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 ).