The direct solar radiation as measured from Manila Observatory from 1972-1980

Abstract

The sun is the source of radiant energy of the earth and also of the other planets. Without the sun, no life could exist on earth. The sun continuously emits radiant energy in space but an insignificant amount of 2.5×10¹⁸ calories per minute penetrates the earth. On its path through the atmosphere the radiant energy undergoes a series of important changes resulting from absorption and scattering. A fraction of this reaches the earth surface as a beam of nearly parallel rays directly from the sun and at the same time a considerable fraction of radiation scattered in the atmosphere reaches the earth's surface. The former is known as the direct solar radiation while the latter is known as the diffuse solar radiation. The sum of the two components comprises the so-called total solar radiation and it could be written as T = D + T_r,
where D is the diffuse solar radiation, T_r is the direct solar radiation.

Recent work of Katsoulis, and Pachristopuolos stated that the variation in the intensity of direct solar radiation cannot be assessed by instant values alone due to the fact that the sun’s altitude varies greatly during the day and from day to day. Barbaro. Coppolino, Tomas & Sinagra proposed an atmosphere model for computing direct and diffuse solar radiation. They used the experimental data of temperature and relative humidity for the model. This model shows a method for the computation of direct and diffuse solar radiation in a cloudy sky. On the other hand, Peterson & Flowers showed that the measurement of direct solar radiation would be used in determining the turbidity of the atmosphere. In their study they used measurements of direct solar radiation to study variations and trends of atmospheric extinction coefficients.
This works aims to find out an estimate of direct solar radiation on a horizontal surface at the Manila Observatory for the following intervals: 6-10, 10-14, and 14-18 hours of the day. Specifically the computed data consists of the following:
a) Comparison of monthly mean values of direct solar radiation from 1972-1980; and
b) Monthly mean percentage of sunshine per hour interval from 1972-1980.