Libya
Libya the State of Libya, is a country in the Maghreb region in North Africa, bordered by the Mediterranean Sea to the north, Egypt to the east, Sudan to the southeast, Chad to the south, Niger to the southwest, Algeria to the west, Tunisia to the northwest. The sovereign state is made of three historical regions: Tripolitania and Cyrenaica. With an area of 1.8 million square kilometres, Libya is the fourth largest country in Africa, is the 16th largest country in the world. Libya has the 10th-largest proven oil reserves of any country in the world; the largest city and capital, Tripoli, is located in western Libya and contains over one million of Libya's six million people. The second-largest city is Benghazi, located in eastern Libya. Libya has been inhabited by Berbers since the late Bronze Age; the Phoenicians established trading posts in western Libya, ancient Greek colonists established city-states in eastern Libya. Libya was variously ruled by Carthaginians, Persians and Greeks before becoming a part of the Roman Empire.
Libya was an early centre of Christianity. After the fall of the Western Roman Empire, the area of Libya was occupied by the Vandals until the 7th century, when invasions brought Islam to the region. In the 16th century, the Spanish Empire and the Knights of St John occupied Tripoli, until Ottoman rule began in 1551. Libya was involved in the Barbary Wars of the 19th centuries. Ottoman rule continued until the Italian occupation of Libya resulted in the temporary Italian Libya colony from 1911 to 1947. During the Second World War, Libya was an important area of warfare in the North African Campaign; the Italian population went into decline. Libya became independent as a kingdom in 1951. A military coup in 1969 overthrew King Idris I; the "bloodless" coup leader Muammar Gaddafi ruled the country from 1969 and the Libyan Cultural Revolution in 1973 until he was overthrown and killed in the 2011 Libyan Civil War. Two authorities claimed to govern Libya: the Council of Deputies in Tobruk and the 2014 General National Congress in Tripoli, which considered itself the continuation of the General National Congress, elected in 2012.
After UN-led peace talks between the Tobruk and Tripoli governments, a unified interim UN-backed Government of National Accord was established in 2015, the GNC disbanded to support it. Parts of Libya remain outside either government's control, with various Islamist and tribal militias administering some areas; as of July 2017, talks are still ongoing between the GNA and the Tobruk-based authorities to end the strife and unify the divided establishments of the state, including the Libyan National Army and the Central Bank of Libya. Libya is a member of the United Nations, the Non-Aligned Movement, the Arab League, the OIC and OPEC; the country's official religion is Islam, with 96.6% of the Libyan population being Sunni Muslims. The Latin name Libya referred to the region west of the Nile corresponding to its central location in North Africa visited by many Mediterranean cultures which referred to its original inhabitants as the "Libúē." The name Libya was introduced in 1934 for Italian Libya, reviving the historical name for Northwest Africa, from the ancient Greek Λιβύη.
It was intended to supplant terms applied to Ottoman Tripolitania, the coastal region of what is today Libya having been ruled by the Ottoman Empire from 1551 to 1911, as the Eyalet of Tripolitania. The name "Libya" was brought back into use in 1903 by Italian geographer Federico Minutilli. Libya gained independence in 1951 as the United Libyan Kingdom, changing its name to the Kingdom of Libya in 1963. Following a coup d'état led by Muammar Gaddafi in 1969, the name of the state was changed to the Libyan Arab Republic; the official name was "Socialist People's Libyan Arab Jamahiriya" from 1977 to 1986, "Great Socialist People's Libyan Arab Jamahiriya" from 1986 to 2011. The National Transitional Council, established in 2011, referred to the state as "Libya"; the UN formally recognized the country as "Libya" in September 2011, based on a request from the Permanent Mission of Libya citing the Libyan interim Constitutional Declaration of 3 August 2011. In November 2011, the ISO 3166-1 was altered to reflect the new country name "Libya" in English, "Libye" in French.
