The Sudan is the largest country in Africa with an area of nearly 2.5 million sq km/1 million sq mi.
It lies entirely within the tropics between 22° and 4°N, and is bordered north by Egypt, northeast by the Red Sea, east by Ethiopia and Eritrea, west by the Central African Republic and Chad, south by Kenya, Uganda, and the Democratic Republic of Congo, and northwest by Libya.
The northern part of the country is desert and has a climate similar to that of the Egyptian, Libyan, and Algerian Sahara. From Khartoum southwards to the southern border there is a progressive increase in the annual rainfall from 150 mm/6 in to over 1,000 mm/40 in.
Rainfall in the north is rare and very sporadic in time and place. The southern margin of the Sahara effectively is where annual rainfall is about 400 mm/16 in, since evaporation is high during the very hot summer.
The rainy season in the Sudan is almost everywhere the period between April and October, although in the extreme south some rain may occur in any month. The length of the rainy season decreases from six to eight months in the south to as little as two months on the southern margins of the desert.
The northern part of the Sudan experiences almost constant northeasterly winds throughout the year. In this dry air-mass humidity is low during the day and this makes the very high daytime temperatures more tolerable.
During the cooler winter months temperatures may occasionally fall quite low and early-morning frost is not unknown in the desert. There is a progressive increase of temperature to the maximum levels reached in July and August when, even at night, the thermometer rarely falls below 24°C/75° F.
During the rainy season in the south and centre of the Sudan, southerly and southwesterly winds replace the northeasterlies. They bring slightly lower temperatures, higher humidity, and more cloud. In the far south monthly temperatures vary little around the year and are highest just before the arrival of the rains.
This moist southerly air has its origins in the South Atlantic or Congo basin and is the source of the Sudan's summer rain. During the rainy season there are spells of dry and sunny weather and even in the wettest areas rain only falls on about one day in three.
The higher humidity during the rainy season does not make the lower temperatures any more comfortable. From April to September some heat stress can be experienced in all parts of the country.
An unpleasant and occasionally dangerous feature of the weather is the haboob, a local Arabic name for a violent but brief squall of wind which can raise a thick pall of dust or sand. Haboobs are most likely to occur in the afternoon and evening before, or at the beginning of, the rainy season. They often precede a thunderstorm which brings rain and lays the dust and sand.
The tables for Khartoum and Port Sudan are representative of the northern desert regions of the Sudan. On the coast of the Red Sea and in the hills behind there is some sporadic rainfall during the months October to December.
The table for Juba shows conditions throughout the year in the extreme south. In the north annual sunshine amounts are almost the maximum possible, ranging from eleven to thirteen hours a day. Sunshine is least in the far south where, during the rainy season, it averages six to seven hours a day.
During the dry season in the south sunshine averages nine to ten hours a day.
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