Correlation of diffuse solar radiation with air mass

Abstract

Measurements of total and diffuse components of solar radiation have been conducted using pyranometers at Zaria (latitude 11°06′N, longitude 07°41′E and altitude of 653.9 m) and Kano (latitude 12°03′N, longitude 8°32′E and altitude of 472.1 m) in northern Nigeria during the period when the dust-bearing north-easterly harmattan winds blow from the Sahara. The measurements have been made for several years and a correlation has been sought for the large amount of data obtained. By means of a simple theoretical consideration, it was shown that the main parameters are the mean optical thickness, the air mass and the diffuse ratio (ratio of the diffuse to the total irradiance). Accordingly, the data for every hour has been processed to give these parameters and the optical thickness was plotted against the diffuse ratio for various ranges of air mass. The results portrayed a satisfactory correlation between the diffuse ratio and the optical thickness as well as the air mass. For the purpose of presentation in more “conventional” form, the results were transposed on to plots of diffuse ratio against clearness factor. In this way, the results become more amenable to comparison with related previous work.