The effects of clouds on solar radiation
Abstract:
Measured solar data for Amman (lat 32°N) were used to compute the empirical constants in the following correlation proposed by Page:
where 
and 
are monthly average daily global, diffuse and extraterrestrial radiation on a horizontal surface, and c and d are empirical constants. The measured solar data were classified into 12 groups of days depending on the ratio S/So, where S is the daily number of sunshine hours and So is the day length. The empirical constants c and d were evaluated for each group. The computed values of the constants were used to develop correlations for both constants. Finally, values for c and d were also obtained under clear sky conditions (S/So = 1.0) and under average sky conditions (0.0 < S/So < 1.0). The obtained values of the diffuse radiation component, using these empirical constants, under both sky conditions showed very good agreement.
URL: http://www.sciencedirect.com/science/article/pii/019689049390005U