Influences of the clearness index for the whole spectrum and of the relative optical air mass on UV solar irradiance for two locations in the Mediterranean area, Valencia and Cordoba

Ultraviolet solar irradiance (290–385 nm) was analyzed in Valencia and Cordoba, Spain, during 1996–1997. Its relation to the instantaneous atmospheric clearness index kt, the relative optical mass mr, the time of the year, and the total or global irradiance on a horizontal surface was established. The maximum values of UV irradiance for each optical mass exponentially diminished with the optical mass, thus for Cordoba and Valencia, Guv = 62.23 mr−1.34 and Guv = 52.84 mr−1.30, respectively. The UV irradiance in Cordoba represented 4.2% of the total irradiance while in Valencia this percentage was 4.9%. In the case of cloudy days (kt<0.4), these percentages ranged between 5.6% and 6.8% in Valencia and between 5.1% and 5.5% in Cordoba.