atmosphere is an extremely thin
sheet of air
extending from the surface of the Earth to the edge of space.
The Earth is a sphere with a roughly 8000 mile diameter;
the thickness of the atmosphere is about 60 miles.
In this picture, taken from a spacecraft orbiting at 200 miles above the surface, we can see
the atmosphere as the thin blue band between the surface and the blackness of space.
If the Earth were the size of a basketball,
the thickness of the atmosphere could be modeled by a thin sheet of
plastic wrapped around the ball.
atmosphere to the Earth's surface. Within the atmosphere, very
complex chemical, thermodynamic, and
fluid dynamics effects occur. The
atmosphere is not uniform; fluid properties are constantly changing
with time and location. We call this change the weather.
At any given location, the air properties also vary with the
distance from the surface of the Earth.
the surface of the Earth, and some of this heat goes into
warming the air near the surface.
The heated air rises and spreads up through the atmosphere.
So the air temperature
is highest near the surface and decreases as altitude increases.
The speed of sound
depends on the temperature and also decreases with increasing altitude.
The pressure of the air can be related to
the weight of the air over a given location. As we increase altitude
through the atmosphere, there is some air below us and some air
above us. But there is always less air above us than was present
at a lower altitude. Therefore, air pressure decreases as we increase
altitude. The air density depends on both
the temperature and the pressure through the equation
and also decreases with increasing altitude.
the air density. To help aircraft designers, it is useful to define a
standard atmosphere model
of the variation of properties
through the atmosphere.
The model was developed from
atmospheric data that was taken by weather balloons released at the surface of
the Earth and allowed to ascend through the atmosphere. The measurements
were then averaged and curve fit to produce
equations for the model. The model assumes that the pressure and
temperature change only with altitude.
There are actually several different
models available--a standard or average day, a hot day, a cold day, and a
tropical day. The models are updated every few years to include
the latest atmospheric data.
for the atmosphere model is available at this web site. With the
simulation, you can change altitude and see the effects on pressure and
The same atmosphere model is also used
in the FoilSim
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