesocyclone), and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gust front. See also pseudo-warm front.

Pseudo-Warm Front - A boundary between a supercell's inflow region and the forward-flank downdraft (or FFD). It extends outward from at or near the mesocyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the mesocyclone. See pseudo-cold front and beaver tail.

*Pulse Storm - A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.

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RADAP II - RAdar DAta Processor II, attached to some WSR-57 and WSR-74 radar units. It automatically controls the tilt sequence and computes several radar-derived quantities at regular intervals, including VIL, storm tops, accumulated rainfall, etc.

Radial Velocity - Component of motion toward or away from a given location. As "seen" by Doppler radar, it is the component of motion parallel to the radar beam. (The component of motion perpendicular to the beam cannot be seen by the radar. Therefore, strong winds blowing strictly from left to right or from right to left, relative to the radar, can not be detected.)

*Rain Foot - [Slang], a horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst.

*Rain-free Base - A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area. Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate.

*Rear Flank Downdraft (or RFD) - A region of dry air subsiding on the back side of, and wrapping around, a mesocyclone. It often is visible as a clear slot wrapping around the wall cloud. Scattered large precipitation particles (rain and hail) at the interface between the clear slot and wall cloud may show up on radar as a hook or pendant; thus the presence of a hook or pendant may indicate the presence of an RFD.

Red Watch or Red Box - [Slang], a tornado watch.

Reflectivity - Radar term referring to the ability of a radar target to return energy; used to derive echo intensity, and to estimate precipitation intensity and rainfall rates. See dBZ, VIP.

Relative Humidity - A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.

Right Entrance Region (or Right Rear Quadrant) - The area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also exit region, left front quadrant.

Ridge - An elongated area of relatively high atmospheric pressure; the opposite of trough.

*Right Mover - A thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers.) See left mover, splitting storm

*Roll Cloud - A low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front (or sometimes with a cold front). Roll clouds are relatively rare; they are completely detached from the thunderstorm base or other cloud features, thus differentiating them from the more familiar shelf clouds. Roll clouds usually appear to be "rolling" about a horizontal axis, but should not be confused with funnel clouds.

*Rope (or Rope Funnel) - A narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado. See rope stage.

Rope Cloud - In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary. The term sometimes is used synonymously with rope or rope funnel.

*Rope Stage - The dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel). Damage still is possible during this stage.

RUC - Rapid Update Cycle, a numerical model run at NCEP that focuses on short-term (up to 12 h) forecasts and small-scale (mesoscale) weather features. Forecasts are prepared every 3 hours for the contiguous United States.

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*Scud (or Fractus) - Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

SELS - SEvere Local Storms Unit, former name of the Operations Branch of the Storm Prediction Center (SPC) in Norman, OK (formerly in Kansas City, MO).

*Severe Thunderstorm - A thunderstorm which produces tornadoes, hail 0.75 inches or more in diameter, or winds of 50 knots (58 mph) or more. Structural wind damage may imply the occurrence of a severe thunderstorm. See approaching (severe).

Shear - Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

*Shelf Cloud - A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

Short-Fuse Warning - A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

Shortwave (or Shortwave Trough) - A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.

Slight Risk (of severe thunderstorms) - Severe thunderstorms are expected to affect between 2 and 5 percent of the area. A slight risk generally implies that severe weather events are expected to be isolated. See high risk, moderate risk, convective outlook.

Sounding - A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature, etc...

SPC - Storm Prediction Center. A national forecast center in Norman, Oklahoma, which is part of NCEP. The SPC is responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding) and severe winter weather over the contiguous United States.

Speed Shear - The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

*Spin-up - [Slang], a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.

*Splitting Storm - A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.

*Squall Line - A solid or nearly solid line or band of active thunderstorms.

Staccato Lightning - A CG lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.

Steering Winds (or Steering Currents) - A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Storm-relative - Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

Storm-scale - Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale, mesoscale.

*Straight-line Winds - Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.

Stratiform - Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).

Stratocumulus - Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.

*Stratus - A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus.

*Striations - Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. Striations often reveal the presence of rotation, as in the barber pole or "corkscrew" effect often observed with the rotating updraft of an LP storm.

Subsidence - Sinking (downward) motion in the atmosphere, usually over a broad area.

Sub-synoptic Low - Essentially the same as mesolow.

*Suction Vortex (sometimes Suction Spot) - A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.

*Supercell - A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm. Storms exhibiting these characteristics often are called classic supercells; however HP storms and LP storms also are supercell varieties.

*Surface-based Convection - Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection

SWEAT Index - Severe Weather ThrEAT Index. A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds as follows: SWEAT = (12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where Td 850 is the dew point temperature at 850 mb, TT is the total-totals index, f 850 is the 850-mb wind speed (in knots), f 500 is the 500-mb wind speed (in knots), and s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer). SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers. The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed sounding analysis programs.

SWODY1, SWODY2, SWODY3 (sometimes pronounced swoe-dee) - The day-1, day-2 and day 3 convective outlooks issued by SPC.

Synoptic Scale (or Large Scale) - Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale, storm-scale.

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*Tail Cloud - A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds. Compare with beaver tail, which is a form of inflow band that normally attaches to the storm's main updraft (not to the wall cloud) and has a base at about the same level as the updraft base (not the wall cloud).

*Tail-end Charlie - [Slang], the thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unimpeded, such a storm often has a higher probability of strengthening to severe levels than the other storms in the line.

Thermodynamic Chart (or Thermodynamic Diagram) - A chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc... if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability.

Thermodynamics - In general, the relationships between heat and other properties (such as temperature, pressure, density, etc...) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Theta-e (or Equivalent Potential Temperature) - The temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.

Theta-e Ridge - An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.

Tilt Sequence - Radar term indicating that the radar antenna is scanning through a series of antenna elevations in order to obtain a volume scan.

Tilted Storm or Tilted Updraft - A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

*Tornado - A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.

Tornado Family - A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

Total-Totals Index - A stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb. The total-totals index is the arithmetic sum of two other indices: the Vertical Totals Index (temperature at 850 mb minus temperature at 500 mb) and the Cross Totals Index (dew point at 850 mb minus temperature at 500 mb). As with all stability indices there are no magic threshold values, but in general, values of less than 50 or greater than 55 are considered weak and strong indicators, respectively, of potential severe storm development.

*Tower - (Short for towering cumulus), a cloud element showing appreciable upward vertical development.

*Towering Cumulus - (Same as congestus.) A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)

Triple Point - The intersection point between two boundaries (dry line, outflow boundary, cold front, etc...), often a focus for thunderstorm development. Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment).

Tropopause - The upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).

Troposphere - The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers - see inversion, cap), vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).

Trough - An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

*Turkey Tower - [Slang], a narrow, individual cloud tower that develops and falls apart rapidly. The sudden development of turkey towers from small cumulus clouds may signify the breaking of a cap.

TVS - Tornadic Vortex Signature. Doppler radar signature in the radial velocity field indicating intense, concentrated rotation - more so than a mesocyclone. Like the mesocyclone, specific criteria involving strength, vertical depth, and time continuity must be met in order for a signature to become a TVS. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it. A TVS is not a visually observable feature.

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UKMET - A medium-range numerical weather prediction model operated by the United Kingdom METeorological Agency.

*Updraft - A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.

*Updraft Base - Alternate term for a rain-free base.

Upper Level System - A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term sometimes is used interchangeably with impulse or shortwave.

Upslope Flow - Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upstream - Toward the source of the flow, or located in the area from which the flow is coming.

UVM (or UVV) - Upward Vertical Motion (or Velocity).

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VAD - Velocity Azimuth Display. A radar display on which mean radial velocity is plotted as a function of azimuth. See VWP.

Vault - Same as BWER.

Veering Winds - Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter example is a form of directional shear which is important for tornado formation. Compare with backing winds.

VIL - Vertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D units that takes into account the three-dimensional reflectivity of an echo. The maximum VIL of a storm is useful in determining its potential severity, especially in terms of maximum hail size.

VIP - Video Integrator and Processor, which contours radar reflectivity (in dBZ) into six VIP levels:

VIP 1 (Level 1, 18-30 dBZ) - Light precipitation

VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.

VIP 3 (Level 3, 38-44 dBZ) - Moderate to heavy rain.

VIP 4 (Level 4, 44-50 dBZ) - Heavy rain

VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.

VIP 6 (Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible.

Vertically-stacked System - A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. Such systems typically are weakening and are slow-moving, and are less likely to produce severe weather than tilted systems. However, cold pools aloft associated with vertically-stacked systems may enhance instability enough to produce severe weather.

*Virga - Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst; see dry microburst.

V Notch - A radar reflectivity signature seen as a V-shaped notch in the downwind part of a thunderstorm echo. The V-notch often is seen on supercells, and is thought to be a sign of diverging flow around the main storm updraft (and hence a very strong updraft). This term should not be confused with inflow notch or with enhanced V, although the latter is believed to form by a similar process.

Volume Scan - A radar scanning strategy in which sweeps are made at successive antenna elevations (i.e., a tilt sequence), and then combined to obtain the three-dimensional structure of the echoes. Volume scans are necessary to determine thunderstorm type, and to detect features such as WERs, BWERs, and overhang.

Vorticity - A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.

Vort Max - (Slang; short for vorticity maximum), a center, or maximum, in the vorticity field of a fluid.

VWP - VAD Wind Profile. A radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar. The display is plotted with height as the vertical axis and time as the horizontal axis (a so-called time-height display), which then depicts the change in wind with time at various heights. This display is useful for observing local changes in vertical wind shear, such as backing of low-level winds, increases in speed shear, and development or evolution of nearby jet streams (including low-level jets).

This product often is referred to erroneously as a VAD.

-W- 

*Wall Cloud - A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion. "Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.

Warm Advection - Transport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.

*Warning - A product issued by NWS local offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning). See short-fuse warning.

*Watch - An SPC product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).

*Watch Box (or Box) - [Slang], a severe thunderstorm or tornado watch.

*Waterspout - In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters. The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.

*Wedge (or Wedge Tornado) - [Slang], a large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base. The term "wedge" often is used somewhat loosely to describe any large tornado. However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least as wide at the ground as it is tall, is very rare. Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many documented wedges have been rated lower. And some violent tornadoes may not appear as wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series of suction vortices without a central condensation funnel). Whether or not a tornado achieves "wedge" status depends on several factors other than intensity - in particular, the height of the environmental cloud base and the availability of moisture below cloud base. Therefore, spotters should not estimate wind speeds or F-scale ratings based on visual appearance alone. However, it generally is safe to assume that most (if not all) wedges have the potential to produce strong (F2/F3) or violent (F4/F5) damage.

WER - Weak Echo Region. Radar term for a region of relatively weak (reflectivity at low levels on the inflow side of a thunderstorm echo, topped by stronger reflectivity in the form of an echo overhang directly above it . The WER is a sign of a strong updraft on the inflow side of a storm, within which precipitation is held aloft. When the area of low reflectivity extends upward into, and is surrounded by, the higher reflectivity aloft, it becomes a BWER.

*Wet Microburst - A microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst. See dry microburst.

Wind Shear - See shear.

*Wrapping Gust Front - A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.

WSR-57, WSR-74 - NWS Weather Surveillance Radar units, replaced by WSR-88D units.

WSR-88D - Weather Surveillance Radar - 1988 Doppler; NEXRAD unit.

-Z- 

Zonal Flow - Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal. Compare with meridional flow. 







 Wind Power Glossary

    AC--See Alternating Current
    Airfoil--The cross section profile of the leeward side of a wind generator blade. Designed to give low drag and good lift. Also found on an airplane wing.
    Air Gap--In a permanent magnet alternator, the distance between the magnets and the laminates.
    Alternating Current--Electricity that changes direction periodically. The period is measured in Cycles per Second (Hertz, Hz).
    Alternator--A device that produces Alternating Current from the rotation of a shaft.
    Amperage--A unit of electrical current, equal to Coulombs per second. This is the flow rate of electrons moving through a circuit, very roughly analogous to gallons per minute flowing from a faucet.
    Ampere-Hour--A measure of energy quantity, equal to amperes times hours. Also used to measure battery capacity.
    Anemometer--A device that measures wind speed.