In December 2017 the Permanent Mission of Libya to the United Nations informed the United Nations that the country's official name was henceforth the "State of Libya". The coastal plain of Libya was inhabited by Neolithic peoples from as early as 8000 BC; the Afroasiatic ancestors of the Berber people are assumed to have spread into the area by the Late Bronze Age. The earliest known name of such a tribe was the Garamantes, based in Germa; the Phoenicians were the first to establish trading posts in Libya. By the 5th century BC, the greatest of the Phoenician colonies, had extended its hegemony across much of North Africa, where a distinctive civilization, known as Punic, came into being. In 630 BC, the ancient Greeks colonized the area around Barca in Eastern Libya and founded the city of Cyrene. Within 200 years, four more important Greek cities were established in the area that became known as
Western Sahara
Western Sahara is a disputed territory on the northwest coast and in the Maghreb region of North and West Africa controlled by the self-proclaimed Sahrawi Arab Democratic Republic and occupied by neighboring Morocco. Its surface area amounts to 266,000 square kilometres, it is one of the most sparsely populated territories in the world consisting of desert flatlands. The population is estimated at just over 500,000, of which nearly 40% live in Laayoune, the largest city in Western Sahara. Occupied by Spain until the late 20th century, Western Sahara has been on the United Nations list of non-self-governing territories since 1963 after a Moroccan demand, it is the most populous territory on that list, by far the largest in area. In 1965, the UN General Assembly adopted its first resolution on Western Sahara, asking Spain to decolonize the territory. One year a new resolution was passed by the General Assembly requesting that a referendum be held by Spain on self-determination. In 1975, Spain relinquished the administrative control of the territory to a joint administration by Morocco and Mauritania.
A war erupted between those countries and a Sahrawi nationalist movement, the Polisario Front, which proclaimed the Sahrawi Arab Democratic Republic with a government in exile in Tindouf, Algeria. Mauritania withdrew its claims in 1979, Morocco secured de facto control of most of the territory, including all the major cities and natural resources; the United Nations considers the Polisario Front to be the legitimate representative of the Sahrawi people, maintains that the Sahrawis have a right to self-determination. Since a United Nations-sponsored ceasefire agreement in 1991, two thirds of the territory has been administered by the Moroccan government, with tacit support from France and the United States, the remainder by the SADR, backed by Algeria. Internationally, countries such as Russia have taken a ambiguous and neutral position on each side's claims, have pressed both parties to agree on a peaceful resolution. Both Morocco and Polisario have sought to boost their claims by accumulating formal recognition from African and Latin American states in the developing world.
The Polisario Front has won formal recognition for SADR from 46 states, was extended membership in the African Union. Morocco has won support for its position from several African governments and from most of the Muslim world and Arab League. In both instances, recognitions have, over the past two decades, been extended and withdrawn according to changing international trends; as of 2017, no other member state of the United Nations has officially recognized Moroccan sovereignty over parts of Western Sahara. However, a number of countries have expressed their support for a future recognition of the Moroccan annexation of the territory as an autonomous part of the Kingdom. Overall, the annexation has not garnered as much attention in the international community as many other disputed annexations. Western Sahara is located on the northwest coast in West Africa and on the cusp of North Africa, bordering the North Atlantic Ocean to the northwest, Morocco proper to the north-northeast, Algeria to the east-northeast, Mauritania to the east and south.
The land is some of the most inhospitable on the planet. The land along the coast is low flat desert and rises in the north, to small mountains reaching up to 600 metres on the eastern side. While the area can experience flash flooding in the spring, there are no permanent streams. At times a cool off-shore current can produce heavy dew; the interior experiences extreme summer heat with average highs reaching as high as 43–45 °C in July and in August. The earliest known inhabitants of Western Sahara were the Gaetuli. Depending on the century, Roman-era sources describe the area as inhabited by Gaetulian Autololes or the Gaetulian Daradae tribes. Berber heritage is still evident from regional and place-name toponymy, as well as from tribal names. Other early inhabitants of Western Sahara may be the Bafour and the Serer; the Bafour were replaced or absorbed by Berber-speaking populations which merged in turn with the migrating Beni Ḥassān Arab tribes. The arrival of Islam in the 8th century played a major role in the development of the Maghreb region.
