Air Masses Uniform bodies of air An air mass is a large body of air that has similar temperature and moisture throughout. The best source regions for air masses are large flat areas where air can be stagnant long enough to take on the characteristics of the surface below. Maritime tropical air masses (mt), for example, develop over the subtropical oceans and transport heat and moisture northward into the U.S.. In contrast, continental polar air masses (cp), which originate over the northern plains of Canada, transport colder and drier air southward. Once an air mass moves out of its source region, it is modified as it encounters surface conditions different than those found in the source region. For example, as a polar air mass moves southward, it encounters warmer land masses and is heated by the ground below. Air masses typically clash in the middle latitudes, producing some very interesting weather. http://www.slideshare.net/radgeog/depression 126545 1
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Five Air Masses 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. 3
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Continental tropical (ct) air masses are hot, dry, unstable at low levels and generally stable aloft 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. 5
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. 6
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. 7
Air Masses Air Mass airmass 8
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Vocabulary Air mass Cold Front Front Warm Front Stationary Front Occluded Front 11
Do air masses stay the same? 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. http://www.teachersdomain.org/resource/mck05.sci.ess.watcyc.fronts/ 12
Fronts Fronts are boundaries between air masses of different temperatures. Fronts are actually zones of transition, but sometimes the transition zone, can occur quickly. The type of front depends on both the direction in which the air mass is moving and the characteristics of the air mass (warm vs cold, moist vs dry) There are four types of fronts that will be described below: cold front, warm front, stationary front, and occluded front. 13
What cloud type is forming? 14
Some of the characteristics of WARM FRONTS include the following: * The slope of a typical warm front is 1:200 (more gentle than cold fronts). * Warm fronts tend to move slowly. * Warm fronts are typically less violent than cold fronts. * Although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of light to moderate, continuous rain). * Warm fronts are usually preceded by cirrus first (1000 km ahead), then altostratus or altocumulus (500 km ahead), then stratus and possibly fog. * Behind the warm front, skies are relatively clear. * Warm fronts are associated with a frontal inversion (warm air overrunning cooler air). 15
What cloud type is forming? 16
Some of the characteristics of COLD FRONTS include the following: * Cold fronts tend to move faster than all other types of fronts. * Cold fronts tend to be associated with the most violent weather among all types of fronts. * Cold fronts tend to move the farthest while maintaining their intensity. * Cold fronts tend to be associated with cirrus clouds well ahead of the front, strong thunderstorms along and ahead of the front, and a broad area of clouds immediately behind the front (although fast moving fronts may be mostly clear behind the front). * Cold fronts can be associated with squall lines (a line of strong thunderstorms parallel to and ahead of the front). 17
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Occluded fronts *Because cold fronts move faster than warm fronts, they can catch up to and overtake their related warm front. When they do, an occluded front is formed. *Occluded fronts are indicative of mature storm systems *The most common type of occlusion in North America is called a cold front occlusion and it occurs when the cold front forces itself under the warm front. 19
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Stationary Fronts * Stationary front a front that does not move or barely moves. * Stationary fronts behave like warm fronts. * Many times the winds on both sides of a stationary front are parallel to the front. * Typically stationary fronts form when polar air masses are modified significantly so as to lose their character (e.g., cold fronts which stall). 21
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