where P and T are the absolute pressure and temperature, kB the Boltzmann constant, ρ the density and m the mass of a single molecule. To be precise, since air is a mixture, equation (4) would represent a partial pressure for one species in the mixture. However, all the results that follow can easily be adapted to the case of a mixture; we simply need to replace single pressure terms by sums. Like any other fluid in a gravitational
field g, the air pressure must vary with height in order to maintain hydrostatic equilibrium. These pressure variations are described by