The influence of cloudiness on UV global irradiance (295–385 nm)

Two years of continuous measurements of UV (295–385 nm) irradiance recorded at Granada (37.18°N, 3.58°W, 660 m a.m.s.l.), Spain, were combined with concurrent synoptic cloud observations to establish the relative influence of clouds on UV irradiance. A marked influence of cloud cover on UV irradiance is evident, but negligible for fractional cloud coverage below 3 octas. The ratio of UV irradiance to the total solar irradiance increases with cloud cover, especially for cloud cover greater than 4 octas, highlighting the different effects of cloud on total solar irradiance and UV irradiance. In order to determine the effect of cloud on UV irradiance, we considered a cloud modification factor, defined as the ratio between the UV measurements and the corresponding clear sky UV irradiance that would be expected for the same time period and atmospheric conditions. It is shown that the effect of cloud for UV wavelengths is less than that for the whole solar spectrum and less than that for the visible part of the spectrum. On the other hand, the small influence of cloud cover for intermediate cloudiness was accompanied by low variability and variability increases with increasing cloud cover.