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. Khartoum has a three-month rainy season (July–September) with an annual average rainfall of ; Atbarah receives showers in August that produce an annual average of only 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 and temperatures can even reach . Northern Sudan, with its short rainy season, has very high daytime temperatures year round, except for winter months in the northwest where there is some precipitation 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 in January and have dropped as low as 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/red wall of sand and clay that can temporarily reduce visibility to zero. Desert regions in central and northern Sudan are among the driest and the sunniest places on Earth : the sunshine duration is always uninterrupted year-round and soar to above 4,000 h in the best cases, or about 91% of the time and the sky is cloudless all