Relationships between UV (0.290–0.385 μm) and broad band solar radiation hourly values in Valencia and Córdoba, Spain

Data were analysed of hourly UV (0.290–0.385 μm) and broad band, G, irradiation on a horizontal plane measured in Córdoba and Valencia, Spain, during the period January 1996 to December 1998. The mean monthly hourly values of the quotient between UV and total global radiation varied within the interval 4.4–5.6% for Valencia with a mean value of 5%, and in Córdoba they ranged from 3.9 to 4.5% with a mean value of 4.2%. If a general linear relation UV=mG is considered, the square of coefficient of correlation r2 is always greater than 0.91 for Valencia and 0.97 for Córdoba. The UV hemispherical transmittance ktUV was modelled as a function of the total clearness index kt, and new correlations were determined permitting improvements in estimation and to reduce the local nature of the correlations. Also, the solar elevation was considered as an additional parameter in the correlation between ktUV and kt, and it was proven that the coefficients of correlation improved for solar elevations over 55°. The ktUV index represented approximately 75% of the kt in Valencia and 71% in Córdoba.