Estimation of the Monthly Mean Values of the Diffused Solar Energies per Hour from the North Sky in Three Spectral Regions

Abstract

The diffused solar energy within solid angle of 0.586π sr from the north sky was measured in each of three spectral regions, that is, ultraviolet region (300-400 nm), visible region (400-700 nm) and infrared region (700-1, 200 nm).
We discussed the correlation between those diffused solar energies per hour and cloud amount or visibility. As a result, following conclusions are obtained.
(1) If the thickness of cloud is constant, each diffused solar energy per hour from the north sky in three spctral regions, divided by the sine of the hourly mean value of solar altitude, is approximately the linear function of cloud amount in the north sky.
(2) The monthly mean value of the diffused solar energy per hour per unit area in each of three spectral regions Q_i, within solid angle of 0.586π sr from the north sky on the north inclined plane of 0.25 π rad, can be estimated by the following approximate formula that is derived from using the monthly mean value of cloud amount in the one hour d and that of solar altitude in the one hour h.
Q_i=(a_i+b_i·d)sin h (W·h/m2·hr)
Here, a_i and b_i are the constants obtained by the regression line in each of three spectral regions. And, this formula can be applied in one hour of any time.