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Climate

East Africa has a diverse climate that consists of hot, dry desert regions, cooler regions, and highlands. Its climate generally is rather atypical of equatorial regions, being mostly arid or semi-arid with rainfall totals across much of the lowland regions below 600 millimetres or 24 inches per year. In fact, on the northern coast of Somalia, annual rainfall is less than 100 millimetres or 4 inches and many years can go by without any rain whatsoever. The causes of the low rainfall totals are not fully understood. One factor is presence of the Somali Jet (a fast wind system) in boreal summer, which brings cool southern hemisphere air into East Africa. A second cause is the presence of east to west river valleys in the East African Rift, which channel strong moisture-laden winds from the Indian Ocean away from East Africa and towards the Congo Basin rainforest.

Rainfall generally increases towards the south and with altitude, being around 400 mm (16 in) at Mogadishu and 1,200 mm (47 in) at Mombasa on the coast, whilst inland it increases from around 130 mm (5 in) at Garoowe to over 1,100 mm (43 in) at Moshi near Kilimanjaro. Rainfall in most of East Africa east of the Rwenzoris and Ethiopian Highlands is characterised by two main rainfall seasons, the long rains from March to May and the short rains from October to December. This is usually attributed to the passage of the Intertropical Convergence Zone across the region in those months, but it may also be analogous to the autumn monsoon rains of parts of Sri Lanka, Vietnam, and the Brazilian Nordeste. West of the mountains, the rainfall pattern is more typically tropical, with rain throughout the year near the equator and a single wet season in most of the Ethiopian Highlands from June to September — contracting to July and August around Asmara. Annual rainfall here ranges from over 1,600 mm (63 in) on the western slopes to around 1,250 mm (49 in) at Addis Ababa and 550 mm (22 in) at Asmara. In the high mountains rainfall can be over 2,500 millimetres or 100 inches.

Rainfall variability is influenced by both El Niño events and a positive Indian Ocean Dipole. El Niño events tend to increase rainfall except in the northern and western parts of the Ethiopian and Eritrean highlands, where they produce drought and poor Nile floods. Similarly, a positive Indian Ocean Dipole result in warm sea-surface temperatures off the coast of East Africa and lead to increased rainfall over East Africa. Temperatures in East Africa, except on the hot and generally humid coastal belt, are moderate, with maxima of around 25 °C (77 °F) and minima of 15 °C (59 °F) at an altitude of 1,500 metres (4,921 ft). At altitudes of above 2,500 metres (8,200 ft), frosts are common during the dry season and maxima typically about 21 °C (70 °F) or less.