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The Sahara covers large parts of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Niger, Western Sahara, Sudan and Tunisia. It covers 9 million square kilometres (3,500,000 sq mi), amounting to 31% of Africa. If all areas with a mean annual precipitation of less than 250 mm were included, the Sahara would be 11 million square kilometres (4,200,000 sq mi). It is one of three distinct physiographic provinces of the African massive physiographic division.
The Sahara is mainly rocky hamada. Ergs (large areas covered with sand dunes) form only a minor part, but many of the sand dunes are over 180 metres (590 ft) high. Wind or rare rainfall shape the desert features: sand dunes, dune fields, sand seas (erg), stone plateaus (hamada), gravel plains (reg), dry valleys (wadi), dry lakes (oued), and salt flats (shatt or chott). Unusual landforms include the Richat Structure in Mauritania.
Several deeply dissected mountains, 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, from its origins in central Africa, crosses the desert underground before emptying into the Mediterranean, although aquifers sometimes surface, forming oases. These include the Bahariya, Ghardaïa, Timimoun, Kufra, and Siwa.
Central Sahara is hyperarid, with sparse 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 region, there are many subdivisions of the great desert: Tanezrouft, the Ténéré, the Libyan Desert, the Eastern Desert, the Nubian Desert and others. These extremely arid areas often receive no rain for years.
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 White and geographer Robert Capot-Rey, 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.
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. 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).
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.