time t. Ej is a function of total sky cover and persistence time. Tabulated values were taken from Lund (9) and parameterized so that persistence times greater than one hour can be considered. The cumulative frequency, i.e., the frequency that the transmission efficiency equals or exceeds Tj(t) for a persistence time t is given by Eq. 2 also. This is due to the fact that C} and E^t) are functions of the total sky cover only and, therefore,is just the cumulative frequency of occurrence of each total sky cover. Note that the occurrence of meteorological phenomena other than clouds is implicitly included in the model. For instance, snow and rain originate within clouds and we have generally assumed zero transmission through precipitating cloud types. Likewise, heavy haze or fog, although not as statistically important as clouds, are reported as sky cover if they obscure visibility in a vertical direction. We have not included any attenuation due to molecular absorption, however, and have assumed that a high transparency window is available.
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