As viewed from the earth the radiation coming from

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As viewed from the earth, the radiation coming from the sun appears to be essentially equivalent to that coming from a back surface at 5762 K. The radiant energy flux received per second by a surface of unit area is termed as I sc and its value is 1367 W/m2. But extraterrestrial radiation I’ sc suffers variation due to the earth’s rotation. The earth’s orbit around the sun is elliptical with small eccentricity and single foci. The distance between the earth and the sun varies a little through the year. Because of this variation, the extra-terrestrial flux also varies. The earth is closest to the sun in the summer and farthest away in the winter. This causes variation in the intensity of solar radiation ( I’ sc ) that reaches the earth. This can be approximated by the equation 𝑰 𝒔𝒄 𝑰 𝒔𝒄 = ? + ?. ?𝟑𝟑 𝒄?𝒔 𝟑??? 𝟑?? where n is the day of the year. As cosine function varies from +1 to -1, the extra-terrestrial radiation flux varies by ±3.3% over a year. 3.3 Spectral distribution of Extraterrestrial distribution The sun does not function as a black body radiator at a fixed temperature. The emitted solar radiation is result of the several layers that emit and absorb radiation of various wavelengths. The photosphere only emits a continuous spectrum of radiation. In addition to the total energy in the solar spectrum (i.e. is the solar constant) it is useful to the spectral distribution of extra-terrestrial solar radiation. The figure 2.2 shows the spectral distribution of extra-terrestrial solar radiation. As cab be seen that the spectral value first increases sharply with wavelength, passes through a maximum at a wavelength of 0.48 μm and then decreases asymptotically to zero. 99% of the sun’s radiation is obtained up to a wavelength of 4 μm. Figure 2.2: Spectral Irradiance of Solar Light
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3 3.4 Terrestrial Solar Radiation or Solar Radiation at Earth’s Surface Solar radiation is received at the earth's surface in an attenuated form because it is subjected to the mechanisms of absorption and scattering as it passes through the earth's atmosphere. Absorption occurs primarily because of the presence of ozone and water vapour in the atmosphere, and to a lesser extent due to other gases (like CO 2 , NO 2 , CO, O 2 and CH 4 ) and particulate matter. It results in an increase in the internal energy of the atmosphere. On the other hand, scattering occurs due to all gaseous molecules as well as particulate matter in the atmosphere. The scattered radiation is redistributed in all directions, some going back into space and some reaching the earth's surface. Figure 2.3: Terrestrial radiation The atmosphere at any location on the earth's surface is often classified into two broad types - an atmosphere without clouds and an atmosphere with clouds. In the former case, the sky is cloudless everywhere, while in the latter, the sky is partly or fully covered by clouds. The mechanisms of absorption and scattering (described in the previous paragraph) are similar with both types of atmosphere. However it is obvious that less attenuation takes place in a cloudless
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