The Sahara ('the Greatest Desert') is the largest hot desert in the world, and the world's third largest desert after Antarctica and the Arctic. Its surface area of 9,400,000 square kilometres (3,600,000 sq mi)—including the Libyan Desert—is comparable to the respective land areas of China or the United States. The desert comprises much of the land found within North Africa, excluding the fertile coastal region situated against the Mediterranean Sea, the Atlas Mountains of the Maghreb, and the Nile Valley of Egypt and Sudan. The Sahara stretches from the Red Sea in the east and the Mediterranean in the north, to the Atlantic Ocean in the west, where the landscape gradually transitions to a coastal plain. To the south, it is delimited by the Sahel, a belt of semi-arid tropical savanna around the Niger River valley and Sudan Region of Sub-Saharan Africa. The Sahara can be divided into several regions, including the western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains, the Ténéré desert, and the Libyan Desert. Its name is derived from the plural Arabic language word for desert).
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The Sahara covers large parts of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Niger, Western Sahara, Sudan and Tunisia, extends over 9 million square kilometres (3,500,000 sq mi) and it covers about 1?4 of the African continent. If all areas with a mean annual precipitation of less than 250 mm were included, the Sahara would be over 11 million square kilometres (4,200,000 sq mi) in area. It is one of three distinct physiographic provinces of the African massive physiographic division.
Most of the Sahara consists of rocky hamada; ergs (large areas covered with sand dunes) form only a minor part. Many of its sand dunes reach over 180 metres (590 ft) in height.6 The desert landforms of the Sahara are shaped by wind or by extremely rare rainfall and include sand dunes and dune fields or sand seas (erg), stone plateaus (hamada), gravel plains (reg), dry valleys (wadi), dry lakes (oued) and salt flats (shatt or chott).7 Unusual landforms include the Richat Structure in Mauritania.
Several deeply dissected mountains and mountain ranges, many volcanic, rise from the desert, including the Aïr Mountains, Ahaggar Mountains, Saharan Atlas, Tibesti Mountains, Adrar des Iforas, and the Red Sea hills. The highest peak in the Sahara is Emi Koussi, a shield volcano in the Tibesti range of northern Chad.
Most of the rivers and streams in the Sahara are seasonal or intermittent, the chief exception being the Nile River, which crosses the desert from its origins in central Africa to empty into the Mediterranean. Underground aquifers sometimes reach the surface, forming oases, including the Bahariya, Ghardaïa, Timimoun, Kufra, and Siwa.
The central part of the Sahara is hyperarid, with little to no vegetation. The northern and southern reaches of the desert, along with the highlands, have areas of sparse grassland and desert shrub, with trees and taller shrubs in wadis where moisture collects. In the central, hyperarid part, there are many subdivisions of the great desert such as the Tanezrouft, the Ténéré, the Libyan Desert, the Eastern Desert, the Nubian Desert and others. These absolute desert regions are characterized by their extreme aridity, and some years can pass without any rainfall.
To the north, the Sahara skirts the Mediterranean Sea in Egypt and portions of Libya, but in Cyrenaica and the Maghreb, the Sahara borders the Mediterranean forest, woodland, and scrub ecoregions of northern Africa, all of which have a Mediterranean climate characterized by hot summers and cool and rainy winters. According to the botanical criteria of Frank White8 and geographer Robert Capot-Rey,910 the northern limit of the Sahara corresponds to the northern limit of date palm cultivation and the southern limit of the range of esparto, a grass typical of the Mediterranean climate portion of the Maghreb and Iberia. The northern limit also corresponds to the 100 mm (3.9 in) isohyet of annual precipitation.11
To the south, the Sahara is bounded by the Sahel, a belt of dry tropical savanna with a summer rainy season that extends across Africa from east to west. The southern limit of the Sahara is indicated botanically by the southern limit of Cornulaca monacantha (a drought-tolerant member of the Chenopodiaceae), or northern limit of Cenchrus biflorus, a grass typical of the Sahel.910 According to climatic criteria, the southern limit of the Sahara corresponds to the 150 mm (5.9 in) isohyet of annual precipitation (this is a long-term average, since precipitation varies annually).11
Important cities located in the Sahara include Nouakchott, the capital of Mauritania; Tamanrasset, Ouargla, Béchar, Hassi Messaoud, Ghardaïa, and El Oued in Algeria; Timbuktu in Mali; Agadez in Niger; Ghat in Libya; and Faya-Largeau in Chad.