This chapter describes some aspects of energy transfer by electromagnetic radiation in the atmosphere. In Section 3.1 we introduce the Planck function, the solar spectrum and the concept of local thermodynamic equilibrium. In Section 3.2 we list some formal definitions of radiometric quantities and then derive and solve the radiative-transfer equation, which describes the way in which radiative power is affected by extinction and emission of radiation. In Section 3.3 we present some basic aspects of molecular spectrosopy and give some of the properties of spectral line shapes. In Section 3.4 we introduce the concept of transmittance, the fraction of radiative power that survives propagation from one point to another. In Section 3.5 we consider the absorption and emission of infra-red radiation and the absorption of ultra-violet radiation by gases in the atmosphere. This absorption and emission lead to heating and cooling; the principles of the calculation of heating rates are outlined in Section 3.6. In Section 3.7, we revisit the greenhouse effect, investigating a more realistic model than that described in Section 1.3.2. Finally, in Section 3.8, we discuss a simple model of atmospheric scattering.

The solution of the radiative transfer equation also plays an important role in certain aspects of atmospheric remote sounding. This will be covered in Chapter 7.

It is an unfortunate fact that quantitative calculations of radiative heating rates, for example, involve considerable geometric and algebraic detail, which tend to distract attention from the basic physics of the processes.