Solar radiation, the electromagnetic radiation emitted by the Sun, is the essential source of energy and life on Earth. It drives the atmospheric and oceanic currents, evaporates the water that later falls as rain and snow, and induces the plant photosynthesis that provides food, fiber, and fuel.

Only about 1/2,000,000 of all the energy emitted by the Sun is received by the Earth, 150 million km (93 million mi) distant. The average amount of energy received in a unit time by a unit area of atmospheric surface is given by a quantity called the solar constant. Satellite measurements have yielded a value for the constant of about (1.373 ± 0.008) X 106 erg/sec/cm2. Ground-level observations indicate that this value varies by 0.2% or less over a 30-year period.

When solar radiation enters the Earth's atmosphere, it is partially absorbed and partially reflected, largely by clouds, snowfields, and deserts. The fraction reflected, called the albedo, is variously estimated at 28% to 35% for the Earth as a whole. Absorption is by ozone in the stratosphere; by carbon dioxide, water vapor, clouds, and dust in the troposphere; and by the Earth's surface. Thus solar radiation is absorbed primarily by water — in the atmosphere, on the surface, and in plants. Nearly half of this energy — a quarter of the total reaching the uppermost level of the atmosphere — goes to evaporate water.

Radiation leaving the Sun has a wavelength of about 0.2 to 10 micrometers. The ozone layer absorbs radiation in the ultraviolet region, while water vapor absorbs radiation in the infrared and several parts of the visible region.

Arnold Court

Bibliography: Barkstrom, Bruce R., ed., Long-Term Monitoring of the Earth's Radiation Budget (1990); Burton, Jane, and Taylor, Kim, The Nature and Science of Sunlight (1997); Ensminger, Peter A., Life under the Sun (2001); Friedman, Herbert, Sun and Earth (1986); Houghton, J. T., ed., The Global Climate (1984); Huebner, W. F., et al., eds., Solar Photo Rates for Planetary Atmospheres and Atmospheric Pollutants (1992); Iqbal, Mohammad, An Introduction to Solar Radiation (1983).