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E. Linacre and B. Geerts |
7/'98 |
Africa extends to 37° N. Eighty percent of Africa lies above 1,000 m above sea-level, with as peak Mt Kilimanjaro at 5895 m. Its climates are largely explained by latitude and topography. Most of the rainfall is associated with the ITCZ, which moves seasonally, except north of the Sahara, where a mediterranean climate exists. Subtropical highs are present over the adjacent Atlantic and Indian oceans. There is a heat low over the Sahara in summer, and a high in winter.
Rainfalls range from an average of less than 1 mm annually in parts of the Sahara, to over 10,000 mm/a at Debundscha (Cameroon). In the Sahel, rainfall totals quickly drop from 1,000 mm/a at the southern margin to 250 mm/a to the north. A feature of the Sahel especially are the dry-month dust-storms, with dust clouds up to 5 km high. These are known as haboobs. Most of the Sahara rainfall comes from an occasional thunderstorm, usually in August or September. Monsoonal rainfall, enhanced by topography, falls in Ethiopia at a latitude where the rest of Africa is arid. Summer rainfall periods in the Sahel region are often associated with ‘easterly waves’, travelling from Ethiopia to West Africa. Much of the Sahel rain comes from long-lived squall lines with a trailing region of stratiform rainfall.
Deserts covered most of Africa about 18,000 years ago, towards the end of the last Ice Age. That dry period was followed by wet times around 5,000 BP until 3,000 BP. In fact fish hooks have been unearthed near former lake shores in the Sahara. There was another moist time between 9th - 14th centuries, when there were major civilisations around Timbouctou and near Lake Chad. The 16th - 19th centuries were relatively moist compared to the 20th century.