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The Atmosphere
Composition
The atmosphere (air) is a mixture of independent gases. Near the
surface of Earth, the percentages by volume of the various constituents
are approximately 78% nitrogen, 21% oxygen, 1% argon, and traces of
other gases such as carbon dioxide, hydrogen, neon, and helium. Water
vapor, which has been omitted from the foregoing list, is found in
relatively small but widely varying amounts; 1% of the total atmosphere
may be taken as the average figure. The quantity of water vapor present
is much greater in equatorial regions than it is in polar regions and
greater over the ocean than over land. The atmosphere has definite
weight, called atmospheric pressure, and is measured by an instrument
called a barometer.
Changes in the
Large-scale changes in temperature, pressure, and water vapor content of
Weather
the atmosphere cause changes in the weather. Warm air is lighter in
weight and can hold more water vapor then cold air. Moist air with a
temperature of 50°F is lighter than drier air of the same temperature
because water vapor is lighter than air. Cold or heavy air has a
tendency to flow toward and take the place of warm or lighter air; and,
as the air begins to move, other forces come in to play, making the
movement of air masses and weather rather complex. You can readily
see that temperature, humidity, and atmospheric pressure are all factors
in considering the weather.
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