Vertical distribution of temperature as shown in Fig. 4.1-1 can be explained by the energy balance of the atmosphere. Nearly all of energy exchange of the earth as a whole consists of absorption of radiation from the sun and emission of radiation by the earth itself, and the two are regarded to be in balance in the annual mean. In this context, `radiation' means energy transfer by means of electromagnetic waves. Among various wavelengths of electromagnetic waves, most of energy in the radiation from the sun (which can be approximated by blackbody radiation of temperature 5700 K) resides in the visible and near infra-red band (0.3 -- 1.5 μm), and most of energy in the radiation from the earth (which can be approximated by blackbody radiation of temperature 255 K) resides in the infra-red band of 7 -- 30 μm. Because the wavelength range of the two overlap very little, we can separately observe balances of solar radiation and of terrestrial radiation.
The material in the atmosphere which absorbs solar radiation most actively in ozone. Ozone absorbs electromagnetic waves in the ultra-violet wavelength band. It mainly resides in the stratosphere. Absorption of solar radiation by other materials are not null, but it can be ignored in the discussion of the outline of energy balance. On the other hand, the principal materials which emit terrestrial radiation are water vapor and carbon dioxide, which are also materials that absorb electromagnetic waves of the same wavelengths. Water vapor is very scarce in stratosphere and above, where carbon dioxide is the most active agent of emission. Let us consider energy balance at each level of height. Emission and absorption of terrestrial radiation occur at any levels, and the amounts are larger as temperature is higher. Absorption of solar radiation, on the other hand, is mostly limited to the ozone layer. Therefore, resulting equilibrium temperature is high in the ozone layer and low elsewhere. The part of solar radiation that transmit through the ozone layer, though somewhat absorbed in atmospheric constituents and clouds, mostly arrives at the surface (of sea and land) and is absorbed there. In the troposphere, the atmosphere tend to lose energy by radiation alone, but it is compensated by the energy transfer from the surface by means of vertical motion of air (i.e. by convection), and relatively high temperature is maintained. The vertical distribution of temperature in the troposphere is essentially determined as the result of convection.
The atmosphere emits terrestrial radiation downward as well as upward. Therefore, terrestrial radiation from the atmosphere arrives at the surface in addition to solar radiation transmitted through the atmosphere. The atmosphere, containing water vapor and carbon dioxide, also absorbs a large part of terrestrial radiation emitted by the surface. The surface air temperature in reality (approximately 14oC, 287 K, as the global mean) is significantly higher than the temperature of the radiation emitted by the earth to space (255 K if approximated by blackbody radiation), because of the effect of the atmosphere absorbing and re-emitting terrestrial radiation. This effect is called the 'greenhouse effect'.