“…It is called the clearness index because it decreases with increasing atmospheric attenuation of solar radiation, mostly determined by cloudiness. The true physical value of the clearness index is instantaneous although it can be used also with hourly values Ogunjobi and Kim, 2004), daily values (Martínez-Lozano et al, 1994a;Kudish and Ianetz, 1996;Ulgen and Hepbasli, 2002;Santos et al, 2003) or monthly values (Udo, 2000;Ulgen and Hepbasli, 2002;Kudish et al, 2005). In order to study the total UV radiation, Martínez-Lozano et al (1994b) defined an ultraviolet clearness index k tUV , with a similar form to k t but referring only to the UV spectral range and using the solar constant for UV radiation, I SCUV (81.13 Wm −2 ) obtained from the SUSIM Atlas 3 extraterrestrial spectrum (http://www.solar.nrl.navy.mil/susim atlas data.…”