Two Methods for Retrieving UV Index for All Cloud Conditions from Sky Imager Products or Total SW Radiation Measurements

Cloud effects on UV Index ( UVI ) and total solar radiation ( TR ) as a function of cloud cover and sunny conditions (from sky images) as well as of solar zenith angle ( SZA ) are assessed. These analyses are undertaken for a southern‐hemisphere mid‐latitude site where a 10‐years dataset is available. It is confirmed that clouds reduce TR more than UV , in particular for obscured Sun conditions, low cloud fraction (<60%) and large SZA (>60°). Similarly, local short‐time enhancement effects are stronger for TR than for UV , mainly for visible Sun conditions, large cloud fraction and large SZA . Two methods to estimate UVI are developed: (1) from sky imaging cloud cover and sunny conditions, and (2) from TR measurements. Both methods may be used in practical applications, although Method 2 shows overall the best performance, as TR allows considering cloud optical properties. The mean absolute (relative) differences of Method 2 estimations with respect to measured values are 0.17 UVI units (6.7%, for 1 min data) and 0.79 Standard Erythemal Dose ( SED ) units (3.9%, for daily integrations). Method 1 shows less accurate results but it is still suitable to estimate UVI : mean absolute differences are 0.37 UVI units (15%) and 1.6 SED (8.0%).