    Angle of Attack--The angle of relative air flow to the blade chord.
    Annealing--A heat treatment process that makes Cold-rolled steel more suitable for forming and bending.
    Area of a Circle--Pi multiplied by the Radius squared.
    Armature--The moving part of an alternator, generator or motor. In many PM alternator designs, it carries the magnets and is attached to the blades and hub. Also called a Rotor.
    Axial Alternator--An alternator design where a flat disc carrying magnets on the face (the Armature) rotates near a flat disc carrying coils (the Stator).
    Axis--The centerline of a rotating object's movement.
    Balancing--With wind turbine blades, adjusting their weight and weight distribution through 2 axes so that all blades are the same. Unbalanced blades create damaging vibration.
    Battery--An electrochemical device for storing energy.
    Battery Bank--An array of Batteries connected in series, parallel, or both.
    Bearing--A device that transfers a force to structural supports. In a wind generator, bearings allow the Shaft to rotate freely, and allow the machine to Yaw into and out of the wind.
    Belt--A device for transferring power from a rotating shaft to a generator. Allows the use of Pulleys to change the ratio of shaft speed to and from the generator.
    Betz Coefficient--59.3 percent. This is the theoretical maximum efficiency at which a wind generator can operate, by slowing the wind down. If the wind generator slows the wind down too much, air piles up in front of the blades and is not used for extracting energy.
    Blade--The part of a wind generator rotor that catches the wind.
    Brakedrum Windmill--A home-built wind generator design by Hugh Piggott of Scotland.
    Braking System--A device to slow a wind turbine's shaft speed down to safe levels electrically or mechanically.
    Bridge Rectifier--An array of diodes used to convert Alternating Current to Direct Current. Single-phase bridge rectifiers use 4 diodes, 3-phase bridge rectifiers use 6 diodes.
    Brushes--Devices for transferring power to or from a rotating object. Usually made of carbon-graphite.
    Ceramic Magnets--See Ferrite Magnets.
    Chord--The width of a wind turbine blade at a given location along the length.
    Coercivity--The amount of power needed to magnetize or demagnetize a permanent magnet. Measured in MegaGauss Oersted (mGO)
    Cogging--The cyclic physical resistance felt in some alternator designs from magnets passing the coils and gaps in the laminates. Detrimental to Start-up.
    Coil--A length of wire wound around a form in multiple turns.
    Cold-Rolled Steel--Steel processed by working at room temperatures. More expensive than hot-rolled steel.
    Commutator--The rotating part of a DC generator.
    Concave--A surface curved like the interior of a circle or sphere.
    Convex--A surface curved like the exterior of a circle or sphere.
    Cowling--See Nacelle.
    Current--See Amperage.
    Cut-In--The rotational speed at which an alternator or generator starts pushing electricity hard enough (has a high enough voltage) to make electricity flow in a circuit.
    Cyanoacrylate--A fast-setting, hard and brittle adhesive. See Superglue®.
    Cycles per Second--Measured in Hertz. In electricity, it is the number of times an AC circuit reaches both minimum and maximum values in one second.
    Darrieus--A Vertical Axis Wind Turbine design from the 1920s and 1930s by F.M. Darrieus, a French wind turbine designer.
    DC--See Direct Current
    Delta--A 3-phase alternator wiring configuration in which all phases are connected in Series.
    Diameter--A straight line passing through the center of a circle, and ending on both edges. Equal to 2 times the Radius.
    Diode--A solid-state device that allows electricity to flow in only one direction.
    Downwind--Refers to a Horizontal Axis Wind Turbine in which the hub and blades point away from the wind direction, the opposite of an Upwind turbine.
    Drag--In a wind generator, the force exerted on an object by moving air. Also refers to a type of wind generator or anemometer design that uses cups instead of a blades with airfoils.
    Dump Load--A device to which wind generator power flows when the system batteries are too full to accept more power, usually an electric heating element. This diversion is performed by a Shunt Regulator, and allows a Load to be kept on the Alternator or Generator.
    Duty Cycle--In a circuit, the ratio of off time to on time.
    Dynamo--A device that produces Direct Current from a rotating shaft. See Generator.
    Eddy Currents--Currents that flow in a substance from variations in magnetic induction. See also Lenz Effect. Laminates are used to prevent eddy currents, which cause physical and electrical resistance in an alternator or transformer, therefore wasting power.
    Efficiency--The ratio of energy output to energy input in a device.
    Electromagnet--A device made of wire coils that produces a magnetic field when electricity flows through the coils.
    Epoxy--A 2-part adhesive system consisting of resin and hardener. It does not start to harden until the elements are mixed together. NOT compatible with Fiberglas® Resin.
    Excitation--Using an electric current to create a magnetic field. See Electromagnet.
    Fatigue--Stress that causes material failure from repeated, cyclic vibration or stress.
    Ferrite Magnets--Also called Ceramic Magnets. Made of Strontium Ferrite. High Coercivity and Curie Temperature, low cost, but brittle and 4-5 times weaker than NdFeB magnets.
    Fiberglas® Resin--Another 2-part adhesive system, NOT compatible with Epoxy. Often used for making castings, since it is much cheaper than Epoxy.
    Field--See Magnetic Field
    Flux--See Magnetic Field
    Freewheeling--a wind generator that is NOT connected to a Load is freewheeling, and in danger of self-destruction from overspeeding.
    Frequency--See Cycles per Second.
    Furling--The act of a wind generator Yawing out of the wind either horizontally or vertically to protect itself from high wind speeds.
    Furling Tail--A wind generator protection mechanism where the rotor shaft axis is offset horizontally from the yaw axis, and the tail boom is both offset horizontally and hinged diagonally, thus allowing the tail to fold up and in during high winds. This causes the blades to turn out of the wind, protecting the machine.
    Gauss--A unit of magnetic induction, equal to 1 Maxwell per square centimeter. Higher Gauss measurements mean more power can be induced to flow in an alternator. Gauss readings can be increased by putting steel behind magnets, stacking magnets, or using larger or higher-grade magnets.
    Gearing--Using a mechanical system of gears or belts and pulleys to increase or decrease shaft speed. Power losses from friction are inherent in any gearing system.
    Generator--A device that produces Direct Current from a rotating shaft.
    Governor--A device that regulates the speed of a rotating shaft, either electrically or mechanically.
    Guy Anchor--Attaches tower guy wires securely to the earth.
    Guy Radius--The distance between a wind turbine tower and the guy anchors.
    Guy Wire--Attaches a tower to a Guy Anchor and the ground.
    H-Rotor--A Vertical Axis Wind Turbine design.
    HAWT--See Horizontal Axis Wind Turbine.
    Hertz--Frequency measurement. See Cycles per Second
    Horizontal Axis Wind Turbine--A "normal" wind turbine design, in which the shaft is parallel to the ground, and the blades are perpendicular to the ground.
    Hub--The center of a wind generator rotor, which holds the blades in place and attaches to the shaft.
    Impedance--See Resistance.
    Induction--The production of a magnetic field by the proximity of a electric charge or the production of a magnetic field by proximity of an electric charge.
    Induction Motor--An AC motor in which the rotating armature has no electrical connections to it (ie no slip rings), and consists of alternating plates of aluminum and steel.
    Kerf--The width of a cut made by a saw.
    Kilowatt--1000 Watts (see Watt)
    kW--See Kilowatt.
    Laminations--Electrical circuit core parts, found in motors, generators, alternators and transformers. When core parts are subjected to alternating electrical or magnetic fields, the buildup of Eddy Currents causes physical and electrical power loss. Laminations are made of thin strips of materials that make good temporary magnets and poor permanent magnets, and each strip is insulated electrically from the next.
    Leading Edge--The edge of a blade that faces toward the direction of rotation.
    Leeward--Away from the direction from which the wind blows.
    Lenz Effect--See also Eddy Currents. From H.F.E Lenz in 1833. Electromotive force is induced with variations in magnetic flux. It can be demonstrated physically in many different ways--for example dragging a strong magnet over an aluminum or copper plate, or shorting the terminals of a PM alternator and rotating the shaft by hand. Laminates are used to reduce power losses from this effect.
    Lift--The force exerted by moving air on asymmetrically-shaped wind generator blades at right angles to the direction of relative movement. Ideally, wind generator blades should produce high Lift and low Drag.
    Live--A circuit that is carrying electricity. When live, it can shock you.
    Load--Something physical or electrical that absorbs energy. A wind generator that is connected to a battery bank is loaded. A disconnected wind generator is NOT loaded, so the blades are free to spin at very high speed without absorbing any energy from the wind, and it is in danger of destruction from overspeeding.
    Losses--Power that is harvested by a wind generator but is not transferred to a usable form. Losses can be from friction, electrical resistance, or other causes.
    Magnet--A body that attracts ferromagnetic materials. Can be a Permanent magnet, Temporary Magnet, or Electromagnet.
    Magnetite--A common Iron-containing mineral with ferromagnetic properties.
    Magnet Wire--The kind of wire always used in making electromagnets, alternators, generators and motors. Uses very thin enamel insulation to minimize thickness and maximize resistance to heat.
    Magnetic Circuit--The path in which magnetic flux flows from one magnet pole to the other.
    Magnetic Field--Magnetic fields are historically described in terms of their effect on electric charges. A moving electric charge, such as an electron, will accelerate in the presence of a magnetic field, causing it to change velocity and its direction of travel. An electrically charged particle moving in a magnetic field will experience a force (known as the Lorentz force) pushing it in a direction perpendicular to the magnetic field and the direction of motion. Also called magnetic flux.
    Maximum Energy Product--Determines how good a magnet that different materials can make. Technically, the amount of energy that a material can supply to an external magnetic circuit when operating within its demagnetization curve.
    MegaGauss Oersted--Magnetic force measurement, see Maximum Energy Product.
    MGOe--See MegaGauss Oersted.
    Moment--A force attempting to produce motion around an axis.
    NdFeB--See Neodymium-Iron-Boron Magnet.
    Nacelle--The protective covering over a generator or motor.
    Neodymium-Iron-Boron Magnet--The composition of the most powerful Permanent Magnets known to man. The materials are mined, processed, and sintered into shape. Then, they are subjected to an extremely strong magnetic field and become Permanent Magnets.
    Ohm's Law--The basic math needed for nearly all electrical calculations. Please see a dictionary or Pocket Ref for all of the variations on Ohm's Law! E=I*R (voltage(E)=amperage(I)*resistance(R)), and all of the algebraic variations of this (I=E/R, R=E/I). Also, for DC circuits, Watts=Volts*Amps. For AC circuits, Watts=Amps * Volts * Cosine of phase angle theta.
    Open-Circuit Voltage--The voltage that a alternator or generator produces when it is NOT connected to a Load.
    Parallel--In DC electrical circuits such as a battery bank or solar panel array, this is a connection where all negative terminals are connected to each other, and all positive terminals are connected to each other. Voltage stays the same, but amperage is increased. In AC circuits such as a wind generator alternator, each parallel coil is connected to common supply wires, again increasing amperage but leaving voltage the same. Opposite of Series. See also Star.
    Permanent Magnet--A material that retains its magnetic properties after an external magnetic field is removed.
    Permanent Magnet Alternator--An Alternator that uses moving permanent magnets instead of Electromagnets to induce current in coils of wire.
    PM--See Permanent Magnet.
    PMA--See Permanent Magnet Alternator.
    Phase--The timing of AC current cycles in different wires. 3-phase alternators produce current that is cyclically timed between 3 different wires and a common wire, while single phase produces it in only 1 wire and a common. In a 3-phase alternator, wire #1 receives a voltage peak, then wire #2 receives a peak, then wire #3, and so on. A diagram is an easier way to explain phase, check out Windstuffnow.com's 3-Phase and 1-Phase Basics page for detailed diagrams.
    Pillow Blocks--Bearings that support a horizontal shaft.
    Pitch--See Setting Angle.
    Poles--A way of picturing magnetic phenomena. All magnets are considered to be "dipoles", having both a North pole (which would point North if used in a compass) and a South pole (which would point South if used in a compass. In an alternator, generator, or motor the number of Poles is a measure of how many coils, permanent magnets or electromagnets are in the armature or stator.
    Prop--Slang term for Propeller.
    Propeller--The spinning thing that makes an airplane move forward. Often incorrectly used (by Otherpower.com also!) to describe a wind turbine Rotor.
    Pulley--A device for transferring power when using Belts as Gearing. Changing to smaller or larger Pulleys changes the gear ratio, and can be used to make a shaft turn faster or slower than the shaft that is providing its power.
    Pulse Width Modulation--(abbrev. PWM) A regulation method based on Duty Cycle. At full power, a pulse-width-modulated circuit provides electricity 100 percent of the time. At half power, the PWM is on half the time and off half the time. The speed of this alternation is generally very fast. Used in both solar wind regulators to efficiently provide regulation.
    PWM--See Pulse Width Modulation.
    Radius--The distance between the center of a circle and the outside.
    Rare-Earth Magnets--See Neodymium-Iron-Boron magnets.
    Rated Power Output--Used by wind generator manufacturers to provide a baseline for measuring performance. Rated output may vary by manufacturer. For example, one manufacturer's 1500 watt turbine may produce that amount of power at a 30 mph windspeed, while another brand of 1500 watt turbine may not make 1500 Watts until it gets a 40 mph windspeed! So read manufacturer's ratings statements very carefully.
    Rectifier--See Diode.
    Radial--An alternator design in which the armature magnets are attached to the outside circumference of a disc, with the stator coils mounted around the outside.
    Regulator--A device to adjust incoming power so as to avoid overcharging a battery bank. In solar power, the regulator generally just turns the solar array off when the batteries are full. With a wind generator, the regulator generally diverts all or part of the incoming power to a Dump Load when the batteries fill, thus keeping a Load on the wind generator so it will not Freewheel.
    Relay--An electromechanical switch that uses a small amount of incoming electricity to charge an electromagnet, which physically pulls down a connecting switch to complete a circuit. This allows a low-power circuit to divert the electricity in a high-power circuit.
    Resistance--The voltage per amp needed to make electricity flow through a wire. See Ohm's Law.
    Root--The area of a blade nearest to the hub. Generally the thickest and widest part of the blade.
    Rotor--1) The blade and hub assembly of a wind generator. 2) The disc part of a vehicle disc brake. 3) The armature of a permanent magnet alternator, which spins and contains permanent magnets.
    RPM--Revolutions Per Minute. The number of times a shaft completes a full revolution in one minute.
    Savonius--A vertical-axis wind turbine design by S.J. Savonius of Finland from the 1920s and 30s. Shaped like a barrel split from end to end and offset along the cut. They are drag machines, and thus give very low rpm but lots of torque.
    Series--In DC electrical circuits such as a battery bank or solar panel array, this is a connection where all the negative terminals are connected to the neighboring positive terminals. Voltage increases, but amperage stays the same. In AC circuits such as a wind generator alternator, each coil is connected to the one next to it, and so on, again increasing voltage but leaving amperage the same. Opposite of Parallel. See also Delta.
    Servo Motor--A motor used for motion control in robots, hard disc drives, etc. Generally designed more like an alternator than a standard motor, most Servos need special control circuitry to make them rotate electrically. Some can be used in reverse to generate alternating current.
    Setting Angle--The angle between the blade Chord and the plane of the blade's rotation. Also called Pitch or blade angle. A blade carved with a Twist has a different setting angle at the Tip than at the Root.
    Shaft--The rotating part in the center of a wind generator or motor that transfers power.
    Short Circuit--1) Parts of a circuit connected together with only the impedance of the leads between them. 2) In wind generators, connecting the output leads directly together so as to heavily load a generator in high winds. This creates a "short" circuit path back to the generator, bypassing all other loads.
    Shunt--An electrical bypass circuit that proportionally divides current flow between the shunt and the shunted equipment. It also allows high current measurements with low-current equipment.
    Shunt Regulator--A bypass device for power not needed for charging batteries. When batteries are full, the regulator shunts all or part of the excess power to a Dump Load to protect the batteries from overcharging damage.
    Slip Ring--Devices used to transfer electricity to or from rotating parts. Used in wound-field alternators, motors, and in some wind generator yaw assemblies.
    Star--A coil connection scheme for 3 phase alternators and generators in which all 3 coil phases are connected in parallel--they all share a common connection.
    Start-Up--The windspeed at which a wind turbine rotor starts to rotate. It does not necessarily produce any power until it reaches cut-in speed.
    Stationary--With wind generator towers, a tower that does not tilt up and down. The tower must be climbed or accessed with a crane to install or service equipment at the top.
    Stator--The part of a motor, generator or alternator that does not rotate. In permanent magnet alternators it holds the coils and laminates.
    SuperGlue®--Cyanoacrylate adhesive. Fast bonding glue, easy to find in different viscosities. Sets on its own, and sets instantly when sprayed with an accelerator chemical. Hard, but somewhat brittle. Does not react adversely with Fiberglas® resin or epoxy.
    Tail--See Vane. The proper term is actually Vane, but Tail is commonly used.
    Tail Boom--A strut that holds the tail (Vane) to the wind generator frame.
    Tape Drive Motor--A type of permanent magnet DC motor often used as a generator in small wind generator systems.
    Taper--The change in wind turbine blade width (chord) along the length.
    Temporary Magnet--A material that shows magnetic properties only while exposed to an external magnetic field.
    Thrust--In a wind generator, wind forces pushing back against the rotor. Wind generator bearings must be designed to handle thrust or else they will fail.
    Thrust Bearing--A bearing that is designed to handle axial forces along the centerline of the shaft--in a wind generator, this is the force of the wind pushing back against the blades.
    Tilt-Up--A tower that is hinged at the base and tilted up into position using a gin pole and winch or vehicle. Wind turbines on tilt-up towers can be serviced on the ground, with no climbing required.
    Tip--The end of a wind generator blade farthest from the hub.
    Tip Speed Ratio--The ratio of how much faster than the windspeed that the blade tips are moving. Abbreviation TSR.
    Torque--Turning force, equal to force times radius. See also Moment.
    Tower--A structure that supports a wind generator, usually high in the air.
    Trailing Edge--The edge of a blade that faces away from the direction of rotation.
    Transformer--Multiple individual coils of wire wound on a laminate core. Transfers power from one circuit to another using magnetic induction. Usually used to step voltage up or down. Works only with AC current.
    TSR--See Tip Speed Ratio.
    Turn--In winding stator coils, this is one loop of wire around a form. A coil will often be referred to by how many turns of a certain gauge wire are in each coil.
    Twist--In a wind generator blade, the difference in Pitch between the blade root and the blade tip. Generally, the twist allows more Pitch at the blade root for easier Startup, and less Pitch at the tip for better high-speed performance.
    Upwind--
    Vane--A large, flat piece of material used to align a wind turbine rotor correctly into the wind. Usually mounted vertically on the tail boom. Sometimes called a Tail.
    Variable Pitch--A type of wind turbine rotor where the attack angle of the blades can be adjusted either automatically or manually.
    VAWT--See Vertical Axis Wind Turbine.
    Vertical Axis Wind Turbine--A wind generator design where the rotating shaft is perpendicular to the ground, and the cups or blades rotate parallel to the ground.
    Voltage--A measure of electrical potential difference. One volt is the potential difference needed in a circuit to make one Ampere flow, dissipating one Watt of heat.
    Volt-Amp--In an AC circuit, this is Volts * Amps, without factoring in the power factor, derived from the phase angle. See also Watt.
    Watt--One Joule of electrical energy per second. In DC circuits, Watts=Volts * Amps. In AC circuits, Watts=Volts * Amps * the cosine of the phase angle. See also Volt-Amp.
    Wild AC--Alternating Current that varies in Frequency.
    Wind Generator--A device that captures the force of the wind to provide rotational motion to produce power with an alternator or generator.
    Windmill--A device that uses wind power to mill grain into flour. But informally used as a synonym for wind generator or wind turbine, and to describe machines that pump water with wind power.
    Wind Turbine--A machine that captures the force of the wind. Called a Wind Generator when used to produce electricity. Called a Windmill when used to crush grain or pump water.
    Windward--Toward the direction from which the wind blows.
    Yaw--Rotation parallel to the ground. A wind generator Yaws to face winds coming from different directions.
    Yaw Axis--Vertical axis through the center of gravity.

