Comparison of the Angstrom-type correlations and the estimation of monthly average daily global irradiation

Linear regression analysis of the monthly average daily global irradiation and the sunshine duration data of eight Zambian locations has been performed using the least square technique. Good correlation (r greater than 0.95) is obtained in all cases showing that the angstrom equation is valid for the Zambian locations. The values of the correlation parameters thus obtained show substantial unsystematic scatter. The analysis was repeated after incorporating the effects of (1) multiple reflections of radiation between the ground and the atmosphere, and (2) not burning the sunshine recorder chart, into the Angstrom equation. The surface albedo measurements at Lusaka were used. The scatter in the correlation parameters was investigated by graphic representation, by regression analysis of the data of the individual stations as well as the combined data of the eight stations. The results show that the incorporation of none of the two effects reduces the scatter significantly.