Measurement of Absorption Characteristics of Water Vapor Near Saturation

It has been suggested that the significant differences that still persist between the measurements of earth’s radiation budget and the predictions are partially due to the inadequacies in the absorption coefficients of water vapor used in the calculations. Water in all its three phases has a profound influence on both the longwave and shortwave streams of radiation, because of its strong absorption over large spectral regions and scattering by the condensed phases. Furthermore, the mixing ratio and the saturation of water vapor can vary over a wide range. Thus, even small errors in the values of absorption properties can have a large influence on the results predicted by atmospheric models. A question that has continued to defy accurate characterization is whether the absorption by water vapor is enhanced when it is close to saturation. These questions are more relevant in the light of observations of anomalously high extinction in fogs or clouds where the partial pressure of the vapor is close to saturation.