Effects of Precipitation on Propagation at 0.63, 3.5, and 10.6 Microns

The attenuation and scattering of laser beams by rain, fog, and snow have been calculated and measured at 0.63, 3.5, and 10.6μ. Attenuation of the infrared wavelengths by light fog is up to one order of magnitude less than at 0.63μ. But for dense fog, calculation shows that the attenuation at 10.6μ can exceed 40 dB per km. It is found that attenuation by rain can be calculated to good accuracy from average path rain rates provided that forward scattering is taken into account; this scattering reduces the attenuation. Measurements of propagation through precipitation over a 2.6 km path are discussed in detail and are found to be consistent with predictions. The wavelength dependence of attenuation is found to vary from one fog to another because of different drop-size distributions. Attenuation of 0.63μ in rain showers is about 20 per cent less than at 3.5μ and is, of course, much less than the attenuation in fog. But even for extremely heavy rain showers, the 0.63μ attenuation never exceeded 20 dB per km, which is less than the attenuation of millimeter waves under such conditions. Both the attenuation and foreward scattering properties of snow appear to be between those of fog and rain.