wx Ed

	Before Passing	While Passing	After Passing
Winds	south-southwest	gusty; shifting	west-northwest
Temperature	warm	sudden drop	steadily dropping
Pressure	falling steadily	minimum, then sharp rise	rising steadily
Clouds	increasing: Ci, Cs and Cb	Cb	Cu
Precipitation	short period of showers	heavy rains, sometimes with hail, thunder and lightning	showers then clearing
Visibility	fair to poor in haze	poor, followed by improving	good, except in showers
Dew Point	high; remains steady	sharp drop	lowering

Before Passing

While Passing

After Passing

Winds

south-southwest

gusty; shifting

west-northwest

Temperature

warm

sudden drop

steadily dropping

Pressure

falling steadily

minimum, then sharp rise

rising steadily

Clouds

increasing: Ci, Cs and Cb

Precipitation

short period of showers

heavy rains, sometimes with hail, thunder and lightning

showers then clearing

Visibility

fair to poor in haze

poor, followed by improving

good, except in showers

Dew Point

high; remains steady

sharp drop

lowering

	This picture shows an example of a high and low pressure system. At the surface, winds flow counterclockwise (cyclonically) around low pressure, and clockwise (anticyclonically) around high pressure. The actual pressure of these systems can be measured in either inches of mercury (e.g., 30.10) or millibars (e.g., 1004 mb). Lines of equal pressure between highs and lows are called "isobars". Surface winds generally flow at an angle to the isobars from high to low pressure.
	Here, a typical surface weather map shows winds rotating counterclockwise around a low pressure system. Each reporting station's observation gives wind direction and speed, temperature, dewpoint, and pressure at that station.

Surface low pressure systems usually have fronts associated with them. A front represents a boundary between two air masses that contain different temperature, wind, and moisture properties. Here, a cold front is shown which can be present any time of the year, but is most pronounced and noticeable during the winter. Air normally is warmer ahead of a cold front and colder behind it. With a cold front, cold air advances and displaces the warm air since cold air is more dense (heavier) than warm air.

This vertical cross-section of a cold front shows cold air behind the cold front (dark blue lines) advancing into warmer air ahead of the front. Where the two air masses meet, convergence often occurs which can result in upward motion of air parcels. If the air contains enough moisture, rain can occur. If the air also is unstable, thunderstorms can develop as well. This is a simplified view of a cold front. Sometimes, fronts aloft (above the surface) can result in precipitation ahead of cold fronts.

Warm fronts also are common, especially from Fall through Spring when larger temperature differences exist across the United States. Relatively cool or cold air is present ahead of a warm front with warmer air behind the front, i.e., the opposite from that of cold fronts. However, while cool air at the surface exists ahead of a warm front, relatively warmer air often is located above it as the warmer surface air behind the front rises up and over the cool air below. If enough moisture is present, this can result in precipitation along and ahead of the front. With a warm front, the cool air ahead of it must retreat before the warm air behind it can advance.

A vertical cross-section of a warm front (dark red lines) shows how surface warm air behind the warm front flows upward in a slantwise manner over top low-level cool or cold air ahead of the position of the surface warm front. This causes clouds and precipitation ahead of the surface warm front, assuming enough upward motion of air parcels and available moisture.

A stationary front is similar to a warm front, i.e., warm air is present behind it (to its south) with cool air ahead of it (to its north). However, while a warm front shows movement, a stationary front shows little or no movement as the cooler, more dense air remains in tact and does not retreat. Notice the winds from the northeast in the picture on the right above. Since these winds are blowing toward the front somewhat, this cooler air is not retreating, so the warmer air to the south of the front cannot lift northward. With a stationary front, a balance usually exists between the warmer and colder air masses on both sides of the front, so that neither air mass can advance on the other one. Thus, the front remains nearly stationary.
	Here, we see what a typical low pressure system might look like on a surface weather map. A cold front extends to the south of the low pressure center, with a warm front to the east. Warm air is located ahead of the cold front and behind the warm front (the so-called "warm sector"), while cool air exists ahead of the warm front and cold air is present behind the cold front. Not all weather systems, however, are as simple as this model.
	This graphic shows how temperature "advection" occurs. To "advect" means to move from one place to another via the wind. At far left, warm air is being advected to the north, i.e., from bottom (warmer) to top (cooler) by a south wind. This is called "warm air advection" and occurs, for example, with warm fronts. Warm air advection can occur at the surface or aloft, and given enough lift and moisture, will result in precipitation. At near left is the opposite. The wind is blowing from north to south moving cooler air toward the warmer air. This is called "cold air advection", and is what usually occurs behind cold fronts.

