Estimation of direct solar beam irradiance from measurements of the duration of bright sunshine

Measurements of the duration of bright sunshine, n, using the Campbell-Stokes sunshine recorder are shown to be highly correlated with those obtained using a normal incidence pyrheliometer to measure direct irradiance, I, at two sites in the very dissimilar radiation climates of Israel and Ireland. The statistical relationships are presented for a variety of time-scales, ranging from annual to hourly totals. For individual mean monthly values the common linear relationship was I=2.209 n-0.955 MJ m(-2) day(-1), with a standard error of estimate of +/-1.368 MJ m(-2) day(-1) and a coefficient of determination of r(2)=0.969. Analysis of long-term series of sunshine hour measurements indicate that reductions in direct irradiance have occurred at both measurement sites. (C) 1997 Royal Meteorological Society.