Although Sudan lies within the tropics, the climate ranges from
arid in the north to tropical wet-and-dry in the far southwest.
Temperatures do not vary greatly with the season at any location;
the most significant climatic variables are rainfall and the length
of the dry season. Variations in the length of the dry season
depend on which of two air flows predominates, dry northeasterly
winds from the Arabian Peninsula or moist southwesterly winds
from the Congo River basin.
From January to March, the country is under the influence of
the dry northeasterlies. There is practically no rainfall countrywide
except for a small area in northwestern Sudan in where the winds
have passed over the Mediterranean bringing occasional light rains.
By early April, the moist southwesterlies have reached southern
Sudan, bringing heavy rains and thunderstorms. By July the moist
air has reached Khartoum, and in August it extends to its usual
northern limits around Abu Hamad, although in some years the humid
air may even reach the Egyptian border. The flow becomes weaker
as it spreads north. In September the dry northeasterlies begin
to strengthen and to push south and by the end of December they
cover the entire country. Yambio, close to the border with Zaire,
has a nine-month rainy season (April-December) and receives an
average of 1,142 millimeters of rain each year; Khartoum has a
three-month rainy season (JulySeptember ) with an annual average
rainfall of 161 millimeters; Atbarah receives showers in August
that produce an annual average of only 74 millimeters.
In some years, the arrival of the southwesterlies and their rain
in central Sudan can be delayed, or they may not come at all.
If that happens, drought and famine follow. The decades of the
1970s and 1980s saw the southwesterlies frequently fail, with
disastrous results for the Sudanese people and economy.
Temperatures are highest at the end of the dry season when cloudless
skies and dry air allow them to soar. The far south, however,
with only a short dry season, has uniformly high temperatures
throughout the year. In Khartoum, the warmest months are May and
June, when average highs are 41° C and temperatures can reach
48° C. Northern Sudan, with its short rainy season, has hot
daytime temperatures year round, except for winter months in the
northwest where there is precipitation from the Mediterranean
in January and February. Conditions in highland areas are generally
cooler, and the hot daytime temperatures during the dry season
throughout central and northern Sudan fall rapidly after sunset.
Lows in Khartoum average 15° C in January and have dropped
as low as 6° C after the passing of a cool front in winter.
The haboob, a violent dust storm, can occur in central Sudan
when the moist southwesterly flow first arrives (May through July).
The moist, unstable air forms thunderstorms in the heat of the
afternoon. The initial downflow of air from an approaching storm
produces a huge yellow wall of sand and clay that can temporarily
reduce visibility to zero.
Data as of June 1991