Sahara
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| Sahara (الصحراء الكبرى) | |
| The Great Desert | |
| Desert | |
A satellite image of the Sahara by NASA World Wind.
|
|
| Countries | Algeria, Chad, Egypt, Eritrea, Libya, Mali, Mauritania, Morocco, Niger, Sudan, Tunisia, Western Sahara |
|---|---|
| Highest point | Emi Koussi 11,204 ft (3,415 m) |
| - coordinates | 19°47′36″N 18°33′6″E |
| Lowest point | Qattara Depression −436 ft (−133 m) |
| - coordinates | 30°0′0″N 27°5′0″E |
| Length | 4,800 km (2,983 mi), E/W |
| Width | 1,800 km (1,118 mi), N/S |
| Area | 9,400,000 km2 (3,629,360 sq mi) |
| Biome | Desert |
This video over the Sahara Desert and the Middle East was taken by the crew of Expedition 29 on board the International Space Station.
The top image shows the Safsaf Oasis on the surface of the Sahara. The bottom (using radar) is the rock layer underneath, revealing black channels cut by the meandering of an ancient river that once fed the oasis.
The Sahara (Arabic: الصحراء الكبرى, aṣ-Ṣaḥrāʾ al-Kubrā , 'the Great Desert') is the largest subtropical hot desert and third largest desert after Antarctica and the Arctic.[1] At over 9,400,000 square kilometres (3,600,000 sq mi), it covers most of North Africa, making it almost as large as China or the United States. The Sahara stretches from the Red Sea to the east, including parts of the Mediterranean coasts to the Atlantic Ocean to the west. To the south, it is delimited by the Sahel, a belt of semi-arid tropical savanna that composes the northern region of central and western Sub-Saharan Africa.
Some of the sand dunes can reach 180 metres (590 ft) in height.[2] The name comes from the plural Arabic language word for desert (صحارى ṣaḥārā [3][4] [ˈsˤɑħɑːrɑː]).[5][6]
Overview
The Sahara was originally occupied by the Amaziɣ peoples whose name for the region was "Tinariwen", meaning deserts a reference to the fact that they differentiated between the different ecoregions of the desert (i.e. Erg, Oasis, Xeric shrublands etc.) The Sahara's boundaries are the Atlantic Ocean on the west, the Atlas Mountains and the Mediterranean on the north, the Red Sea on the east, and the Sudan and the valley of the Niger River on the south. The Sahara is divided into western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains (a region of desert mountains and high plateaus), Ténéré desert and the Libyan Desert (the most arid region). The highest peak in the Sahara is Emi Koussi (3,415 metres (11,204 ft)) in the Tibesti Mountains in northern Chad.The Sahara is the largest desert on the African continent. The southern border of the Sahara is marked by a band of semiarid savanna called the Sahel; south of the Sahel lies Southern Sudan and the Congo River Basin. Most of the Sahara consists of rocky hamada; ergs (large areas covered with sand dunes) form only a minor part.
People lived on the edge of the desert thousands of years ago[7] since the last ice age. The Sahara was then a much wetter place than it is today. Over 30,000 petroglyphs of river animals such as crocodiles[8] survive, with half found in the Tassili n'Ajjer in southeast Algeria. Fossils of dinosaurs, including Afrovenator, Jobaria and Ouranosaurus, have also been found here. The modern Sahara, though, is not lush in vegetation, except in the Nile Valley, at a few oases, and in the northern highlands, where Mediterranean plants such as the olive tree are found to grow. The region has been this way since about 1600 BCE, after shifts in the Earth's axis increased temperatures and decreased precipitation.[9]
The peoples of the Sahara are of various origins. Among them the Amaziɣ including the Turūq, various Arabized Amaziɣ groups such as the Hassaniya-speaking Sahrawis, whose populations include the Znaga a tribe whose name is a remnant of the pre-historic Zenaga language. Other major peoples include the Toubou, Nubians, Zaghawa, Kanuri, Hausa, Songhai, and Fula/Fulani (French: Peul; Fula: Fulɓe). 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.
Geography
A geographical map of Africa, showing the ecological break that defines the Saharan area.
