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## The Atmosphere

#### Glenn Research Center

The Earth's 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. Gravity holds the 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 sun heats 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 of state and also decreases with increasing altitude.

Aerodynamic forces depend on 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.

An interactive simulation for the atmosphere model is available at this web site. With the simulation, you can change altitude and see the effects on pressure and temperature. The same atmosphere model is also used in the FoilSim and EngineSim computer simulators.

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