A method was developed to produce hourly updated nowcasts of recommended sun protection factors (SPF) for different photobiological skin types. Actual UV (250-400 nm) measurements and a forecast of the UV index provide the basis of the method. The method, used operationally for the urban area of Vienna, provides a useful tool to inform people about the use of sunscreens, thereby lowering the risk of health damage from overexposure to solar ultraviolet radiation. The validation of the nowcast shows that deviations from the 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 hours after noon were, in every case, within +/-0.5 SPF of the observed value.