The following image describes the annual pattern of solar radiation absorption at the Earth's surface for the year 1987. Within this zone there are localized maximums over the tropical oceans and deserts where the atmosphere has virtually no cloud development for most of the year. Outside the tropics, annual receipts of solar radiation generally decrease with increasing latitude.
Minimum values occur at the poles.
This pattern is primarily the result of Earth-sun geometric relationships and its effect on the duration and intensity of solar radiation received. Greater cloudiness over land surfaces accounts for this variation. NASA's Surface Radiation Budget Project has used satellite data, computer models, and meteorological data to determine shortwave surface radiation fluxes for the period July 1983 to June 1991.