The distribution of global radiation reaching sea-level (Figure 2.11) is the result of the variations of extra-terrestrial radiation, cloudiness and turbidity. Relatively high values are measured near the Tropics at about 23°, because of the clear skies there, especially over land, and intensities are greater in summer than in winter, because of the seasonal variation of the extraterrestrial radiation (Note 2.F). At Darwin at 12°S in Australia, the global radiation is higher in July (Table 2.2), when skies tend to be clear, than in January, during the rainy season (Chapters 12 and 16). The annual swing is greatest at high latitudes, and there is great similarity of the ranges across Australia in January.
The amount of radiation falling on a particular surface depends on its aspect, i.e. its orientation to the Sun. This is governed by the steepness and the direction of slope of the ground surface, e.g. a slope facing north in the southern hemisphere receives more insolation than the opposite side of a valley or mountain. Mountains may cut off direct sunshine into valleys, while insolation on top of high mountains is increased by the lower horizon, the reduced thickness of the atmosphere above, and sometimes by fewer clouds.
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