The weather on the surface of the earth is controlled significantly by what occurs in the atmosphere above. Therefore, meteorologists always analyze "upper air charts" for weather disturbances, moisture, temperatures, etc. and their expected changes to determine what weather likely will occur at the surface. The pictures above show what a very basic upper air weather map might look like (in this example, at 500 mb or about 18,000 feet). At left, we see a "trough" of low pressure, which may be associated with a low pressure system (red "L"), fronts, and precipitation at the surface ahead of the upper trough location. At right, we see a "ridge" of high pressure aloft. A surface high pressure system and often fair weather typically are located ahead (east) of an upper ridge axis with lower surface pressure behind (west of) the ridge. Troughs and ridges can be weak or quite strong.

In looking at upper air data, we also closely examine the "jet stream" which is a relatively high speed ribbon of air between 25,000 and 40,000 feet above the ground. The jet stream is strongest in the winter when the greatest temperature differences exist across the Northern Hemisphere. In a general sense, warmest air masses are located south of the jet with cold air to its north (left picture). Changes in the jet stream pattern and different wind speeds within the jet play a major role in surface temperature and precipitation patterns. At right is a typical upper air chart (300 or 200 mb, i.e., about 30,000 or 40,000 feet) showing height (white) lines revealing troughs across southern California/Baja of Mexico and the eastern United States, with a ridge across much of the Rocky Mountains. In addition, wind vectors and colored isotachs (lines of equal wind speed) show the location of the jet stream (green colors). Yellow, red, and pink colored isotachs represent a ribbon of higher speed winds within the jet. Notice that within the jet stream, wind speeds usually differ. It is these differences in wind speeds within the jet stream that can cause significant weather. In this example, the highest winds are located along the East Coast. This is called a "jet streak" within the overall jet stream.
	This picture is a satellite image showing clouds associated with a major low pressure system. Satellite pictures are taken by weather satellites located about 22,000 miles above the earth. The surface low center in this case likely would be located in western Iowa with a cold front extending south through Missouri into eastern Texas along the cloud band. Rain showers may be occurring within this band. Meanwhile, across Nebraska, South Dakata, Minnesota, and northern Wisconsin, steadier rain or snow may be occurring to the north and west of the surface low and warm front (which probably extends northeast from the low across central Wisconsin).
	Now we will look at different precipitation types. The red line above represents a vertical profile of temperature in the atmosphere. Notice that temperatures (the red line) remain colder than 0 deg C (32 deg F) throughout the atmosphere. Thus, snow will form aloft and remain as snow as it falls to the ground.

Now we see a different temperature profile. The air is colder than 0 deg C aloft where snow forms, but then warms to just above freezing in a small layer. In this layer, the falling snow partially melts so that the precipitation is now partly snow and partly water. As it continues to fall toward the surface where cold air again exists (temperature below freezing), the precipitation refreezes to form sleet or ice pellets.
	This temperature profile is fairly similar to that for sleet, although a warmer and deeper warm layer exists with a shallower cold layer near the surface. Therefore, snow falls from above where the temperature is cold enough, then it completely melts to rain within the warm layer. As the rain then falls into the shallow cold layer at the surface, it does not have time to refreeze until it hits the surface. Upon doing so, the "supercooled" raindrops freeze on exposed surfaces. This can be a very dangerous situation as ice is more difficult to walk and drive on than snow or sleet.

