A region may be stormy at the meeting place of two air masses, known as a front, especially if the two air masses are very different. There
Feet Meters Altostratus Altocumulus Cirrus 40,000 T 12,200 [\ A I
30,000 4- 9J50
Cumulus 1 Nimbostcatus
Fog or drizzle
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Heavy rain, thunderstorms
In a warm front, a warm air mass rises over a cold front, producing cumulus and cumulonimbus clouds and violent precipitation. In a cold front, warm air slides upward over cold air at a warm front. Stratus and cirrus clouds form, the former producing gentle rain.
are several types of fronts. For example, stationary fronts are those in which the air masses do not move—rain, drizzle, and fog can sit over the same area for several days. Occluded fronts form when a fast-moving cold air mass traps a warm air mass against a second cold air mass and lifts it off the ground, resulting in precipitation where the air masses of different temperatures meet.
As a front travels over a region, one air mass replaces the other. A warm front is where the warm air takes over the position of the cold air. The warm air rises, producing clouds and precipitation, occasionally even thunderstorms. Warm fronts advance slowly, at only about 15 miles (25 km) per hour. A cold front is where cold air takes over the position of the warm air and pushes it upward. Cold fronts advance faster, at about 22 miles (35 km) per hour, and are associated with violent weather such as intense thunderstorms.
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