Solar Radiation

The seasons on the Earth are not caused by how close the Earth is to the Sun. The Earth is closest to the Sun on or about January 1, and farthest away on or about July 1 each year. The reason for the seasons lies in the amount of the Sun's radiation that reaches the Earth.

The amount of energy put out by the Sun is a constant. The incoming solar radiation is known as insolation. The amount of solar energy reaching the Earth is 70 percent. The surface of the Earth absorbs 51 percent of the insolation. Water vapor and dust account for 16 percent of the energy absorbed. The other 3 percent is absorbed by clouds. Of the 30 percent that is reflected back into space, 6 percent is reflected by air and dust. Clouds reflect 20 percent, and the remaining 4 percent is reflected by the surface. The energy that was absorbed can be reradiated. Of the reradiated energy, 70 percent is lost to outer space. The surface is responsible for 21 percent of this, and the remaining 49 percent is lost by the atmosphere. The remaining 30 percent is transferred by the surface to the atmosphere.