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Heat balance, terrestrial atmospheric

The balance of various types of energy in the atmosphere and at the Earth's surface. At the top of the atmosphere, the incoming solar radiation that is absorbed by the Earth-atmosphere system is approximately balanced by the terrestrial radiation emitted from this system over long periods of time. The flux of solar energy (energy per time) across a surface of unit area normal to the solar beam at the mean distance between the Sun and the Earth is referred to as the solar constant. Based on recent satellite measurements, a value of 1365 watts per square meter (W/m2) for the solar constant 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 341 W/m2. Based on measurements from satellite radiation budget experiments, 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 various types of clouds, as well as reflection by different surfaces. Thus, only about 70% of the incoming solar flux, that is, about 239 W/m2, is absorbed within the Earth-atmosphere system. For this system to be in thermodynamic equilibrium or balance, it must emit the same amount of thermal infrared radiation.Solar energy

For the presentation of internal heat balance components, the effective solar constant of 341 W/m2 may be arbitrarily represented by 100 units . Of these units, roughly 26 are absorbed within the atmosphere, including 22 by clear column and 4 by clouds. A total of 30 units are reflected back to space, including about 7 from clear column, 17 from cloudy atmospheres, and 6 directly from the Earth's surface. The remaining 44 units are absorbed by the surface. The Earth-atmosphere system emits terrestrial radiation according to its temperature and composition distributions. The upward flux from the warmer surface accounts for about 115 units. The colder troposphere emits both upward and downward fluxes, with about 70 and 100 units at the top and surface, respectively. The clear and cloudy portions are 34 and 36 at the top and 33 and 67 at the surface, respectively. The net upward flux at the surface, representing the difference between the flux emitted by the surface and the downward flux from the atmosphere reaching the surface, is about 15 units.Terrestrial radiation

As a result of thermal emission, the atmosphere loses 55 units. With absorption of the incoming solar flux contributing only 26 units, the net radiative loss from the atmosphere amounts to about 29 units. This deficit is balanced by convective fluxes of sensible and latent heat associated with temperature gradient and evaporation. Based on statistical analyses, the average annual ratio of sensible to latent heat loss at the surface has a global value of about 0.27. It follows that the latent and sensible heat components are about 23 and 6 units, respectively, in order to produce an overall heat balance at the surface . The atmosphere experiences a net radiative cooling that must be balanced by the latent heat of condensation released in precipitation processes and by the convection and conduction of sensible heat from the underlying surface.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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