Vertical structure

Nomenclature of atmospheric vertical structure

The static atmosphere is described by the four properties, pressure (P), density (ρ), temperature (T) and composition. Between them these properties determine much of the atmosphere's behaviour. They are not independent, being related by the universal gas law which may be written in various forms (Equations 2.5–2.7). For our purposes the form

P = nkT, (Equation 2.7)

where n is the number of molecules per unit volume, is particularly useful. The quantity ‘n’ is properly called the concentration or the number density, but density alone is often used when the sense is clear.

The regions of the neutral atmosphere are named according to various schemes based in particular on the variations with height of the temperature, the composition, and the state of mixing. Figure 4.1 illustrates the most commonly used terms. The primary classification is according to the temperature gradient. In this system the regions are ‘spheres’ and the boundaries are ‘pauses’. Thus the troposphere, in which the temperature falls off at 10 K/km or less, is bounded by the tropopause at a height of 10–12 km. The stratosphere above was originally thought to be isothermal, but in fact is a region where the temperature increases with height. A maximum, due to heating by ultra-violet absorption in ozone, appears at about 50 km and this is the stratopause.