The Atmospheric Layers
The Earth’s atmosphere is a complex system comprising a set of layers, which differ in their temperature gradient with respect to altitude. Figure 1-2 shows typical temperature and pressure profiles for mid-latitudes. The rate of temperature change in the atmosphere as a function of height can be used to define regions of positive and negative gradient or lapse rate. Starting at the Earth’s surface, the temperature decreases up to the region known as the tropopause. The latter separates the troposphere, which is vertically well mixed, from the stratosphere, which is characterised by slow vertical mixing. In the stratosphere the temperature increases from the tropopause to the stratopause, which separates the stratosphere from the mesosphere. In the thermosphere, above the mesosphere, the temperature increases again.
The increase in temperature in the stratosphere results mainly from the absorption of solar radiation between 200-300 nm by the stratospheric ozone layer. In the thermosphere a different but related mechanism results in a temperature increase, caused by the absorption of short wavelength solar radiation typically below 200 nm by molecules, atoms and ions. The temperature of the thermosphere is modulated significantly by the solar cycle.
The pressure of the atmosphere is highest at the Earth’s surface and decreases with height according to the barometric formula. The height of the tropopause varies between about 8 km at the poles and 16 km in the tropics. The stratopause occurs at typically about 45-50 km and the mesopause typically at 85-90 km. Between 80 and 90% of the atmospheric mass is contained within the troposphere. The troposphere and stratosphere contain over 95% of the mass of the atmosphere.