FIGURE 2.3. The energy emitted at different wavelengths for blackbodies at several temperatures. The function Bx (T), Eq. A-1, is plotted.

blackbody spectrum. (A brief theoretical background to the Planck spectrum is given in Appendix A.1.1.) It is plotted as a function of temperature in Fig. 2.3. Note that the hotter the radiating body, the more energy it emits at shorter wavelengths. If the observed radiation spectrum of the Sun is fitted to the blackbody curve by using T as a free parameter, we deduce that the blackbody temperature of the Sun is about 6000 K.

Let us consider the energy balance of the Earth as in Fig. 2.4, which shows the Earth intercepting the solar energy flux and radiating terrestrial energy. If at the location of the (mean) Earth orbit, the incoming solar energy flux is S0 = 1367 Wm-2, then, given that the cross-sectional area of the Earth intercepting the solar energy flux is na2, where a is the radius of the Earth (Fig. 2.4), solar power incident on the Earth = Sona2 = 1.74 X 1017 W, using the data in Table 1.1. Not all of this radiation is absorbed by the Earth; a significant fraction is reflected. The ratio of reflected to incident solar energy is called the albedo, a. As set out in Table 2.2 and the map of surface albedo shown in Fig. 2.5, a depends on the nature of the reflecting surface and is large for clouds, light surfaces such as deserts, and (especially) snow and ice. Under the present terrestrial conditions of cloudiness and snow and ice cover, on

TABLE 2.2. Albedos for different surfaces. Note that the albedo of clouds is highly variable and depends on the type and form. See also the horizontal map of albedo shown in Fig. 2.5.

Type of surface

Albedo (%)


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