A glance will show that precipitation is not only unevenly distributed over the earth’s surface, but the patterns of distribution are also complicated. The livelihood and amount of precipitation is controlled by two groups of factors.
First, precipitation depends upon the vertical motions of the atmosphere. Vertical motion may be due to horizontal convergence and divergence of different air masses. It may also be produced by the atmospheric disturbances and highland barriers.
Precipitation also depends on a second group of factors, i.e. internal characteristics of the air itself. These characteristics include the degree of instability, temperature of the air and its humidity.
Areas of convergence and divergence as controlling factors of precipitation are zonal in character to a large extent. But there are other factors like the distribution of land and water, and the highland barriers and their arrangement which are non-zonal. The zonal patterns of precipitation distribution are disturbed by these non-zonal controls and made complicated.
The meridional profile of the mean annual precipitation demonstrates the more important aspects of world distribution of precipitation by latitude zones. It should be borne in mind that variations in the amount of precipitation in the east-west direction are not shown in this profile.
(1) Maximum precipitation occurs in a belt about 10 to 20 degrees wide near the equator. The average precipitation is about 160 centimeters.
However, in the northern hemisphere the belt extending from the equator to 10°N latitude receives more precipitation than the corresponding belt in the southern hemisphere. The principal reason for this difference is the mean position of the doldrums to the north of the equator.
(2) At latitudes about 20 to 30° North and South are found the belts of lower precipitation. The mean annual precipitation is about 80 to 90 centimeters. In this belt of subtropical anticyclones, the air subsides and so it becomes warm and dry.
(3) In latitude zones extending from 40 to 55° North and South the mean annual precipitation varies from 80 to 120 centimeters. This is the secondary belt of maximum precipitation in both the hemispheres.
In this belt the maximum cyclonic activity accounts for the heavy amount of precipitation. Another interesting feature is that this belt in the southern hemisphere receives more rainfall than its counterpart in the northern hemisphere. The extensive middle- latitude deserts in the interior of Eurasia and North America are responsible for this precipitation anomaly.
(4) There is an abrupt diminution in the amount of precipitation in both the hemispheres from about 50 to 55° latitudes reaching the primary zonal minima of less than 15 centimeters in the Polar Regions.
(5) Total annual precipitation is the same for both the hemispheres. However, there are certain dissimilarities in the zonal distribution of precipitation in the northern and southern hemispheres: (a) Latitudes 0 to 10° north have more precipitation than latitudes 0 to 10°S.
This is because of the fact that intertropical convergence is more extensive in the north than in the south of the equator.
Besides, for a greater part of the year, the ITC is located to the north of the equator, (b) the belt extending from 40 to 60° South latitude receives much greater precipitation than the corresponding belt in the northern hemisphere. This is because of the larger proportion of ocean in this latitudinal belt in the southern hemisphere.
(6) Despite the preponderance of land masses in the northern hemisphere, the average annual precipitation for both the hemispheres remains the same. Almost 50 per cent of the total precipitation falls between 20″ N and 20° S latitudes. In this extensive belt the proportion of land in both the hemispheres is the same.
However, because the doldrums is positioned north of the equator for a greater part of the year, the northern half of this belt is wetter than the southern half. But because of greater proportion of ocean in the southern hemisphere, the mid-latitudinal belt is rainier than its northern counterpart. Thus, the total precipitation remains the same in both the hemispheres.