4.2 - Air Masses and Fronts
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Definition of an Air Masses
We have been talking about masses of air rising over mountains or descending in high pressure but we haven't really defined what these air masses are and how they form. Air masses are parcels of air that are distinct from one another with horizontally uniform temperature, humidity and pressure. In order to be an air mass three requirements must be met:
- It must be large. A typical air mass is more that 1,000 miles across and 1 to 2 miles high
- It must have uniform temperature, humidity, and stability characteristics at each altitude within the air mass
- It must travel, remaining distinct from neighboring air and when it moves, it must retain its original characteristics
Source Regions
Areas where air masses originate are called source regions. To be a source region an area has to be large, physically uniform and associated with air that sits, such as areas of persistent high pressure. It must be an area with uniform topography - fairly flat, all water or all land. Ideal source regions are ocean surfaces or extensive flat land areas that have uniform covering of snow, forest or desert.
An air mass develops its characteristics by remaining over a uniform land or sea surface long enough to pick up the characteristics from that surface - warm and moist, cold and dry etc. An air mass only has to stick around in one place for a couple of days to pick up those characteristics. For air to sit or stagnate, it has to be relatively stable. Most air masses, form in areas with high pressure or anticyclonic conditions.
The graphic above shows the major sources regions for air masses that affect North America. Warm-air masses can form any season over the waters of the southern North Atlantic, the Gulf of Mexico and the Caribbean, and the southern North Pacific and in summer over the deserts of the U.S. South West and northwestern Mexico. Cold-air masses develop over the northern portions of the Atlantic and Pacific oceans and over the snow-covered interior of north central Canada.
Continental Polar air masses develop in central and northern Canada, and Arctic air masses originate farther north. They are cold, dry and stable and are dominate in the winter.
Maritime Polar air that affects North America originates over the North Pacific or North Atlantic. These air masses are cool, moist and relatively unstable. The Pacific air masses bring widespread cloudiness and heavy precipitation to the mountainous coastal regions.
Air masses that develop over the North Atlantic are also cool, moist, and unstable. Since the prevailing wind direction is from the west, these air masses only impact the U.S. and Canada when conditions are just right. The famed "Nor'easter" is an example.
Maritime Tropical air from the Gulf of Mexico is warm, moist, and unstable. It has a strong influence on weather east of the Rockies in the U.S., in Southern Canada and much of Mexico, providing most of the precipitation for these areas. In summer it brings humid moist air into the region.
Pacific Maritime Tropical air masses originate over cold water so they are cooler, drier, and more stable than those from the Gulf. These air masses only impact the southwestern U.S. and northwestern Mexico. In winter they result in coastal fog and orographic precipitation. In summer, they can bring thunderstorm activity to the SW and are responsible for the Arizona "monsoons".
Lastly, there is the Continental Tropical air mass that forms over north central Mexico. It brings dry, hot air with it.
Now look at the map above again. Notice that all across the U.S. unlike air masses are likely to collide with no topographic barriers preventing it. When these unlike air masses meet the potential for significant weather events is high.
Fronts
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When unlike air masses meet, they do not mix. Instead a boundary zone call a front develops between them. A front is a relatively narrow zone where atmospheric conditions change rapidly. The idea of air masses advancing and fronts forming sounds faintly militaristic and in fact it is. The concept was developed by meteorologists in WW I and they definitely drew an analogy with confrontation of opposing armies.
Fronts form whenever unlike air masses collide. Fronts are named based on which front is advancing faster and "invading" the other air mass. In a warm front (see above) warm air is advancing into an area dominated by an air mass of cooler air. As in all fronts, the warm air rises over the colder air which stays in contact with the ground. In a warm front this rise is very gradual, like the air is riding up a ramp. As the air rises over the colder air, it expands, cools adiabatically and clouds and precipitation are likely. Cirrus clouds are often way out ahead of the front, warning that it is coming. 
As the front moves in, altocumulus and altostratus clouds dominate. Precipitation is often long lasting and gentle. Most rain falls ahead of the surface location of the front. On a weather map a warm front is depicted as a line with solid semicircles pointing in the direction the front is moving.
A front formed by advancing cold air is called a cold front (see diagram above) . Cold fronts are often fast moving. They are much steeper than a warm front, plowing into warm air forcing it to rise abruptly. The steep slope and rapid movement of cold fronts leads to a rapid uplift of warm air often making that air very unstable. The result is blustery and more violent weather along the cold front. Cumuliform clouds are common (as opposed to the more stratus form, horizontal clouds associated with warm fronts). Precipitation can be intense but is often of shorter duration than in a warm front. Both clouds and precipitation are behind the surface location of the front. On a map, as you can see above, a cold front is depicted by a line with solid triangles, the points of which show the direction the front is moving.
Two other fronts are stationary and occluded fronts. A stationary front occurs when neither air mass displaces the other. Their boundary is a stationary front. Generally, these fronts have limited precipitation and conditions similar to a warm front. Occluded fronts are formed when a cold front overtakes a warm front and all of the warm air has been pushed aloft.