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Air consists of gas molecules, which are combinations of two or
more atoms. Although you cannot see them with your eyes, the molecules
are constantly moving this way and that at very high speeds. As
they move, they collide with one another and with solid surfaces.
The temperature of the air is a measure of how quickly the molecules
are moving. The more energy of motion the molecules have, the higher
the temperature you feel in the air.
Air temperature is measured with thermometers. Common thermometers
consist of a glass rod with a very thin tube in it. The tube contains
a liquid that is supplied from a reservoir, or "bulb,"
at the base of the thermometer. Sometimes the liquid is mercury,
and sometimes it is red-colored alcohol. As the temperature of the
liquid in the bulb rises, the liquid expands. As the liquid expands,
it rises up in the tube. The tube is marked with a scale, in degrees
Fahrenheit or in degrees Celsius.
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 half-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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