ABSORPTION OF ATMOSPHERIC RADIATION BY WATER FILMS AND WATER CLOUDS

Abstract

Spectrally averaged infrared absorptivities of thin water films are computed using recent spectrometric results. A water film 5 µ thick irradiated at normal incidence with 0C blackbody radiation is found to have a total absorptivity of 0.49, while, for thicknesses of 10, 50, and 100 µ, the absorptivities are 0.67, 0.92, and 0.95, respectively. Spectrally averaged reflectivity at normal incidence for bulk water is 0.04. Change in the shape of the blackbody emission curve with a change in temperature from 0C to 40C produces negligibly small effect on the spectrally averaged absorptivity and reflectivity. A simple bulk-water method yields infrared cloud absorption half-depths of 17 m for 0.25 g m⁻³ (and 5 m for 1 g m⁻³) liquid water content. Only 0.5 per cent is transmitted deeper than 400 m for 0.25 g m⁻³ or deeper than 100 m for 1 g m⁻³. Evidence for concluding that Mie-theory effects will tend to reduce these absorption depths to still lower values is presented.