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Air Masses 4.4 Air masses & Fronts • An air mass is a large body of air of similar temperature, humidity, & air pressure. • Four major types of air masses influence the weather in North America: maritime tropical, continental tropical, maritime polar, & continental polar. • Tropical: warm air mass, forms in the tropics, low air pressure • Polar: cold air mass, forms in the poles, high air pressure. Air masses & Fronts • Maritime: form over oceans, can be very humid. • Continental: forms over land, can be very dry. • In the continental US, air masses are commonly moved by the prevailing westerlies (pushing air masses from west to east) & jet streams (bands of highspeed winds about 10km above Earth’s surface). Fronts-the boundary where the air masses meet. • Cold front: a fast-moving cold air mass overtakes a warm air mass. (represented by triangles) • Warm front: a warm air mass overtakes a slowmoving cold air mass. (represented by half circles) • Stationary front: cold & warm air masses meet, but neither can move the other. (represented by half circles and triangles on opposite sides of a line) • Occluded front: a warm air mass is caught between 2 cooler air masses. (represented by half circles and triangles on the same side of a line) Air Masses & Fronts • When a cold front passes through an area, colder, drier air moves in, often bringing clear skies, a shift in wind, & lower temps. • After a warm front passes through an area, the weather is likely to be humid & warm. Air Masses & Fronts • If a stationary front remains stalled over an area, it may bring many days of clouds & precipitation. • As the warm air of an occluded front cools and its water vapor condenses, the weather may turn cloudy & rain or snow may fall. • http://www.fcgov.com/oem/oem-images/warm-front.jpg Air masses & Fronts • A swirling center of low pressure is a cyclone. • Cyclones & decreasing air pressure are associated with clouds, wind, & precipitation. • A swirling center of high pressure is an anticyclone. • The descending air in an anticyclone generally causes dry, clear weather. Anticyclones & Cyclones
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