An intense Saharan dust storm sent a massive dust plume northwestward over the Atlantic Ocean on 2 March 2003
The Sahara covers large parts of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Niger, Western Sahara, Sudan and Tunisia, extends over 9,000,000 km² (over 3,475,000 sq mi) and it covers about 1/4 of the African continent. If all areas with a mean annual precipitaion of less than 250mm were included, the Sahara desert would be over 11,000,000 km² (over 4,100,000 sq mi)in area. 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).[10] 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 is 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 reaches to the Mediterranean Sea in Egypt and portions of Libya, but in Cyrenaica and the Maghreb, the Sahara borders Mediterranean forest, woodland, and scrub ecoregions of northern Africa, which have a Mediterranean climate characterized by a winter rainy season. According to the botanical criteria of Frank White[11] and geographer Robert Capot-Rey,[12][13] the northern limit of the Sahara corresponds to the northern limit of date palm cultivation and the southern limit 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.[14]
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.[12][13] 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).[14]
Climate
The Sahara is a harsh environment with extreme conditions. It is the world's largest subtropical hot desert, and the world's hottest desert. The Sahara has mainly a subtropical, hot desert climate (Köppen climate classification BWh) with long, prolonged, extremely hot to scorching summers while the winters stay short, brief, extremely warm to truly very hot. The climate of this desert is also characterized by a perpetual clear sky, fair weather and by very low, and even almost non-existent rainfall but the precipitation is also very irregular and sporadic. Although the Sahara is located under the Tropic of Cancer in the most part, his climate is said to be subtropical due to the subtropical high pressure belt, which is mainly responsible of the hot desert climate. The northern fringe of the GreatDesert receives very low winter rainfall, where low pressure systems associated with the polar front arrive as being very weak and very attenuated. The southern fringe of the Great Desert receives summer irregular rainfall but relatively low which can only occur when the Intertropical Convergence Zone moves up far enough northerly or when the tropical low pressure systems are strong enough to cause precipitation. In both cases, the climate is very arid and the rainfall only occur on a few days per year. Between the two parts, the central Sahara has an extremely arid climate, with the influence of the continental trade wind. The western coastal desert zone bask under the cool ocean current, the Canary Current which is responsible of a higher clouds and fog formation. According to the dryness of the air, the climate of the Sahara can not not be extreme, since any place where visible and invisible cloud cover as well as the water vapor contained within the atmosphere don't act as temperature regulator, the day between sunlight and ground in order to reduce the roasting, and the night between the Earth surface and space in order to reduce cooling process by sending the absorbed infrared radiation of contracted heat during the day towards the sky, any place having that feature will be characterized by large temperature variations and therefore by an extreme climate. The climates of the Sahara possess relatively high diurnal temperature ranges (between days and nights), and in some rare cases, brutal temperatures variations due to the extremely dry and pure air as well as the clarity of the desert skies. The presence of slow but constant winds make the dryness and the aridity of the Sahara even worse, by enabling a greater evaporation. The climate oscillates between the extreme heat during a typical summer day and the coolness of the winter nights. This desert is noted for his weather and climate extremes : indeed the Sahara contains the places, making part of the hottest, driest and sunniest places found in the entire world. The climate has mainly oppressively hot, sunny, windy and dry conditions all year long. The Sahara is the perfect model of the hot deserts as well as their climate. Carl Ritter liked to say that the Sahara was "the South of the World". This apparently paradoxical reflection means that the Sahara was the hottest and the driest region on Earth. The annual mean temperature of the Great Desert, reduced to the sea level is higher than the one of all other deserts found on the planet[15]
Precipitation
The Sahara is one of the driest deserts in the world and receives a very to an extremely low annual average precipitation. The precipitation in the desert are also very irregular : it may not rain for some years and it may rain more than the annual amount in only one day. Some parts of the Sahara, especially deserts along Libya, Egypt and Sudan where many decades may pass without seeing any rainfall at all, have the lowest rain amount recorded on Earth, less than 10 mm of rain (0.39 in) and are as dry as the Atacama Desert, considered as the driest place. Sometimes, the precipitation evaporates before reaching the ground, when falling into the low-level hot, dry desert atmosphere.The Sahara desert is considered as a "real desert" because it has arid, between 50 mm (1.96 in) and 250 mm of rain annually (9.84 in) and hyperarid zones, between 0 mm (0.0 in) and 50 mm of rain annually (1.96 in), and even a semi-arid zone which receives between 250 mm (9.84 in) and 500 mm of rain (19.68 in), called the Sahel. The major part of the Sahara desert receives less than 100 mm of rain annually, and even less than 50 mm in a great portion. The Sahara is the desert and the Sahel is the steppe. The driest part of the desert is the central, hyperarid area and is one of the least rainy places found on the planet, rivaling the famous Atacama Desert. Nevertheless, it's very important to note that the Atacama Desert of Chile and Peru is much smaller than the large hyperarid core of the Sahara, and high air moisture does occur at least for part of the annual cycle, in contrast with the central and eastern Sahara.[16] In the central, hyperarid part of the Great Desert, the driest regions are the Tanezrouft, the Ténéré, the Libyan Desert, the Nubian Desert and the Eastern Desert. That's why the rainfall amount is very low on the coast, around 30 mm (1.18 in) and 50 mm (1.96 in). For example, it falls on annual average 1 mm of rain in Aswan (0.04 in), Egypt; 1.1 mm of rain in Luxor (0.04 in), Egypt; 5,3 mm of rain in Hurghada (0.21 in), Egypt; 3 mm of rain in Wadi Halfa (0.12 in), Sudan; 18 mm in Dongola, Sudan (0.71 in); 15 mm in Adrar, Algeria (0.59 in); 16 mm in In Salah, Algeria (0.63 in); 30 mm in Tindouf, Algeria (0.75 in); 131 mm in Ouarzazate, Morocco (5.16 in); 61 mm in Zagora, Morocco (2.40 in); 59 mm in Merzouga, Morocco (2.32 in); 38 mm in Smara, Western Sahara (5.43 in); 33 mm (1.3 in) in Dakhla, Western Sahara; 68 mm in Tessalit, Mali (2.68 in); 11 mm in Taoudenni, Mali (0.43 in); 12 mm in Bilma, Niger (0.47 in); 111 mm in Agadez, Niger (4.37 in); 12 mm in Faya-Largeau, Chad (0.47 in); 15 mm in Bardai, Chad (0.59 in); 73 mm in Atar, Mauritania (2.87 in); 47 mm in Bir Moghrein, Mauritania (1.85 in); 1 mm in Kufra, Libya (0.04 in); 8 mm in Ghat, Libya (0.31 in); 9 mm in Sebha, Libya (0.35 in). In order to compare, Paris receives on annual average 637 mm of rain (25 in) while Marseille receives 515 mm of rain (20.2 in). The permanent drought, more so than the heat, makes the Sahara a desert. In addition to the extremely low rainfall amount that reaches the desert surface, the major part of the Sahara only sees the rain some days in an entire year. On average, it rains about 10 or 15 days per year and there are many locations that only see the rain less than 5 days a year.