Trade developed further, the territory may have been one of the routes for caravans between Marrakesh and Tombouctou in Mali. In the 11th century, the Maqil Arabs settled in Morocco. Towards the end of the Almohad Caliphate, the Beni Hassan, a sub-tribe of the Maqil, were called by the local ruler of the Sous to quell a rebellion. During Marinid dynasty rule, the Beni Hassan rebelled but were defeated by the Sultan and escaped beyond the Saguia el-Hamra dry river; the Beni Hassan were at constant war with the Lamtuna nomadic Berbers of the Sahara. Over
Lead
Lead is a chemical element with symbol Pb and atomic number 82. It is a heavy metal, denser than most common materials. Lead is soft and malleable, has a low melting point; when freshly cut, lead is silvery with a hint of blue. Lead has the highest atomic number of any stable element and three of its isotopes each include a major decay chain of heavier elements. Lead is a unreactive post-transition metal, its weak metallic character is illustrated by its amphoteric nature. Compounds of lead are found in the +2 oxidation state rather than the +4 state common with lighter members of the carbon group. Exceptions are limited to organolead compounds. Like the lighter members of the group, lead tends to bond with itself. Lead is extracted from its ores. Galena, a principal ore of lead bears silver, interest in which helped initiate widespread extraction and use of lead in ancient Rome. Lead production declined after the fall of Rome and did not reach comparable levels until the Industrial Revolution. In 2014, the annual global production of lead was about ten million tonnes, over half of, from recycling.
Lead's high density, low melting point and relative inertness to oxidation make it useful. These properties, combined with its relative abundance and low cost, resulted in its extensive use in construction, batteries and shot, solders, fusible alloys, white paints, leaded gasoline, radiation shielding. In the late 19th century, lead's toxicity was recognized, its use has since been phased out of many applications. However, many countries still allow the sale of products that expose humans to lead, including some types of paints and bullets. Lead is a toxin that accumulates in soft tissues and bones, it acts as a neurotoxin damaging the nervous system and interfering with the function of biological enzymes, causing neurological disorders, such as brain damage and behavioral problems. A lead atom has 82 electrons, arranged in an electron configuration of 4f145d106s26p2; the sum of lead's first and second ionization energies—the total energy required to remove the two 6p electrons—is close to that of tin, lead's upper neighbor in the carbon group.
This is unusual. The similarity of ionization energies is caused by the lanthanide contraction—the decrease in element radii from lanthanum to lutetium, the small radii of the elements from hafnium onwards; this is due to poor shielding of the nucleus by the lanthanide 4f electrons. The sum of the first four ionization energies of lead exceeds that of tin, contrary to what periodic trends would predict. Relativistic effects, which become significant in heavier atoms, contribute to this behavior. One such effect is the inert pair effect: the 6s electrons of lead become reluctant to participate in bonding, making the distance between nearest atoms in crystalline lead unusually long. Lead's lighter carbon group congeners form stable or metastable allotropes with the tetrahedrally coordinated and covalently bonded diamond cubic structure; the energy levels of their outer s- and p-orbitals are close enough to allow mixing into four hybrid sp3 orbitals. In lead, the inert pair effect increases the separation between its s- and p-orbitals, the gap cannot be overcome by the energy that would be released by extra bonds following hybridization.
Rather than having a diamond cubic structure, lead forms metallic bonds in which only the p-electrons are delocalized and shared between the Pb2+ ions. Lead has a face-centered cubic structure like the sized divalent metals calcium and strontium. Pure lead has a silvery appearance with a hint of blue, it tarnishes on contact with moist air and takes on a dull appearance, the hue of which depends on the prevailing conditions. Characteristic properties of lead include high density, malleability and high resistance to corrosion due to passivation. Lead's close-packed face-centered cubic structure and high atomic weight result in a density of 11.34 g/cm3, greater than that of common metals such as iron and zinc. This density is the origin of the idiom to go over like a lead balloon; some rarer metals are denser: tungsten and gold are both at 19.3 g/cm3, osmium—the densest metal known—has a density of 22.59 g/cm3 twice that of lead. Lead is a soft metal with a Mohs hardness of 1.5. It is somewhat ductile.
The bulk modulus of lead—a measure of its ease of compressibility—is 45.8 GPa. In comparison, that of aluminium is 75.2 GPa. Lead's tensile strength, at 12–17 MPa, is low; the melting point of lead—at 327.5 °C —is low compared to most metals. Its boiling point of 1749 °C is the lowest among the carbon group elements; the electrical resistivity of lead at 20 °C is 192 nanoohm-meters an order of magnitude higher than those of other industrial metals. Lead is a superconductor at temperatures lower than 7.19 K.