 







 
Glossary

    aggregation — Bundling several wind energy projects together so that they are treated as one larger project (for example, when purchasing turbines, interconnecting or maintaining a project) to distribute costs among more turbines or projects.  This practice can improve project economics.
    airfoil — The cross section profile of the leeward side of a wind generator blade, designed to provide low drag and good lift. Also found on an airplane wing.
    anemometer — An instrument used to measure the velocity, or speed, of the wind.
    angle of attack — The angle of relative air flow to a wind turbine's blade.
    armature — The moving part of an alternator, generator or motor. In many alternator designs, it carries the magnets and is attached to the blades and hub. Also called a Rotor.
    availability factor — The percentage of time that a wind turbine is able to operate and is not out of commission due to maintenance or repairs.
    average capacity (also known as capacity factor) — A measure of a wind turbine's productivity, calculated by the amount of power that a wind turbine produces over a set time period, divided by the amount of power that would have been produced if the turbine had operated at full capacity during that same time interval.
    average wind speed (velocity) — The mean wind speed over a specified period of time. 
    balancing — Adjusting wind turbine blades' weight and weight distribution through two axes so that all blades are the same. Unbalanced blades create damaging vibration.
    Betz Coefficient — 59.3 percent. This is the theoretical maximum efficiency at which a wind generator can operate, by slowing the wind down.  If the wind generator slows the wind down too much, air piles up in front of the blades and is not used for extracting energy.
    blades — The flat panels on a wind turbine that are connected to a center shaft that converts the push of the wind into a circular motion in a wind turbine.
    braking system — A device to slow a wind turbine's shaft speed down to safe levels (electrically or mechanically).
    capacity factor — see average capacity. 
    chord — The width of a wind turbine blade at a given location along the length.
    commercial scale wind — Wind energy projects greater than 100 kilowatts in which the electricity is sold rather than used on-site. This category includes large arrays of 100 or more turbines owned by large corporations and a single locally-owned wind turbine greater than 100 kilowatts in size.
    community wind —Locally-owned, commercial-scale wind projects that optimize local benefits. Locally-owned means that one or more members of the local community has a significant direct financial stake in the project other than through land-lease payments, tax revenue, or payments in lieu of taxes.
    cut-in speed — The wind speed at which the turbine blades begin to rotate and produce electricity, typically around 10 miles per hour.
    cut-out speed — The wind speed, usually around 55 to 65 miles per hour, at which some wind turbines automatically stop the blades from turning and rotates out of the wind to avoid damage to the turbine.
    distributed generation — A small-scale power generation technology that provides electric power at a site closer to customers than central power plant generation. The term is commonly used to indicate non-utility sources of electricity, including facilities for self-generation.
    downwind turbine — Refers to a horizontal-axis wind turbine in which the hub and blades point away from the wind direction; the opposite of an upwind turbine.
    furling — The act of a wind generator yawing out of the wind, either horizontally or vertically, to protect itself from high wind speeds.
    gigawatt — A unit of power equal to 1 billion Watts, 1 million kilowatts, or 1,000 megawatts.
    green pricing — A practice utilized by some power providers in which electricity produced from clean, renewable resources is sold at a higher cost than electricity produced from conventional fuels to buyers willing to pay a premium for clean power.
    grid — An electricity transmission and distribution system.
    grid-connected system — A residential electrical system, such as solar panels or wind turbines, which is connected to the electric utility system. The utility system serves as a backup source of electricity if the residential system is not producing power.
    guy anchor — A concrete or metal base that secures wind tower guy wires to the earth.
    guy wire — A strong metal cable or wire that attaches some towers (typically those of small residential wind turbines) to the ground.
    horizontal axis wind turbine — A wind turbine design in which the shaft (axis of rotation) is parallel to the ground and the blades are perpendicular to the ground.
    hub — The central part of the wind turbine, which supports the turbine blades on the outside and connects to the low-speed rotor shaft inside the nacelle.
    hub height — Measuring from the ground, the tower height of the hub, or central part of a horizontal-axis wind turbine.
    hybrid system — The combination of multiple energy-producing technologies such as photovoltaic solar electric systems combined with small wind turbine systems.
    independent power producer — An electricity generator that sells electricity but is not owned by a utility.
    installed capacity — The total capacity of electrical generation devices in a system. Learn more about the installed capacity of wind energy in the United States here.
    inverter — A device that converts direct current electricity to alternating current, either for stand-alone systems or to supply power to an electric utility system.
    Investment Tax Credit — A tax credit granted for specific investment types, such as investment in wind projects.
    investor-owned utility (IOU) — A power provider owned by stockholders or other investors rather than government agencies or cooperatives.
    kilowatt — A standard unit of electrical power, equal to 1,000 Watts.
    kilowatt-hour — A unit or measure of electricity supply or consumption of 1,000 Watts for a period of 1 hour.
    megawatt — A standard measure of electric power plant generating capacity equal to 1,000 kilowatts or 1 million watts.
    met tower — A tower with a group of instruments (including anemometers and wind vanes) attached that collectively measure various meteorological parameters such as wind speed, wind direction, and temperature at various heights above the ground. The term met is short for meteorological.
    nacelle — The structure at the top of the wind turbine tower just behind (or, in some cases, in front of) the wind turbine blades. It houses the key components of the wind turbine, including the rotor shaft, gearbox and generator.
    net metering — Measuring both consumption and generation of electricity by a small generation facility (such as a small wind system). The net energy produced or consumed is purchased from or sold to the power provider.
    payback period — The amount of time required for the savings resulting from a renewable energy system to equal the system cost.
    peak wind speed — The maximum instantaneous wind speed (or velocity) that occurs within a specific time period.
    power curve — A graphic displaying the instantaneous power output of a specific turbine design at various wind speeds; used with wind resource data to determine the potential for electricity generation at a project site.
    Production Tax Credit (PTC) — A provision included in the Energy Policy Act of 1992: a commercial tax credit that applies to wholesale electrical generators of wind energy facilities based on the amount of energy generated.
    renewable energy certificates — Also known as green tags, certificates representing the environmental attributes of power produced from renewable resources. By separating the environmental attributes from the power, clean power generators are able to sell the electricity they produce to power providers at a competitive market value. The additional revenue generated by the sale of the certificates can be applied to the above-market costs associated with producing power made from renewable energy sources.
    Public Utilities Regulatory Policy Act (PURPA) — A 1978 law that requires electric utilities to purchase electricity produced from qualifying power producers that use renewable energy resources or are co-generators.
    rated wind speed — The wind speed at which a wind turbine reaches its nameplate-rated level of power production. For most small wind turbines, this is around 30 to 35 miles per hour. 
    rotor — The visible spinning parts of a wind turbine, including the turbine blades and the hub.
    start-up speed — The wind speed at which a wind turbine rotor starts to rotate. The turbine does not necessarily produce any power until the wind reaches cut-in speed. 
    thrust bearing — A bearing that is designed to handle axial forces along the centerline of the shaft; in a wind generator, the axial force is the force of the wind pushing back against the blades.
    tower — The base structure that supports and elevates a wind turbine rotor and nacelle.
    turbine — A device for converting the flow of a fluid (air, steam, water or hot gases) into mechanical motion that can be utilized to produce electricity.
    twist — In a wind generator blade, the difference in pitch between the blade root and the blade tip. Generally, the twist allows more pitch at the blade root for easier startup and less pitch at the tip for better high-speed performance.
    utility-scale wind — Wind energy projects greater than 100 kilowatts in capacity in which the electricity is sold rather than used on-site. This category includes large arrays of turbines owned by corporations and a single locally-owned wind turbine greater than 100 kilowatts in size.
    vane — A large, flat piece of material used to align a wind turbine rotor correctly into the wind. Usually mounted vertically on the tail boom. Sometimes called a tail.
    variable pitch turbine — A type of wind turbine rotor where the attack angle of the blades can be adjusted either automatically or manually.
    vertical axis wind turbine — A wind generator design in which the rotating shaft (axis of rotation) is perpendicular to the ground and the cups or blades rotate parallel to the ground. 
    watt — The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).
    watt-hour — A unit of electricity supply or consumption of one Watt over the period of one hour.
    waveform — The shape of the phase power at a certain frequency and amplitude.
    wind monitoring system — An instrument or group of instruments (including anemometers and wind vanes) that collectively measure various meteorological parameters, such as wind speed, wind direction and temperature at various heights above the ground.
    wind power class — A system designed to rate the quality of the wind resource in an area, based on the average annual wind speed. The scale ranges from 1 to 7 with 1 being the poorest wind energy resources and 7 representing exceptional wind energy resources.
    wind resource — The wind energy available for use based on historical wind data, topographic features and other parameters.
    wind resource assesment — The process of characterizing the wind resource and its energy potential for a site of geographical area. Wind resource maps for the U.S. are available here.
    wind rose — A circular plot used to portray certain characteristics about wind speed and direction observed at a monitoring location.
    wind shear — A term and calculation used to describe how wind speed increases with height above the surface of the earth. The degree of wind shear is a factor of the complexity of the terrain as well as the actual heights measured. Wind shear increases as friction between the wind and the ground becomes greater. Wind shear is not a measure of the wind speed at a site.
    wind speed — The rate at which air particles move through the atmosphere, commonly measured with an anemometer.
    wind vane — A device used to measure wind direction.
    windmill — A device that uses wind power to mill grain into flour. Informally used as a synonym for wind generator or wind turbine, and to describe machines that pump water with wind power.
    yaw — The rotation of a horizontal-axis wind turbine about its tower or vertical axis

Source: The Energy Bible and the U.S. Department of Energy's Energy Basics glossary
 







 
Accelerated Depreciation

With accelerated depreciation, wind energy projects can reduce the assessed value of their equipment on their financial balance sheets over a shorter period of time than other real assets.
Access Roads

