The all-sky meteorological radiation model is a broadband solar-radiation estimation model that uses synoptic and sunshine information. The original model due to Muneer-Gul-Kambezidis was improved using regressions based on the sunshine fraction to increase the accuracy of the estimation of diffuse horizontal irradiation, thus achieving an accuracy increase for the estimation of the global horizontal irradiation. The improved model was validated using data from ten worldwide sites and using three statistical indicators:-coefficient of determination between computed and measured global irradiation data and the relevant, mean bias error and the root mean square error of the computed global irradiation. The performance of the new model was improved when compared to that of the original model. The new regression coefficients were found to be more accurate in estimating global horizontal radiation for both fine and coarse datasets. (c) 2005 Elsevier Ltd. All rights reserved.