Any substance that possesses heat sends out energy of electromagnetic radiation from its surface. The amount of radiant energy thus sent out is directly proportional to the fourth power of the absolute temperature of the substance. Also, the lower the temperature of the radiating material, the longer is the wavelengths of the rays emitted.
The ground or ocean surface, possessing heat derived originally from absorption of the sun's rays, continually radiates this energy back into the atmosphere, a process known as ground radiation or terrestrial radiation. This infrared radiation consists of wavelengths longer than 3 to 4 microns and is referred to here as long-wave radiation. The atmosphere also radiates energy both toward the earth and outward j into space where it is lost. Note that long-wave radiation is quite different from reflection in which the rays are turned back directly without being absorbed. Long-wave radiation from both ground and atmosphere continues during the night, when no solar radiation is being received.
Energy radiated from the ground is easily absorbed by the atmosphere because it consists largely of very long wavelengths (4 to 30 microns), in contrast to the visible light rays (0.4 to 0.7 microns) and shorter infrared rays (0.7 to 3.0 micron) which make up almost all of the entering solar radiation. Absorption of long-wave radiation by water vapour and carbon dioxide takes place largely in wavelengths from 4 to 8 microns and 12 to 20 microns. However, radiation in the range of wavelengths between 8 to 12 microns passes freely through the earth's atmosphere and into outer space.
Of the long-wave energy radiated from the ground, a portion is radiated back to the earth's surface, a process called counter radiation. Thus the atmosphere receives heat by an indirect process in which the radiant energy in shortwave form is permitted to pass through, but that in long-wave form is delayed in making its escape. For this reason, the lower atmosphere with its water vapour and carbon dioxide acts as a blanket, which returns heat to the earth and helps to keep surface temperatures from dropping excessively during the night or in winter at middle and high latitudes. Somewhat the same principle is employed in greenhouses and in homes using the solar-heating method. Here the glass permits entry of shortwave energy. Accumulated heat cannot escape by mixing with cooler air outside. The expression greenhouse effect has been used by meteorologists to describe the atmospheric heating principle.
The total long-wave radiation leaving the earth's land and ocean surface is equivalent in amount to 98 percentage units. Of this, 8 units are lost to space, while 90 units are absorbed by the atmosphere. In turn, the atmosphere emits long-wave radiation. The total of this radiation is equivalent in amount to 137 percentages units of the insolation at the top of the atmosphere. This radiation is divided into two parts, one of which goes out into space (60 units) and the other of which is absorbed by the earth's surface as counter radiation (77 units). The earth's surface has a net outgoing long-wave radiation of 21 units, while the atmosphere has a net outgoing long-wave radiation of 47 units. Combining these two figures gives 68 units for the net outgoing radiation from the entire earth-atmosphere system, which equals the total energy absorbed by that same system. Two other mechanisms transfer the missing energy back into the atmosphere. One is as latent heat, through the evaporation of surface water. A second is the mechanical transfer of heat from surface to air. The heat is first conducted as sensible heat from water or soil into the overlying air layer, then carried upward in turbulent eddies. (The reverse flow of heat can also occur in this way) Evidently about 60 percent of the incoming energy is returned to the atmosphere by the combined processes of evaporation and sensible heat transfer. Of this 60 percent, about 2/3 returns as latent heat, and about 1/3 as sensible heat.
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