Temperatures
The Sahara is characterized by intense heat and a scorching sun, especially during the hot season, the summertime when the desert becomes dangerously hot. The thermal climate is quite uniform during summer but is more variable during winter. Mainly having a low elevation, and located close of the equator where the sun beats down always on high angles, the African desert beats heat records.The desert possesses long, prolonged, extremely hot and even scorching summers, with average high temperatures often between 38 °C (100.4 °F) and 46 °C (114.8 °F), sometimes for more than 3 consecutive months, while the average low temperatures mainly stays around 21 °C (69.8 °F) and 29 °C (84.2 °F). All the Sahara reaches at least once 40 °C (104 °F), and even 45 °C (113 °F) in the most part. For example, the averages highs and the averages lows for the hottest month of the year (generally July) are respectively 40.1 °C (104.2 °F) and 26.8 °C (80.2 °F) in Biskra, Algeria; 41.6 °C (106.9 °F) and 25.1 °C (77.2 °F) in Touggourt, Algeria; 45.1 °C (113.2 °F) and 27°6 °C (81.7 °F) in Aoulef, Algeria; 45.3 °C (114.2 °C) and 27.5 °C (81.5 °F) in In Salah, Algeria; 42.5 °C (108.5 °F) and 25.8 °C (78.4 °F) in Dongola, Sudan; 42.2 °C (107.9 °F) and 25.1 °C (7.1 °F) in Wadi Halfa, Sudan; 41.9 °C (107.4 °F) and 26 °C (78.8 °F) in Aswan, Egypt; 41.4 °C (106.5 °F) and 23.6 °C (74.5 °F) in Luxor, Egypt; 42.4 °C (108.3 °F) and 24.3 °C (75.3 °F) in Bilma, Niger; 41.2 °C (106.1 °F) and 26.2 °C (79.1 °F) in Agadez, Niger; 44.5 °C (112.1 °F) and 24.4 °C (75.9 °F) in Faya-Largeau, Chad; 45.9 °C (113.9 °F) and 26.9 °C (80.4 °F) in Adrar, Algeria; 42.7 °C (108.9 °F) and 28.9 °C (84 °F) in Tessalit, Mali; 42 °C (107.6 °F) and 27 °C (80.6 °F) in Atar, Mauritania; 41.9 °C (107.4 °F) and 27.1 °C (80.8 °F) in Khartoum, Sudan; 43.3 °C (109.9 °F) and 26.1 °C (80 °F) in Tindouf, Algeria; 42.6 °C (108.7 °F) and 22.2 °C (72 °F) in Ghadames, Libya; 40.2 °C (104.4 °F ) and 26.3 °C (79.3 °F) in Ghat, Libya. The Libyan Desert is nearly as scorching as the rest of the Sahara, and the averages highs for the hottest month are uniformly around 40 °C (104 °F) and can soar to 43 °C (109.4 °F) or even more. During heat waves or frequently in the hottest desert spots, the absolute highs can soar up to more than 50 °C (122 °F). For example, in In Salah during the 1931 summer, the mercury climbed up to 48 °C (118.4 °F) or more for nearly 45 consecutive days (1 month and half a month) and the low temperatures have never been under 21 °C (69.8 °F). This is a record which surpasses the one of Death Valley, California. In the most part of the Great Desert, the record absolute high temperature are over 50 °C (122 °F). There are also some places in the Sahara that have more moderated temperatures than the rest of the desert, especially in summer either because of the relative high elevation above sea level or because of the influence of the cool Atlantic Ocean or even the Mediterranean Sea, which tend to cool off the hot climate. For example, Tamanrasset, Algeria or Kufra, Libya has an average high temperature which does not reach 40 °C (104 °F) during their hottest month although their average high temperature still remains around 37 °C (98.6 °F) but the daytime temperatures can easily soar to 42 °C (107.6 °F) or more[15] The summer heat is the most extreme in low valleys of the Tanezrouft and in desert depressions and plateaus such as the Tademaït Plateau and the Tidikelt Depression.
The desert possess brief, short, extremely mild to moderately hot winters, with average high temperatures often between 16 °C (60.8 °F) and 29 °C (84.2 °F) (and even more in the warmest tropical regions), while the average low temperatures is around 4 °C (39.2 °F) and 15 °C (59 °F). For example, the averages highs and the averages lows for the coldest month of the year (generally January) are respectively 16.5 °C (61.7 °F) and 6.4 °C (43.5 °F) in Biskra, Algeria; 16.6 °C (61.9 °F) and 3.8 °C (38.8 °F) in Touggourt, Algeria; 17.8 °C (64 °F) and 2.9 °C (37.2 °F) in Ghadames, Libya; 20.4 °C (68.7 °F) and 6.8 °C (44.2 °F) in Ghat, Libya; 21.6 °C (70.9 °F) and 4.9 °C (40.8 °F) in In Salah, Algeria; 20.6 °C (69 °F) and 4.1 °C (39.4 °F) in Adrar, Algeria; 28 °C (82.4 °F) and 10.5 °C (50.9 °F) in Dongola, Sudan; 28.2 °C (82.8 °F) and 12.3 °C (54.1 °F) in Abu Hamad, Sudan; 30.5 °C (86.9 °F) and 14.6 °C (58.3 °F) in Atbara, Sudan; 23 °C (73.4 °F) and 7 °C (44.6 °F) in Luxor, Egypt; 22.9 °C (73.2 °F) and 8.7 °C (47.7 °F) in Aswan, Egypt; 21.5 °C (70.7 °F) and 11 °C (51.8 °F) in Hurghada, Egypt; 21 °C (69.8 °F) and 5 °C (41 °F) in Kufra, Libya; 26.8 °C (80.2 °F) and 12.6 °C (54.7 °F) in Tessalit, Mali; 26.2 °C (79.7 °F) and 9.9 °C (49.8 °F) in Taoudenni, Mali; 25.6 °C (78 °F) and 8.4 °C (47.1 °F) in Bilma, Niger; 28.1 °C (82.6 °F) and 11.5 °C (55.7 °F) in Agadez, Niger; 28.3 °C (82.9 °F) and 13.7 °C (56.7 °F) in Faya-Largeau, Chad; 24.9 °C (76.8 °F) and 8.1 °C (46.6 °F) in Wadi Halfa, Sudan; 31.2 °C (88.1 °F) and 15.6 °C (60 °F) in Khartoum, Sudan. The desert climate becomes more and more continental in the north, while the continentality of the Great Desert is weakened by the latitude (closer to the equator) at the south. So, in the continental northern part, it is not rare for temperatures to drop to around 0 °C (32 °F) at night due to the strong infrared radiation emitted by the Earth surface in a clean, dry air and below clear skies. Besides the record absolute low temperatures in the Great Desert are very often around 0 °C (32 °F) or below but this feature is rarer in the tropical southern part[15] Though being possible during night, the freezing is completely impossible during the day as the desert is located under the hot zone where the daytime temperatures cannot drop to the freezing point or below.
