Within this zone there are localized maximums over the tropical deserts where the atmosphere has virtually no cloud development for most of the year. Insolation quantities at the equator are approximately the same as values found in the middle latitudes. Considerable energy from the sun is trapped in the atmosphere above the equator because of the absorption of sunlight by greenhouse gases and water. Most of this energy is radiated as longware radiation to the Earth's surface where it creates heat. 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.