Far Infrared and Submillimeter Wave Attenuation by Clouds and Rain

Newly determined optical constants for water at far infrared and submillimeter wavelengths, as revealed by the author's recent survey, are used to estimate water cloud and rain attenuation over the wavelength range between 12 micrometers and 2 centimeters. For this purpose new analytic drop-size distribution models simulating fog, nimbostratus cloud, and rain corresponding to rainfall rates of 10 and 50 mm/hr are set up. The corresponding volume extinction and absorption coefficients are computed according to polydisperse Mie scattering theory at specific wavelengths and are presented in tables and graphs. It is found that cloud extinction may exceed 50 per kilometer for wavelengths less than 100 micrometers, whereas for wavelengths longer than 200 micrometers, under near saturated conditions, water vapor absorption should be the dominant attenuator. The greatest attenuation by heavy rain may be expected around 5 mm with a value of about 5 per kilometer. In the presence of nonprecipitating water clouds or fog, there may be a relative transmission "window" centered around 1.3 mm.