The mean annual temperatures in the Sahara desert are generally between 20 °C (68 °F) and 30 °C (86 °F), and even more in the Saharan-Sahelian border. For example, the mean annual temperature is 25.9 °C (78.2 °F) in Aswan, Egypt; 24,6 °C (76.3 °F) in Luxor, Egypt; 26.2 °C (79.2 °F) in Wadi Halfa, Sudan; 27.5 °C (81.5 °F) in Dongola, Sudan; 28.6 °C (83.5 °F) in Abu Hamad, Sudan; 28.7 °C (83.7 °F) in Tessalit, Mali; 28.1 °C (82.6 °F) in Taoudenni, Mali; 26 °C (78.8 °F) in In Salah, Algeria; 24.4 °C (75.9 °F) in Adrar, Algeria; 23.8 °C (74.8 °F) in Tindouf, Algeria; 23.3 °C (73.9 °F) in Kufra, Libya; 23.5 °C (74.3 °F) in Sebha, Libya; 21.9 °C (71.4 °F) in Ghadames, Libya; 28.7 °C (83.7 °F) in Faya-Largeau, Chad; 27.8 °C (82 °F) in Fada, Chad; 26.8 °C (80.2 °F) in Bilma, Niger; 28.3 °C (82.9 °F) in Agadez, Niger; 23 °C (73.4 °F) in Kufra, Libya; 23.8 °C (74.8 °F) in Illizi, Algeria; 20.5 °C (68.9 °F) in Dakhla, Western Sahara; 22.7 °C (72.9 °F) in Smara, Western Sahara; 27.9 °C (82.2 °F) in Atar, Mauritania; 27.2 °C (80.1 °F) in Ouadane, Mauritania. The previously mentioned temperatures are taken in the shade, the felt temperatures for a human body exposed to the direct sunlight in the desert can be 15 °C (59 °F), even 20 °C (68 °F) higher than the official temperatures.
Sunshine
The Sahara contains the sunniest places worldwide, with a very to an extremely high sunshine duration year-round, between 3,000 hours (about 68.4% of the daylight hours) and 4,000 hours (about 91.3% of the daylight hours). The Eastern Desert (Egypt, Sudan, Chad, Libya) is the sunniest part of the Sahara, with some desert areas which could have a sunshine duration well above 4,000 hours per year (91.3%).[17][18] According to a NASA study, the sunniest spot in the world would be a ruined fort in Agadem, located in southeastern Niger where the solar energy productivity would be unmatched and this desert area would see even fewer clouds than the rest of the desert[19] The theoretical maximum level of annual sunshine is the amount of daylight hours, that is to say 4,380 hours. The hyperarid part of the Sahara gets always over 3,500 hours of sun per year everywhere (79.9%). For example Aswan, Egypt, has 3,864 hours of sunshine (88.2%); Luxor, Egypt, has 3,833 hours of sunshine (87.5%); Hurghada, Egypt has 3,844 hours of sunshine (87.7%); Faya-Largeau, Chad, has about 3,792 hours of sunshine (86.5%); Bilma, Niger, has 3,699 hours of sunshine (84.4%); Agadez, Niger has 3,493 hours of sunshine (79.7%); Kufra, Libya, has 3,723 hours of sunshine (85.2%); Tindouf, Algeria, has 3,603 hours of sunshine (82.2%); In Salah, Algeria has 3,688 hours of sunshine (84.2%); Djanet, Algeria, records nearly 3,800 hours of sunshine (86.7%); Taoudeni, Mali has 3,671 hours of sunshine (83.9%); Tessalit, Mali has 3,496 hours of sunshine (79.8%); Ouarzazate, Morocco records 3,465 hours of sunshine (78.7%); Tamanrasset, Algeria has 3,383 hours of sunshine (77.2%%); Bechar, Algeria possesses 3,559 hours of sunshine (81.2%); Wadi Halfa, Sudan, has around 3,900 hours of sunshine (89%); Khartoum, Sudan, has 3,707 hours of sunshine (84.6%); Atbara, Sudan has 3,739 hours of sunshine (85.3%); Abu Hamad, Sudan, has 3,763 hours of sunlight annually (85.9%); Dongola, Sudan, has an amount of 3,814 sunny hours (87%), Abu Simbel, Egypt has approximatively 3,880 hours of sunshine (88.5%) and Marsa Alam, Egypt has even 3,958 hours of sunshine (90.3%)[20][21][22][23][24] The Libyan Desert receives uniformly more than 3,700 hours of annual bright sunshine (84.4%) and the sunniest areas could receive up to 3,800 sunny hours or more (86.7%). For comparison, Marseille, the sunniest major city in France, has an average of 2,838 hours of sunshine annually (65%), Paris receives 1,689 hours of annual sunshine (38.5%); Berlin receives 1,625 hours of bright sunshine (37.1%) and Dunkerque has about 1,961 sunny hours (44.7%). Days with no sun are scarce in the Sahara and there may be several sunny months passing without any cloudy day. The only area which has a comparable sunshine duration is in the Mojave Desert and Sonora Desert region of the Southwestern United States. Theclear weather with bright sunshine prevails very much all year-long in the whole desert and there cannot be a month when the cloudy hours are above the sunny hours for any location in the Sahara.
