Estimation of sunshine duration from global radiation measurements for general application purposes

A Simplified Radiation Transfer Model is used to compute sunshine values from 10 minutes global radiation data. The algorithm used consists in the correction of the modeled diffuse clear sky radiation using the measured to clear sky model ratios. Direct radiation is then computed from the measured global radiation and the estimated diffuse radiation. If the direct radiation is greater than 120 Wm(-2) the 10 minutes are added to the sunshine value. The algorithm was applied to 4 stations located in Portugal, Azores and Madeira Islands. Results shown an acceptable errors of +/- 0.1 hour for 55% to 75% of hourly data +/- 1 hour for 70% to 80% of daily data.