The Sahara is the biggest 'traditional' desert (apart from the polar deserts) in the world and stretches across most of Northern Africa from the Atlantic Ocean in the west to the Red Sea in the east, and from the Mediterranean coastline in the north to the Sahel in the south. Its name is derived from the plural Arabic language word for desert (صحارى ṣaḥārā).
The classic pictures of endless sanddunes are true to a certain point, as there are larges areas of 'ergs' (sandsea in Arabic) to be found mainly in Algeria and Libya. Many of its sand dunes reach over 180 metres in height. Most of the Sahara is gravel and rocks though and there are several mountain ranges within its boundaries as well, including the Ahaggar Mountains in Algeria, the Aïr Mountains in Niger and the Tibesti Mountains in Chad. The latter also has the highest point at 3,415 metres above sea level.
The Sahara extends over 9 million square kilometres and it covers about 1⁄4 of the African continent. It is one of three distinct physiographic provinces of the African massive physiographic division. 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).
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 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 line 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 line of annual precipitation (this is a long-term average, since precipitation varies annually).
The Sahara comprises several distinct ecoregions, and with their variations in temperature, rainfall, elevation, and soil, they harbor distinct communities of plants and animals.
The Atlantic coastal desert is a narrow strip along the Atlantic coast, where fog generated offshore by the cool Canary Current provides sufficient moisture to sustain a variety of lichens, succulents, and shrubs. It covers 39,900 square kilometres in the south of Morocco and Mauritania.
The North Saharan steppe and woodlands is along the northern desert, next to the Mediterranean forests, woodlands, and scrub ecoregions of the northern Maghreb and Cyrenaica. Winter rains sustain shrublands and dry woodlands that form a transition between the Mediterranean climate regions to the north and the hyper-arid Sahara proper to the south. It covers 1,675,300 square kilometres in Algeria, Egypt, Libya, Mauritania, Morocco,and Tunisia.
The Sahara Desert ecoregion covers the hyper-arid central portion of the Sahara where rainfall is minimal and sporadic. Vegetation is rare, and this ecoregion consists mostly of sand dunes (erg, chech, raoui), stone plateaus (hamadas), gravel plains (reg), dry valleys (wadis), and salt flats. It covers 4,639,900 square kilometres of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Niger, and Sudan.
The South Saharan steppe and woodlands ecoregion is a narrow band running east and west between the hyper-arid Sahara and the Sahel savannas to the south. Movements of the equatorial Intertropical Convergence Zone (ITCZ) bring summer rains during July and August which average 100 to 200 mm but vary greatly from year to year. These rains sustain summer pastures of grasses and herbs, with dry woodlands and shrublands along seasonal watercourses. This ecoregion covers 1,101,700 km2 in Algeria, Chad, Mali, Mauritania, and Sudan.
In the West Saharan montane xeric woodlands, several volcanic highlands provide a cooler, moister environment that supports Saharo-Mediterranean woodlands and shrublands. The ecoregion covers 258,100 km2, mostly in the Tassili n'Ajjer of Algeria, with smaller enclaves in the Aïr of Niger, the Dhar Adrar of Mauritania, and the Adrar des Iforas of Mali and Algeria.
The Tibesti-Jebel Uweinat montane xeric woodlands ecoregion consists of the Tibesti and Jebel Uweinat highlands. Higher and more regular rainfall and cooler temperatures support woodlands and shrublands of palms, acacias, myrtle, oleander, tamarix, and several rare and endemic plants. The ecoregion covers 82,200 km2 in the Tibesti of Chad and Libya, and Jebel Uweinat on the border of Egypt, Libya, and Sudan.
The Saharan halophytics is an area of seasonally flooded saline depressions which is home to halophytic (salt-adapted) plant communities. The Saharan halophytics cover 54,000 km2, including the Qattara and Siwa depressions in northern Egypt, the Tunisian salt lakes of central Tunisia, Chott Melghir in Algeria, and smaller areas of Algeria, Mauritania, and The southern part of Morocco.
The Tanezrouft is one of the harshest regions on Earth as well as one of the hottest and driest parts of the Sahara, with no vegetation and very little life. It is along the borders of Algeria, Niger and Mali, west of the Hoggar mountains.
Travelling across the Sahara is possible and there are even possibilities to travel almost exclusively along tarmac roads if you choose to travel along the coastline of Morocco, Western Sahara and Mauritania. There are other routes though there are more demanding, like the ones across Algeria into Niger. For more information on this and other aspects check the Sahara Overland website.
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