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The Sudan, entirely within the tropics from latitudes 22°N. to 3°N. and with a simple topography from the meterological aspect, is almost entirely landlocked and possesses a continental climate. Maritime characteristics are confined to the narrow eastern coastal plain and eastern slopes of the Red Sea Hills. Elsewhere the vast plain is broken only by the Marra Mountains of Darfur, the Nuba Mountains of southern Kordofan, (and the few small ranges of Equa toria). The swampy Sudd of the Upper Nile is the only inland body of water large enough to influence climate. The Sudanese plain extends far west and north, but eastward and southward it is lim ited by the Ethiopian, East Africa, and Congo highlands.
Mean annual isotherms, or lines of equal temperature, in the Sudan (Figure 323) range from 25°C. to 29oc. Temperatures in this figure are not reduced to a standard level and caution is necessary in dealing with these due to relatively small amount of available data.
In the north and the Red Sea area, the highest mean maxima occur in June or July; these are in the range of 41.1°C. to 43.8°C. Elsewhere they precede the rains and vary from May at Khartoum to January in Equatoria; these range from 36.9°C. to 40.9°c. in the plains and less in highland localities. The lowest mean maxima occur in January in the north and during the rains, July or August, south of about 140N. Seasonal variation decreases from north to south. The highest daily maxima (37.5oC. to 52.5°C. at Wadi Halfa) occur in the north, the lowest in the south (36.7°c. to 43.7°C. at Juba).
*This section is abstracted from Ireland (1948) in Agriculture in the Sudan.
MEAN ANNUAL RAINFALL (MM)
MEAN ANNUAL RAINFALL DISTRIBUTION
The lowest mean minima throughout the Sudan occur in the win ter. Mean daily minima are lowest in the extreme north (7.8°c. to 23.8°c. at Wadi Halfa) and in the west and highest in the Red Sea area (19.2°C. to 28.8°c. at Port Sudan). At Juba they range from 19.8°c. to 22.0°c.; at Wau from 17.6°c. to 22.4°C.; and at Wad Medani from 14.3°c. to 24.3°C. The seasonal variation decreases from north to south and is very small south of Malakal. Numerous other temperature data may be found in Ireland's (1948) original summary
Rainfall in the Sudan (Figures 324 and 325) is characterized by a remarkably regular decrease in mean annual total from 1400 mm. in the south (1500 mm. in eastern Equatoria mountains) to 25 mm. or less in the northern deserts. Figure 325 shows the annual rainfall distribution expressed as monthly percentages of the total.
Except coastally, diagrams represent means over four degree squares. Inland increase in rainy season length from north to south and the anomalous Red Sea area régime are clear. ly shown, as is the notable latitudinal uniformity and the equa torial double maximum in the extreme south, See Ireland (1948) for further details.
Climatically the Sudan may be divided into three regions:
1. North of about latitude 19°N.
North of about latitude 19°N. the desert receives little, infrequent rain or none at all. Strong winter winds with sand storms and occasional frontal rain occur with an influx of cold air behind a vigorous Mediterranean depression. The desert climate characteristically experiences wide diurnal and annual temperature variations,
South of about latitude 19on. the typical tropical conti. nental climate is dominated by the annual movement of the boundary between dry northerlies and moist southerlies, a boundary reaching its northern limit in midsummer and its southern limit in mid winter. The southerlies bring rain that extends five hundred miles or so south from the actual boundary. The rainy season
is shortest in the north and longest in the south. Host of the rain is convectional with a marked afternoon and evening maximum. Dry winter weather is very stable but intense thunderstorms occur in summer. In the semiarid north, early rainy season winds associated with thunderstorms bring dust storms or ho boobs.
The Red Sea coast and the eastern slopes of the Red Sea Hills, influenced by the Red Sea, have northerlies throughout the year. These bring rain and clouds, rain falling chiefly in winter. The higher relative humidity of this narrow coastal strip markedly influences the flora and fauna of the area.
The vegetation of the Sudan (Andrews 1948) is divided into seven principal Districts from north to south (Figure 326):
Isolated areas unrelated to their immediate surroundings are the Red Sea Hills (Erkowit) and Gebel Elba (southeastern Egypt adjacent to the Sudan frontier and administered by the Sudan Government).
The following brief abstract provides a generalized picture of the chief floral aspects of the Sudan. For further details, see Andrews (1948) and various chapters in Tothill (1948).