About 6 per cent of the extra-terrestrial radiation is reflected to space by the atmosphere and 20 per cent by clouds, and around 23 per cent is scattered downwards to the surface of the Earth as diffuse radiation. This is part of the global radiation, the solar radiation reaching a horizontal surface on the ground (Figure 2.6). (The adjective 'global' is used because it consists of solar radiation from all directions.) The other part comes straight from the Sun, without scattering, and is called direct radiation.
Direct radiation leads to shadows, while diffuse radiation provides what illumination there is in the shadow. So shadows are very black in the rarefied clean air of high mountains or on the Moon, where there is little atmosphere and few aerosols to cause scattering. On the other hand, the scattering within clouds increases diffuse radiation and therefore reduces the intensity of shadows, or even eliminates them.
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