The purpose of this chapter is to review the current state and recent developments in the modelling of microwave absorption by atmospheric gases and in the validation of those models by radiometric measurements. Local thermodynamic equilibrium implies that the absorption coefficient also determines the thermal energy emitted by the clear atmosphere. Absorption models generally consist of (1) a database of line parameters (e.g. transition frequencies, intensities, widths), which may be specific to the model or drawn from a large general-purpose database such as HITRAN (high resolution transmission) or GEISA, (2) shape factors for the lines and (3) ancillary model components such as continuum absorption.

Inspec keywords: radiometry; atmospheric thermodynamics; absorption coefficients; electromagnetic wave absorption; atmospheric chemistry; gases; atmospheric techniques; atmospheric optics

Other keywords: shape factors; absorption coefficient; continuum absorption; clear atmosphere; local thermodynamic equilibrium; atmospheric spectroscopy; microwave absorption; thermal energy emission; absorption models; GEISA; HITRAN; radiometric measurements

Subjects: Atmospheric spectral energy distribution, spectral absorption