Introduction the Solar Spectrum

An examination of the spectrum of the solar radiation received at the earth's surface shows that it departs significantly from the theoretical curve, shown in Fig. 6.2, in respect of radiation from a black body at the temperature of the sun that is at about 6,000 K. The difference becomes evident from Fig. 8.1, reproduced from Brunt (1944), which presents the intensity of solar radiation at different wavelengths received at surface at Washington with the sun at different solar altitudes (curves II, III, IV and V at solar altitudes 90°, 30°, 19.3° and 11.3° respectively) and also at the outer boundary of the atmosphere in the latitude of Washington, after allowing for estimated absorption and scattering of the atmosphere (Curve I). Curve VI gives the relative brightness of the different parts of the spectrum. Curve VII gives the intensity distribution of the skylight at different wavelengths.

Two aspects of the departure from Fig. 6.2 stand out in Fig. 8.1. These are:

(a) A sharp cut-off of energy in the short-wave part of the spectrum below about 0.3 and

(b) Appreciable loss of energy in passing through the terrestrial atmosphere at all wave-lengths. The loss becomes more and more pronounced with decrease of solar altitude.

The intensity of the net solar radiation that we receive at the earth's surface is just about half of what is intercepted by the earth at the outer limit of its atmosphere. The short wave part of the solar radiation does not reach the earth's surface, since it is totally absorbed by the outer layers of the earth's atmosphere which may be called the upper atmosphere. So, it is the radiation from the sun's surface at wavelengths above 0.3| that comes down to the lower layers in the form of heat and light. But, before solar radiation can reach the earth's surface, it has to pass through the different layers of the atmosphere. Several important effects are produced by the physical and chemical processes that occur in these layers as the radiation passes through them to the earth's surface. Some of these are the following:

Fig. 8.1 Spectrum of the sun at Washington (Reproduced from Brunt, 1944, with kind permission of Cambridge University Press)

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Fig. 8.1 Spectrum of the sun at Washington (Reproduced from Brunt, 1944, with kind permission of Cambridge University Press)

(a) Absorption, heating and ionization in the upper atmosphere (above about 80 km) at wavelengths < 0.2|);

(b) Absorption and heating by ozone in the stratosphere, between about 10 and 50 km, at wavelengths between about 0.2| and 0.3|; and

(c) Scattering, reflection and absorption by air molecules, particulate matter suspended in the atmosphere and by clouds;

The above-mentioned processes are briefly described in the sections that follow.

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