A park is an area of natural, semi-natural or planted space set aside for human enjoyment and recreation or for the protection of wildlife or natural habitats. Urban parks are green spaces set aside for recreation inside cities. National parks and Country parks are green spaces used for recreation in the countryside. State parks and Provincial parks are administered by sub-national government agencies. Parks may consist of grassy areas, rocks and trees, but may contain buildings and other artifacts such as monuments, fountains or playground structures. Many parks have fields for playing sports such as soccer and football, paved areas for games such as basketball. Many parks have trails for walking and other activities; some parks are built adjacent to bodies of water or watercourses and may comprise a beach or boat dock area. Urban parks have benches for sitting and may contain picnic tables and barbecue grills; the largest parks can be vast natural areas of hundreds of thousands square kilometers, with abundant wildlife and natural features such as mountains and rivers.
In many large parks, camping in tents is allowed with a permit. Many natural parks are protected by law, users may have to follow restrictions. Large national and sub-national parks are overseen by a park ranger or a park warden. Large parks may have areas for canoeing and hiking in the warmer months and, in some northern hemisphere countries, cross-country skiing and snowshoeing in colder months. There are amusement parks which have live shows, fairground rides and games of chance or skill. English deer parks were used by the aristocracy in medieval times for game hunting, they had walls or thick hedges around them to keep game animals in and people out. It was forbidden for commoners to hunt animals in these deer parks; these game preserves evolved into landscaped parks set around mansions and country houses from the sixteenth century onwards. These may have served as hunting grounds but they proclaimed the owner's wealth and status. An aesthetic of landscape design began in these stately home parks where the natural landscape was enhanced by landscape architects such as Capability Brown.
As cities became crowded, the private hunting grounds became places for the public. With the Industrial revolution parks took on a new meaning as areas set aside to preserve a sense of nature in the cities and towns. Sporting activity came to be a major use for these urban parks. Areas of outstanding natural beauty were set aside as national parks to prevent their being spoiled by uncontrolled development. Park design is influenced by the intended purpose and audience, as well as by the available land features. A park intended to provide recreation for children may include a playground. A park intended for adults may feature walking paths and decorative landscaping. Specific features, such as riding trails, may be included to support specific activities; the design of a park may determine, willing to use it. Walkers may feel unsafe on a mixed-use path, dominated by fast-moving cyclists or horses. Different landscaping and infrastructure may affect children's rates of use of parks according to sex.
Redesigns of two parks in Vienna suggested that the creation of multiple semi-enclosed play areas in a park could encourage equal use by boys and girls. Parks are part of the urban infrastructure: for physical activity, for families and communities to gather and socialize, or for a simple respite. Research reveals that people who exercise outdoors in green-space derive greater mental health benefits. Providing activities for all ages and income levels is important for the physical and mental well-being of the public. Parks can benefit pollinators, some parks have been redesigned to accommodate them better; some organisations, such as Xerces Society are promoting this idea. City parks play a role in improving cities and improving the futures for residents and visitors - for example, Millennium Park in Chicago, Illinois or the Mill River Park and Green way in Stamford, CT. One group, a strong proponent of parks for cities is The American Society of Landscape Architects, they argue that parks are important to the fabric of the community on an individual scale and broader scales such as entire neighborhoods, city districts or city park systems.
Parks need to feel safe for people to use them. Research shows that perception of safety can be more significant in influencing human behavior than actual crime statistics. If citizens perceive a park as unsafe, they might not make use of it at all. A study done in four cities. There are a number of features. Elements in the physical design of a park, such as an open and welcoming entry, good visibility, appropriate lighting and signage can all make a difference. Regular park maintenance, as well as programming and community involvement can contribute to a feeling of safety. While Crime Prevention Through Environmental Design has been used in facility design, use of CPTED in parks has not been. Iqbal and Ceccato performed a study in Stockholm, Sweden to determine if it would be useful to apply to parks, their study indicated that while CPTED could be useful, due to the
North Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, it is sometimes defined as stretching from the Atlantic shores of Morocco in the west, to Egypt's Suez Canal and the Red Sea in the east. Others have limited it to top North-Western countries like Algeria and Tunisia, a region, known by the French during colonial times as "Afrique du Nord" and is known by all Arabs as the Maghreb; the most accepted definition includes Algeria, Morocco, Tunisia and Egypt, the 6 countries that shape the top North of the African continent. Meanwhile, "North Africa" when used in the term North Africa and the Middle East refers only to the countries of the Maghreb and Libya. Egypt, being part of the Middle East, is considered separately, due to being both North African and Middle Eastern at the same time. North Africa includes a number of Spanish and Portuguese possessions, Plazas de soberanía, Ceuta and Melilla and the Canary Islands and Madeira.
