Long-Wave Radiation
Long-Wave Radiation
(in the atmosphere), the infrared (thermal) radiation of the earth’s surface, atmosphere, and clouds. For the temperatures existing at the earth’s surface, in the troposphere, and in the stratosphere (approximately 200°-330°K), the greater proportion (up to 99 percent) of the long-wave radiation lies in the wavelength range from 4 to 40 microns (μm).
Atmospheric radiation and absorption are sharply selective. The principal gases that make up air (nitrogen, oxygen, and argon) virtually do not radiate or absorb in this range. The chief sources of long-wave radiation—water vapor, carbon dioxide gas, and ozone—are concentrated in the troposphere and stratosphere. The radiation spectrum of these gases is extremely complex. The most intensive radiation and absorption are in the wavelength range from 5 to 8μmn and above 18μm (water vapor), from 13 to 17μim (carbon dioxide), and from 9 to 10μm (ozone). The atmosphere is most transparent in the wavelength range from 8 to 12μm and from 17 to 18μm (the so-called atmospheric windows). The long-wave radiation spectrum of clouds (water drops) is close to the spectrum of water vapor but is more intense. Dense clouds are virtually opaque to long-wave radiation.
Long-wave radiation plays an important part in atmospheric processes, since it is the means by which the earth and the atmosphere exchange heat and lose it to outer space. The various forms of long-wave radiation are distinguished according to the source and the direction: the intrinsic radiation of the earth’s surface; counterradiation (the intrinsic radiation of the atmosphere that is directed towards the earth’s surface); outgoing radiation (the earth’s radiation as a planet together with the atmosphere into outer space); and the effective radiation (the difference between the earth’s radiation and the counterradiation of the atmosphere). The amount of long-wave radiation varies substantially in time and space with changes in the factors that control one or another form.
At night the long-wave radiation close to the earth’s sur-face may be measured, for example, by a radiation balance gauge or a pyrgeometer. The methods of measurement in the daytime are imperfect because of the difficulty of distinguishing long-wave radiation from short-wave radiation; therefore, the long-wave radiation is often determined by various computational methods.