Zinc
Zinc is a chemical element with symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. In some respects zinc is chemically similar to magnesium: both elements exhibit only one normal oxidation state, the Zn2+ and Mg2+ ions are of similar size. Zinc has five stable isotopes; the most common zinc ore is sphalerite, a zinc sulfide mineral. The largest workable lodes are in Australia and the United States. Zinc is refined by froth flotation of the ore and final extraction using electricity. Brass, an alloy of copper and zinc in various proportions, was used as early as the third millennium BC in the Aegean, the United Arab Emirates, Kalmykia and Georgia, the second millennium BC in West India, Iran, Syria and Israel/Palestine. Zinc metal was not produced on a large scale until the 12th century in India, though it was known to the ancient Romans and Greeks; the mines of Rajasthan have given definite evidence of zinc production going back to the 6th century BC. To date, the oldest evidence of pure zinc comes from Zawar, in Rajasthan, as early as the 9th century AD when a distillation process was employed to make pure zinc.
Alchemists burned zinc in air to form what they called "philosopher's wool" or "white snow". The element was named by the alchemist Paracelsus after the German word Zinke. German chemist Andreas Sigismund Marggraf is credited with discovering pure metallic zinc in 1746. Work by Luigi Galvani and Alessandro Volta uncovered the electrochemical properties of zinc by 1800. Corrosion-resistant zinc plating of iron is the major application for zinc. Other applications are in electrical batteries, small non-structural castings, alloys such as brass. A variety of zinc compounds are used, such as zinc carbonate and zinc gluconate, zinc chloride, zinc pyrithione, zinc sulfide, dimethylzinc or diethylzinc in the organic laboratory. Zinc is an essential mineral, including to postnatal development. Zinc deficiency affects about two billion people in the developing world and is associated with many diseases. In children, deficiency causes growth retardation, delayed sexual maturation, infection susceptibility, diarrhea.
Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans. Consumption of excess zinc may cause ataxia and copper deficiency. Zinc is a bluish-white, diamagnetic metal, though most common commercial grades of the metal have a dull finish, it is somewhat less dense than iron and has a hexagonal crystal structure, with a distorted form of hexagonal close packing, in which each atom has six nearest neighbors in its own plane and six others at a greater distance of 290.6 pm. The metal is hard and brittle at most temperatures but becomes malleable between 100 and 150 °C. Above 210 °C, the metal can be pulverized by beating. Zinc is a fair conductor of electricity. For a metal, zinc has low melting and boiling points; the melting point is the lowest of all the d-block metals aside from cadmium. Many alloys contain zinc, including brass. Other metals long known to form binary alloys with zinc are aluminium, bismuth, iron, mercury, tin, cobalt, nickel and sodium.
Although neither zinc nor zirconium are ferromagnetic, their alloy ZrZn2 exhibits ferromagnetism below 35 K. A bar of zinc generates a characteristic sound when bent, similar to tin cry. Zinc makes up about 75 ppm of Earth's crust. Soil contains zinc in 5–770 ppm with an average 64 ppm. Seawater has only 30 ppb and the atmosphere, 0.1–4 µg/m3. The element is found in association with other base metals such as copper and lead in ores. Zinc is a chalcophile, meaning the element is more to be found in minerals together with sulfur and other heavy chalcogens, rather than with the light chalcogen oxygen or with non-chalcogen electronegative elements such as the halogens. Sulfides formed as the crust solidified under the reducing conditions of the early Earth's atmosphere. Sphalerite, a form of zinc sulfide, is the most mined zinc-containing ore because its concentrate contains 60–62% zinc. Other source minerals for zinc include smithsonite, hemimorphite and sometimes hydrozincite. With the exception of wurtzite, all these other minerals were formed by weathering of the primordial zinc sulfides.
Identified world zinc resources total about 1.9–2.8 billion tonnes. Large deposits are in Australia and the United States, with the largest reserves in Iran; the most recent estimate of reserve base for zinc was made in 2009 and calculated to be 480 Mt. Zinc reserves, on the other hand, are geologically identified ore bodies whose suitability for recovery is economically based at the time of determination. Since exploration and mine development is an ongoing process, the amount of zinc reserves is not a fixed number and sustainability of zinc ore supplies cannot be judged by extrapolating the combined mine life of today's zinc mines; this concept is well supported by data from the United States Geol
Tell Atlas
The Tell Atlas is a mountain chain over 1,500 km in length, belonging to the Atlas mountain ranges in North Africa, stretching from Morocco, through Algeria to Tunisia. The ranges of this system have average elevations of about 1,500 m and form a natural barrier between the Mediterranean and the Sahara, its highest summit is the 2,308 m high Lalla Khedidja in the Jurjura Range. Several large cities such as the Algerian capital, with ~1,500,000 residents and Oran with ~770,000 residents lie at the base of the Tell Atlas; the Algerian city Constantine with 505,000 residents lies 80 km inland and directly in the mountains at 650 meters in elevation. A number of smaller towns and villages are situated within the Tell; the Tell Atlas runs parallel to the Mediterranean coast. Together with the Saharan Atlas to the south it forms the northernmost of two more or less parallel ranges which approach one another towards the east, remaining quite distinct from one another in Western Algeria and merging in Eastern Algeria.
