Air Masses
	Principle	Air masses are relatively large bodies of air that are fairly horizontally uniform in characteristics. They may extend across an entire continent and are relatively uniform in temperature and moisture content. The boundary separating two different air masses is called a front, and it is along these fronts that a substantial amount of our weather occurs.
	Figure 1 - Air Mass Source Regions
	Air Masses That Affect the Continental U.S.	Air masses are relatively large bodies of air that are fairly horizontally uniform in characteristics. They have relatively uniform temperature and moisture content; the region separating two different air masses is called a front. Air masses form in "source regions" where there is little topography and relatively stagnant winds near the surface. The air mass takes on the properties of the surface of the source region (e.g., dry, hot, moist, etc.). It takes several days for an air mass to "form", so they generally form in areas of high pressure (light winds). Five air masses affect the United States during the course of a typical year: continental polar, continental arctic, continental tropical, maritime polar, and maritime tropical. Continental air masses are characterized by dry air near the surface while maritime air masses are moist. Polar air masses are characterized by cold air near the surface while tropical air masses are warm or hot. Arctic air masses are extremely cold.   Continental polar (cP) or continental arctic (cA) air masses are cold, dry, and stable. These air masses originate over northern Canada and Alaska as a result of radiational cooling. They move southward, east of Rockies into the Plains, then eastward. Continental polar or continental arctic air masses are marked by surface high pressure, cold temperatures, and low dew points. Maritime polar (mP) air masses are cool, moist, and unstable. Some maritime polar air masses originate as continental polar air masses over Asia and move westward over the Pacific, collecting warmth and moisture from the ocean. Some mP air masses originate from the North Atlantic and move southwestward toward the Northeast States. The latter air mass generally is colder and drier than the mP off of the Pacific. Maritime tropical (mT) air masses are warm, moist, and usually unstable. Some maritime tropical air masses originate in the subtropical Pacific Ocean, where it is warm and air must travel a long distance over water. These rarely extend north or east of southern California. Some maritime tropical air masses originate over the Gulf of Mexico and Caribbean Sea. They can be associated with fog and low clouds as they moves northward. In the spring and summer, this air mass accounts for the thunderstorms in the Great Plains and elsewhere. Continental tropical (cT) air masses are hot, dry, unstable at low levels and generally stable aloft (upper-level ridge) Continental tropical air masses originate in northern Mexico. They are characterized by clear skies and negligible rainfall. If one moves into the Great Plains and stagnates, a severe drought can result. Air masses can be modified significantly as they pass over regions with different characteristics. When air masses are modified, they are renamed according to their new characteristics. Topography can play a crucial role in the modification of air masses. For example, the Rocky Mountains cause flow from the west to be lifted over the mountains. The originally mP air loses its moisture as it precipitates, leaving dry air to move eastward. Hence, mP air becomes cP air after it is forced over the Rockies. Most large-scale weather events occur at the boundary of two or more air masses.
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