Access roads allow access to a wind energy project site and individual wind turbine location during the development, construction, and the operational phases of a project. Access roads are constructed for Access roads – Access roads allow access to a wind energy project site and individual wind turbine location during the development, construction, and the operational phases of a wind energy project. Access roads are constructed for long-term use and built to accommodate heavy equipment and maintenance vehicles throughout the life of the project.
Air Pollution

The addition of harmful chemicals to the atmosphere that often result from the burning of fossil fuels, especially in internal-combustion engines.
Alternative Energy

Energy that is produced from alternative energy sources such as solar, wind, or nuclear energy that serve as alternative energy forms that produce traditional fossil-fuel sources such as coal, oil, and natural gas.
American Wind Energy Association (AWEA)

Formed in 1974, American Wind Energy Association (AWEA) is a Washington, D.C.-based national trade association representing wind power project developers, equipment suppliers, service providers, parts manufacturers, utilities, researchers, and others involved in the wind energy industry.
Anemometer

An instrument that measures the speed of wind. Meteorological towers used for wind assessment have anemometers attached to the end of booms that extend from the tower in different directions. The anemometer collects data including wind speed and direction that can be used to determine the long-term alternative energy potential and seasonal attributes of the wind at a specific location.
Base Load Power

The minimum amount of power that a utility or distribution company must make available to its customers. Base load power could also be described as the amount of power required to meet minimum demands based on reasonable expectations of customer requirements. Base load values typically vary from hour to hour in most commercial and industrial areas. Also known as base load demand.
Battery

For most wind energy projects, battery systems are cost-prohibitive and not considered commercially viable to include as part of a commercial or utility-scale wind farm development project for alternative energy. Also called galvanic battery or voltaic battery.
Beaufort Scale

A scale used to classify wind speeds, devised in 1805 by Admiral Francis Beaufort of the British Navy.* *Carl Wozniak
Blades

The large “arms” of wind turbines that extend from the hub of a generator. Most turbines have either two or three blades. Wind blowing over the blades causes the blades to “lift” and rotate. Blades vary in length by turbine rating (MW) and manufacturer (i.e. GE, Siemens, Vestas). The most common turbine blades in the United States range between 116’ and 164’ in length and are made of composite fiberglass or aluminum. Blades are designed to maximize the efficiency of wind energy production from a wind turbine generator.
Brake

Wind turbine generators contain a disc brake that can be applied mechanically, electrically, or hydraulically to stop the rotor for maintenance, repair, and in emergency situations.
Carbon Dioxide (CO2)

A heavy colorless gas that does not support combustion, dissolves in water to form carbonic acid, is formed especially in animal respiration and in the decay or combustion of animal and vegetable matter, is absorbed from the air by plants in photosynthesis, and is used in the carbonation of beverages.* *Merriam-Webster
Carbon Monoxide (CO)

A colorless, odorless, highly poisonous gas, formed by the incomplete combustion of carbon or a carbonaceous material, such as gasoline.
Circuit

The complete path of an electric current, including the generating apparatus, intervening resistors, or capacitors. Also known as an electric circuit.
Climate

The meteorological conditions, including temperature, precipitation, and wind, that characteristically prevail in a particular region.* *The Free Dictionary by Farlex
Conductor

The material through which electricity is transmitted. Wind turbine generators require electric lines with a conductor material that is almost always an aluminum alloy to transmit the wind energy produced to a location where the wind energy can be consumed and used as alternative energy.
Controller

Wind turbine generators contain a controller that starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Generally, wind turbine generators do not operate at wind speeds above 55 mph because they might be damaged by the high winds.
Converter

A piece of equipment found within wind turbine generators that converts a direct current (dc) voltage to another dc voltage.* *Energy Bible
Coverage Ratio

A type of accounting ratio that helps measure a company's ability to meet its obligations satisfactorily. A coverage ratio encompasses many different types of financial ratios. Typically, these kinds of ratios involve a comparison of assets and liabilities. The better the assets "cover" the liabilities, the better off the company is.
Curtailment

The forced shut-down of some or all of the wind turbine generators within a wind farm to mitigate issues associated with turbine loading export to the grid, or certain planning conditions. Curtailment is controlled by the regional transmission operator.
Deregulation

The process of changing policies and laws of regulation in order to increase competition among suppliers of commodities and services. The Energy Policy Act initiated deregulation of the electric power industry in 1992.
Direct Current

A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low-voltage and high-current. To be used for typical 120-volt or 220-volt household appliances, DC must be converted to alternating current, its opposite.* *Energy Bible
Distributed Energy Resources

Commonly referred to as “distributed generation”, and includes a variety of small, modular power-generating technologies that are used to create alternative energy and can be combined with wind energy management and storage systems used to improve the operation of the electricity delivery system.
Distributed Generation

Localized, on-site power, or “behind the meter” generation. Also known as distributed energy resources.
Distribution Electric Cooperatives

Service end-users, such as residences and businesses, that make up their membership and are often located in rural areas. Distribution Electric Cooperatives commonly purchase the majority of their power from Generation and Transmission Cooperatives that typically purchase power centrally for multiple Distribution Electric Cooperatives. Also known as Distribution & Transmission “D&T” Cooperatives.
Electric Cooperative

A type of electric utility for alternative energy in which all users are shareholders. This is common in rural areas that are expensive to serve because of the long distances between users. Frequently, the government contributes in various ways to rural cooperatives to reduce costs to individual owner/users. The two types of electric cooperatives are Distribution and Transmission “D&T” Cooperatives and Generation and Transmission “G&T” Cooperatives.
Electric Reliability Council of Texas (ERCOT)

Formed in 1970; one of eight Independent System Operators in North America, and the successor to the Texas Interconnected System (TIS). TIS originally formed in 1941 when several power companies banded together to provide their excess generation capacity to serve industrial loads on the Gulf Coast supporting the U.S. World War II effort. ERCOT is one of nine regional electric reliability councils under North American Electric Reliability Corporation (NERC) authority. NERC and the regional reliability councils were formed following the Northeast Blackout of 1965.* *Wikipedia
Electric Utility

An organization responsible for the installation, operation, or maintenance of an electric supply system. Electric utilities are generally owned by investors, municipalities, or cooperatives.
Electricity

The energy of moving electrons, the current of which is used as a source of power. Electricity can be used as alternative energy.
Electricity Generation

Wind turbines extract or generate wind energy and convert that wind energy into electricity that is measured in kilowatt hours (kWh).
Energy

The capacity for work. Energy can be converted into different forms, but the total amount of energy remains the same. Wind turbine generators extract wind energy and convert the energy into electricity.
Energy Policy Act of 1992 (EPAct)

A federal statute that established additional forms of non-utility generators. It also permitted non-generator-owning municipalities to purchase wholesale electricity to use as alternative energy.
Energy Storage

The process of storing or converting energy from one form to another for later use. An example of an energy storage device is a battery.
Environment

All the natural and living things around us including the earth, air, weather, plants, and animals.
Environmental Impact Statement (EIS)

Environmental Impact Statements are required in various parts of the United States as part of the permitting stage of wind project development. EIS studies review proposed electric utility projects and the potential for significant environmental impacts, including evaluation of alternatives and mitigation.
Federal Energy Regulatory Commission (FERC)

The Federal agency with jurisdiction over interstate electricity sales, wholesale electric rates, hydroelectric licensing, natural gas pricing, oil pipeline rates, and gas pipeline certification. FERC is an independent regulatory agency within the Department of Energy and is the successor to the Federal Power Commission.* *U.S. Energy Information Administration
Feed-In Tariff (FIT)

Feed-In Tariffs, also known as renewable energy payments, are often included in national, state, or local policy to encourage the adoption of renewable energy sources. FITs typically includes three key provisions: 1) guaranteed grid access, 2) long-term contracts for the electricity produced, and 3) purchase prices that are methodologically based on the cost of renewable energy generation and tend towards grid parity.
Fossil Fuel

Fuel sources or “feed-stocks”, including coal, oil, and gas, burned with oxygen to produce energy. Fossil fuels are the most common fuel source for the production of electricity in the United States.
Fuel

Any material that can be consumed to make energy such as alternative energy.
Gearbox

A wind turbine generator’s protective casing for the system of gears.
Generation and Transmission Cooperative (G&T)

G&T Cooperatives are power supply cooperatives owned by a group of distribution cooperatives. Generally, G&Ts generate power or purchase it from municipal, investor-owned utilities, or independent power producers.
Generator

A device for converting mechanical energy to electrical energy.
Gigawatt (GW)

A unit of power equal to 1 billion watts; 1 million kilowatts, or 1,000 megawatts.
Global Warming

A term used to describe the increase in average global temperatures due to the increase in greenhouse gases found within the Earth’s atmosphere.
Green Credit

One way to purchase renewable electric generation. Green credits divide alternative energy generation into two separate products: the commodity energy and the renewable attributes associated with the generation of the commodity energy. The green credit represents the renewable attributes of a single megawatt of renewable energy. Also known as green tags, renewable energy credits, or renewable energy certificates.
Green Power

A popular term for alternative energy produced from renewable energy resources such as wind energy.
Greenfield

A Greenfield site is a site on which a power plant has not previously existed. A Greenfield project is an alternative energy project in the early stages of development. Alternative energy projects advance past the Greenfield stage when they enter into power purchase agreement negotiations, construction, or commercial operations.
Greenhouse Effect

The heating effect resulting from long-wave radiation from the sun being trapped by greenhouse gases that have been produced from natural and human sources.
Greenhouse Gases

Gases that trap the heat of the sun in the Earth's atmosphere, producing the greenhouse effect; the two major greenhouse gases are water vapor and carbon dioxide; lesser greenhouse gases include methane, ozone, chlorofluorocarbons, and nitrogen oxides.* *Occupational Safety & Health Administration
Grid

A common term referring to an electricity transmission and distribution system. See also “power grid.”
High-Speed Shaft

The shaft within a wind turbine generator that is driven by the rotation of the turbine blades when propelled by wind to convert into wind energy.
Horizontal-Axis Wind Turbines

On wind farms there are wind turbine generators on which the axis of the rotor’s rotation is parallel to the wind stream and the ground to produce alternative energy like wind energy.
Hub

The central part of the wind turbine generator, which supports the turbine blades on the outside and connects to the low-speed rotor shaft inside the nacelle.* *Energy Bible
Independent Power Producers (IPPs)

Private or public companies that independently generate capacity or wholesale power for sale to utilities. IPPs are not utilities and do not own transmission lines or sell electricity on the retail market. IPPs are authorized by FERC to sell electricity at market-based rates.
Independent System Operator (ISO)

An organization formed at the direction or recommendation of the Federal Energy Regulatory Commission (FERC). In the areas where an ISO is established, it coordinates, controls, and monitors the operation of the electrical power system, usually within a single U.S. state, but sometimes encompassing multiple states. RTOs typically perform the same functions as ISOs, but cover a larger geographic area.* *Wikipedia
Inductive Reactance

Electrical current produces heat and/or a magnetic field (such as in the windings of a motor). We refer to the tendency for current flow and changes in flow to be influenced by magnetic fields as inductance. An AC circuit that contains only inductance, capacitance or a combination of the two is defined by the total opposition to current flow expressed in reactance. Inductance only affects current flow when the current is changing. Inductance produces a self-induced voltage (called a counter emf) that opposes changes in current. Obviously, the current changes constantly in an AC circuit. Inductance in an AC circuit, therefore, causes a continual opposition. This opposition to current flow is called inductive reactance.* *Occupational Safety & Health Administration
Installed/Rated/Nameplate Capacity

A wind farm’s capacity is the most common term used to describe the maximum possible electricity output from the wind farm. Generally, wind farms operate at capacity approximately 30%-40% of the time, depending on the average wind speed and profile of the wind at a specific site.
Interconnection

The process of linking a wind farm to the electric grid for the purpose of distributing electricity to a purchaser of the power produced. Interconnection rules vary by region and require permission from the local utility and regional transmission operator.
Inverter

An electrical device that converts direct current electricity to alternating current electricity. The common United States household consumes energy in the form of kilowatt hours provided by alternating current electricity.
Investment Tax Credit (ITC)

A form of financial incentive that a state or federal government can implement to help consumers reduce the up-front (“investment”) cost of an expensive one-time purchase, such as a small wind system. Typically, a portion of the system’s purchase price can be taken as a credit against (subtracted from) the purchaser’s income tax payment.* *AWEA
Kilowatt (kW)

A measure of the rate of electricity production. A wind farm's wind turbine generator’s size or production capacity is measured in megawatts and represents the rate at which the turbine can produce electricity at a given wind speed.
Kilowatt-Hour (kWh)

A measure of the amount of electricity produced over time. A wind farm's wind turbine generator that is rated at 1 MW (1,000 kw), operating for 1 year (8,760 hours), with a net generating capacity of 40%, produces 3,504 megawatt hours (MWhrs) per year. (1 MW x 8,760 hours x 0.40 = 3,504 MWhrs) The average United States home electric bill is expressed in kWh to reflect the amount of electricity consumed during the previous month.
Leading Edge

The surface part of a wind turbine blade on a wind farm that first comes into contact with the wind.
Levelized Costs of Energy (LCOE)

Levelized cost of electricity (LCOE) provides a common way to compare the cost of energy across technologies. LCOE takes into account the installed system price and associated costs such as financing, land, insurance, transmission, operation and maintenance, and depreciation. The LCOE is a true apples-to-apples comparison of electricity costs and is the most common measure used by electric utilities or purchasers of power to evaluate the financial viability and attractiveness of a wind energy project.
Mean Power Output (of a wind turbine)

The average power output of a wind farm at the mean wind speed of the wind farm.
Mean Wind Speed

The average wind speed over a time period at a specific height. The mean wind speed is used to determine the average amount of electricity produced over a time period for alternative energy potential.
Mechanical Energy

The energy possessed by an object due to its motion (kinetic energy) or its potential energy.
Median Wind Speed

The wind speed with 50% probability of occurring.
Megawatt (MW)

The standard measure of electric power plant generating capacity. One megawatt is equal to one thousand kilowatts or 1 million watts.
Megawatt-hour (MWh)

