Emission and spectroscopy of the clear atmosphere

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.