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 Cord oba, Spain, during 1996-199'7. Its relation to the instantaneous atmospheri c clearness index k(t), the relative optical mass m(r), the time of the yea r, and the total or global irradiance on a horizontal surface was establish ed. The maximum values of UV irradiance for each optical mass exponentially diminished with the optical mass, thus for Cordoba and Valencia, G(uv) = 6 2.23 m(r)(-1.34) and G(uv) = 52.84 m(r)(-1.30), respectively. The UV irradi ance in Cordoba represented 4.2% of the total irradiance while in Valencia this percentage was 4.9%. In the case of cloudy days (k(t) < 0.4), these pe rcentages ranged between 5.6% and 6.8% in Valencia and between 5.1% and 5.5 % in Cordoba.