Precipitation over Equatorial east Africa is quite different from that over other equatorial regions. East Africa itself, along with Somalian coast, is an area of low rainfall and is almost arid in some parts. Thus, the most notable characteristic of this region is very low amount of rainfall in virtually equatorial situation. During the northern summer (April to October) the winds come from the south and in southern summer from the north. The equatorial double maxima is separated by drought months, many areas are semi arid. Much of the rainfall, which does fall occurs in hilly regions, or where local convergence takes place. The rain generating processes of the tropics which give rise to such heavy rainfall elsewhere, are absent here.
This is because of a number of factors—
- The east African plateau relief cuts down the overall thickness of the troposphere affected by the surface flow. This reduces the effectiveness of convective activity and of the air that is forced to rise up the coastal scarp.
- Both seasonal flows are largely over land. This means that there is only a small proportion of ocean to land moisture transport, and most rains come in the transitional seasons when this is in force.
- The disturbances, which form over the oceans rarely penetrate more than 80 km inland, so even when the atmosphere is disturbed, the rainfall is low.
- The upper air flow is north – easterly with a very low moisture content and a low-level inversion, thus, only stratus clouds form.
- Malagasy lies in the path of South Easterlies, imparting a rain shadow effect.
- Along the Somali coasts, the orientation of the coastlines become different. The African coast is parallel to both the north-east winds of winter and south-west winds of summers.
- Most of the winds are offshore, and this causes upwelling. Upwelling reduces eveporation and reduces moisture content of the air.