The solar radiation is in the form of short waves. These waves strike the outer surface of the atmosphere. About 53% of the energy is lost through scattering by gas molecules, reflection by clouds and absorption by water vapor and other particles. Only about 47 percent of the energy reaches the surface.

Controlling factors: Insolation is not uniform on all parts of the earth’s surface. It differs from one region to another and also from one season to another. The important factors determining the amount of solar radiation are:

a. The angle of incidence of the sun’s rays: The sun rays do not strike the earth surface at an. equal angle. They are almost perpendicular as well as concentrated near the equator and slanting as well as spread over a large area at the poles. 

b. Atmosphere: The amount of insolation also depends on the atmosphere through which it passes. The amount of cloud cover, its thickness,, dust, water vapour absorbs temperature.

c. Duration of Day: The duration of sunshine or day varies with the seasons and latitudes. Longer the duration of sunshine, more will be the insulation and vise versa, d. Distance between the earth and the sun: The earth revolves around the sun on its orbit. It is nearest to the sun on January 3 and far away on July Thence the solar radiation is more in January than in the month of July.