Quoted from:[1] Swartman R K ,  Ogunlade O . Solar radiation estimates from common parameters[J]. Solar Energy, 1967, 11(3):170-172.

https://www.sci-hub.ren/10.1016/0038-092x(67)90026-6

ABSTRACT

In tropical countries, precipitable water, W, is not a good basis for predicting solar radiation because of the narrow range of fluctuation in this parameter. Relative humidity, however, does fluctuate over a larger range and has been used in this study. The following empirical relationships between solar-radiation intensity, H, ratio of hours of bright sunshine to twelve hours, S, and relative humidity of the environment, R, have been obtained by statistical methods applied to the available data.These equations give better estimates of solar-radiation intensity than the single-parameter relationship like H=a+bS Charts have been constructed from the empirical relations to simplify the use of the expressions.

BACKGROUND

In tropical countries, precipitable water fluctuation is very small, whereas relative humidity varies over quite a sizeable range. In the literature, 2 water vapor has been shown to exert a considerable influence on both direct and sky radiation. From the thermodynamic definition, relative humidity can be taken as a measure of the amount of water vapor in the earth's atmosphere at the existing temperature. By virtue of the engineer's familiarity with relative humidity, its introduction as an estimator of solar radiation looks quite attractive.