Cumulative distribution curves of daily clearness index in a southern tropical climate

Most previous works, involving the analysis of the statistical properties of solar radiation, have considered places situated in the Northern Hemisphere, generally at latitudes above 30°. In this work, the cumulative distribution curves for 23 sites located in the Southern Hemisphere, in the tropical and inter-tropical regions of Brazil, between the Equator and 30° latitude were calculated, trying to enlarge the available information on the statistical properties of solar radiation towards locations in sub-equatorial regions of the world. Results are compared with four distinct models of daily cumulated distribution functions (F(Kt,K¯t)), proposed by Bendt et al., Hollands and Huget, Saunier et al., and Babu and Satyamurty. A first comparison shows that the function proposed by Saunier et al. is well adjusted to our experimental data for the greater part of the Brazilian locations, except for places with temperate climates, in which the Hollands and Huget model, with the maximum clearness index equal to 0.864, as proposed by these authors, is superior. By substituting the maximum clearness index 0.864 of the Hollands and Huget model for a local maximum, as obtained experimentally in this work, results improve considerably. This study ratifies the conclusions previously obtained by other authors such as Saunier et al., and Babu and Satyamurty on the non-universal character of the Liu and Jordan cumulative distribution functions (CDFs).