The Sahara also has the highest insolation result worldwide, which actually represents the total amount of solar radiation energy that reaches the ground on a given area and on a given duration, amounting from 2,200 kWh/(m² year) to more than 2,850 kWh/(m² year) in the best cases because the desert sun shines nearly every day and nearly always vertically (high sun angle).[25] The average annual solar radiation is about 2,533 kWh/(m² year) in the Sahara desert and this is the only desert which receives on average more than 2,400 kWh/(m² year)[26] For comparison, the major part of Europe has an insolation around 1,100 kWh/(m² year) while the Arabian Desert located in Middle East has an average insolation around 2,200 kWh/(m² year). When the sun is well above the horizon and shines through clear skies, the direct solar radiation represents between 85% and 90% of the total global solar radiation[27] The annual average sun altitude (angle) at solar noon on the 21st day of each month is generally between 60 ° and 70 ° in the Sahara and even more in the most southern areas, while this average is only 41 ° in Paris and 47 ° in Marseille, which means the solar radiation intensity will be higher, much higher in the Great Desert[28][29] This very high sun angle combined with the lack of cloud cover are synonymous of a maximum solar energy gain. These numbers make the Sahara Desert an ideal location for installing large-scale solar farms and solar plants. The quality, the quantity, the frequency and the intensity of the sunlight that bakes the desert day after day are nearly unmatched on the planet. This sunshine record helps to explain the typical extreme heat of the desert.
Humidity
The relative humidity of the air is generally extremely low to very low during summer and very low to low during winter, knowing the relative humidity of the air directly depends of the air temperature. The major part of the Sahara has an average relative humidity between 10% and 25% during summer and between 25% and 50% during winter. The northern Sahara is more favorised, with a humidity between 20% and 35% in summer and 35% and 55% during winter. We have sometimes recorded some extremely low absolute relative humidity values, like in Tamanrasset, Algeria where the humidity dropped at an extreme of 2% during summer at several occasions. The Nigerian desert is the part of the Sahara which has the least water vapor amount in the air : the air is even drier than other parts and the annual verage relative humidity is lower than 20% and even reaches 15% in some areas of the Ténéré. For example, the annual average relative humidity is 26.2% in Aswan, Egypt; 29.9% in Luxor, Egypt; 28.8% in Khartoum, Sudan; 28.6% in Wadi Halfa, Sudan; 25.1% in Dongola, Sudan; 22.8% in Faya-Largeau, Chad; 15.3% in Bilma, Niger; 19.6% in Agadez, Niger; 27.6% in Atar, Mauritania; 33.7% in Bir Moghrein, Mauritania; 29.3% in Kufra, Libya; 33.9% in Sebha, Libya; 33.8% in Ghadames, Libya; 27.3% in In Salah, Algeria; 22.3% in Tamanrasset, Algeria. In order to compare, Paris has an annual average relative humidity of 78.1%. It's also because of the excessively dry air that the diurnal temperature variations can be high. The only exceptions to the hygrometric regime of the Great Desert are the coastal areas or the desert zones close to a water source, such as lakes or rivers. According to the ones of the temperatures, some diurnal variations of the relative humidity occur : the hygrometric degree is the lowest at about 3 p.m. and is the highest at about 6 a.m., during the sunrise.Winds
The Sahara is also extremely windy, although the winds rarely blow at very high speeds. Located in the trade winds belt, the region is subject to winds that are moderately 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 hotter and drier. The scorching, desiccating and sometimes dust-laden winds are sometimes felt north and south of the desert, where they are variously known as sirocco, khamsin, simoom, chergui and harmattan and some others. When these winds blow, they bring typical desert climate conditions, which means extremely high temperatures especially at summertime where highs can be easily around 40 °C (104 °F) and very often soar to over 45 °C (113 °F), an extremely dry and sometimes dusty atmosphere with a relative humidity nearly always under 20% and even 10% as well as clear skies and sunny weather. The northern slopes of the Atlas Mountains and some other minor mountain ranges intercept most of the moisture from winds blowing inshore from the Mediterranean Sea and from the Atlantic Ocean. The effects of these winds can be made even worse : for example, the chergui which is an already very hot and very dry wind, and which blows easterly or southeasterly on Morocco heats up and dries out even more, like the foehn when this one passes over the high peaks of the Atlas Mountains and descend even more hotter and especially much drier, hence the extreme absolute high temperatures, that are normally around 50 °C (122 °F), particularly in Marrakech which is located in the leeward side of the this foehn-like wind.Evaporation
All the climate features of the Sahara, this harsh, hot desert are responsible of the highest theoretical potential evaporation rate on the world. The extremely high temperatures all-year-long, the strong energetic and constant sunshine, the lack of clouds, the lack of rainfall, the excessively low humidity and the dessicating winds that aggravate the dryness all cause very high theoretical levels of potential evaporation, between 3,000 mm (3 m) and 6,000 mm (6 m). The Atlantic coastal Sahara zone has an annual theoretical potential evaporation between 1,500 mm (1,5 m) and 3,000 mm (3 m). Though the potential evaporation of the Sahara is extremely high, the real evaporation is very low, because the lack of available water for the ground and for the vegetation (extreme drought) is a limiting factor, that prevents large amounts of liquid water to be evaporated by the desert heat and the desert sun.History
