Proceedings PaperComparison Of Near-Millimeter Wave And Infrared Propagation Under Conditions Of Low Visibility
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Water in the atmosphere is the principal natural impediment to propagation of both infra-red and near-millimeter wave electromagnetic energy. Attenuation by water in the vapor phase is greater near one millimeter than near 10 μm. When fog is present, extinction by the water or ice particles depends upon the wavelength of the propagating energy, the complex index of refraction of the particles for that wavelength, and the particle-size distribution. This paper summarizes a thorough survey of the literature on fog drop-size distributions throughout the world. A representative sample of data is selected for use in computations of the extinction of 10.6, 870, and 1250 μm by numerous fogs. Attenuation by fog drops is much greater near 10 μm than near one millimeter. Computations made with German data show that attenuation of liquid plus vapor is smallest for 1250 μm in fogs of different densities in all seasons. In very dense German fogs, 10.6 μm has the largest attenuation in all seasons. In spring, summer, and autumn attenuations of 10.6 μm and 870 μm in moderate fogs are the same order of magnitude.