This chapter outlines the basic principles of energy transfer by electromagnetic radiation in the atmosphere. First, in Section 3.1, we introduce the Planck function, the solar spectrum and the concept of local thermodynamic equilibrium. Then in Section 3.2 we list some formal definitions of radiometric quantities and 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 key facts of molecular spectroscopy 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 apply the concepts introduced in earlier sections to 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 two models that are slightly more realistic than that described in Section 1.3.2. Finally, in Section 3.8, we discuss a simple model of atmospheric scattering.

Radiative heating and cooling play crucial roles in the physics of climate change: more details will be given in Chapter 8. The solution of the radiative-transfer equation also underlies certain techniques of atmospheric remote sounding: see Chapter 7.