At the western end, it ends at the Middle Atlas ranges in Morocco. The Tell Atlas are a distinct physiographic section of the larger Atlas Mountains province, which in turn is part of the larger African Alpine System physiographic division; the Tell Atlas and the Saharan Atlas form two natural barriers, the first against the Mediterranean and the second against the Sahara. Between them lies the valley of the Chelif and various lesser rivers. South of the Tell Atlas is the high plateau of the Hautes Plaines with level terrain where water collects during the wet season, forming large shallow salt lakes which become salt flats as they dry. Agriculture includes grazing of sheep and goats on grass in better-watered high plateau areas and some farming; the Chelif is a 725 km long river with headwaters in the Tell Atlas to its discharge into the Mediterranean. The Chelif is characterized by an fertile valley. Other noteworthy rivers having their sources in this range are the Seybouse River. Only seasonal streams are found flowing south from the Tell Atlas.
The Tell Atlas enjoys a typical Mediterranean climate, warm with dry summers and mild, rainy winters with snow at upper elevations. As a consequence, the northern slopes of the Tell Atlas are forested with the endemic Abies numidica, Atlas cedar and cork oak. In the summer a hot, dry wind, the Sirocco, blows north from the Sahara across the Tell Atlas, causing dusty, dry conditions along the northern coast of Africa. Despite the arid climate, some agriculture for barley and wheat farming is found in the Tell Atlas region; the Chiffa gorge is situated within the Tell Atlas. List of mountains in Algeria Saharan Atlas Media related to Tell Atlas at Wikimedia Commons Tell Atlas - The Great Soviet Encyclopedia
Ontario
Ontario is one of the 13 provinces and territories of Canada and is located in east-central Canada. It is Canada's most populous province accounting for 38.3 percent of the country's population, is the second-largest province in total area. Ontario is fourth-largest jurisdiction in total area when the territories of the Northwest Territories and Nunavut are included, it is home to the nation's capital city and the nation's most populous city, Ontario's provincial capital. Ontario is bordered by the province of Manitoba to the west, Hudson Bay and James Bay to the north, Quebec to the east and northeast, to the south by the U. S. states of Minnesota, Ohio and New York. All of Ontario's 2,700 km border with the United States follows inland waterways: from the west at Lake of the Woods, eastward along the major rivers and lakes of the Great Lakes/Saint Lawrence River drainage system; these are the Rainy River, the Pigeon River, Lake Superior, the St. Marys River, Lake Huron, the St. Clair River, Lake St. Clair, the Detroit River, Lake Erie, the Niagara River, Lake Ontario and along the St. Lawrence River from Kingston, Ontario, to the Quebec boundary just east of Cornwall, Ontario.
There is only about 1 km of land border made up of portages including Height of Land Portage on the Minnesota border. Ontario is sometimes conceptually divided into Northern Ontario and Southern Ontario; the great majority of Ontario's population and arable land is in the south. In contrast, the larger, northern part of Ontario is sparsely populated with cold winters and heavy forestation; the province is named after Lake Ontario, a term thought to be derived from Ontarí:io, a Huron word meaning "great lake", or skanadario, which means "beautiful water" in the Iroquoian languages. Ontario has about 250,000 freshwater lakes; the province consists of three main geographical regions: The thinly populated Canadian Shield in the northwestern and central portions, which comprises over half the land area of Ontario. Although this area does not support agriculture, it is rich in minerals and in part covered by the Central and Midwestern Canadian Shield forests, studded with lakes and rivers. Northern Ontario is subdivided into two sub-regions: Northeastern Ontario.
