Penetrability of Haze, Fog, Clouds and Precipitation by Radiant Energy over the Spectral Range 0.1 Micron To 10 Centimeters

摘要

To determine attenuation values over a wide range of wavelengths, an analytical methodology is developed to accommodate the population of droplets according to size in unit volume of several models of water-occluded atmospheres. The extent of penetrability of cloudy and rainy atmospheres is then demonstrated analytically as a function of wavelength. Unique phenomena appear at wavelengths from about 100 microns to 2 millimeters, in part due to the population of droplets by size in clouds and rain but also due to the strong wavelength dependence of the complex index of refraction of liquid water. The submillimeter band is accordingly given special emphasis. Most layer-type water clouds, especially due to rain of moderate intensity is found to decrease slowly with decreasing wavelength below 2 millimeters. Further, pronounced forward scatter in moderate rain, adding to the forward transmission, first begins to appear at a wavelength of 2 millimeters and becomes increasingly more pronounced the shorter the wavelength. None of these trends would be predicted by simple extrapolation from experience at microwave frequencies. The analysis of attenuation by water droplet atmospheres draws on the Mie theory of absorption and scatter by spherical droplets. The absorption profile arising from atmospheric gases is structured in fine detail from 0.4 micron to 3.2 centimeters by extensive search of the literature. Because of requirements forged by its scope, the study provides an extensive data base on the population of droplets by size in various water-occluded atmospheres. (Author)