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
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.
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).