Source: Adapted from Kiehl and Trenberth, 1997.

Source: Adapted from Kiehl and Trenberth, 1997.

major processes, selective scattering and nonselective scattering, are related to the size of the particles in the atmosphere. In selective scattering, the shorter wavelength of UV energy and blue light are scattered more severely than that in longer wavelengths (red) and infrared (IR) energy. Selective scattering is caused by fumes and by gases such as nitrogen, oxygen, and carbon dioxide. This is known as Rayleigh scattering and is the primary cause of the blue color of the sky. For larger sizes of particles, scattering is independent of the wavelength, i.e., white light is scattered. The phenomenon is known as Mei scattering. As the path length increases, the percentage of solar energy in the visible part decreases. Within the visible part itself, the ratio of the blue to the red part decreases with increased path length. This is because the part of the spectrum with higher frequency is scattered to a greater extent than the part with lower frequency. The red color of the sky at sunrise and sunset is because of increased path length in the atmosphere which scatters blue and green wavelengths so only red light reaches the viewer (Sabins, 1997).

The atmosphere absorbs about 20 percent of the solar radiation. The constituents of the atmosphere that absorb the solar radiation significantly are oxygen, ozone, carbon dioxide, and water vapors. This absorption is of great importance to life on the earth's surface, because only a very small amount of this radiation can be tolerated by living organisms.

Oxygen and ozone: Solar radiation in the wavelengths <0.3 ^m is not observed on the ground. It is absorbed in the upper atmosphere. Energy of <0.1 ^m is highly absorbed by the atomic and molecular oxygen and also by nitrogen in the ionosphere. Energy of 0.1 to 0.3 ^m is absorbed efficiently by ozone in the ozonosphere. Further but less complete ozone absorption occurs in the 0.32 to 0.36 ^m region and at minor levels around 0.6 fxm (visible part) and 4.75 ^m, 9.6 ^m, and 14.1 ^m (infrared part).

Carbon dioxide: This gas is of chief significance in the lower part of the atmosphere. Carbon dioxide has a weak absorption band at about 4 ^m and 10 ^m and a very strong absorption band around 15 ^m.

Water vapor: Among the atmospheric gases, water vapors absorb the largest amount of solar radiation. Several weak absorption bands occur below 0.7 ^m, while important broad bands of varying intensity exist between 0.7 and 0.8 ^m. The strongest water absorption is around 6 ^m, where almost 100 percent of longwave radiation may be absorbed if the atmosphere is sufficiently moist (Barrett, 1992).

Thus, after reflection, scattering, and absorption in the atmosphere, about half of the solar radiation reaches the earth's surface. Out of this, about 6 percent is reflected back to outer space. This is known as albedo. The albedo is defined as the fraction of incoming shortwave radiation that is reflected by the earth's surface. The albedo varies with the color and composition of the earth's surface, the season, and the angle of the sun's rays. The values are higher in winter as well as at sunrise and sunset. The albedo also varies with the wavelength of the incident radiation (Roberto et al., 1999). Very small values have been recorded in the ultraviolet part of the spectrum and higher values in the visible part. The albedo values of some selected surfaces are given in Table 2.5.

Outgoing Longwave Radiation

The surface of the earth after being heated by the absorption of solar radiation becomes a source of radiation itself (Figure 2.2). Because the average temperature of the earth's surface is about 285°K, 99 percent of the radiation is emitted in the infrared range from 4 to 120 ^m, with a peak near 10 |.im, as indicated by Wein's displacement law. This is longwave radiation and is also known as terrestrial radiation. The average annual global disposal of infrared radiation is represented by equations 2.9, 2.10, and 2.11.

where I(e) is infrared radiation emitted by the earth's surface; Ia is infrared radiation from the earth's surface absorbed by the atmosphere; Is is infrared radiation from the earth lost to space; I(a) is infrared radiation from the atmosphere; I4- is counter radiation; I(a)s is infrared radiation from the atmosphere lost in space; and I is the effective outgoing radiation from the earth. The quantitative disposal of longwave radiation (W m-2 per year) from the earth-atmosphere system is summarized in Table 2.6.

The earth's atmosphere absorbs about 90 percent of the outgoing radiation from the earth's surface. Water vapors absorb in wavelengths of 5.3 to 7.7 |j,m and beyond 20 |j,m; ozone in wavelengths of 9.4 to 9.8 |j,m; carbon dioxide in wavelengths of 13.1 to 16.9 nm; and clouds in all wavelengths. Longwave radiation escapes to space between 8.5 and 11.0 ^m, known as the atmospheric window. A large part of the radiation absorbed by the atmosphere is sent back to the earth's surface as counter radiation. This counter radiation prevents the earth's surface from excessive cooling at night.

TABLE 2.5. Albedo of shortwave radiation



Albedo (%)

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