The Sahara is the largest single arid region on Earth. For most times of the year, the Sahara desert is under the dominance of northeasterly dry and hot airflows from the anticyclonic system centered over the North Atlantic. This northeasterly trade wind, the Harmattan, is persistent in the western part of the Sahara extending to about 10°N. The Harmattan season lasts up to eight months from October to May. In summer, at the end of it, warm and humid southwesterly air flows influence the southern fringes of the Sahara.
Major Source areas of Saharan dust. Solid circles indicate the location of stations
in the African turbidity network. The locations Bilma and some mountains and mountain chains
Tassali N' Ajjer (TN), Tibesti (T), Hoggar (H), Air (AIR), Adardes Iforhas (AI), and Ennedi (E) have
strategic importance for the dust transport (modified from D'Almeida, 1986)
Sahara and Sahel regions have four major source areas. Source 1 extends from the Spanish Sahara to North Mauritania; Source 2 is located in Algeria in the triangle formed by the Hogger, Adar des Iforhas and Air mountains, northeast of Gao(Mali); source 3 is situated north to northeast of Dirku, north of Milna(Niger) off the west side of the Tibesti Mountains in Chad and Source 4 is located in the northern part of the Sudan and the southern part of Egypt. The meteorological conditions associated with dust storms in this region are depressions in the westerlies, such as the Mediterranean depressions and the Atlantic depressions in the winter months.
The precise dust-source locations in the Sahara region is a subject of controversial discussions in the literature. This is because the geomorphology of the Sahara is rather complex, comprising rock deserts, gravely soils, loamy soils, salt desert, sandy desert and a mixture of these different types. The most important dust origins are Source 1, 2 and 3, each having a dust production of over 200 Mt per year. Dust production from Source 4 is relatively weak. The transport of dust from the Saharan and Sahel regions takes place throughout the year, but is particularly strong between March and June. The transport direction is determined by the general circulation pattern. The southward transport toward the Sahel region and the Gulf of Guinea originates from all four source regions and is of frequent occurrence in wintertime. The total annual transport in the southward direction is approximately 380 Mt. The westward transport to the Atlantic Ocean is about half the strength of the southward transport, being around 190 Mt. The northward transport is around 100 Mt, while the eastward transport is small. Only about 5% (16 Mt of the 380 Mt) of the southward-moving dust reaches 5°N. In contrast, about 75% of the westward-moving dust (143 Mt of the 190 Mt) is deposited in the North Atlantic Ocean. (Shao, 2000)