The graph shows the forecasted and measured UV index at Uccle for today. The forecast is based on a model with the ephemerides and the ozone column as main input parameters. The observations (made each half hour) are performed with a Brewer spectrophotometer at RMI. The actual values of the UV index at other sites in the country can deviate significantly from those at Uccle, mainly due to differences in the cloud cover which can attenuate the UV radiation of about 80%. The forecasted value is always for clear-sky conditions. At (heavy) cloud cover the measured values are necessary (much) lower than the forecasted ones.