Figure 2 – average annual insolation on the surface of the earth

7. Topographical variations: Earth does not have a featureless surface. The topographical variations are the major factors modifying the distribution of insolation. Variability in elevation, surface orientation (slope and aspect), and obstruction by surrounding topographic features creates strong local gradients of insolation. Similarly, in the northern hemisphere a south-facing slope (more open to sunlight and warm winds) will therefore generally be warmer and dryer due to higher levels of evapotranspiration than a north- facing slope.[1] This can be seen in the Swiss Alps, where farming is much more extensive on south-facing than on north-facing slopes. In the Himalayas, this effect can be seen to an extreme degree, with south-facing slopes being warm, wet and forested, and north-facing slopes cold, dry but much more heavily glaciated. Vegetation, human activities are more visible on the slopes where insolation is more relatively.

Under combined effect of the above discussed factors, the amount of total annual insolation received by different regions is different. The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles. Maximum insolation is received over the subtropical deserts. Equator receives comparatively less insolation than the tropics due to presence of clouds. Generally, at the same latitude the insolation is more over the continent than over the oceans because more clouds over the oceans reflect sun rays back into space. Isohels are lines connecting points on the earth surface that receive equal amounts of sunshine. Isohels are more or less parallel to latitudes, especially in southern hemisphere (figure 2).