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© NASA
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Air pressure is all around us.
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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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