This figure shows the prediction (grey line and light colors), as well as the observation (black line with marks, dark colors) of the UV index of today.
The forecast is based on a model with the efemerides and the ozone column as input parameters. The observations (made each half hour) are performed with a UV spectrophotometer at the RMI in Uccle. 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. The forecasted value is always for clear-sky conditions. At (heavy) cloud cover the measured values are necessary (much) lower than the forecasted ones.
You will find more information about the UV-index on the pages of the "ozone research group" of RMI.