An investigation of estimating clear sky insolation has been conducted. Some discrepancies have been found between the conventional method of estimation and the data. It has been noticed that the conventional method underestimates the insolation. A simpler, more elegant and more accurate "universal" relationship has been derived for estimating the annual mean daily clear sky insolation as a function of the latitude and altitude of a given location. As part of this derivation, two simple and accurate correlations have been obtained, i.e. for extraterrestrial insolation and for atmospheric attenuation. (C) 2003 Elsevier Ltd. All rights reserved.