Naivasha Virtual Fieldcourse

Rainfall patterns in East Africa comprise Long Rains (April/May) and a Short Rains (Oct/Nov) interrupted by dry seasons. The regularity of the East African Monsoon winds is related to the annual rotation of the Earth around the Sun; the same processes which causes Summer and Winter seasons in the northern and in the southern hemisphere.

Changes between rainy seasons and dry seasons are mainly due to changes in temperature over the land mass and over the sea. Land heats and cools more rapidly than the Ocean and these differences in temperature generate wind.

During the early part of the year (December-February), when it is warm in the southern hemisphere, the warm air heated by the land mass rises towards the upper atmosphere. This builds up high pressure in the southern tropics while it is winter (low pressure) in the northern hemisphere; monsoon winds blow in a Southwest direction during this dry season.  Shortly after the Equinox (siku ambazo usiku na mchana huwa sawa), the winds weaken as the Indian Ocean becomes warmer; during this time, cold air charged with water vapour from the evaporation of the Indian Ocean has accumulated in the Central Highlands of Kenya. Here, as winds decrease in intensity, cool high latitude forests trap clouds favouring the condensation of moisture and activating the Long Rains.

As the Earth turns around the Sun and exposes the northern hemisphere to direct sun rays (Summer in northern hemisphere), a high pressure tends to occur over the Indian Ocean and winds blowing towards North-East become dominant. They bring moisture from the southern Atlantic Ocean that in October, when the winds cease, condenses over the Central Highlands causing the Short Rains.