Fig. 6. Calculated extinction coefficients for clouds with marked vertical extent. Particle distribution data were taken from the following references: cumulus humulis I and II IWarner (35)]; cumulus congestus and cumulonimbus (Carrier et al. (34)). using the water mode only. The assumption that cloud particles are homogeneous and composed entirely of pure water is perfectly acceptable in the middle- and far-infrared spectral regions and is subject to question only for UV, visible, and nearinfrared wavelengths. A significant reduction in ;8ex occurs around 11 /am for clouds in which the large particle distribution decays more rapidly than a 2. This effect is particularly noticeable in calculations for altostratus, stratocumulus, and stratus clouds as shown in Fig. 5. Clouds characterized by a greater proportion of larger particles, e.g., nimbostratus and cumulonimbus (Figs. 5 and 6) show little improvement in /3ex at 11 jam. Alternately, operation at a laser wavelength near 2.25 jam may offer improved transmission through thin clouds because of a minimum in pn. Operation at even shorter wavelengths is undesirable because of the increased attenuation due to haze and molecular scattering. Tabulated absorption and extinction coeffi-
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