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Sahara[suhAr´u] Pronunciation Key [Arab.,=desert], world's largest desert, c.3,500,000 sq mi (9,065,000 sq km), N Africa; the western part of a great arid zone that continues into SW Asia. Extending more than 3,000 mi (4,830 km), from the Atlantic Ocean to the Red Sea, the Sahara is bounded on the N by the Atlas Mts., steppelands, and the Mediterranean Sea; it stretches south c.1,200 mi (1,930 km) to the Sahel, a steppe in W and central Africa that forms its southern border. The desert includes most of Western Sahara, Mauritania, Algeria, Niger, Libya, and Egypt; the southern portions of Morocco and Tunisia; and the northern portions of Senegal, Mali, Chad, and Sudan. The E Sahara is usually divided into three regions : the Libyan Desert, which extends west from the Nile valley through W Egypt and E Libya; the Arabian Desert, or Eastern Desert, which lies between the Nile valley and the Red Sea in Egypt; and the Nubian Desert, which is in NE Sudan.
Regions of sand dunes (erg) occupy only about 15% of the Sahara; "stone deserts," consisting of plateaus of denuded rock (hammada) or areas of coarse gravel (reg), cover about 70% of the region; mountains, oases, and transition zones account for the remainder. Sparse vegetation is found in most parts of the Sahara, with the exception of the sand dune regions. High mountain massifs rise in the central regions; they are the Ahagger (Hoggar) in S Algeria, which rises to more than 9,000 ft (2,740 m); the Tibesti Massif in N Chad, which rises to more than 11,000 ft (3,350 m); and the AIr Mountains (Azbine) in N Niger, which rise to more than 6,000 ft (1,830 m). The mountains are deeply dissected and were in the past infamous for the shelter they provided to marauders preying on desert traffic. From west to east the four principal land routes across the desert are from Colomb-Bechar to Dakar; from Colomb-Bechar to Gao and Timbuktu by way of Reganne; from Touggourt to Agadez and Kano by way of In-Salah; and from Tripoli to Ghat.
The Sahara has one of the harshest climates in the world. Located in the trade winds belt, the region is subject to winds that are frequently strong and that blow constantly from the northeast between a subtropical high-pressure cell and an equatorial low-pressure cell. As air moves downward from the high-pressure into the low-pressure cell, it becomes warmer and drier. The desiccating and dust-laden winds are sometimes felt north and south of the desert, where they are variously known as sirocco, khamsin, simoom, and harmattan. The northern slopes of the Atlas Mts. intercept most of the moisture from winds blowing inshore from the Mediterranean Sea.

Border zones on the north and south, where the desert merges with the steppe, receive about 10 in. (25 cm) of rain a year with some seasonal regularity, but over most of the region rainfall is sparser, with an average annual total of less than 5 in. (12.7 cm); rainfall is usually torrential when it occurs after long dry periods that sometimes last for years. The region's low relative humidity rarely exceeds 30% and is often in the 4% to 5% range.

Daytime temperatures are high; Azizia, Libya, recorded the world's highest official temperature in the shade (136°F/58°C) in Sept., 1922. Heat loss is rapid at night and a diurnal range of 86°F (30°C) is common. Freezing temperatures are not uncommon at night from December to February

Water and Other Resources
The Nile and Niger rivers, both fed by rains outside the desert, are the only permanent rivers in the region. Water is present at or just below the surface gravel in wadis (intermittent streams) that radiate from the mountain massifs, in scattered oases where the water table comes to the surface, and at greater depths in huge underground aquifers. The aquifers are believed to be filled with water dating from the Pleistocene epoch, when the Sahara was much wetter than it is today. The more than 20 lakes (called chotts in the north) and areas of salt flats and boggy salt marshes are also considered relics from this pluvial period.

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Important discoveries of minerals, oil, and gas have been made in the Sahara. There are huge oil and gas deposits in Algeria and Libya, but in most cases, inaccessibility has delayed exploitation. In ing for oil reserves, underground deposits of water also have been found. Extensive iron ore deposits are worked in the Fort Gouraud area of Mauritania. Salt is still mined, as in the past, at Taoudenni, Mali, and at Bilma, Niger, and is transported, as in the days of the great medieval kingdoms of W Africa, by camel caravans across the desert.
Two thirds of the Sahara's estimated 2 million inhabitants (excluding those in the Nile valley) are concentrated in oases where date palms, fruits, vegetables, grains, and other crops are produced under irrigation. Nomads, with herds of sheep and goats and with camels for transportation, predominate in drier areas and continue to use oases (including modern oases created by the drilling of wells), as in centuries past, for water, trade, and provisioning stops. The principal ethnic groups of the Sahara are the Tuareg (of Berber origin), who dominate the mountains of the central Sahara; the peoples of mixed Berber and Arab origin in W Sahara; and the Tibu (TEbu), who dominate the Tibesti Massif.
The Sahara has undergone a series of wet periods, the most recent occurring c.5,00010,000 years ago; it was not until c.3000 B.C. that the Sahara transformed into its present arid state. There is dispute as to whether the desertification of the region has continued into historic time. Those who support this theory contend that increasing aridity is the reason for the recorded advance of desert conditions into areas under cultivation in Roman times in the north and more recently (since the late 1960s) in the south. Opponents of this view explain such changes as being the result of alterations in land-use practices and neglect of water-supply and irrigation systems.

The camel was introduced probably in the 1st cent. A.D. and facilitated occupation by nomads (first the Berbers, later the Arabs), who lived in interdependence with the oasis dwellers, providing protection against enemies in exchange for supplies of food and water. A profitable trans-Saharan trade in gold and slaves from W Africa, salt from the desert, and cloth and other products from the cities on the Mediterranean coast was carried on by the nomads. The first European explorers to travel in the Sahara were Friedrich Horneman in 1805 and Mungo Park in 1806. Some areas of the Sahara remain virtually unexplored, although a network of air and automobile routes now crosses the desert and links the major oases and mining areas.

Sahel[sAhel´] Pronunciation Key, name applied to the semiarid region of Africa between the Sahara to the north and the savannas to the south, extending from Senegal, on the west, through Mauritania, Mali, Burkina Faso, Niger, N Nigeria, Sudan, to Ethiopia on the east. Beginning in the late 1960s the Sahel was afflicted by a prolonged and devastating drought that further reduced the region's normally meager water supplies, shattered its agricultural economy, contributed to the starvation of an estimated 100,000 people, and forced the mass migration southward of many people. Although rainfall and international relief efforts helped, drought and famine affected the Sahel again in the mid-1980s and early 1990s.
(Information excerpts from emcyclopedia)
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