Measurements and attenuation of erythemal radiation in Central Spain.

Biologically active ultraviolet (UVER, 280-400 nm) and global (300-3000 nm) solar irradiances have been recorded and analysed in a rural area in Valladolid (Spain) for a period of 49 months between June 2002 and December 2008. UVER daily measurements follow the pattern of the solar elevation angle and maximum hourly average irradiances occur in July, 0.20 Wm−2 and minimum, 0.03 Wm−2, in December at noon; the total accumulated UVER along a year reaches 1470 kJm−2. The temporal evolution of the erythemal ultraviolet radiation and its distribution under different sky conditions have been studied, being 25% clear, 57% partly cloudy and 18% overcast days. The most representative statistical characteristics of the hourly and daily UVER data have been evaluated for different sky conditions and the fundamental indices show symmetry with respect to solar noon. Maximum values may be considered as representative of the UVER characteristics in the area, whereas the minimum values are not significant, mainly during summertime. The UVER records show quite stable values in the analysed period, particularly along the months of July. The isoline-set of the first four moment distribution of the hourly UVER values has been plotted and the relative gradients are shown. The results are compared with the values obtained in other stations. The UVER cloud attenuation through the atmosphere shows that the transmitted percentage increases from winter to summer and a maximum value of 10% was achieved in July and a minimum of 30% in winter and spring. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]