Atmospheric density (as distinct from atmospheric pressure) decreases with increasing altitude. This is because of the accompanying reduction in pressure caused by the reducing mass of air as altitude increases.
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There are a number of physical factors that can be used to determine air density, including those derived from the ideal gas law (pressure, volume, gas constant, temperature), and the partial pressures of dry air and water vapour.
For many purposes, an approximation will suffice. Michael Richmond, a scientist with the Rochester Institute of Technology offers this example:
density = (1.21kg/m^3) * exp(-height/8000m)
The approximation does not take into account levels of water vapour and temperature, for example. Where these are likely to significantly affect the reliability of the results, other methods should be used.
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