A worldwide forecast of the global solar erythemally effective UV irradiance, also called UV Index, under clear skies is presented. The forecast was established to inform the public about the expected amount of erythemally effective ultraviolet radiation for the next day. The forecast enables the user to choose the appropriate sun protection tool and to adopt the personal behavior to reduce the risk of health damage. The forecast as well as the radiation model are validated using measurements from a Robertson-Berger meter made at four continents (Antarctica, Australia, America, and Europe). The measurements cover the latitudinal range from 67.4 degreesN to 60.1 degreesS. Since the UV Index was determined to quantify UV radiation protection, the goal of a forecast scheme is to avoid underestimation. Therefore the frequency of underestimation was used as quality criteria also. The Austrian forecast model shows less than 7.3% underestimation in all cases, whereas 5.0% result from the radiation model and 2.3% from the used value of the total ozone content of the atmosphere. (C) 2002 Society of Photo-Optical Instrumentation Engineers.