Operational nowcasting of the sun protection factor for the urban area of Vienna

A method was developed to produce hourly updated nowcasts of recommended su n protection factors (SPF) for different photobiological skin types. Actual UV (250-400 nm) measurements and a forecast of the UV index provide the ba sis of the method. The method, used operationally for the urban area of Vie nna, provides a useful tool to inform people about the use of sunscreens, t hereby lowering the risk of health damage from overexposure to solar ultrav iolet radiation. The validation of the nowcast shows that deviations from t he measured daily dose depend on the time of the nowcast. Forecasts made 2 h before solar noon produce deviations of the daily dose of less than +/-0. 5 SPF (for skin type I) in 68% of all cases; forecasts made two or more hou rs after noon were, in every case, within +/-0.5 SPF of the observed value.