Heat balance of the Earth
Liou, Kuo-Nan Department of Atmospheric Sciences, University of California, Los Angeles, California.
Last reviewed:November 2019
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The balance of various types of energy in the atmosphere and at the Earth's surface. Essentially all the energy that the Earth–atmosphere system receives comes from the Sun. This energy is conventionally referred to as the solar constant, and is defined as the flux of solar energy (energy per time) available on 1 square meter facing the Sun at the top of the atmosphere when the Earth is at its mean distance from the Sun. On the basis of recent satellite observations, a value of about 1336 watts per square meter (W/m2) has been suggested. Because the area of the spherical Earth is four times that of its cross section facing the parallel solar beam, the top of the Earth's atmosphere receives an average of about 342 W/m2. Based on analysis of the observed data from satellite radiation budget experiments in the last 40 years, about 30% of this is reflected back to space and is referred to as the global albedo. The reflecting power of the Earth–atmosphere system includes the scattering of molecules, aerosols, and clouds, as well as reflection of different types of surfaces. As a consequence of this global albedo, only about 70% of the incoming solar flux (that is, about 239 W/m2) is available on average to warm the Earth–atmosphere system. For this system to be in thermodynamic equilibrium or balance so that an equilibrium temperature can be defined, it must radiate the same amount of energy (239 W/m2) back to space. The emitted (or outgoing) terrestrial radiation from the Earth and the atmosphere having an equilibrium temperature of about 254 K (−2.5°F) is in the infrared portion of the electromagnetic spectrum and is called the thermal infrared radiation or longwave radiation. This is differentiated from the solar radiation or shortwave radiation from the Sun, which has an effective temperature of about 5800 K (10,000°F). See also: Albedo; Atmosphere; Heat balance; Solar constant; Solar energy; Terrestrial radiation
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