The solar-radiation variation over horizontal surfaces calculated by the ASHRAE clear-sky model is compared with measurements for Riyadh, Saudi Arabia. Both model results and measurements are averaged on an hourly basis for all days in each month of the year to get a monthly-averaged hourly variation of the solar flux. The measured data are further averaged over the years 1996-2000. The ASHRAE model implemented utilizes the standard values of the coefficients proposed in the original model. Calculations are also made with a different set of coefficients proposed in the literature. The results show that the ASHRAE model calculations generally over-predict the measured data particularly for the months of October May. A daily total solar-flux is obtained by integrating the hourly distribution. Based on the daily total flux, a factor Phi (<1) is obtained for every month to adjust the calculated clear-sky flux in order to account for the effects of local weather-conditions. When the ASHRAE model calculations are multiplied by this factor, the results agree very well with the measured monthly-averaged hourly variation of the solar flux. It is recommended that these adjustment factors be employed when the ASHRAE clear-sky model is used for solar radiation calculations in Riyadh and localities of similar environmental conditions. Instantaneous, daily and yearly solar-radiation on various surfaces, such as building walls and flat-plate solar collectors, can then be conveniently calculated using the adjusted model for different orientations and inclination angles. The model also allows the beam, diffuse and ground-reflected solar-radiation components to be determined separately. Sample results characterizing the solar radiation in Riyadh are presented by using the "adjusted" ASHRAE model. (C) 2004 Elsevier Ltd. All rights reserved.