1,000 kilowatt-hours or 1 million watt-hours.
Merchant Power Plant

Merchant power plants differ from traditional rate-based power plants as to: 1) how they are financed and 2) where they sell the electricity they generate. A merchant power plant is funded by investors and sells electricity in the competitive wholesale power market. Since a merchant plant is not required to serve any specific retail consumers, consumers are not obligated to pay for the construction, operations, or maintenance of the plant.* *B.S. Meel
Met Tower

Metal structures that extend 60 meters above ground and have booms that extend in various directions and heights. The booms have anemometers located approximately 10 feet away from the tower that measure and verify the wind resource found within a certain area of land. Met towers commonly host a group of instruments, including anemometers and wind vanes, that measure wind speed, wind direction, and temperature at various heights above the ground.
Midwest Independent Transmission System Operator, Inc. (Midwest ISO or MISO)

An Independent System Operator (ISO) and the Regional Transmission Organization (RTO) that provides open-access transmission service and monitors the high-voltage transmission system throughout the Midwest United States and Manitoba, Canada. The Midwest ISO operates one of the world’s largest real-time energy markets and has 93,600 miles of transmission lines under its direction.* *Wikipedia
Nacelle

The nacelle sits atop the tower and contains the gearbox, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on and have the appearance of a small mobile home or travel trailer at ground level. Wind turbine generators contain a rotor, located at the base of the blades, that is attached to the nacelle, which sits atop the tower and includes the gearbox, low- and high-speed shafts, generator, controller, and brake.
National Environmental Policy Act (NEPA)

A United States environmental law that established a U.S. national policy promoting the enhancement of the environment and also established the President's Council on Environmental Quality (CEQ). NEPA's most significant effect was to set up procedural requirements for all federal government agencies to prepare Environmental Assessments (EAs) and Environmental Impact Statements (EISs). EAs and EISs contain statements of the environmental effects of proposed federal agency actions. NEPA’s procedural requirements apply to all federal agencies in the executive branch. NEPA does not apply to the President, to Congress, or to the federal courts.* *Wikipedia
National Renewable Energy Laboratory (NREL)

A United State Department of Energy research facility funded to research renewable and alternative energy technologies including  solar, biomass, hydro, geothermal, and wind energy.
Natural Gas

A hydrocarbon gas obtained from underground sources, often in association with petroleum and coal deposits. Natural gas is used as a heating fuel and for electricity generation and is generally thought of as the most direct competition to wind generated electricity. Both natural gas and wind energy are considered intermittent and not base load generation sources.
Negative Power Prices

Wind energy generators face very small costs of shutting down and starting back up, but they do face another cost when shutting down: loss of the Production Tax Credit and state Renewable Energy Credit revenue which depend upon generator output. It is economically rational for wind power producers to operate as long as the subsidy exceeds their operating costs plus the negative price they have to pay the market. Even if the market value of the power is zero or negative, the subsidies encourage wind energy power producers to keep churning the megawatts out.* *Industrial Wind Action Group
Net Metering

A term used to describe grid-connected alternative energy generation in which the local electrical source, including wind turbines or solar panels, is connected to the electrical meter so that excess generated electricity passes to the grid and causes the meter to run backwards.
Non-Renewable Fuels

Fuel sources or feed-stocks that cannot be easily renewed or reproduced and commonly include oil, natural gas, and coal.
North American Electric Reliability Corporation (NERC)

A non-profit corporation formed in 2006 as the successor to the North American Electric Reliability Council, established to develop and maintain mandatory reliability standards for the bulk electric system, with the fundamental goal of maintaining and improving the reliability of that system. NERC consists of regional reliability entities covering the interconnected power regions of the contiguous United States, Canada, and Mexico.* *U.S. Energy Information Administration
Open Access Same-Time Information System (OASIS)

An Internet-based system for obtaining services related to electric power transmission in North America. It is the primary means by which high-voltage transmission lines are reserved for moving wholesale quantities of electricity. The OASIS concept was originally conceived with the Energy Policy Act of 1992, and formalized in 1996 through Federal Energy Regulatory Commission (FERC) Orders 888 and 889.* *Wikipedia
Peak Demand/Load

The maximum energy demand or load in a specified time period. Electric utilities constantly manage the electric grid to prepare for peak energy demand/load condition
Peak Wind Speed

The maximum instantaneous wind speed that occurs within a specific period of time.
Pitch

Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.* *U.S. Department of Energy
PJM Interconnection, LLC (PJM)

A Regional Transmission Organization (RTO) which is part of the Eastern Interconnection grid operating an electric transmission system serving all or parts of Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, and the District of Columbia. PJM, headquartered in Valley Forge, Pennsylvania, is currently the world's largest competitive wholesale electricity market. Over 500 companies are members of PJM, which serves 51 million customers and has 165 gigawatts of generating capacity. With 1,271 generation sources, 56,000 miles of transmission lines and 6,038 transmission substations, PJM covers 13 states, delivering 700 terawatt-hours of electricity every year. * *Wikipedia
Power

Energy that is capable or available for doing work such as wind energy and alternative energy.
Power Grid

A common term referring to an electricity transmission and distribution system. Also known as utility grid.
Power Marketers

Companies that buy and sell electricity, but usually do not own or operate generation, transmission, or distribution facilities. Independent power producers often utilize the services of power marketers to move power from the point of generation to the purchaser of power.
Power Purchase Agreement (PPA)

A legal contract between an electricity generator (provider) and a power purchaser (host).The power purchaser purchases energy, and sometimes also capacity and/or ancillary services, from the electricity generator. Such agreements play a key role in the financing of independently owned (i.e. not owned by a utility) electricity generating assets.
Prevailing Wind Direction

The direction from which the wind predominantly blows as a result of the seasons, high and low pressure zones, the tilt of the earth on its axis, and the rotation of the earth.
Production Tax Credit (PTC)

The result of the Energy Policy Act of 1992, a commercial tax credit that applies to wholesale electrical generators of wind energy facilities based upon the amount of energy generated in a year. The renewable energy Production Tax Credit (PTC), a credit of 2.1 cents per kilowatt-hour, is the primary federal incentive for wind energy and has been essential to the industry’s growth. Other electricity generation technologies have their own forms of federal support, often permanent in tax law, so wind power would be disadvantaged in the absence of a PTC or other comparable incentive. Since its establishment in 1992, the PTC has undergone a series of short-term extensions, and has been allowed to lapse in three different years: 1999, 2001 and 2003. In February 2009, through the American Recovery and Reinvestment Act, Congress acted to provide a three-year extension of the PTC through December 31, 2012. Additionally, wind project developers can choose to receive a 30% Investment Tax Credit (ITC) in place of the PTC for facilities placed in service in 2009 and 2010, and also for facilities placed in service before 2013 if construction begins before the end of 2010. The ITC then qualifies to be converted to a grant from the Department of Treasury. The Treasury Department must pay the grant within 60 days of an application being submitted. This policy is designed to help the wind energy industry continue to finance projects during these challenging economic times. * ** * Energy Bible ** AWEA
Public Utility Regulatory Policies Act of 1978 (PURPA)

Opened up competition in the generation market with the creation of qualifying facilities. Later, the Energy Policy Act of 1992 (EPACT) removed some constraints on ownership of electric generation facilities and encouraged increased competition in the wholesale electric power business.* *U.S. Energy Information Administration
Public Utility Regulatory Policies Act of 1978 (PURPA)

Opened up competition in the generation market with the creation of qualifying facilities. Later, the Energy Policy Act of 1992 (EPACT) removed some constraints on ownership of electric generation facilities and encouraged increased competition in the wholesale electric power business.* *U.S. Energy Information Administration
Qualifying Facilities

PURPA facilitated the emergence of a group of non-utility electricity-generating companies called qualifying facilities or QFs. Under PURPA, small power producers receive status as a QF by meeting certain requirements for ownership, operating methods, and efficiency. Those requirements were established by the Federal Energy Regulatory Commission (FERC).* *U.S. Energy Information Administration
Regional Transmission Organization (RTO)

An organization that is responsible for moving electricity over large interstate areas. Like a transmission system operator (TSO), an RTO coordinates, controls and monitors an electricity transmission grid that is larger with much higher voltages than the typical power company's distribution grid. TSOs in Europe cross state and provincial borders like RTOs.* *Wikipedia
Renewable Energy

Energy that is derived from resources that are regenerative or that cannot be depleted including wind energy, solar, biomass, geothermal, and moving water. Also known as alternative energy.
Renewable Energy Credits (RECs)

A REC represents the property rights to the environmental, social, and other non-power qualities of renewable electricity generation. A REC can be sold separately from the electricity associated with a renewable energy generation source.
Renewable Energy Standard

The Renewable Electricity Standard (RES), also known as a Renewable Portfolio Standard (RPS), uses the free market to ensure that an increasing percentage of alternative energy and electricity are produced from renewable sources, like wind energy.The RES provides a predictable, competitive market, within which renewable energy generators compete with each other to lower prices. RES policies currently exist in 28 U.S. states, but not at the national level.
Resistance (R)

The property of a conductor, which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.* *Energy Bible
Restructuring (Electric Utility Restructuring)

Introducing competition into the generation phase of electricity production, with a corresponding decrease in regulatory control.
Rotor

The part of a wind turbine generator where blades and the hub come together.
Shadow Flicker

Shadow flicker is the term used to describe what happens when rotating turbine blades come between the viewer and the sun, causing a moving shadow. Shadow flicker is almost never a problem for residences near new wind farms, and in the few cases where it could be, it is easily avoided. For some who have homes close to wind turbines, shadow flicker can occur under certain circumstances and can be disruptive when trying to read or watch television. However, the effect can be precisely calculated to determine whether a flickering shadow will fall on a given location near a wind farm, and how many hours in a year it will do so. Potential problems can be easily identified using these methods, and solutions range from providing an appropriate setback from the turbines to planting trees to disrupt the effect. Normally, shadow flicker should not be a problem in the U.S. because at U.S. latitudes (except Alaska) the sun’s angle is not very low in the sky. If any effect is experienced, it is generally short-lived, as in a few hours over a year’s time.* *AWEA
Southwest Power Pool (SPP)

Eleven regional utilities who entered into an Inter-Company Agreement on December 14, 1941. The eleven companies were: Arkansas Power & Light (a subsidiary of Entergy), Louisiana Power & Light (a subsidiary of Entergy), Mississippi Power & Light, Southwestern Gas and Electric, Public Service Company of Oklahoma, Nebraska Power, Texas Power & Light, Southern Light and Power, Oklahoma Gas and Electric, Kansas Gas and Electric, and Empire District Electric.* *Wikipedia
Step-up Gearbox

Increases wind turbine electricity production in stages by increasing the number of generator revolutions produced by the rotor revolutions.
Tower

The physical structure in wind farms that supports the nacelle and is most often a steel cylinder with an internal ladder which provides access for maintenance and repairs. As wind speed increases with height, taller towers enable turbines to capture more wind energy and generate more electricity.
Transformer

Wind farms often include an electromagnetic device that changes the voltage of alternating current electricity. A transformer is the electrical equipment that allows for a wind farm to interconnect to electric lines.
Transmission Utility

The regulated owner/operator of only a transmission system as opposed the generation and transmission.
Tres Amigas Project

Focuses on uniting North America’s two major power grids (the Eastern Interconnection and the Western Interconnection) and one minor grid (the Texas Interconnection) to enable faster adoption of renewable energy and alternative energy (like wind energy) and increase the reliability of the U.S. grid.* *Wikipedia
Turbine

A term used for a wind energy conversion device that produces electricity. See also "wind turbine."
Turbulence

A swirling motion of the atmosphere that interrupts the flow of wind. Wind turbulence has a direct impact on the siting of a wind farm and the layout of turbines across the project footprint.
Unbundling

The process of separating a service into component parts (generation, transmission, distribution, ancillary services, etc) to allow customers to choose where to buy each service separately. Utility unbundling, overseen by regulators, generally requires utilities to ensure that the price of each service accurately reflects the cost of that service (plus a margin for profit).
Variable-Speed Wind Turbines

Wind turbine generators in which the rotor speed increases and decreases with changing wind speeds. Complex power control systems are required on variable speed turbines to insure that their power maintains a constant frequency compatible with the grid.
Volt (V)

A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.* *Energy Bible
Watt

The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).* *Energy Bible
Watt-hour (Wh)

A unit of electricity consumption of one watt over the period of one hour.
Wind

Moving air. The wind's movement is caused by the sun's heat, the earth, and the oceans, forcing air to rise and fall in cycles.
Wind Class

A system designed to rate the quality of the wind resource in an area, based on the average annual wind speed. The scale ranges from 1 to 7 with 1 being the poorest wind energy resources and 7 representing exceptional wind energy resources.* *Energy Bible
Wind Energy

Power generated by converting the mechanical energy of the wind into electrical energy through the use of a wind generator. See also "wind power."
Wind Farm

Wind farm is used in reference to the land, wind turbine generators, electrical equipment, and transmission lines for the purpose of generating wind energy and alternative energy.
Wind Power

Power generated by converting the mechanical wind energy into electrical energy through the use of a wind generator. See also "wind energy."
Wind Powering America (WPA)

A U.S. Department of Energy initiative designed to promote the use of wind energy across the country, with the goal of quadrupling U.S. wind energy capacity by 2010.* *Energy Bible
Wind Resource Assessment

The process of characterizing the wind resource and its wind energy potential for a specific site or geographical area.
Wind Rose

A diagram that indicates the average percentage of time that the wind blows from different directions, on a monthly or annual basis.
Wind Speed

The rate of flow of wind when it blows undisturbed by obstacles.
Wind Speed Frequency Curve

A curve that indicates the number of hours per year that specific wind speeds occur.
Wind Speed Profile

A profile of how the wind speed changes at different heights above the surface of the ground or water.
Wind Turbine Generator

A term used to describe the installed wind turbine including the tower, nacelle, rotor, hub, blades, foundation, and related components designed to convert wind energy into electricity.
Wind Turbine Noise

An operating modern wind farm at a distance of 750 to 1,000 feet is no noisier than a kitchen refrigerator or a moderately quiet room. The sound turbines produce is similar to a light whooshing or swishing sound, and is quieter than other types of modern-day equipment. Even in rural or low-density areas, where there is little additional sound to mask that of the wind turbines, the sound of the blowing wind is often louder. Exceptions to quiet operating turbines can occur in two instances: with older turbines from the 1980s and with contemporary turbines in some types of hilly terrain. Modern wind turbines have been designed to drastically reduce the noise of mechanical components so the most audible noise is the sound of the wind interacting with the rotor blade. However, in some hilly terrain where residences are located in sheltered dips or hollows downwind from turbines, turbine sounds may carry further and be more audible. This effect can generally be anticipated and avoided in the development process through adequate setbacks from homes.* *AWEA
Wind Turbine Rated Capacity

The amount of wind energy a wind turbine can produce at its rated wind speed.
Wind Velocity

The wind speed and direction in an undisturbed flow.
Windmill

A wind energy conversion system that is used to grind grain. However, the word windmill is commonly used to refer to all types of wind energy conversion systems.
Windpower Profile

The change in the power available in the wind due to changes in the wind speed or velocity.
Yaw Drive

Upwind turbines on a wind farm that face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind.* *U.S. Department of Energy
Yaw Motor

Powers the yaw drive.
 







