In this approach, relation between extraterrestrial variables (length of day and solar irradiation) ratio and terrestrial variables (measured sunshine duration and global solar irradiation) ratio are taken into consideration. This approach has not any restrictive assumptions and the main idea is to suggest a practical formulation for solar irradiation and sunshine duration estimations.
As mentioned before, solar irradiation and sunshine duration records depend on combined effects of astronomical and meteorological events. Meteorological events effects on the solar energy calculations introduce random behaviors. Meteorological solar irradiation (terrestrial) H and sunshine duration S variables have randomness in their temporal and spatial evolutions due to the shortening effect, in other words, the reduction amount of solar irradiation and sunshine duration is not definite but might be in the form of random amounts during a day or month depending on the atmospheric composition, climate and weather conditions.