Analysis of zenith luminance data for all sky conditions

Two different procedures are explored in the present work to estimate the z enith luminance at the International Daylight Measurement Program station i n Madrid, when all the data available are used independently of the sky typ e. On the basis of the results obtained, correlations for the seasonal depe ndence of the zenith luminance on solar elevation are reported. However, it is observed that polynomials obtained for different ranges of the luminous clearness index, the ratio of global to extraterrestrial illuminance, can predict the zenith luminance with a more reasonable accuracy. The results s how that the luminous clearness index is an important parameter for zenith luminance characterization, to be used when global illuminance values on ho rizontal surfaces are available, but diffuse illuminance is not measured (C ) 2001 Elsevier Science Ltd. All rights reserved.