Calculation of monthly mean solar radiation for horizontal and inclined surfaces

The use of daily rather than hourly time intervals in the calculation of the monthly mean values of solar radiation for both horizontal and inclined surfaces has for long been a desirable objective due to a reduction in the required computational effort and more general availability of daily data. This paper presents a method for such calculations, with bright sunshine hours and surface albedo being the only required input data. The incorporation of the effects of multiple reflection between the ground and atmosphere results in relationships used to calculate the diffuse and total solar radiation for a horizontal surface which are largely independent of season and location. The present paper shows that, despite the use of a daily time interval, the solar radiation incident on both horizontal and south-facing surfaces may be calculated with an accuracy generlly associated with instrumental measurements. This result is achieved without the necessity of arbitrarily varying the empirical coefficients to account for variations in climatological conditions at the stations being studied.