Analysis and cloudiness influence on UV total irradiation

Ultraviolet (UV) total solar irradiation, 290–385 nm (UVT), at ground level in Valladolid, Spain (lat. 41°40′N, long. 4°50′W and 840 m a.s.l.), has been recorded from February 2001 to June 2008. The temporal evolution of the UVT irradiation follows a sinusoidal pattern with maximums in summer and minimums in winter. A complete statistical study of hourly and daily values of UVT solar irradiation has been carried out. The results show that the stability is greater in the summer months and it can be concluded that the maximum values can be considered as representative of UVT irradiation. During an average year, the accumulated UVT solar irradiation is 244.5 MJ m−2. The relationships between UVT and global solar irradiation have been analysed finding a relationship of 4.2% between them. A potential relationship between both also provides good results. The dependence of UVT solar radiation with solar elevation and cloudiness has been studied. It can be concluded that solar elevation is the main factor to take into account for modelling the UV solar radiation. The dependence of UV/G ratio with cloudiness has been shown introducing the clearness index or global hemispherical transmittance (kt). This ratio has an increasing trend with the cloud cover and hence, it can be deduced that the presence of clouds reduces the global radiation more strongly than the UV component does. UV hemispherical transmittance (ktuv) has been evaluated. It has been found that ktuv values represent almost 70% of the global hemispherical transmittance in Valladolid city. With a potential relationship between both transmittances coefficient of determination values higher than 0.9 are also achieved. Copyright © 2010 Royal Meteorological Society

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