Numerically, the third cloud transmission model assumes [1] zero transmission efficiency for cumuliform clouds, [2] the transmission efficiency for cirriform and middle clouds decreases from 95% to 80% as the total sky cover increases from 0 to 10 tenths, i.e., 5% to 20% of the transmitted power is lost to aerosol vaporization, [3] the transmission efficiency for stratiform clouds decreases from 90% to 60% over the same sky-cover range, and |4] for mixed cloud forms, the transmission efficiency decreases from 80% to 30% as the total sky cover increases from 0 to 8 tenths; overcast conditions (9-10 tenths sky cover) with mixed-form clouds are impenetrable. Also, to account for statistical variations in the persistence probabilities, Ej values used in model 3 were increased by 9%, consistent with the observational results of Lund (9) and the optimistic nature of the model. The weighted cloud transmissivity for a given sky cover is calculated by multiplying the probability of occurrence of a cloud form, if a cloud is present, by the respective transmission efficiency, and summing over all cloud-form categories. Details of this procedure are given in Ref. (10). The occurrence probabilities of the various cloud forms as a function of sky cover were inferred from data of Lund and Shanklin (7). These data are observational results for Columbia, Missouri, although Lund and Shanklin suggest that they can be generalized to other continental mid-latitude sites without substantial error. Occurrence probabilities for each site should be used but, as discussed previously, such statistical data are not routinely available. Histograms of the weighted cloud transmissivity Tj as a function of total sky cover j for the three models are shown in Fig. 1. Statistical Results and Analysis Statistical calculations of the seasonal and annual power availabilities, transmis-
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