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Sahara Desert

The Sahara (Arabic: الصحراء الكبرى‎, the Great Desert') is the largest hot desert and the third largest desert in the world after Antarctica and the Arctic. Its area of 9,200,000 square kilometers (3,600,000 sq mi) is comparable to the area of China or the United States.

The desert comprises much of North Africa, excluding the fertile region on the Mediterranean Sea coast, the Atlas Mountains of the Maghreb, and the Nile Valley in Egypt and Sudan. It 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 changes from desert to coastal plains. To the south, it is bounded by the Sahel, a belt of semi-arid tropical savanna around the Niger River valley and the Sudan Region of Sub-Saharan Africa.

The Sahara can be divided into several regions including Western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains, the Ténéré desert, and the Libyan Desert.

The name 'Sahara' is derived from the dialectal Arabic word for "desert", ṣaḥra.

Geography
A geographical map of Africa, showing the ecological break that defines the Saharan area.
An oasis in the Ahaggar Mountains. Oases support some life forms in extremely arid deserts.
An intense Saharan dust storm sent a massive dust plume northwestward over the Atlantic Ocean on 2 March 2003
Rocky mountains naturally sculpted by the wind

The Sahara covers large parts of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Niger, Western Sahara, Sudan and Tunisia. It covers 9 million square kilometers (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 kilometers (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 (stone plateaus), Ergs (sand seas - large areas covered with sand dunes) form only a minor part, but many of the sand dunes are over 180 meters (590 ft) high.Wind or rare rainfall shape the desert features: sand dunes, dune fields, Sand seas, stone plateaus, gravel plains (reg), dry valleys (wadi), dry lakes (Ouad), and salt flats (shaft 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.

The 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: the 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.

Climate
Main articles: Sahara pump theory and Green Sahara

The Sahara is the world's largest low-latitude hot desert. The area is located in the horse latitudes under the subtropical ridge, a significant belt of semi-permanent subtropical warm-core high pressure where the air from upper levels of the troposphere tends to sink towards the ground. This steady descending airflow causes a warming and a drying effect in the upper troposphere. The sinking air prevents evaporating water from rising and, therefore, prevents the adiabatic cooling, which makes cloud formation extremely difficult to nearly impossible.[13]

The permanent dissolution of clouds allows unhindered light and thermal radiation. The stability of the atmosphere above the desert prevents any convective overturning, thus making rainfall virtually non-existent. As a consequence, the weather tends to be sunny, dry and stable with a minimal risk of rainfall. Subsiding, diverging, dry air masses associated with subtropical high-pressure systems are extremely unfavorable for the development of convectional showers. The subtropical ridge is the predominant factor that explains the hot desert climate (Köppen climate classification BWh) of this vast region. The lowering of air is the strongest and the most effective over the eastern part of the Great Desert, in the Libyan Desert which is the sunniest, driest and the most nearly "rain-less" place on the planet rivaling the Atacama Desert, lying in Chile and Peru.

The rainfall inhibition and the dissipation of cloud cover are most accentuated over the eastern section of the Sahara rather than the western. The prevailing air mass lying above the Sahara is the continental tropical (cT) air mass, which is hot and dry. Hot, dry air masses primarily form over the North-African desert from the heating of the vast continental land area, and it affects the whole desert during most of the year. Because of this extreme heating process, a thermal low is usually noticed near the surface, and is the strongest and the most developed during the summertime. The Sahara High represents the eastern continental extension of the Azores High,[citation needed] centered over the North Atlantic Ocean. The subsidence of the Sahara High nearly reaches the ground during the coolest part of the year while it is confined to the upper troposphere during the hottest periods.

The effects of local surface low pressure are extremely limited because upper-level subsidence still continues to block any form of air ascent. Also, to be protected against rain-bearing weather systems by the atmospheric circulation itself, the desert is made even drier by his geographical configuration and location. Indeed, the extreme aridity of the Sahara can not be only explained by the subtropical high pressure. The Atlas Mountains, found in Algeria, Morocco and Tunisia also help to enhance the aridity of the northern part of the desert. These major mountain ranges act as a barrier causing a strong rain shadow effect on the leeward side by dropping much of the humidity brought by atmospheric disturbances along the polar front which affects the surrounding Mediterranean climates.

The primary source of rain in the Sahara is the Intertropical Convergence Zone, a continuous belt of low-pressure systems near the equator which bring the brief, short and irregular rainy season to the Sahel and southern Sahara. Rainfall in this giant desert has to overcome the physical and atmospheric barriers that normally prevent the production of precipitation. The harsh climate of the Sahara is characterized by: extremely low, unreliable, highly erratic rainfall; extremely high sunshine duration values; high temperatures year-round; negligible rates of relative humidity; a significant diurnal temperature variation; and extremely high levels of potential evaporation which are the highest recorded worldwide.[14] Temperature

The sky is usually clear above the desert and the sunshine duration is extremely high everywhere in the Sahara. Most of the desert enjoys more than 3,600 h of bright sunshine annually or over 82% of the time, and a wide area in the eastern part experiences in excess of 4,000 h of bright sunshine a year or over 91% of the time. The highest values are very close to the theoretical maximum value. A value of 4,300 h or 98% of the time would be recorded in Upper Egypt (Aswan, Luxor) and in the Nubian Desert (Wadi Halfa).[15] The annual average direct solar irradiation is around 2,800 kWh/(m2 year) in the Great Desert. The Sahara has a huge potential for solar energy production.
Sahara desert

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