	The Highs and Lows of the NAO
	For those who still don't know what those "Hs" and "Ls" stand for on the nightly weather report, air pressure is a measure of how much air is pushing down on the surface of the Earth at a given point. Generally, high- and low-pressure systems form when air mass and temperature differences between the surface of the Earth and the upper atmosphere create vertical currents. In a low-pressure system, these vertical winds travel upwards and suck air away from the surface of the Earth like a giant vacuum cleaner, decreasing the air pressure above the ground or sea. This decrease in surface air pressure in turn causes atmospheric currents moving parallel to the surface of the Earth near the base of the low to spin counter clockwise (clockwise in the Southern Hemisphere). Conversely, in a high-pressure system, air is being pushed down on the ground like a vacuum put in reverse. The downward vertical winds cause an increase in air pressure on the ground and force atmospheric currents to spin clockwise (counter clockwise in the Southern Hemisphere). Both lows and highs function like giant slow-moving hurricanes and anti-cyclones, respectively. The higher in pressure a high-pressure system gets or the lower in pressure a low-pressure system gets, the more robust and larger this spinning circulation pattern becomes.
			A low pressure system will pull in air from the surrounding area. Winds around a low spiral counter-clockwise (in the Northern Hemisphere, clockwise in the Southern Hemisphere) and upwards towards the center of the system.
			Air is pushed away from a high pressure system. The winds rotate clockwise (in the Northern Hemisphere, counter-clockwise in the Southern Hemisphere) and away from the system's center.
	“Generally speaking the NAO is an oscillation in atmospheric mass between a low around Greenland and Iceland and a high over the Azores west of Portugal,” says Vikram Mehta. He is an atmospheric scientist at NASA’s Goddard Space Flight Center who has been studying Atlantic climate anomalies for over 10 years.
	He explains that a permanent low-pressure system exists over Greenland and Iceland, and a permanent high-pressure system exists over a group of islands roughly 900 miles (1400 kilometers) west of Portugal, known as the Azores. For most of the year, the high and the low are mild, and their influence on the Atlantic basin climate is minimal. When winter hits, however, all of this begins to change. Both pressure systems grow much more intense and begin to fluctuate from week to week between two different states. In one state, which scientists call a positive NAO, the high-pressure system grows especially high, while the low-pressure system grows especially low, creating a large pressure difference between the Azores and Iceland. In the other state, known as a negative NAO, the high-pressure system weakens and the low becomes shallow, creating a milder pressure difference between the two regions of the Atlantic. As the low and high intensify and relax, the winds revolving around their centers increase and decrease in both strength and in extent. During a strong positive NAO, the two pressure systems can just about cause all the currents in the northern half of the northern Atlantic to spin counterclockwise and all those currents in the southern half to spin clockwise. Though the impact of the NAO and its phases can be felt across the entire Atlantic and the surrounding continents, its greatest effect is on the storms passing into Europe. Between the two swirling, clockwise and counterclockwise circulation patterns created by the high and low, there is an area where they come together and form a steady, forward-moving current that channels weather systems from the United States to Europe. “As the pressure systems vary, they modulate the winds along this track and change the number of storms and the amount of moisture over Europe coming from the Atlantic and the Gulf Stream,” says Mehta. Like two wheels of a printing press, the high and low systems can increase or decrease the strength of the winds along this channel. When the pressure difference between the two systems is large (a positive NAO index), the winds along this conduit pick up, and they push the storms north towards Scandinavia and northern France. When the pressure difference is small (a negative NAO index), the storms take a more direct course from the southern United States to southern Europe, the Middle East, and northern Africa. Jim Hurrell is an atmospheric scientist at the National Center for Atmospheric Research who spent a number of years analyzing the connection between the North Atlantic Oscillation and winter weather. He says, “The direction these storms take as a result of NAO can cause remarkable changes in the temperature and the weather over Europe from December through March.” A positive NAO on average can increase rainfall in northern Europe by a little over an eighth of an inch per day and warm the air there by roughly 5 degrees Fahrenheit (2.8 degrees Celsius). If the condition persists for most of the winter, it can lengthen the growing season by 20 days in Sweden, lower reindeer populations in Norway, lead to water shortages in the Fertile Crescent, and provide sunnier, drier conditions for tourists on the French Riviera. A negative NAO, on the other hand, will bring rain to southern Europe, drop the temperatures in northern Europe, and maintain the already warm climate across the Mediterranean. If the negative state persists, it will increase the production of olives and grapes in Greece, put Denmark in a deep freeze, and create ideal skiing conditions in Austria.		The positive and negative phases of the North Atlantic Oscillation are defined by the differences in pressure between the persistent low over Greenland and Iceland and the persistent high off the coast of Portugal. During a positive NAO, both systems are stronger than usual. That is, the low has a lower atmospheric pressure and the high has a higher atmospheric pressure. During the negative phase of the NAO, both systems are weaker, lowering the difference in pressure between them. (Images by Robert Simmon)
	Hurrell said the NAO’s effects could also be felt to a lesser degree in the United States. When the NAO is classically positive, the high-pressure system residing near the Azores strengthens. The winds rotating around the system expand and push warm air from the tropical Atlantic and the Caribbean northward. “So on the west side of the Azores high you have warm air being advected on to the Caribbean and up onto the East Coast, creating a warmer winter along the mid-Atlantic States,” says Hurrell. The result is typically less snowfall for the Washington-New York corridor. During a negative NAO, the high-pressure system grows weak and winter storms, and cold weather that are normally meant for Boston and Maine, head south. “As to this winter, though the NAO was slightly negative this year and has contributed to the winter weather, it does not appear to be a classic NAO pattern,” he says. He explains that the El Niño has led to a slight deepening of the low-pressure system that typically sits over the southeastern United States during the winter, which has brought colder temperatures and more precipitation to the mid-Atlantic. If the NAO was strongly positive on average, as it has been in recent years, then the warm temperatures from the Azores high would likely have counteracted these colder temperatures and the weather would have been much milder along the East Coast. But this year the NAO was neither particularly strong nor particularly weak, and the dominant low-pressure system in the Atlantic was a bit south and east of Greenland. The net effect has been the cold weather and winter storms in the northeast United States. Hurrell adds that there weren’t any pronounced impacts on the European spring or winter either and that storms have tended towards the northern and southern Europe largely without bias. Relying on the Ocean’s Long Term Memory Searching for Atlantic Rhythms		The maps at left show the relationship between a strong positive NAO and precipitation and temperature. Positive correlation means that an area is wetter or warmer than normal, negative correlation means an area is drier or colder than normal, and no correlation means the area is unaffected by the NAO. (Images courtesy Lamont-Doherty Earth Observatory )