Correlations to estimate monthly mean daily solar global irradiation: application to Romania

Seven existing relationships between monthly mean clearness index and the number of bright sunshine hours are tested under the climate and latitude of Romania. New (best-fit) correlations are derived. They use bright sunshine hour number or fractional total cloud amount as input. The dependence of the new sunshine-based regressions on month is stronger than their dependence on latitude. They generally underestimate the clearness index in the first half-year and overestimate it in the rest of the year. The accuracy of the new regression seems not to depend significantly on the year. (C) 1999 Elsevier Science Ltd. All rights reserved.