2.14 Characteristics of Rain in Tropical and Temperate Climate
2.14.1 South Africa Climate and Seasonal Variability
South Africa is located on latitude 29o00'S and longitude 24o00'E in the most southern tip of the African continent [7]. With this positioning on the globe, South Africa can be classified as a sub- tropical region, because the subtropics include all parts of the world just outside 23.4°N and 23.4°S latitude [8]. South Africa borders other countries like Botswana and Zimbabwe in the central north, Mozambique in north-east, Namibia in north-west. Swaziland and Lesotho are embedded in the country (see Fig. 2-6 below) [9]. South Africa is climatically moderated by its surrounding oceans; the Atlantic ocean in the western part and the Indian ocean in the eastern part. The cold Benguela current pushes northwards from the Antarctic along the Atlantic coastline up to the west coast, and a much warmer stream, the Agulhas current also moves southwards from the Indian Ocean. The eastern seas' steady evaporation provides generous rainfall while the Benguela current retains its moisture to cause desert conditions in the west [South Africa year book, 2006; [2]].
Rainfall is highly seasonal over most of southern Africa. South Africa can be classified into four seasons: summer which occurs in mid-October to mid-February; autumn which is in mid- February to April; winter occurs in May to July; and spring which occurs in August to mid- October. These seasons have varying temperatures ranging from -2oC in the winter to 36oC in the summer [2]. Rainfall over the interior northern regions of South Africa follows an annual cycle
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Fig. 2- 6: Map of South Africa
and is almost entirely a summer phenomenon [Tyson, 1986]. South Africa has an average annual precipitation of 502 mm, as against a world mean of 857 mm. Twenty-one per cent of the country receives less than 200 mm annually, 48 per cent between 200 and 600 mm, while 31 per cent records a total above 600 mm [South Africa year book, 2006]. The 400 mm rainfall line divides the country into a wet and a dry western half.
The moist Indian Ocean air masses, which are the chief source of rain over most of the country, gradually lose their moisture as they move towards the western interior. The rainfall distribution pattern in South Africa is also influenced by the orographic3 effect. The coastal plain has abundant low stratus cloud and fog, particularly during the summer. At night fog tends to advance on to the coastal plain to a depth of 30 – 50 km, withdrawing again as the land heats up during the day [South Africa year book, 2006].
3 Caused mostly by the forced uplift of air over high grounds [Ajayi, 1989]
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The winter rainfall region is a relatively small area along the Cape West and South West Coast and has a rainfall regime of the Mediterranean type with a conspicuous winter maxima. The rain is often long lasting and not very intense, except along the mountains, where the orographic effect may induce heavy showers. Snow occurs on the mountains four to six times during winter, but it usually melts within a few days. Hail and thunderstorm are both rare [South Africa year book, 2006]. The summer rainfall region covers most of the rest of the country, with common light orographic rains along the windward slopes of the eastern escarpment. Over most of the summer months, violent convection storms accompanied by thunder, lightning, sudden squalls and often hail are associated with the rainfall [2].
Between the winter and summer rainfall regions lies a transitional area, where rain comes in all seasons, that is, neither in winter nor in summer, but in autumn and spring. This transitional area can be divided into two sub-regions: a southern coastal strip with annual total of 375 to 875 mm, and a drier inland corridor behind the coastal ranges with an annual total of 50 to 250 mm [Brasseur et al., 1999]. In general, the climatic conditions in South Africa ranges from mediterranean in the south-western corner of the country to temperate in the interior plateau, subtropical in the northeast, and a small area in the northwest with a desert climate. Most rainfall in South Africa normally comes in the summer and autumn months. The spring and winter months are drier months; with the exception of Western Cape (Cape Town), because of its location and mediterranean nature of climate gets most of its rain in winter [[10];Odedina and Afullo, 2008].
It can be seen that rainfall in South Africa is highly variable and unpredictable throughout the country. Large fluctuations around the mean annual figure are the rule rather than exception. Like other countries in similar latitudes, South Africa, is periodically afflicted by severe and prolonged droughts. However, the droughts often end with in severe floods especially during summer [South Africa year book, 2006]. Tyson et al., [1976] observed that over the period of 1910-1972, much of the summer rainfall area of South Africa experienced a quasi 20-year oscillation rainfall [Tyson et al., 1976]. This rainfall spectrum shows a clear peak at about twenty years as well as peaks in 2-3 and 3-4 years bands [Fashuyi, 2006; Fashuyi et al., 2006].
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