Kenya is generally a dry country; more than 75% of its area is classified as arid or semi-arid and only about 20% is viable for agriculture. Inland, precipitation and temperature are closely related to changes in altitude, with variations induced by local topography. Generally, the climate is hot and humid on the coast, cool and humid in the central highlands, and hot and dry in the north and east.

In most of the country, the rains are strongly seasonal, although their pattern, timing and extent vary greatly from place to place and from year to year. The relatively wet coastal belt along the Indian Ocean receives 1,000 mm or more of rain per year. Most of the rain falls from April to July as a result of the southeast monsoon.

Another wet belt occurs in the Lake Victoria Basin and its surrounding escarpments and uplands, primarily due to moist westerly winds originating over the Atlantic Ocean and the Congo Basin. Except in areas immediately adjacent to the lake, rainfall reliably occurs from March to November. The highland plateaus adjacent to this area are less influenced by the lake and the rains fall mainly in March-May and July-September. In much of the central highlands, there is also a bimodal rainfall pattern, with rainy seasons in March-May and October-December.

About the remaining 70% of the land area falls in the ‘arid lowland’ zone (NRI 1996), with mean rainfall less than 500 mm and varying greatly from year to year. Rain peaks in most areas in November and April. About 30% of this area can be classified as semi-desert, with average rainfall less than 300mm per year and evaporation often in excess of 3,000mm.

With the exception of the coast and the Lake Victoria region, altitude is the main determinant of precipitation. High-altitude areas (above about 1,500 m) in the central Kenyan highlands typically experience considerable rainfall, reaching more than 2,000 mm per year in parts of the Mau Escarpment.

However, topography also has a large influence, with strong rain shadow effects to the east of Mount Kenya and the Aberdare Mountains. Here, even areas above 1,800m can be relatively dry. In the arid lowlands, isolated mountain peaks attract clouds and mist, and can support vegetation very different from that of the surrounding plains.

Temperature differences vary predictably with altitude. Frosts occur regularly at 3,000 m and occasionally to at least 2,400 m, and there is permanent snow and ice on the summit of Mount Kenya at 5,200 m. The warmest areas are in the arid northeast and west of Lake Turkana, where maximum temperatures average more than 34 C.

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