Atmospheric back-radiation

Fig. 2.13 Radiation and heat balance of the system earth-atmosphere. Percentages are given in relation to the incoming solar radiation (100% = 343 W m ). I interception of solar radiation by molecules/particles (p) clouds (c), and earth surface (e), D diffuse radiation by molecules/particles (p) clouds (c), R reflexion (albedo) by molecules/particles (p) clouds (c), and earth surface (e), IR infrared dissipation to space by molecules/particles (p) clouds (c), T terrestrial radiation back to space (without absorption), A absorption of terrestrial radiation by molecules, AB atmospheric back-radiation, SH and LH sensible and latent heat, resp.

In the terrestrial radiation (budget -19%) at the earth s surface, one has to consider a split between absorption of terrestrial radiation by molecules (99%) and so-called atmospheric back-radiation (95%), while 20% is radiated back into space. The budget (-4%) must be added to the transmission of the 15% of terrestrial radiation which can pass the atmosphere without absorption (through the atmospheric windows) to space. This back-radiation is the reason that the earth s surface has a higher temperature than would be expected from the effective radiation temperature, as mentioned above. 

The temperature of Earth is determined by the balance between solar radiation input (see Fig. 2-30) and reflection and reradiation of energy from Earth and its atmosphere back into space. Recall from Section 4.3.1 that the solar constant is approximately 1400 W/m2. The projected area intercepting the solar radiation is 7rr2, where r is Earth s radius. Given that Earth s surface area is 47rr2, the average solar radiation received outside Earth s atmosphere is approximately one-fourth the solar constant, or 350 W/m2. 

One design for a low temperature convection furnace shown in Figure 4 utilizes an external circulating fan, heating chamber, and duct system. The fan draws air (or a protective atmosphere) from the furnace and passes through the external heating chamber and back into the furnace past the work. This system minimizes the chance that the work receives any direct heat radiation. In theory it is less efficient because the external blower, heating chamber, and ductwork add external surfaces that are subject to heat losses. 

Fig. 17-4. Radiation heat balance. The 100 units of incoming shortwave radiahon are distributed reflected from earth s surface to space, 5 reflected from cloud surfaces to space, 20 direct reaching earth, 24 absorbed in clouds, 4 diffuse reaching earth through clouds, 17 absorbed in atmosphere, 15 scattered to space, 9 scattered to earth, 6. The longwave radiation comes from (1) the earth radiating 119 units 101 to the atmosphere and 18 directly to space, and (2) the atmosphere radiating 105 units back to earth and 48 to space. Additional transfers from the earth s surface to the atmosphere consist of latent heat, 23 and sensible heat, 10. Source After Lowry (4).

Greenhouse effect The retention of heat by the earth and the atmosphere due to certain gases being transparent to incoming solar radiation but opaque to the longer-wave radiation back from the earth. 

Considerable energy is radiated back from Earth s surface into space as long-wave infrared radiation. The atmosphere absorbs some of this infrared radiation, preventing its loss to space. This trapping is sometimes referred to as the Greenhouse Effect. ... 

A non-uegligible fraction of the solar radiation incident on the earth is lost by reflection from the top of the atmosphere and tops of clouds back into outer space. For the radiation penetrating the earth s atmosphere, some of the incident energy is lost due to scattering or absorption by air molecules, clouds, dust and aerosols. The radiation that reaches the earth s surface... 

Fig. 17-1 The global climate system, (a) Energy fluxes, including incoming solar radiation, reflected radiation, emitted longwave radiation (from an effective altitude of ca. 6 km), and atmospheric and oceanic heat flux toward the polar regions, (b) The atmospheric circulation corresponding to part (a). Refer back to Fig. 7-4 and associated text for a discussion of the general circulation.

The greenhouse effect of the atmosphere a fraction of the infrared radiation is emitted from the surface, absorbed by the atmosphere and reflected back to the surface. 

The black body radiation profiles for a planet and the Sun have significantly different maximum temperatures with different spectral characteristics planetary emission is principally in the infrared whereas stellar emission is dominated by the visible. Molecules present in the atmosphere may absorb the infrared radiation and re-radiate the radiation back to Earth. This was thought to be the role of glass... 

Particles and gases in the earth s atmosphere absorb about 25% of this energy and 25% is reflected back to space by the atmosphere, mostly from clouds. About 5% of the incoming solar radiation is reflected back to space from the surface of the earth, mostly from bright regions such as deserts and ice fields. A 1-square-meter surface (39 inches by 39 inches), placed above the atmosphere will collect about 1,370 watts of radiant... 

The amount of energy the earth absorbs from the sun is the same amount it radiates back to space on average over the 500 trillion square meters of surface area. Satellites above the earth s atmosphere can measure the outgoing thermal radiation and show this balance to a high degree of precision. 

The latitudinal heat gradient in the atmosphere and ocean remains relatively constant over time despite the short-term and spatial variations in insolation. This steady state is maintained by the net transport of heat from low to high latitudes where it is radiated back into space. Atmospheric currents (winds) are responsible for about half of this meridional net transport of heat. The rest is accomplished by water movement in the... 

Blackbody Radiation The process by which solar energy absorbed by the Earth is transformed into longer wavelengths and reradiated back into the atmosphere. In physics, a blackbody is a perfect adsorber of electromagnetic radiation that can be released at other wavelengths with no loss of total energy. 

Greenhouse effect The warming of Earth s atmosphere as a result of the retention of solar radiation. This retention is possible because insolation absorbed by the land and ocean is radiated back to the atmosphere as IR energy. This energy is absorbed by atmospheric gases and then radiated from them as heat. 

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