The Earth atmosphere can be broadly divided into three distinct regimes, as shown in Figure G-1:

• the homosphere comprises the regions of the troposphere (surface up to ~ 10 - 12 km altitude), the stratosphere (~ 10 - 12 km up to 50 km), and the mesosphere (~50 km up to 90 km),

• the thermosphere extends from about 90 km altitude to approx 400 km, depending on solar and geomagnetic activity,

• the exosphere extends from the top of the thermosphere and extends into space.

The boundaries between these regions are classically defined by relatively sharp and consistent changes in the temperature profile as a function of altitude, with the exception of the base of the exosphere_.The exosphere base is defined by that altitude where the mean free path equals the scale height. In practice, all of these boundaries, whether determined in altitude or in a pressure co-ordinate system, vary with solar, seasonal, latitudinal and other conditions.

Due to vertical winds and turbulent mixing the homosphere has a nearly uniform composition of about 78,1% N₂, 20,9% O₂, and 0,9% Ar. The temperature profile of the heterosphere shows alternating gradients with (normally) the maximum temperature at the surface (global average ~288K), a local minimum at the tropopause (global average ~218K), a local maximum at the stratopause (global average ~280K), and another local minimum at the mesopause (≈150K – 250K).