Global solar irradiation and sunshine duration on the horizontal surface are measured with radiometers, pyranometers and sunshine recorders. Depending on atmospheric conditions, differences occur between total extraterrestrial and measured
global solar irradiation for the same time and unit area. The meteorological events affect directly on the solar energy calculations in a random manner due to weather conditions which give rise to temporal and spatial variations. For these reasons, randomness occurs in the solar irradiation and sunshine duration evolutions. The meteorological variability reduces the astronomical daily H0 and S0 values in two ways.
1. The astronomical H0 and S0 are shortened due to meteorological events, which are measured at a solar station as global solar irradiation, H, and sunshine duration, S. In other words, S < S0 and H < H0,
2. The shortening effect is not definite but might be in the form of different random amounts during a day or month depending on the climate and atmospheric conditions. These are solar irradiation scattering by air molecules, water, dust and aerosols as well as absorption by O3, H2O, CO2 and other greenhouse gasses.
Consequently, ratios of measured solar energy variables at surface to their astronomical counterparts H/H0 and S/S0 assume values between zero and one in a random manner depending on the cloud cover and atmospheric turbidity of the period concerned.