Pressure
Atmospheric pressure
is the force per unit area that is always applied perpendicularly to a surface
by the surrounding gas. It is determined by a planet's gravitational force in combination with the
total mass of a column of gas above a location. Units of air pressure are based on the
internationallyrecognized
standard atmosphere
(atm), which is defined as 101,325
Pa
(or
1,013,250
dynes
per cm
2
).
The pressure of an atmospheric gas decreases with altitude due to the diminishing mass of gas
above each location. The height at which the pressure from an atmosphere declines by a factor of
e
' (an
irrational number
with a value of 2.71828.
.) is called the
scale height
and is denoted by H.
For an atmosphere with a uniform temperature, the scale height is proportional to the
temperature and inversely proportional to the mean
molecular mass
of dry air times the planet's
gravitational acceleration. For such a model atmosphere, the pressure declines exponentially with
increasing altitude. However, atmospheres are not uniform in temperature, so the exact
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 Fall '10
 EmilyHoward
 Astronomy, Atmosphere, Gravitational Force, Pressure Atmospheric pressure, internationallyrecognized standard atmosphere, atmospheric gas decreases

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