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Air has weight. That idea might seem strange to you, because air
seems very thin, even at sea level. Remember however, that the atmosphere
extends to great altitudes. The pressure of the air is equal to
the weight of a column of air above a unit area on the land surface.
The column of air above a square area that is one foot on a side
is about 14.7 lb., at sea level. In the metric system, that's about
72 kg per square meter. If you try to pump the air out of a closed
container, the container will collapse inward from the outside air
pressure, unless it is very strong. The reason you don't feel the
air pressure is that the pressure inside your body is adjusted to
be exactly the same! Air pressure decreases upward in the atmosphere.
That's because at higher levels in the atmosphere there is less
air above to cause the pressure.
Detailed weather maps show the atmospheric pressure by means of
curved lines called isobars. As with an isotherm for temperature,
an isobar connects all points with the same atmospheric pressure.
There is one difference with isobars, however. The pressure at the
land surface is less where the elevation of the surface is high,
so the pressure is "corrected" to sea level. The corrected
pressure is what you would measure at the place if you could dig
a very deep mine all the way down to sea level and put your barometer
at the bottom of the hole. The corrected pressure is used on weather
maps.
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Most weather maps show areas, labeled with an H, where the atmospheric
pressure is relatively high, and areas labeled with an L where the
atmospheric pressure is relatively low. The isobars around such
areas are closed curves with the approximate shape of circles. High-pressure
areas are places where the atmosphere is relatively thick. Winds
blow outward from these areas, although in a spiraling way. As air
leaves the high-pressure area, the remaining air sinks slowly downward
to take its place. That makes clouds and precipitation scarce, because
clouds depend on rising air for condensation. High-pressure areas
usually are areas of fair, settled weather. Low-pressure areas are
places where the atmosphere is relatively thin. Winds blow inward
toward these areas. This causes air to rise, producing clouds and
condensation. Low-pressure areas tend to be well-organized storms.
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