The climate of the Sahara has undergone enormous variations between wet and dry over the last few hundred thousand years.[30] This is due to a 41,000 year cycle in which the axial tilt of the earth changes between 22° and 24.5°.[31] At present (2000 CE), the Sahara is in a dry period, but it is expected that the Sahara will become green again in 15,000 years (17,000 CE).During the last glacial period, the Sahara was even bigger than it is today, extending south beyond its current boundaries.[32] The end of the glacial period brought more rain to the Sahara, from about 8000 BCE to 6000 BCE, perhaps because of low pressure areas over the collapsing ice sheets to the north.[33]
Once the Ice Age ended, the northern Sahara dried out. In the southern Sahara though, the drying trend was soon counteracted by the monsoon, which brought rain further north than it does today. In this period, there was still a monsoon climate in the Sahara. Monsoons form by heating of air over the land during summer. The hot air rises and pulls in cool, wet air from the ocean, which causes rain. Thus, though it seems counterintuitive, the Sahara was wetter when it received more insolation in the summer. This was caused by a stronger tilt in Earth's axis of orbit than today (24.5 degree tilt vs the 23.4° tilt today[31]), and perihelion occurred at the end of July around 7000 BCE.[34]
By around 4200 BCE, the monsoon retreated south to approximately where it is today,[9] leading to the gradual desertification of the Sahara.[35] The Sahara is now as dry as it was about 13,000 years ago.[30] These conditions are responsible for what has been called the Sahara Pump Theory.
The Sahara has one of the harshest climates in the world. The prevailing north-easterly wind often causes sand storms and dust devils. When the wind blows towards the Mediterranean, it is known as sirocco and often reaches hurricane speeds in North Africa and southern Europe. Half of the Sahara receives less than 20 mm (0.79 in) of rain per year, and the rest receives up to 100 mm (3.9 in) per year.[36] The rainfall happens very rarely, but when it does it is usually torrential when it occurs after long dry periods. The southern boundary of the Sahara, as measured by rainfall, was observed to both advance and retreat between 1980 and 1990. As a result of drought in the Sahel, the southern boundary moved south 130 kilometers (81 mi) overall during that period.[37]
Recent signals indicate that the Sahara and surrounding regions are greening because of increased rainfall. Satellite imaging shows extensive regreening of the Sahel between 1982 and 2002, and in both Eastern and Western Sahara a more than 20-year-long trend of increased grazing areas and flourishing trees and shrubs has been observed by climate scientist Stefan Kröpelin.[38]
Snow and ice
On 18 February 1979, snow fell in several places in southern Algeria, including a half-hour snowstorm that stopped traffic in Ghardaïa, and was reported as being "for the first time in living memory".[39] The snow, however, was gone within hours.[40] Several Saharan mountain ranges, however, receive snow more regularly. Although relative humidity is low in the arid environment, the absolute humidity is high enough for moisture to condense when driven up a mountain range. In winter, temperatures drop low enough on the Tahat summit to cause snow on average every three years; the Tibesti Mountains receive snow on peaks over 2,500 meters (8,200 ft) once every seven years on average.[41][42]On 18 January 2012, snow fell in several places in western Algeria. Strong winds blew the snow across roads and buildings in Béchar Province.[43]
Ecoregions
The major topographic features of the Saharan region.
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 kilometers (15,400 sq mi) in Western Sahara and Mauritania.[44]
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 kilometers (646,800 sq mi) in Algeria, Egypt, Libya, Mauritania, Morocco, Tunisia, and Western Sahara.[45]
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 km (1,791,500 square miles) of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Niger, and Sudan.[10]
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 (3.9 to 7.9 in) 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 (425,400 mi2) in Algeria, Chad, Mali, Mauritania, and Sudan.[46]
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 (99,700 mi2), 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.[47]
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 (31,700 mi2) in the Tibesti of Chad and Libya, and Jebel Uweinat on the border of Egypt, Libya, and Sudan.[48]
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 (20,800 mi2), 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 Western Sahara.[49]
The Tanezrouft is one of the harshest regions on Earth and the driest in the Sahara, with no vegetation and very little life. It is along the borders of Algeria, Niger and Mali, west of the Hoggar mountains.
Flora and fauna
The Flora of the Sahara is highly diversified based on the bio-geographical characteristics of this vast desert. Floristically, the Sahara has three zones based on the amount of rainfall received – the Northern (Mediterranean), Central and Southern Zones. There are two transitional zones – the Mediterranean-Sahara transition and the Sahel transition zone.[50]The Saharan flora comprises around 2800 species of vascular plants. Approximately a quarter of these are endemic. About half of these species are common to the flora of the Arabian deserts.[51]
The central Sahara is estimated to include five hundred species of plants, which is extremely low considering the huge extent of the area. Plants such as acacia trees, palms, succulents, spiny shrubs, and grasses have adapted to the arid conditions, by growing lower to avoid water loss by strong winds, by storing water in their thick stems to use it in dry periods, by having long roots that travel horizontally to reach the maximum area of water and to find any surface moisture and by having small thick leaves or needles to prevent water loss by evapo-transpiration. Plant leaves may dry out totally and then recover.
