Modified daylength and the computation of global insolation

Global irradiation and sunshine-duration data reported for 14 locations in the Southern African region are employed to develop correlations for estimating the regression parameters a and b used by Hay for incorporating the effects of multiple reflection and performance characteristics of a sunshine-recorder chart for small elevation of the sun. The developed correlations allow us to estimate the constants a and b from the latitude, the elevation and sunshine duration for a given location. A new correlation is also suggested to evaluate H′ directly from the latitude, elevation and sunshine duration without calculating the regression parameters. The applicability of the developed equations is tested by evaluating global radiation data for four new locations. Excellent agreement between experimental and estimated data shows the wide applicability of the model to all locations in the Southern African region.