Correlations between values of daily horizontal beam and global radiation for Beer Sheva, Israel

Individual monthly and seasonal correlations have been developed for Beer Sheva, Israel, for calculating daily values of horizontal beam radiation from measured global radiation via the clearness index. The data base comprises normal incidence and global radiation measurements. The dependent variable in the correlations was either the beam fraction or the beam transmittance; the latter is defined as the ratio of the horizontal-beam to the extraterrestrial radiation. Three different correlation formats were tested, based on the linear beam fraction and the exponential and linear beam transmittances. The most recent 12 months of data were reserved to validate the empirical regression correlations and to compare their predictive utilities. The results of our analysis for this site showed that replacing the beam fraction with beam transmittance as the dependent variable did not improve the predictive capability of the correlations significantly. The linear beam-transmittance correlations were somewhat inferior to the usual linear beam fraction format. Also, the performance of the exponential-beam transmittance relative to the linear-beam-fraction correlations was erratic (viz. superior for some months and inferior for others). The predictive capabilities for the seasonal correlations were comparable to those for the individual monthly correlations. The correlations for both the linear-beam-fraction and exponential-beam-transmittance formats varied significantly throughout the year, with values of the average monthly coefficients of variation varying from < 10% to > 20%.