Various physical processes modify a radiation field as it propagates through an atmosphere. The rate at which the atmosphere emits depends on its composition and thermal structure, while its absorption and scattering properties are defined by the prevailing molecular opacity and cloud structure.

Independently of whether the radiation field is generated internally or is imposed externally, the study of how it interacts with the atmosphere is embodied in the theory of radiative transfer. Many authors have dealt with this theory in various contexts. Monographs include those by Kourganoff (1952), Woolley & Stibbs (1953), Goody (1964), and Goody & Yung (1989). A standard text is by Chandrasekhar (1950), which treats the subject as a branch of mathematical physics. The emphasis is on scattered sunlight in planetary atmospheres and on various problems of astrophysical interest.

Our own approach is somewhat different and emphasizes spectra produced by thermal emission from planetary atmospheres, especially as observed from space platforms. In order to demonstrate the connection between the thermal radiation giving rise to these spectra and the physical state of the atmosphere under consideration, it is necessary to examine how the transport of this radiation is effected. Only then is it possible to have a clear understanding of how the structure of an atmosphere leads to its spectral appearance, a topic considered at length in Chapter 4.