DIRECT ABSORPTION OF SOLAR RADIATION BY ATMOSPHERIC WATER VAPOR

Abstract

A review of Fowle's solar-absorption studies reveals that no pressure corrections must be applied to the familiar Fowle band-absorptivity curves since these were obtained by methods that automatically corrected them to sea-level pressures. Using these absorptivities, along with recent solar-energy-distribution data, total insolational absorptivities are obtained. These are fitted (with relative accuracy of about 1 per cent over the range 0.3 to 8.0, precipitable water) to an exponential function similar to that used by Mügge and Möller to obtain working relations for use in calculating daily heating amounts due to insolational absorption by water vapor. It is found that variation of the pressure exponent from zero to unity in trial pressure-correction laws yields only a six-per-cent variation in total columnar absorption for a concrete case. Estimates accurate to within about ten per cent with respect to the Fowle absorption data are attainable for levels below about 400 mb in humid regions. However, clarification of the method employed by Houghton in arriving at his roughly thirty per cent greater absorptivities leaves doubt as to the status of the familiar Fowle absorptions, so it is concluded that we cannot now claim to be able to predict insolational heating rates to better than about thirty per cent absolute accuracy.