 
Glossary of Wind Energy Terms

aggregation — Bundling several wind energy projects together so that they are treated as one larger project (when purchasing turbines, interconnecting, or maintaining a project, for example,) in order to spread out costs over more turbines or projects. This can have the effect of improving project economics.

airfoil — The cross section profile of the leeward side of a wind generator blade. Designed to give low drag and good lift. Also found on an airplane wing.

anemometer — An instrument used to measure the velocity, or speed, of the wind.

angle of attack — The angle of relative air flow to the blade in a wind turbine.

armature — The moving part of an alternator, generator or motor. In many alternator designs, it carries the magnets and is attached to the blades and hub. Also called a Rotor.

availability factor — The percentage of time that a wind turbine is able to operate and is not out commission due to maintenance or repairs.

balancing — With wind turbine blades, adjusting their weight and weight distribution through 2 axes so that all blades are the same. Unbalanced blades create damaging vibration.

Betz Coefficient — 59.3 percent. This is the theoretical maximum efficiency at which a wind generator can operate, by slowing the wind down. If the wind generator slows the wind down too much, air piles up in front of the blades and is not used for extracting energy.

blades— The flat panels on a wind turbine that are connected to a center shaft that converts the push of the wind into a circular motion in a wind turbine.

braking system— A device to slow a wind turbine's shaft speed down to safe levels electrically or mechanically.

capacity factor — A measure of the productivity of a wind turbine, calculated by the amount of power that a wind turbine produces over a set period of time, divided by the amount of power that would have been produced if the turbine had been running at full capacity during that same time interval.

chord — The width of a wind turbine blade at a given location along the length.

commercial scale wind — Wind energy projects greater than 100 kW where the electricity is sold rather than used on-site. This category can include large arrays of 100 or more turbines owned by large corporations, a single locally-owned wind turbine greater than 100 kW in size, or anything in between.

community wind —Locally-owned, commercial-scale wind projects that optimize local benefits. Locally-owned means that one or more members of the local community has a significant direct financial stake in the project other than through land lease payments, tax revenue, or other payments in lieu of taxes.

cut-in speed — The wind speed at which the turbine blades begin to rotate and produce electricity, typically around 10 miles per hour.

cut-out speed — The wind speed at which the turbine automatically stops the blades from turning and rotates out of the wind to avoid damage to the turbine, usually around 55 to 65 miles per hour.

distributed generation —A small-scale power generation technology that provides electric power at a site closer to customers than the central station generation. The term is commonly used to indicate non-utility sources of electricity, including facilities for self-generation.

downwind turbine —Refers to a Horizontal Axis Wind Turbine in which the hub and blades point away from the wind direction, the opposite of an Upwind turbine.

furling —The act of a wind generator yawing out of the wind either horizontally or vertically to protect itself from high wind speeds.

grid-connected system — A residential electrical system such as solar panels or wind turbines which are connected to the electrical grid. In such systems the grid serves as a battery backup for the residential system for those times when there is no sun or wind.

guy anchor — A concrete or metal base which attaches windtower guy wires securely to the earth.

guy wire — A strong metal cable or wire which attaches a tower to the ground.

horizontal axis wind turbine —A "normal" wind turbine design, in which the shaft is parallel to the ground, and the blades are perpendicular to the ground

hub — The central part of the wind turbine, which supports the turbine blades on the outside and connects to the low-speed rotor shaft inside the nacelle.

hybrid system — The combination of  multiple energy producing technologies such as photovoltaic systems combined with small wind turbine systems.

inverter — A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.

met Tower — A group of instruments (including anemometers and wind vanes) that collectively measure various meteorological parameters such as wind speed, wind direction, and temperature at various heights above the ground. met is short for meteorological.

nacelle — The structure at the top of the wind turbine tower just behind (or in some cases, in front of) the wind turbine blades that houses the key components of the wind turbine, including the rotor shaft, gearbox, and generator.

power curve	— The instantaneous power output of a specific turbine design at various wind speeds. Used with wind resource data to determine the potential for electricity generation at a project site.

Production Tax Credit (PTC— The result of the Energy Policy Act of 1992, a commercial tax credit that applies to whole-sale electrical generators of wind energy facilities based upon the amount of energy generated in a year.

rated wind speed — The wind speed at which the turbine is producing its nameplate-rated power production.  For most small wind turbines this is around 30 to 35 miles per hour.

rotor — Comprises the spinning parts of a wind turbine, including the turbine blades and the hub.

start-up speed — The windspeed at which a wind turbine rotor starts to rotate. It does not necessarily produce any power until it reaches cut-in speed.

tail boom — A strut that holds the tail (Vane) to the wind generator frame.

thrust bearing — A bearing that is designed to handle axial forces along the centerline of the shaft--in a wind generator, this is the force of the wind pushing back against the blades.

tower — The base structure that supports and elevates a wind turbine rotor and nacelle.

turbine — A device for converting the flow of a fluid (air, steam, water, or hot gases) into mechanical motion that can be utilized to produce electricity.

twist — In a wind generator blade, the difference in pitch between the blade root and the blade tip. Generally, the twist allows more pitch at the blade root for easier startup, and less pitch at the tip for better high-speed performance.

vane — A large, flat piece of material used to align a wind turbine rotor correctly into the wind. Usually mounted vertically on the tail boom. Sometimes called a Tail.

variable pitch turbine — A type of wind turbine rotor where the attack angle of the blades can be adjusted either automatically or manually.

vertical axis wind turbine — A wind generator design where the rotating shaft is perpendicular to the ground, and the cups or blades rotate parallel to the ground.

Watt — The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

waveform — The shape of the phase power at a certain frequency and amplitude.

wind monitoring system — Also termed met tower, a group of instruments (including anemometers and wind vanes) that collectively measure various meteorological parameters such as wind speed, wind direction, and temperature at various heights above the ground..

Wind Powering America (WPA) — A U.S. Department of Energy initiative designed to promote the use of wind energy across the country, with the goal of quadrupling U.S. wind capacity by 2010.

wind power class — A system designed to rate the quality of the wind resource in an area, based on the average annual wind speed. The scale ranges from 1 to 7 with 1 being the poorest wind energy resources and 7 representing exceptional wind energy resources

wind resource — The wind energy available for use based on historical wind data, topographic features, and other parameters.

wind rose — A circular plot used to define certain characteristics about wind speed and direction observed at a monitoring location.

wind shear — A term and calculation used to describe how wind speed increases with height above the surface of the earth. The degree of wind shear is a factor of the complexity of the terrain as well as the actual heights measured. Wind shear increases as friction between the wind and the ground becomes greater. Wind shear is not a measure of the wind speed at a site.

wind speed — The rate at which air particles move through the atmosphere, commonly measured with an anemometer.

wind vane — A device used to measure wind direction.

windmill — A device that uses wind power to mill grain into flour. But informally used as a synonym for wind generator or wind turbine, and to describe machines that pump water with wind power.

Back to Top
 







 Wind Energy Basics

Basic information on wind energy and wind power technology, resources, and issues of concern.
Wind Energy and Wind Power

Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetative cover. This wind flow, or motion energy, when "harvested" by modern wind turbines, can be used to generate electricity.
How Wind Power Is Generated

The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity to power homes, businesses, schools, and the like.
Wind Turbines

Wind turbines, like aircraft propeller blades, turn in the moving air and power an electric generator that supplies an electric current. Simply stated, a wind turbine is the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity.
Wind Turbine Types

Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the traditional farm windmills used for pumping water, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor. Most large modern wind turbines are horizontal-axis turbines.
Turbine Components

Horizontal turbine components include:

    blade or rotor, which converts the energy in the wind to rotational shaft energy;
    a drive train, usually including a gearbox and a generator;
    a tower that supports the rotor and drive train; and
    other equipment, including controls, electrical cables, ground support equipment, and interconnection equipment.

Wind turbine diagram - click for enlarged image.

Wind turbine diagram
Turbine Configurations

Wind turbines are often grouped together into a single wind power plant, also known as a wind farm, and generate bulk electrical power. Electricity from these turbines is fed into a utility grid and distributed to customers, just as with conventional power plants.

See Wind Energy Photos page for wind farm photographs.
Wind Turbine Size and Power Ratings

Wind turbines are available in a variety of sizes, and therefore power ratings. The largest machine has blades that span more than the length of a football field, stands 20 building stories high, and produces enough electricity to power 1,400 homes. A small home-sized wind machine has rotors between 8 and 25 feet in diameter and stands upwards of 30 feet and can supply the power needs of an all-electric home or small business. Utility-scale turbines range in size from 50 to 750 kilowatts. Single small turbines, below 50 kilowatts, are used for homes, telecommunications dishes, or water pumping.

See Wind Energy Photos page for wind turbine photographs.
Wind Energy Resources in the United States

Wind energy is very abundant in many parts of the United States. Wind resources are characterized by wind-power density classes, ranging from class 1 (the lowest) to class 7 (the highest). Good wind resources (e.g., class 3 and above, which have an average annual wind speed of at least 13 miles per hour) are found in many locations (see United States Wind Energy Resource Map). Wind speed is a critical feature of wind resources, because the energy in wind is proportional to the cube of the wind speed. In other words, a stronger wind means a lot more power.
Advantages and Disadvantages of Wind-Generated Electricity
A Renewable Non-Polluting Resource

Wind energy is a free, renewable resource, so no matter how much is used today, there will still be the same supply in the future. Wind energy is also a source of clean, non-polluting, electricity. Unlike conventional power plants, wind plants emit no air pollutants or greenhouse gases. According to the U.S. Department of Energy, in 1990, California's wind power plants offset the emission of more than 2.5 billion pounds of carbon dioxide, and 15 million pounds of other pollutants that would have otherwise been produced. It would take a forest of 90 million to 175 million trees to provide the same air quality.
Cost Issues

Even though the cost of wind power has decreased dramatically in the past 10 years, the technology requires a higher initial investment than fossil-fueled generators. Roughly 80% of the cost is the machinery, with the balance being site preparation and installation. If wind generating systems are compared with fossil-fueled systems on a "life-cycle" cost basis (counting fuel and operating expenses for the life of the generator), however, wind costs are much more competitive with other generating technologies because there is no fuel to purchase and minimal operating expenses.
Environmental Concerns

Although wind power plants have relatively little impact on the environment compared to fossil fuel power plants, there is some concern over the noise produced by the rotor blades, aesthetic (visual) impacts, and birds and bats having been killed (avian/bat mortality) by flying into the rotors. Most of these problems have been resolved or greatly reduced through technological development or by properly siting wind plants.
Supply and Transport Issues

The major challenge to using wind as a source of power is that it is intermittent and does not always blow when electricity is needed. Wind cannot be stored (although wind-generated electricity can be stored, if batteries are used), and not all winds can be harnessed to meet the timing of electricity demands. Further, good wind sites are often located in remote locations far from areas of electric power demand (such as cities). Finally, wind resource development may compete with other uses for the land, and those alternative uses may be more highly valued than electricity generation. However, wind turbines can be located on land that is also used for grazing or even farming.
For More Information

Much additional information on wind energy science and technology and wind energy development issues is available through the Web. Visit the Wind Energy Links page to access sites with more information. In particular, the DOE Wind Energy Technologies page has good information on wind energy basics, and is the source for much of the information presented here. The American Wind Energy Association web site has an excellent FAQ page with information about wind technology, and the The Danish Wind Industry Association web site has extensive information about wind energy and technology, including a 28-minute video introducing wind technology.
 







 Wind Energy: Glossary

Latest Publications
Basol A., Raheem A., Abhari R.S., Huber M.,
Full-Annular Numerical Investigation of the Rim Seal Cavity Flows Using GPU’S
Creation date/time: 22-04-2014 07:53
Chokani N., Subramanian B., Abhari R. S.
Instrumented Drone Measurements of 3D Flow Structure of Multi-MW Wind Turbines (Invited Paper)
Creation date/time: 26-03-2014 08:25

 

A | B | C | D | E | F | G | H | I | J | K  | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Active power:
Also known as ‘Real Power’ or ‘Power’, is a real component of the apparent power. Measured in watts and often-expressed in kW or MW.

Armature:
The moving part of an alternator, generator or motor. In many alternator designs, it carries the magnets and is attached to the blades and hub. Also called a Rotor.

Ampere-Hour:
A measure of energy quantity, equal to amperes times hours. Also used to measure battery capacity.

Anemometer:
A devise used to measure the velocity, or speed, of the wind. Ultrasonic, laser, hot wire and cup varieties are available. They are used in wind resource assessment, installed on masts and provide measurement inputs to wind turbine control systems, installed on nacelle of wind turbines.

Availability factor:
The percentage of time that a wind turbine is able to operate and is not out of commission
Betz Coefficient:

The theoretical maximum power efficiency of a wind turbine (0.593).

Blade:

The blades of a wind turbine are the wing-like structures attached to the hub.  As the air flow passes the blade, it generates aerodynamic lift that causes the rotor to rotate and makes power generation possible.

Blade Passing Frequency:
The frequency at which the blades of a wind turbine pass the tower.

