Tests of the Suerhcke relationship have been made with a properly selected database of global irradiation and sunshine hours. A simple clear sky model, based on a generalisation of Beer's law using a unique dispersion parameter for the entire wavelength spectrum, has been used. It is shown that the Suerhcke relationship forecasts the solar radiation with a mean quadratic error of slightly better than the precision given by the Angstrom-Prescott model. Moreover. the Suerhcke model offers a great potential for improving the estimation process since. being based on the, physics of the underlying fundamental phenomena, it can be used with more elaborated and/or more precise clear sky models. It should be remarked that this procedure, by converting the large database of sunshine hours, existing in the world, into global irradiation, might expand considerably the information about the earth solar resource at a negligible cost, when compared to the use of satellite-based procedures or earth-based pyranometers networks. (C) 2004 Elsevier Ltd. All rights reserved.