In studies, mainly by experimental method, it is common practice to predict a luminous quantity for partly cloudy skies from values for cloudless and overcast skies using parameters such as the sunshine fraction or the cloud ratio. In the present work, it is shown that simple linear relations with high correlation coefficients can be used to predict mean zenith luminance values for each of the three mentioned sky conditions from mean zenith luminance values for any of the other two sky conditions. (C) 2003 Published by Elsevier Ltd.