Blade Element-Momentum Theory (BEM):
An aerodynamic theory linking the drag and lift forces experienced by each section of a wind turbine blade to the change in momentum of air passing through the rotor disc.


Braking System:

A device to slow a wind turbine’s shaft speed down to safe levels electrically or mechanically.

Brushes:

Devices for transferring power to or from a rotating object. Usually made of carbon-graphite.
Capacity Factor:
The amount of energy that a wind turbine actually produces in a year, divided by the amount of energy that would be theoretically produced if it was operating at its rated power 24 hours a day, 365 days a year.

Computational fluid dynamics:

Often abbreviated to CFD. A branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems involving fluid flows. Computers are used to perform the calculations required to simulate the interaction of liquids and gases with surfaces defined by boundary conditions.

Chord:

The width of a wind turbine blade at a given location along the length.

Cogging:

The cyclic physical resistance felt in some alternator designs from magnets passing the coils and gaps in the laminates. Detrimental to Start-up.

Cut-In Speed:
Minumum wind speed that a wind turbine starts operating.

Cut-out speed:

The wind speed at which the turbine automatically stops the blades from turning and rotates out of the wind to avoid damage to the turbine

Cycles per Second:

Measured in Hertz. In electricity, it is the number of times an AC circuit reaches both minimum and maximum values in one second.
Dump Load:

A device to which wind generator power flows when the system batteries are too full to accept more power, usually an electric heating element. This diversion is performed by a Shunt Regulator, and allows a Load to be kept on the Alternator or Generator.

Distributed generation:

The concept of using smaller, more dispersed generation facilities to produce power, rather than larger, centrally located power plants.

Direct drive:

A new generation of wind tubines that has recently emerged where the rotor is connected directly on a single shaft to a special high-torque, low speed generator without the use of a gear box.

direct-drive-wind-turbine

1. Main carrier

2. Yaw drive

3. Annular generator

4. Blade adapter

5. Rotor hub

6. Rotor blade

7. Anemometer



Dynamically-scaled Wind Energy Research:

The LEC facility is comprised of a water-towing tank, having dimensions of 40 x 1 x 1m and a carriage that tows a model wind turbine through the stationary water.
Econometric modelling:

Advanced wind simulation techniques aimed to improve performance, reduce costs and maintain reliability of wind energy projects.

Eddy Currents:

Currents that flow in a substance from variations in magnetic induction. See also Lenz Effect. Laminates are used to prevent eddy currents, which cause physical and electrical resistance in an alternator or transformer, therefore wasting power.

Efficiency:

The ratio of energy output to energy input in a device. For a wind turbine, it describes the amount of active electrical power generated as a percentage of the wind power incident on the rotor area.

Electromagnet:

A device made of wire coils that produces a magnetic field when electricity flows through the coils.
Fatigue:

The phenomenon by which a repeated loading and unloading of a structure causes its various components to gradually weaken and eventually fail. It is critical issue for a wind turbine due to the cyclic load it is exposed to.

Freewheeling:

a wind generator that is NOT connected to a Load is freewheeling, and in danger of self-destruction from overspeeding.

Full-scale Wind Turbine Research:

The LEC research approach uses an instrumented UAV, incorporating the use of fast response aerodynamic probe technology provides high spatial resolution (~5m) measurements of wind (speed, direction and turbulence).

Furling:

The act of a wind generator Yawing out of the wind either horizontally or vertically to protect itself from high wind speeds.

Furling Tail:

A wind generator protection mechanism where the rotor shaft axis is offset horizontally from the yaw axis, and the tail boom is both offset horizontally and hinged diagonally, thus allowing the tail to fold up and in during high winds. This causes the blades to turn out of the wind, protecting the machine.
Gearing:

Using a mechanical system of gears or belts and pulleys to increase or decrease shaft speed. Power losses from friction are inherent in any gearing system.
wind-turbine

1. Blade hub
2. Blade
3. Rotor locking system
4. Main shaft
5. Oil cooler
6. Hydraulic unit
7. Gearbox
8. Mechanical disk brake
9. Generator
10. Air cooler for generator
11. Ultrasonic sensors
12. High voltage transformers
Gearbox:
The gearbox in a wind turbine is used to convert the low speed, high torque rotation of the rotor to a high speed, low torque rotation suitable for driving the generator to produce alternating current at the correct grid frequency. Not all modern wind turbines have gearboxes (see Direct Drive).

Generator:
The generator is an electrical machinery that  converts  rotating motion of rotor into alternating current electrical power. Most fixed (or partially variable) speed wind turbines have induction generators, whereas variable speed wind turbines have synchronous generators.


Governor:

A device that regulates the speed of a rotating shaft, either electrically or mechanically.

Grid:

Also termed transmission system, the network of power lines and associated equipment required to deliver electricity from generators to consumers.
High Voltage Direct Current (HVDC):
It is proposed that in linking offshore wind farms to land by buried cable, HVDC links will be used instead of alternating current power transmission.

High-Speed Shaft:
In wind turbines with gearboxes, the high-speed shaft is the shaft connecting the gearbox to the generator.


Hub:
The center of a wind generator rotor, which supports the turbine blades on the outside and connects to the low-speed rotor shaft inside the nacelle.
Hybrid systems are especially popular in remoteareas that are not connected to the electricity grid.
Immersed boundary method:
An approach used to model and simulate mechanical systems in which elastic structures (or membranes) interact with fluid flows.

Inverter:
A power electronic device used to convert direct current to alternating current. An inverter will often form part of the power electronics used with variable speed wind turbine along with a rectifier.
KWh (kilowatt-hour):
A unit of energy that measures the amount of power produced or used over a 1-hour time interval.
Leading Edge:

The edge of a blade that faces toward the direction of rotation.

Levelised costs:

the present-day average cost per kWh produced by the turbine over its entire lifetime, including all costs – (re-)investments, operation and maintenance. Levelised costs are calculated using the discount rate and the turbine lifetime.

Lift:

The force exerted by moving air on asymmetrically-shaped wind generator blades perpendicular to the direction of relative movement. Ideally, wind generator blades should produce high Lift and low Drag.


Low-Speed Shaft:
Low-speed shaft connects the rotor to the gearbox in wind turbines with gearboxes.
M
Nacelle: 
The structure at the top of the wind turbine tower just behind (or in some cases, in front of) the wind turbine blades that houses the key components of the wind turbine, including the rotor shaft, gearbox, and generator.
O
Park Effect:
The effect whereby wind turbines positioned together in large wind parks each produce less energy than they would if in the same position on their own, due to the wind shadows of the other wind turbines in the park.

Power Curve:
A plot of power output vs. wind speed that is typical for a specific wind turbine model

Q
Rated Power:
The maximum power output that can be generated by a wind turbine. This is dictated by the generator size and loads that the wind turbine can bear. Choice of rated power for a site is a balance dictated by the amount of energy available in the wind at different wind speeds and the cost of increasingly large and powerful WTs.

Root:

The area of a blade nearest to the hub. Generally the thickest and widest part of the blade.
Start-Up:

The windspeed at which a wind turbine rotor starts to rotate. It does not necessarily produce any power until it reaches cut-in speed.


Swept Area:
The area swept out by the wind turbine blades as the rotor rotates - the area of the rotor disc - is also known as the rotor area.
Tail boom:

A strut that holds the tail (Vane) to the wind generator frame.

Thrust bearing: 

A bearing that is designed to handle axial forces along the centerline of the shaft--in a wind generator, this is the force of the wind pushing back against the blades.

Tip:

The end of a wind generator blade farthest from the hub.


Tip Speed Ratio:
Ratio of blade tip speed to speed of incoming wind speed.

Tower:

A structure that supports a wind generator, usually high in the air.

Turbine:

A device for converting the flow of a fluid (air, steam, water, or hot gases) into mechanical motion that can be utilized to produce electricity.

Turbine lifetime:

This is the expected total lifetime of the turbine (normally 20 years).
UAV:
The UAV is an acronym for Unmanned Aerial Vehicle, an aircraft with no pilot on board.  UAVs can be flown remotely by a pilot at a ground control station, or can fly autonomously on pre-programmed flight plans to map technical data over a fixed location.
The UAV is often referred to as a drone, however the term drone actually refers to an unmanned air vehicle that is not generally intended to be reused, such as a vehicle used for target practice or a vehicle that is intended to be crashed.
Vertical Axis Wind Turbine (VAWT):
An alternative design of wind turbine where the rotor rotates around a vertical axis, rather than a horizontal one.
Wind power density:

A way to define the amount of wind power contained in a given area for use by a wind turbine, measured in watts per square meter.

Wind resource:

The wind energy available for use based on historical wind data, topographic features, and other parameters.

Wind rose:

A circular plot used to define certain characteristics about wind speed and direction observed at a monitoring location. The wind rose plot is a circle divided into 8, 12, or 16 “pie wedges” that represent different directions, such as on a compass.

Wind speed:

Wind speed, or velocity, is simply the rate at which air particles move through the atmosphere, commonly measured with an anemometer.
X
Yaw:

Rotation parallel to the ground. A wind generator Yaws to face winds coming from different directions.

Yaw Axis:

Vertical axis through the center of gravity.
Z
Alternative LEC Glossaries:
General Glossary | Wind Energy Glossary | Turbomachinery Glossary | Plasma Science Glossary |Instrumentation Glossary 







 16 point compass rose 	To describe the location of wind farms, in relation to well known towns and cities, I have used the 16 point compass rose. In this system, north-east (NE) is the direction half way between north and east (45° 'true') and nor-nor-east (NNE) is the direction half way between north and north-east (22.5° true), etc.
Putting it another way, starting at north and moving clockwise we have: N, NNE, NE, ENE, E, ESE, SE, SSE, S, SSW, SW, WSW, W, WNW, NW, NNW, and back to N.
AEMO 	The Australian Energy Market Operator "delivers an array of gas and electricity market, operational, development and planning functions". The data provided on power generation are difficult to make use of, see ALG, below. AEMO replaced NEMMCO.
ALG 	Australian landscape guardians (ALG), a group opposed to wind power and apparently not much concerned about any other 'threats to Australian landscapes'. They have connections to the Waubra Foundation, the mining industry, and the right wing think tank the Institute of Public Affairs and its misleadingly named off-shoot the Australian Environment Foundation.
Amp or Ampere 	The SI unit of electric current; the symbol is A or I. Compare to volt.
Anemometer 	A device used to measure wind speed
Annual production 	As used in these pages, the annual average energy production of a particular wind farm. Generally measured in GWh.
Attenuation 	Air absorbs some energy from sound waves (this is on top of the reduction in sound energy due to the inverse-square-law). Higher frequencies are attenuated much more than lower frequencies; thus the sound of a nearby lightning strike is a sharp 'crack', while distant thunder is a low rumble. The rate of attenuation will be greater if there is dust or mist in the air. A calculator for attenuation of sound in air is at Tontechnik-Rechner.
Availability	The percentage of time that the particular wind farm, or wind turbine, is in an operational condition.
Axial-flow turbine 	A turbine in which the air moves in the direction of the axis of rotation of the turbine. All utility scale wind turbines in Australia are axial flow. Compare to cross-flow turbine.
Base load 	The exact meaning of Base-load seems ill-defined. The term base-load power was used to mean the load that was always present and could be supplied by power that comes from generators such as coal-fired stations, which can only very slowly vary their generation, and therefore were suited to running night and day at a steady rate.

It has often been said that wind power cannot provide base-load power. What seems to be meant by this is that wind power is not dispatchable; that is, it is not available on demand. See also Timing of wind power generation.
Betz Limit 	The maximum theoretical power that can be captured by a wind turbine from the wind. Equal to 59.3% of the wind energy.
Blade 	
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The part of a turbine that air reacts against to cause the turbine to spin. Most electricity-generating turbines have two or three blades (all utility-scale turbines in Australia have three), whereas water-pumping wind turbines – the traditional windmills – will usually have around 20.
Capacity factor 	Sometimes called load factor; the percentage of potential generation that is actually achieved. (See also Wind is variable.) For example; A wind farm consisting of ten 2 MW turbines could theoretically generate 175 200 MWh of electricity per year (10×2×24×365=175 200) if all the turbines were to work at 100% of their capacity 100% of the time. In practice turbines do not work at full capacity all the time (no power generation system does) and such a wind farm in Australia would probably generate around 60 000MWh per year (a capacity factor of 34%). See also notes on actual capacity factors in Australian wind farms.

A typical capacity factor for a small photovoltaic system, for comparison, is around 18%. The capacity factor used for wind farms is very similar to the load factor used in electrical engineering.
Cross-flow turbine 	A turbine where the flow of air is at right angles to the axis of rotation of the turbine. Also see vertical axis turbine and compare to axial-flow turbine.
Current 	The rate at which electricity flows in a conductor. Analogous to the volume of water flowing through a pipe. Measured in Amperes, or Amps. Compare to volt.
Darrius rotor or turbine 	A form of vertical-axis wind turbine that uses thin blades
dB, Decibel 	
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The decibel is a unit of sound volume, or more accurately, sound pressure. It is a logarithmic scale, so 40dB is ten times as 'loud' as 30dB, 50dB is ten times as loud as 40dB and 100 times as loud as 30dB, etc. With exposure over a very long period a sound level of 85dB or over can cause permanent damage to hearing; higher volumes can cause damage with shorter exposure. (Also see dB(A), below.)
dB(A) 	Sound volume, as perceived by humans, is dependent on the pitch of the sound. A sound reading shown as dB(A), or dBA, indicates that the scale is adjusted to match human perception. The dB(A) scale adjusts absolute dB readings of sound levels from very low frequency sounds, infrasound, downward because of this human perception. So an infrasound level of, say 110dB on the absolute scale, would be adjusted to something well below 110 on the dB(A) scale.
Diameter 	When applied to a wind turbine it is the diameter of the area swept by the turbine blades: the diameter of the Swept area. The blade length will be less than half of the diameter.
Dispatchability 	This term is used in the electricity supply industry to describe how readily power generation is increased or decreased to follow changes in demand. Some forms of gas-fired generators are the most dispatchable (flexible), coal-fired generators are only slowly varied, while nuclear is usually run at a constant rate. Output from wind and solar PV installations are normally entirely dependent on how much wind there is or the brightness of the light.

