Fig. 7. Measured (36-40, 42, 43) and calculated (41) transmissivities of cirriform clouds for X = 11 /im. cients for the two spectral windows of interest are given in Ref. (31). Statistical variations in the particle distributions and concentrations can result in up to a factor of 3 difference in the predicted value of /3ex as illustrated by the curves labelled cumulus humulis I and II in Fig. 6. For example, particle concentrations, N„, for cumulus clouds range from about 100 cm'3 to greater than 400 cm-3. All of the thicker cloud types (especially cumuliform types, Fig. 6) are highly attenuating and are impenetrable unless hole boring at very high intensities is employed. Those cloud types which are characteristically thinner, such as middle and stratiform types, can be partially transparent, as shown by observational measurements (31). Calculation of the transmission efficiency through such formations should use forward-scattering corrections since Mie scattering from cloud particles is predominately in a forward direction. While this correction may result in a change in the transmission efficiency of perhaps 20%, this effect is unimportant compared with statistical uncertainties inherent in any model developed to predict the temporally- dependent laser power received at terrestrial sites.
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