A model for climatological estimations of global, diffuse, and direct solar radiation on a horizontal surface

Abstract

A simple model based on the intensity of direct and diffuse radiation expressed as fractions of the intensity of the extraterrestrial radiation has been developed. Using these expressions and assuming a random distribution of bright sunshine hours with not too wide a variation in the transmission coefficient values, a number of equations for estimating the climatological values of global and diffuse irradiation are derived. Four of the relationships, including Angstrom's correlation for estimating global irradiation, are already known empirically while several new ones are formulated. The empirically known relationships are consequently provided with a theoretical basis and the estimation techniques are made more flexible with the new equations. This study also clarifies the nature of the constants appearing in the various equations. Three independent basic parameters in the model are identified and the constants appearing in the various equations are simple functions of these three basic parameters, and thus the interrelationships between the various constants are unified. The experimental data for diffuse irradiation, global irradiation and bright sunshine duration from various locations throughout the world between latitudes 20°S and 50°N correlate well with the linear equations, and the nature and interrelationships of the constants are found to be as predicted by the theory.