Data were analysed of hourly UV (0.290-0.385 mum) and broad band, G, irradiation on a horizontal plane measured in Cordoba 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 Cordoba 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 r(2) is always greater than 0.91 for Valencia and 0.97 for Cordoba. The UV hemispherical transmittance k(tUV) was modelled as a function of the total clearness index k(t) 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 k(tUV) and k(t), and it was proven that the coefficients of correlation improved for solar elevations over 55degrees. The k(tUV) index represented approximately 75% of the kt in Valencia and 71% in Cordoba.