Many forms of generation (or forms of energy storage) can be varied, but there is an economic compromise between producing the maximum amount of power as often as posible or producing a smaller amount of power when prices are higher.
Efficiency of wind turbines 	The Betz Limit gives a theoretical maximum to wind turbine efficiency of 59.3%. The efficiency of wind turbines varies greatly – must vary greatly – depending on wind speed; it is discussed in more depth elsewhere on these pages.
Energy 	Energy in physics is the capacity for doing work. Compare to Power. In the SI metric units energy is measured in Watt-hours (Wh), kilowatt-hour (kWh), etc. As examples, an amount of energy is used to pump a quantity of water from a low place to a high place; an amount of energy is required to move a vehicle from point A to point B; an amount of energy is required to boil a litre of water. Also see Units of energy. Many people, even in the energy business, confuse power and energy.
Energy return on investment 	Defined as the ratio between the useful energy got out of a process against the energy needed for that process; in simple terms, energy out against energy in.
EPC 	Engineering, Procurement and Construction; refers to the major sections in the setting-up of a wind farm.
ESIPC (SA) 	The Electricity Supply Industry Planning Council has been established to monitor the electricity supply industry in South Australia. At 1 July 2009 ESIPC became a part of AEMO.
Exawatt-hour, EWh 	A unit of energy equal to one billion billion (1018) Watt-hours. Also see Metric system multipliers.
Expected life 	A wind turbine and a wind farm has a limited life expectancy. Parts wear out and, in a fast developing field such as wind power, machinery becomes out-dated. Underground electrical cabling deteriorates with time. Some parts can be replaced as they wear or fail, but there comes a time when the most economic option is to replace, or scrap, the whole wind farm. We in Australia must be careful that failed turbines never litter our ridge-lines.
Footing 	The footing is the base, usually concrete, that secures the turbine in place. This is sometimes called the foundation, but that term is more accurately applied to the geological formation around the footing. There are two main types of wind turbine footings. If a turbine is built on bed-rock it can make use of rock anchors to secure a relatively small concrete footing (about 220 tonnes for a 2MW turbine) to the underlying bed-rock. If there is no shallow bed-rock, or the bed-rock is shattered, then heavier footings (gravity footings: about 800 tonnes for a 2MW turbine), that are capable of holding the turbine in place without any attachment to underlying materials, must be used.
Furling 	A method of preventing damage to horizontal-axis wind turbines by automatically turning them out of the wind using a spring-loaded tail or other device. Utility-scale wind turbines do not use such devices; they are computer-controlled.
Gigawatt, GW 	A unit of power equal to one billion (109) Watts. Also see Metric system multipliers.
Greenhouse gas saving 	
  	
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The amount of carbon dioxide that would have been released into the atmosphere by a fossil fuel fired power station had the particular wind farm not been generating power. Every MWh of electricity generated by a wind farm replaces a MWh that would have been generated by some other means. In the case of a coal fired power station around one tonne of carbon dioxide is released into the atmosphere for every MWh or electricity generated. See CO2 abatement for more information.
Greenhouse intensity 	
  	
Fuel	Kg CO2
Natural gas	0.45
Oil	0.5
Black coal	0.8
Brown coal	1.2
Alternatively Carbon intensity. A measure of the amount of greenhouse gas released into the atmosphere in relation to the amount of useful energy (usually electricity) produced. Some typical values for common fossil fuels, in kilograms of CO2 released per kWh of power generated, are given in the table on the right.

The higher the number for greenhouse intensity, the 'dirtier' the fuel is considered to be in relation to climate change.

The greenhouse intensity of wind power is less than 0.05.

(Also see Carbon intensity.)
Gigawatt-hour, GWh 	A unit of energy equal to one billion (109) Watt-hours. Also see Metric system multipliers.
HVDC 	High voltage direct current is used to transmit large amounts of power over long distances; there are smaller power losses and the construction cost of a HVDC line is less than that of a more conventional high voltage alternating current line. HVDC could be used to advantage for some of the longer transmission lines in Australia, especially if full use it to be made of Australia's great wind power potential. Also see Wikipedia.
Horizontal-axis turbine 	The most common form of wind turbine, in which the axis is parallel to the direction of the wind. Another name for a axial-flow turbine.
Hub 	The section which connects the turbine blades to the main shaft. At construction it is usually attached to the blades at the base of the turbine tower and then the whole assembly is lifted in one piece.
Infrasound 	According to the International Electrotechnical Commission's (IEC's) IEC 1994, infrasound is: Acoustic oscillations whose frequency is below the low frequency limit of audible sound (about 16 Hz). However this definition is incomplete as infrasound at high enough levels is audible at frequencies below 16 Hz. Infrasound in relation to health is discussed elsewhere on this site. Also see Wind turbine noise: Infrasound.
Installed capacity 	The amount of electricity that will be generated by a wind farm when all its turbines are generating at their full capacity.
Inverse square law 	This physical law has been known since the seventeenth century and applies to things like gravitation, electrostatics, light and sound. It describes how the strength of something such as sound decreases with the distance from the source; putting it simply, doubling the distance from the source causes the strength (loudness) to decrease to a quarter, trebling the distance reduces the strength to a ninth, four times the distance a sixteenth the strength, etc.

The inverse square law applies to anything that radiates from a distinct source.
Katabatic wind 	A cold wind that flows downhill and is powered by gravity. The term is generally used for winds that blow off the Antarctic Plateau toward the coast, although it could also be applied to gully winds.
kilowatt, kW 	A unit of power equal to one thousand Watts. Also see Metric system multipliers.
kilowatt-hour, kWh 	A unit of energy equal to one thousand Watt-hours. One kWh is sufficient to heat about 11 litres of water from room temperature (20°) to boiling point, or to run a 20 Watt light bulb for 50 hours. Also see Metric system multipliers.
Load factor 	An electrical engineering term very similar to capacity factor.
Latitude 	Distance south of the equator expressed in degrees. In these pages I have used decimal degrees rather than degrees, minutes and seconds because I believe the latter to be archaic: as pounds, shillings and pence are archaic currency.
Longitude 	Distance east of the Prime Meridian expressed in degrees. See also latitude, above.
Minimum operational wind speed 	
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The lightest breeze that a particular turbine can use. See also Productive wind speeds and wind speed range of turbines.
Minimum wind speed for full output 	The lightest wind sufficient for a particular turbine to produce its maximum rated electricity generation. Also see wind speed range of turbines.
MRET 	Mandatory Renewable Energy Target; (as of March 2015 Australia's MRET is in question because of the Abbott government's opposition to renewable energy).
Megawatt, MW 	A unit of power equal to one million Watts. One MW is enough power for around 430 electric kettles (2300W) or 45 000 compact fluorescent light bulbs (22W each). Also see Metric system multipliers.
Megawatt-hour, MWh 	A unit of energy equal to one thousand kWh one million Watt-hours. Also see Metric system multipliers.
Nacelle 	That part of the turbine that houses the gearbox (not all wind turbines have a gearbox), electrical generator, cooling system etcetera, at the top of the tower.
NEMMCO 	The National Electricity Market Management Company is the market operator of the National Electricity Market (NEM) and the system operator of the national grid. Has been replaced by AEMO.
Nocebo effect 	The placebo effect is when people who undergo some treatment that could not physically or chemically alleviate their symptoms experience an improvement just because they believe that the treatment is helping them. The Nocebo effect is the opposite. It is when people become ill because of an unfounded belief that something is causing them to be ill. The Skeptic's Dictionary has a fuller explanation.
Peak load 	Peak load (or peak demand) is that time when the demand for electricity is at its greatest. In Australia it tends to come at around 6pm on exceptionally hot days, when many people are coming home from work, switching on air conditioners, and preparing dinner. It is important because it is the time when both the electrical generation system and transmission system is under greatest stress. They are largely designed with the aim of coping as well as possible with peak demand. The wide-spread take-up of household solar PV power has tended to make the time of peak load come later in the day than previously.
Petawatt-hour, PWh 	A unit of energy equal to one million billion (1015) Watt-hours. Also see Metric system multipliers.
Power 	Compare to energy. Power is a flow of energy; an amount of energy per unit time. In the SI metric units, it is measured in Watts (W), kilowatts (kW), etc. Also see Units of Power. As examples, an amount of power is required to push a car at a given speed (under specific conditions) and an amount of power is required to run an electric jug. Many people, even in the energy business, confuse power and energy.
Power purchase agreement 	
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The operator of a wind farm makes an agreement to sell power to a distributor of electricity. This is a Power purchase agreement. In these pages I have usually given only the name of the distributor who has agreed to purchase the power against this phrase.
Price elasticity of electricity demand 	A measure of how electricity demand responds to changes in the price of electricity. For example the AEMO in 2011 estimated the price elasticity in South Australia "to be -0.25, with slightly less than half this applying to peak demand (that is, a 4% real rise in prices is expected to lead to a 1% reduction in sales and a 0.5% reduction in peak demands)".
Productive wind speeds 	That range of wind speeds that are useable by a particular wind turbine for electricity generation. The power available from wind is proportional to cube of the wind's speed: double the speed, eight times the energy. So as the speed of the wind falls the amount of energy that can be got from it falls very rapidly. On the other hand, as the wind speed rises, so the amount of energy in it rises very rapidly; very high wind speeds can overload a turbine. Productive wind speeds for a modern turbine might be from around 2.5m/sec to 35m/sec (9km/hr to 125km/hr). See also Survival wind speed
Renewable energy 	Energy that is produced from a renewable source, such as sunlight, flows of wind or water, or sustainably grown plants.
Rock anchor 	If suitable rock is situated beneath the turbine footing steel rods are used to anchor the turbine and footing to the underlying bedrock, reducing the amount of concrete that would otherwise be necessary. If there is no bedrock within a few metres of the surface, or if the bedrock is highly weathered or fractured, then gravity footing are needed.
Rotor 	The blades and hub at the centre of the blades - the part that rotates in front of the Nacelle.
Rotor diameter 	The diameter of the circle swept-out by the tips of the turbine's blades.
Savonius turbine or rotor 	A type of vertical-axis turbine that uses half-drum shaped 'blades' to catch the wind and turn a shaft. Generally a low-speed turbine with high torque, usually used for water pumping.
Shut down wind speed 	The maximum wind speed at which a particular turbine can generate electricity. With higher wind speeds the turbine must be shut-down to avoid damage.
SOO; Statement of opportunities, NEMMCO 	
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NEMMCO state that "The SOO is intended to assist existing and potential National Electricity Market (NEM) participants when assessing the future need for:
- electricity supply capacity;
- demand-side participation (DSP); and - transmission network augmentation in ssupport of NEM operations.
The SOO incorporates the Annual National Transmission Statement (ANTS). The SOO is published each year in October and can be downloaded from the NEMMCO Net site.
Spinning reserve 	In order to assure electrical supply it is necessary to keep some generation plant (usually fossil-fuelled) running, but not generating, so that it will be able to be brought into production at short notice. (The need for spinning reserve would be greatly reduced by introducing Supply Dependent Load, which is discussed in my Sustainable Electricity page.)
Survival wind speed 	The maximum wind speed that a turbine is designed to withstand before sustaining damage. See also Productive wind speeds
Swept circle and swept area 	The circle through which the turbine blades rotate and the area of that circle.
Terawatt-hour, TWh 	A unit of energy. One TWh is a million-million Watt-hours. Also see Metric system multipliers.
Tip-speed ratio 	The ratio of the blade tip speed to wind speed
Turbine 	A device that converts the energy in a stream of moving fluid into mechanical energy.
Turbulence 	Airflow that varies in speed and direction rapidly and violently that can cause damage to wind turbines. Often caused by objects such as trees or buildings.
Vertical-axis turbine 	A wind turbine with the axis or main shaft mounted vertically. This type of turbine does not have to turn to face the wind. Types include the Darrius and Savonius.
Volt, voltage 	The volt is the SI unit of electric potential. Voltage is analogous to the pressure of water in a pipe. Compare to Amp and current.
Watt 	The basic SI metric unit of power; equal to one Joule of work performed per second; also, in electricity, the power dissipated in an electrical conductor carrying one ampere current between points at one volt potential difference. Also see Units of power.
Watt-hour, Wh 	A unit of energy, generally electrical energy, equal to a flow of power of one Watt for a period of one hour.
Wind farm 	An integrated group of wind turbines that feed electricity into one or more electrical sub-stations and thence, usually, into the electricity grid.
Wind turbine 	A turbine designed to convert the energy in a stream of moving air into mechanical, and then electrical, energy.

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Index
16 point compass rose
AEMO
ALG
Amp
Anemometer
Annual production
Attenuation
Availability
Axial-flow turbine
Base load
Betz Limit
Blade
Capacity factor
Carbon intensity
Cross-flow turbine
Current
Darrius rotor
dB
dB(A)
Diameter
Dispatchability
Energy return on investment
EPC
ESIPC
EWh
Efficiency of turbines
Energy
Exawatt-hour
Expected life
Footing
Furling
GWh
Gigawatt
Gigawatt-hour
Gravity footing
Greenhouse gas saving
Greenhouse intensity
HVDC
High voltage direct current
Horizontal-axis turbine
Hub
Infrasound
Installed capacity
Inverse square law
Katabatic wind
Kilowatt
Kilowatt-hour
Latitude
Load factor
Longitude
MRET
MWh
Megawatt
Megawatt-hour
Minimum operational wind speed
Minimum wind speed for full output
NEMMCO
Nacelle
Nocebo effect
Peak demand
Petawatt-hour
Power
Power purchase agreement
Price elasticity
Productive wind speeds
PWh
Renewable energy
Rock anchor
Rotor
Rotor diameter
SOO
Savonius turbine
Shut down wind speed
Spinning reserve
Survival wind speed
Swept area
Swept circle
TWh
Terawatt-hour
Tip-speed ratio
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Turbine
Turbine footing
Turbulence
Vertical-axis turbine
Volt
Watt
Watt-hour
Wind farm
Wind turbine
kWh