The average annual precipitation over the whole earth is about 80 centimeters (30 inches), but this is distributed very unevenly. Equatorial regions and the monsoon areas of Southeast Asia record the highest rainfall.
Temperate regions, on the other hand, receive moderate amounts of precipitation. The dry regions of subtropical high pressure belt and Polar Regions receive little precipitation.
If the earth’s surface were perfectly uniform, the mean annual rainfall would be distributed in distinct latitudinal zones. But the pattern of global circulation of wind, the distribution of land and water, and the mountains make the distribution pattern of precipitation a little more complicated.
Since precipitation occurs mainly from the ascent and adiabatic cooling of the moist air masses, the areas of rising air get heavy rains, whereas the deserts are situated in regions in which descending air masses get warmed and their capacity to hold moisture is further increased.
In subtropical regions, the trade winds blowing from across the warm tropical oceans bring abundant precipitation to the east coasts of the continents.
On the contrary, the west coasts remain practically dry. In high latitudes, the west coasts get more precipitation than the east coasts. But even in these latitudes, precipitation tends to be high on the windward slopes of mountain ranges and low on the leeward slopes.