Dependence on latitude of the relation between the diffuse fraction of solar radiation and the ratio of global-to-extraterrestrial radiation for monthly average daily values

An approach for the prediction of the monthly average daily diffuse radiation, Hd, was proposed by Page in 1961. The Page method is based on the use of the linear correlation HdH = c + dHHo, where H and Ho are, respectively, the monthly average daily values of global and extraterrestrial radiation, both on a horizontal surface. The values of c and d are a function of atmospheric conditions. cloud cover conditions/types, as well as latitude. In the present work we study the dependence on latitude of c and d for European locations with 36°N < (γ) < 61°N (longitudes between 29°E and 11°W). The dependence is first studied using 28 values of c and d obtained using experimental values of H and Hd. Next the dependence on γ is studied using experimental values of H for 64 European locations, obtained for the period 1966–1975, and corresponding values of Hd estimated using the European Community Solar Radiation Model (E.C.S.R.M.). In both cases a minimum for c vs. γ and a maximum for d vs. γ are obtained for similar values of γ. Using the E.C.S.R.M. it is shown that both, the minimum and the maximum can be explained by the way HdH and HHo vary with γ.