Atmospheric measurements of infrared water-vapor absorption indicate that there is more absorption occurring than can be explained by radiative transfer theory for single water-vapor molecules. The anomalous water-vapor absorption, as it has been called, has been attributed to dimors, trimers, or larger clusters of water-vapor molecules and inaccurate treatment of water-vapor absorption in the far infrared.
Results of calculations are presented describing the importance of including the anomalous water vapor absorption in radiative–convective atmospheric temperature calculations, as first noted by Ramanathan (1976). Since anomalous water-vapor absorption depends on the square of the water vapor partial pressure, any process that leads to an increase in the water vapor enhances the surface temperature sensitivity. For instance, the surface-temperature sensitivity to a change in relative humidity is increased by up to a factor of three when anomalous water vapor absorption is included. This result highlights one inadequacy of some present models and the continued need for their improvement.