Zenith luminance and total horizontal illuminance of the tropical clear sky

Data on zenith luminance and total horizontal illuminance obtained for tropical clear sky at Delhi showed strong turbidity-related seasonal dependance. A comparison of the variability of these parameters with solar height for the Delhi sky with variabilities for other locations, as given by data-based empirical equations and as calculated theorectically, revealed significant differences. The best approximation to zenith luminance and its variability for different locations was obtained with the formula of Dogniaux. A similar approximation was obtained for the total horizontal illuminance of different locations with the values and variabilities as derived, combining Dogniaux's formula for zenith luminance with the luminance distribution of Kittler. Exceptions to this type of approximation may be noticed for locations with special atmospheric conditions and ground reflectance.