Since 2000, a double system of cloud determination is installed at the Uccle (Belgium) UV-Visible monitoring station: it consists in a Total Sky Imager - TSI (Yankee Environmental System, US) operating with a visible CCD camera, and a Cloud Infrared Radiometer - CIR-7 (Atmos, F) operating with 7 IR (9-14 µm) pyrometers. From these two instruments, accurate measurements of the nebulosity and the ceiling heights are obtained and integrated in the “atmospheric condition” determination indispensable for a detailed interpretation of the UV-Visible solar irradiances measurements.

The attenuation of the global UV (280-400 nm) solar spectral irradiances will be discussed for different type of clouds conditions (nebulosity, low altitude, middle altitude and high altitude; multiple layers…) and for different conditions of observation (various Solar Zenith Angle, different total ozone column…)

The better understanding of the cloud effect on the radiation transfer processes permits to propose an “empirical cloud model” for the prediction of the UV indices. This last aspect will be also illustrated and discussed.