The unpopulated Hudson Bay Lowlands in the extreme north and northeast swampy and sparsely forested. Southern Ontario, further sub-divided into four regions. Despite the absence of any mountainous terrain in the province, there are large areas of uplands within the Canadian Shield which traverses the province from northwest to southeast and above the Niagara Escarpment which crosses the south; the highest point is Ishpatina Ridge at 693 metres above sea level in Temagami, Northeastern Ontario. In the south, elevations of over 500 m are surpassed near Collingwood, above the Blue Mountains in the Dundalk Highlands and in hilltops near the Madawaska River in Renfrew County; the Carolinian forest zone covers most of the southwestern region of the province. The temperate and fertile Great Lakes-Saint Lawrence Valley in the south is part of the Eastern Great Lakes lowland forests ecoregion where the forest has now been replaced by agriculture and urban development. A well-known geographic feature is part of the Niagara Escarpment.
The Saint Lawrence Seaway allows navigation to and from the Atlantic Ocean as far inland as Thunder Bay in Northwestern Ontario. Northern Ontario occupies 87 percent of the surface area of the province. Point Pelee is a peninsula of Lake Erie in southwestern Ontario, the southernmost extent of Canada's mainland. Pelee Island and Middle Island in Lake Erie extend farther. All are south of 42°N – farther south than the northern border of California; the climate of Ontario varies by location. It is affected by three air sources: cold, arctic air from the north; the effects of these major air masses on temperature and precipitation depend on latitude, proximity to major bodies of water and to a small extent, terrain relief. In general, most of Ontario's climate is classified as humid continental. Ontario has three main climatic regions; the surrounding Great Lakes influence the climatic region of southern Ontario. During the fall and winter months, heat stored from the lakes is released, moderating the climate near the shores of the lakes.
This gives some parts of southern Ontario milder winters than mid-continental areas at lower latitudes. Parts of Southwestern Ontario have a moderate humid continental climate, similar to that of the inland Mid-Atlantic states and the Great Lakes portion of the Midwestern United States; the region has warm to cold winters. Annual precipitation is well distributed throughout the year. Most of this region lies in the lee of the Great Lakes. In December 2010, the snowbelt set a new record when it was h
Sahara
The Sahara is a desert located on the African continent. It is the largest hot desert in the world, the third largest desert overall after Antarctica and the Arctic, its area of 9,200,000 square kilometres is comparable to the area of the United States. The name'Sahara' is derived from a dialectal Arabic word for ṣaḥra; the desert comprises much of North Africa, excluding the fertile region on the Mediterranean Sea coast, the Atlas Mountains of the Maghreb, 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 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: the western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains, the Ténéré desert, the Libyan Desert.
For several hundred thousand years, the Sahara has alternated between desert and savanna grassland in a 41,000 year cycle caused by the precession of the Earth's axis as it rotates around the Sun, which changes the location of the North African Monsoon. The area is next expected to become green in about 15,000 years. There is a suggestion that the last time that the Sahara was converted from savanna to desert it was due to overgrazing by the cattle of the local population; the Sahara covers large parts of Algeria, Egypt, Mali, Niger, Western Sahara and Tunisia. It covers 9 million square kilometres, 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, it is one of three distinct physiographic provinces of the African massive physiographic division. The Sahara is rocky hamada. Wind or rare rainfall shape the desert features: sand dunes, dune fields, sand seas, stone plateaus, gravel plains, dry valleys, dry lakes, salt flats.
Unusual landforms include the Richat Structure in Mauritania. Several dissected mountains, many volcanic, rise from the desert, including the Aïr Mountains, Ahaggar Mountains, Saharan Atlas, Tibesti Mountains, Adrar des Iforas, 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: Tanezrouft, the Ténéré, the Libyan Desert, the Eastern Desert, the Nubian Desert and others; these arid areas 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 and scrub eco-regions 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 corresponds to the 100 mm 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, 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 isohyet of annual precipitation. Important cities located in the Sahara include the capital of Mauritania; the Sahara is the world's largest low-latitude hot desert. It 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 drying effect in the upper troposphere. The sinking air prevents evaporating water from rising, therefore prevents adiabatic cooling, which makes cloud formation difficult to nearly impossible; the permanent dissolution of clouds allows thermal radiation. The stability of the atmosphere above the desert prevents any convective overturning, thus making rainfall non-existent; as a consequence, the weather tends to be sunny and stable with a minimal chance of rainfall. Subsiding, dry air masses associated with subtropical high-pressure systems are unfavorable for the development of convectional showers; the subtropical ridge is the predominant factor that explains the hot desert climate (Köppen climate classifica
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