Several species of fox live in the Sahara, including the fennec fox, pale fox and Rüppell's fox. The addax, a large white antelope, can go nearly a year in the desert without drinking. The dorcas gazelle is a north African gazelle that can also go for a long time without water. Other notable gazelles include the Rhim gazelle and dama gazelle.
The Saharan cheetah (northwest African cheetah) lives in Algeria, Togo, Niger, Mali, Benin, and Burkina Faso. There remain fewer than 250 mature cheetahs which are very cautious, fleeing any human presence. The cheetah avoids the sun from April to October, seeking the shelter of shrubs such as balanites and acacias. They are unusually pale.[52][53]
Other animals include the monitor lizards, hyrax, sand vipers, and small populations of African wild dog,[54] in perhaps only 14 countries[55] and ostrich. There exist other animals in the Sahara (birds in particular) such as African silverbill and black-faced firefinch, among others. There are also small desert crocodiles in Mauritania and the Ennedi Plateau of Chad.[56]
The deathstalker scorpion can be 10 cm (3.9 in) long. Its venom contains large amounts of agitoxin and scyllatoxin and is very dangerous; however, a sting from this scorpion rarely kills a healthy adult. The Saharan silver ant is unique in that due to the extreme high temperatures of their habitat and the threat of predators, the ants are active outside their nest for only about ten minutes per day.[57]
Dromedary camels and goats are the domesticated animals most commonly found in the Sahara. Because of its qualities of endurance and speed, the dromedary is the favourite animal used by nomads.
Human activities are more likely to affect the habitat in areas of permanent water (oases) or where water comes close to the surface. Here, the local pressure on natural resources can be intense. The remaining populations of large mammals have been greatly reduced by hunting for food and recreation. In recent years development projects have started in the deserts of Algeria and Tunisia using irrigated water pumped from underground aquifers. These schemes often lead to soil degradation and salinization.
History
Nubians
Beni Isguen, a holy city surrounded by thick walls in the Algerian Sahara.
During the Neolithic Era, before the onset of desertification, around 9500 BCE the central Sudan had been a rich environment supporting a large population ranging across what is now barren desert, like the Wadi el-Qa'ab. By the 5th millennium BCE, the peoples who inhabited what is now called Nubia, were full participants in the "agricultural revolution", living a settled lifestyle with domesticated plants and animals. Saharan rock art of cattle and herdsmen suggests the presence of a cattle cult like those found in Sudan and other pastoral societies in Africa today.[58] Megaliths found at Nabta Playa are overt examples of probably the world's first known archaeoastronomy devices, predating Stonehenge by some 2,000 years.[59] This complexity, as observed at Nabta Playa, and as expressed by different levels of authority within the society there, likely formed the basis for the structure of both the Neolithic society at Nabta and the Old Kingdom of Egypt.[60]
Egyptians
By 6000 BCE predynastic Egyptians in the southwestern corner of Egypt were herding cattle and constructing large buildings. Subsistence in organized and permanent settlements in predynastic Egypt by the middle of the 6th millennium BCE centered predominantly on cereal and animal agriculture: cattle, goats, pigs and sheep. Metal objects replaced prior ones of stone. Tanning of animal skins, pottery and weaving were commonplace in this era also. There are indications of seasonal or only temporary occupation of the Al Fayyum in the 6th millennium BCE, with food activities centering on fishing, hunting and food-gathering. Stone arrowheads, knives and scrapers from the era are commonly found.[61] Burial items included pottery, jewelry, farming and hunting equipment, and assorted foods including dried meat and fruit. Burial in desert environments appears to enhance Egyptian preservation rites, and dead were buried facing due west.[62]By 3400 BCE, the Sahara was as dry as it is today, due to reduced precipitation and higher temperatures resulting from a shift in the Earth's orbit,[9] and it became a largely impenetrable barrier to humans, with only scattered settlements around the oases but little trade or commerce through the desert. The one major exception was the Nile Valley. The Nile, however, was impassable at several cataracts, making trade and contact by boat difficult.
Phoenicians
The people of Phoenicia, who flourished from 1200–800 BCE, created a confederation of kingdoms across the entire Sahara to Egypt. They generally settled along the Mediterranean coast, as well as the Sahara, among the people of Ancient Libya, who were the ancestors of people who speak Berber languages in North Africa and the Sahara today, including the Tuareg of the central Sahara.
The Phoenician alphabet seems to have been adopted by the ancient Libyans of north Africa, and Tifinagh is still used today by Berber-speaking Tuareg camel herders of the central Sahara.
Sometime between 633 BCE and 530 BCE, Hanno the Navigator either established or reinforced Phoenician colonies in Western Sahara, but all ancient remains have vanished with virtually no trace.
Greeks
By 500 BCE, Greeks arrived in the desert. Greek traders spread along the eastern coast of the desert, establishing trading colonies along the Red Sea. The Carthaginians explored the Atlantic coast of the desert, but the turbulence of the waters and the lack of markets caused a lack of presence further south than modern Morocco. Centralized states thus surrounded the desert on the north and east; it remained outside the control of these states. Raids from the nomadic Berber people of the desert were a constant concern of those living on the edge of the desert.Urban civilization
An urban civilization, the Garamantes, arose around 500 BCE in the heart of the Sahara, in a valley that is now called the Wadi al-Ajal in Fezzan, Libya.[30] The Garamantes achieved this development by digging tunnels far into the mountains flanking the valley to tap fossil water and bring it to their fields. The Garamantes grew populous and strong, conquering their neighbors and capturing many slaves (which were put to work extending the tunnels). The ancient Greeks and the Romans knew of the Garamantes and regarded them as uncivilized nomads. However, they traded with the Garamantes, and a Roman bath has been found in the Garamantes capital of Garama. Archaeologists have found eight major towns and many other important settlements in the Garamantes territory. The Garamantes civilization eventually collapsed after they had depleted available water in the aquifers and could no longer sustain the effort to extend the tunnels further into the mountains.[63]
Berbers
The Berber people occupied (and still occupy) much of the Sahara. The Garamantes Berbers built a prosperous empire in the heart of the desert.[64] The Tuareg nomads continue, to the present day, to inhabit and move across wide Sahara surfaces.
