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Large masses of air, as much as a thousand miles across, take on certain
weather characteristics when they stay at high latitudes (near the poles)
or at low latitudes (near the equator) for several days at a time. They
may be very cold or very warm, or they may be very humid or very dry. Then,
as they move into other areas, they can affect the weather there very strongly.
The coldest winter weather in much of the United States is at times when
a bitter cold air mass from the high arctic regions of northeastern Asia,
Alaska, or northern Canada sweeps down into the southern parts of North
America. At other times, a flow of warm and humid air from the tropics causes
unusually warm weather in the eastern United States.
The boundaries between air masses are often zones of very rapid changes
in temperature and moisture. Enormous swirling storms tend to develop
along these zones of rapid change. On weather maps, cold fronts are shown
as lines with triangular teeth. These show where the cold air mass is
wedging under the warm air mass. As the warm air is lifted along the front,
heavy rain from thunderstorms is common. Warm fronts are shown on weather
maps as lines with circular teeth. These show where the warm air masses
are moving up over the cold air masses. Broad areas of rain are often
associated with warm fronts.
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© NASA
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A diagram showing two air masses in proximity with their fronts
designated Cold (top panel) where the cold air mass is steep
and Warm where the front is more gentle. Please note that
the cold, heavier air mass, associated with a High, is typically
below the lighter warm air.
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