The rains on the Upper Nile in the equatorial regions from the Albert Nyanza to the Bahr El Ghazal last from February or March to October or the middle of November.
As one proceeds northwards the heavy rains come later; the “Kherif,” or rainy season, in the neighbourhood of Kodok, lasts, as a rule, from the middle of May to the end of October, whilst at Khartoum it may be said to be during July, August, and September. At Halfa there is no rainy season.
At Gondokoro the hot weather commences middle of November and lasts till the end of March.
Colonel Stewart states (1883):—“The rains are very heavy, lasting 10 to 12 hours at a time. From Gondokoro south to the Equator, rainy seasons increase in length, till on the Equator it may be said that rain and sunshine succeed each other in rapid succession all the year round.”
Heavy thunderstorms and rains occur at intervals during the rest of the year, especially from October to January in the hilly regions round Rejaf and the Sudd district to the north of it.
Winds.
During late autumn winds are very variable, blowing from all points of the compass. East and south-east winds at this period predominate, especially in the upper reaches. During the rest of the year the north wind, varying from north-west to north-east, is fairly constant, except during the summer rains, when the wind shifts to the south and south-east.
Temperature.
The average maximum and minimum shade temperatures on the Upper Nile during the hot months are 99° and 85° respectively, and during the rest of the year 88° and 74°. Between Khartoum and Halfa the maximum average is a good deal higher, and the minimum rather lower.
In the Sudd region and south the maximum averages about 85°, but, owing to the dampness of the district, from April to December fevers are rife, and the heat and mosquitos are difficult to bear with equanimity.