Latitudinal belts of atmospheric convergence and divergence exercise maximum control over the distribution of precipitation. As noted earlier, adiabatic cooling of air masses is the most important causative factor of precipitation and condensation.
Upward movement of air is possible in the zones of convergence of air streams. That is why the convergence zones are belts of maximum precipitation. On the contrary, in latitudes where subsidence and divergence are pronounced, areas of minimum precipitation are found.
Since there are other factors besides horizontal convergence which force the air masses to rise, the zonal arrangement of the distribution of precipitation is disturbed. These additional factors include convection, land form barriers, etc.
Since the equatorial belt of calm or the doldrums represents the convergence zone of north and south trade winds, the latitudinal belt of maximum precipitation is found near the equator. Along with the seasonal shifting of doldrums, the trade wind convergence zone moves north or south of the equator.
In the subtropical high pressure belts, there is subsidence of air which gives rise to anticyclonic conditions on the surface of the earth. This is, therefore, the zone of minimum rainfall. In these latitudes, the atmospheric stability does not encourage the cloud formation, so that there is no precipitation.
From the subtropical high pressure belt pole-ward there are belts of westerlies in both the hemispheres. In these belts the cold polar winds meet the warm moist tropical air masses along the polar fronts.
These fronts witness the formation of a number of cyclones and anticyclones. Annual average precipitation in the mid-latitude convergence zone varies from 100 to 125 centimeters. In the belt of polar easterlies, because of subsidence there is much less precipitation.
Moreover, low temperature diminishes the moisture holding capacity of air which also accounts for the lesser amount of precipitation. From 75° north and south latitudes pole-ward the mean annual precipitation decreases to about 25 centimeters.