Atmospheric models

Static atmospheric models describe how the ideal gas properties (namely: pressure, temperature, density, and molecular weight) of an atmosphere change, primarily as a function of altitude.

For example, the US Standard Atmosphere is essentially a table of values for air temperature, pressure, and mass density, as a function of altitude above sea level.

Other static atmospheric models may have other outputs, or depend on inputs besides altitude.

Basic assumptions

The gas which comprises an atmosphere is usually assumed to be an ideal gas, which is to say:

Where ρ is mass density, M is average molecular weight, P is pressure, T is temperature, and R is the ideal gas constant.

The gas is held in place by so-called "hydrostatic" forces. That is to say, for a particular layer of gas at some altitude: the downward (towards the planet) force of its weight, the downward force exerted by pressure in the layer above it, and the upward force exerted by pressure in the layer below, all sum to zero. Mathematically this is: