The equatorial zone is generally an area of abundant precipitation (over 200 centimeters annually). This high level of precipitation is due to the equatorial belt’s high temperature, high humidity and highly unstable air.
The trade winds from both the hemispheres converge and give rise to a general upward motion of air. The ascending air currents become intensified locally in the tropical storms that produce abundant precipitation in the Caribbean, the southwest Pacific Ocean, the Indian Ocean and the China Sea.
Over the land areas, most of the precipitation occurs from the thunderstorms that are very frequent and active in the equatorial regions. It is to be noted that a larger part of the precipitation in this region falls as heavy intermittent convective showers accompanied by thunder and lightning.
During the course of a year, the doldrums is displaced toward the summer hemisphere with the result that outside a central region near the equator, which gets abundant precipitation at all seasons, there is a zone where summer is rainier than the winter.
The result is a rainy summer and a dry winter. The dry season towards the pole-ward side of the equatorial belt becomes progressively longer and more severe. These latitudinal contrasts are less prominent on the eastern side of a continent.
It is in the subtropical high pressure belts that the most notable breakdown in the zonal distribution of precipitation occurs. Here are found not only many of the world’s largest hot deserts, but also the regions of abundant rainfall.
This is so because the eastern and western parts of the semi-permanent subtropical anticyclones have different characteristics. Dry areas of the subtropics, such as the desert regions of North Africa, South Africa, the Arabian Peninsula, Australia and Central South America owe their existence due to these anticyclones in which the air subsides and is adiabatically warmed.
On the eastern side of the subtropical oceanic highs, subsidence is more pronounced and a strong inversion exists very near the surface which results in atmospheric stability.
Besides, the upwelling of cold water along the west coasts of the continents cools the air from below and increases the stability on the eastern sides of these anticyclones.
Since these anticyclones are found on the eastern side of an ocean, the western sides of the continents in the vicinity of these subtropical highs tend to become very arid. The Sahara Desert (North Africa), the Atacama (South America), and the Great Desert (Australia) are typical examples.
On the contrary, the eastern regions of the continents in the sub-tropical high pressure belt receive abundant precipitation throughout the year. The reason is simple ; on the western side of these subtropical anticyclones, subsidence is not so marked as on the eastern side, and the convergence with associated ascending air is more pronounced.
In addition to the above, the trade winds blowing over a vast expanse of warm water pick up more moisture from the ocean surface and this makes them more unstable. The interiors of continents in the subtropics which are sheltered by mountain ranges are also very dry.