Page 169 - Handbook of Gold Exploration and Evaluation
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Gold deposition in the weathering environment  147

            going radiation exceeds insolation thereby creating a heat deficit. Partial
            explanations for the non-uniform distribution of heat are given by:
            · the oblate spherical shape of the Earth
            · the elliptical orbit of the Earth around the Sun, which provides seasonal
              changes in orbital distance and axial tilt relative to the Sun
            · the rotation of the Earth, which causes local changes in insolation throughout
              each day.
            Incoming solar radiation is intercepted at different rates over different parts of
            the Earth in its passage through the atmosphere. Some 50% of the total annual
            incident radiation is lost due to scattering and diffuse reflection from solid
            pollutants in the atmosphere and from cloud reflection back into space. Of the
            50% or so of energy received at the Earth's surface, the amount absorbed varies
            according to the nature of the surface and vegetal cover. The remaining heat is
            reflected outwards from the Earth's surface by mechanisms involving conduc-
            tion, radiation and latent heat of evaporation, thereby providing an effective
            means of heating the air in lower levels of the atmosphere. An estimate of the
            absorption of solar energy by the atmosphere and at the Earth's surface is given
            in Table 3.3. According to this estimate, the total absorption (atmosphere and
            surface) is some 4.7 times greater at the equator than at the poles. The latitudinal
            variation of insolation (about 2.6 times greater at the equator than at the poles)
            provides energy differences that are vital elements of atmospheric circulation
            and terrestrial weathering. The Earth's radiation balance is also developed from
            other components of the total energy flow including energy absorbed by the
            Earth, energy transmitted in the form of reflection and long-wave radiation from
            the Earth into space. Figure 3.7 is a simplified diagram of the Sun's radiant
            energy.
              Geographical and seasonal variations of climate are fundamentally supported
            by modification of solar radiant energy (insolation) intercepted in the


                   Table 3.3 Absorption of solar energy at Earth's surface (annual mean, Ly/
                   day)* (after Fritz, 1987)

                                            0ë N     30ë N    60ë N     90ë N
                   Latitude
                   Extraterrestrial         850      740       470      350
                   AE surface
                   With clear sky           570      520       320      220
                   With normal cloud cover  410      440       200      150
                   Total absorption
                   Atmosphere and surface   570      530       260      120

                                          2
                   * Langleys per day (1Ly = 1cal/cm ).
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