Islamic expansion
The Byzantine Empire ruled the northern shores of the Sahara from the 5th to the 7th century. After the Muslim conquest of Arabia (Arabian peninsula) the Muslim conquest of North Africa began in the mid 7th to early 8th centuries, Islamic influence expanded rapidly on the Sahara. By the end of 641 all of Egypt was in Muslim hands. The trade across the desert intensified. A significant slave trade crossed the desert. It has been estimated that from the 10th to the 19th century some 6,000 to 7,000 slaves were transported north each year.[65]
This trade through Sahara persisted for several centuries until the development in Europe of the caravel allowed ships, first from Portugal and soon from all of Western Europe, to sail around the desert and gather the resources from the source in Guinea. The Sahara was rapidly marginalized.
Ottoman Turkish era
In the 16th century the northern fringe of the Sahara, such as coastal regencies in present day Algeria and Tunisia, as well as some parts of present-day Libya, together with the semi-autonomous kingdom of Egypt, were occupied by the Ottoman Empire. From 1517 Egypt was a valued part of the Ottoman Empire, ownership of which provided the Ottomans with control over the Nile Valley, the east Mediterranean and North Africa. The benefit of the Ottoman Empire was the freedom of movement for citizens and goods. Trade exploited the Ottoman land routes to handle the spices, gold and silk from the East, manufactures from Europe, and the slave and gold traffic from Africa. Arabic continued as the local language and Islamic culture was much reinforced. The Sahel and southern Sahara regions were home to several independent states or to roaming Tuareg clans.European colonialism
European colonialism in the Sahara began in the 19th century. France conquered the regency of Algiers from the Ottomans in 1830, and French rule spread south from Algeria and eastwards from Senegal into the upper Niger to include present-day Algeria, Chad, Mali then French Sudan including Timbuktu, Mauritania, Morocco (1912), Niger, and Tunisia (1881). By the beginning of the twentieth century, the trans-Saharan trade had clearly declined because goods were moved through more modern and efficient means, such as airplanes, rather than across the desert.[66]The French Colonial Empire was the dominant presence in the Sahara. It established regular air links from Toulouse (HQ of famed Aéropostale), to Oran and over the Hoggar to Timbuktu and West to Bamako and Dakar, as well as trans-Sahara bus services run by La Companie Transsaharienne (est. 1927).[67] A remarkable film shot by famous aviator Captain René Wauthier documents the first crossing by a large truck convoy from Algiers to Tchad, across the Sahara.[68]
Egypt, under Muhammad Ali and his successors, conquered Nubia in 1820–22, founded Khartoum in 1823, and conquered Darfur in 1874. Egypt, including the Sudan, became a British protectorate in 1882. Egypt and Britain lost control of the Sudan from 1882 to 1898 as a result of the Mahdist War. After its capture by British troops in 1898, the Sudan became an Anglo-Egyptian condominium.
Spain captured present-day Western Sahara after 1874, although Rio del Oro remained largely under Tuareg influence. In 1912, Italy captured parts of what was to be named Libya from the Ottomans. To promote the Roman Catholic religion in the desert, Pope Pius IX appointed a delegate Apostolic of the Sahara and the Sudan in 1868 ; later in the 19th century his jurisdiction was reorganized into the Vicariate Apostolic of Sahara.
Breakup of the empires and afterwards
A natural rock arch in south western Libya.
The Sahara today.
Egypt became independent of Britain in 1936, although the Anglo-Egyptian treaty of 1936 allowed Britain to keep troops in Egypt and to maintain the British-Egyptian condominium in the Sudan. British military forces were withdrawn in 1954.
Most of the Saharan states achieved independence after World War II: Libya in 1951, Morocco, Sudan, and Tunisia in 1956, Chad, Mali, Mauritania, and Niger in 1960, and Algeria in 1962. Spain withdrew from Western Sahara in 1975, and it was partitioned between Mauritania and Morocco. Mauritania withdrew in 1979, and Morocco continues to hold the territory.
In the post-World War II era, several mines and communities have developed to utilize the desert's natural resources. These include large deposits of oil and natural gas in Algeria and Libya and large deposits of phosphates in Morocco and Western Sahara.
A number of Trans-African highways have been proposed across the Sahara, including the Cairo–Dakar Highway along the Atlantic coast, the Trans-Sahara Highway from Algiers on the Mediterranean to Kano in Nigeria, the Tripoli – Cape Town Highway from Tripoli in Libya to N'Djamena in Chad, and the Cairo – Cape Town Highway which follows the Nile. Each of these highways is partially complete, with significant gaps and unpaved sections.
People and languages
A 19th-century engraving of an Arab slave-trading caravan transporting black African slaves across the Sahara.
Arabic dialects are the most widely spoken languages in the Sahara, from the Atlantic to the Red Sea. Berber people are found from western Egypt to Morocco, including the Tuareg pastoralists of the central Sahara. The Beja live in the Red Sea Hills of southeastern Egypt and eastern Sudan. Arabic, Berber and its variants now regrouped under the term Amazigh (which includes the Guanche language spoken by the original Berber inhabitants of the Canary Islands) and Beja languages are part of the Afro-Asiatic or Hamito-Semitic family.[citation needed]