The countries of North Africa share a common ethnic and linguistic identity, unique to this region. Northwest Africa has been inhabited by Berbers since the beginning of recorded history, while the eastern part of North Africa has been home to the Egyptians. Between the A. D. 600s and 1000s, Arabs from the Middle East swept across the region in a wave of Muslim conquest. These peoples, physically quite similar, formed a single population in many areas, as Berbers and Egyptians merged into Arabic and Muslim culture; this process of Arabization and Islamization has defined the cultural landscape of North Africa since. The distinction between North Africa, the Sahel and the rest of the continent is as follows: Nineteenth century European explorers, attracted by the accounts of Ancient geographers or Arab geographers of the classical period, followed the routes by the nomadic people of the vast "empty" space, they documented the names of the stopping places they discovered or rediscovered, described landscapes, took a few climate measurements and gathered rock samples.
A map began to fill in the white blotch. The Sahara and the Sahel entered the geographic corpus by way of naturalist explorers because aridity is the feature that circumscribes the boundaries of the ecumene; the map details included topographical relief and location of watering holes crucial to long crossings. The Arabic word "Sahel" and "Sahara" made its entry into the vocabulary of geography. Latitudinally, the "slopes" of the arid desert, devoid of continuous human habitation, descend in step-like fashion toward the northern and southern edges of the Mediterranean that opens to Europe and the Sahel that opens to "Trab al Sudan." Longitudinally, a uniform grid divides the central desert shrinks back toward the Atlantic Ocean and the Red Sea. The Sahara-Sahel is further divided into a total of twenty sub-areas: central, southern, eastern, etc. In this way, "standard" geography has determined aridity to be the boundary of the ecumene, it identifies settlements based on visible activity without regard for social or political organizations of space in vast, purportedly “empty” areas.
It gives only cursory acknowledgement to what makes Saharan geography, for that matter, world geography unique: mobility and the routes by which it flows. The Sahel or "African Transition Zone" has been affected by many formative epochs in North African history ranging from Ottoman occupation to the Arab-Berber control of the Andalus; as a result, many modern African nation-states that are included in the Sahel evidence cultural similarities and historical overlap with their North African neighbours. In the present day, North Africa is associated with West Asia in the realm of geopolitics to form a Middle East-North Africa region; the Islamic influence in the area is significant and North Africa is a major part of the Muslim world. Some researchers have postulated that North Africa rather than East Africa served as the exit point for the modern humans who first trekked out of the continent in the Out of Africa migration. North Africa has three main geographic features: the Sahara desert in the south, the Atlas Mountains in the west, the Nile River and delta in the east.
The Atlas Mountains extend across much of northern Algeria and Tunisia. These mountains are part of the fold mountain system that runs through much of Southern Europe, they recede to the south and east, becoming a steppe landscape before meeting the Sahara desert, which covers more than 75 percent of the region. The tallest peaks are in the High Atlas range in south-central Morocco, which has many snow-capped peaks. South of the Atlas Mountains is the dry and barren expanse of the Sahara desert, the largest sand desert in the world. In places the desert is cut by irregular watercourses called wadis—streams that flow only after rainfalls but are dry; the Sahara's major landforms include large seas of sand that sometimes form into huge dunes. The Sahara covers the southern part of Algeria and Tunisia, most of Libya. Only two regions of Libya are outside the desert: Tripolitania in the northwest and Cyrenaica in the northeast. Most of Egypt is desert, with the exception of the Nile River and the irrigated land along its banks.
The Nile Valley forms a narrow fertile thread. Sheltered valleys in the Atlas Mountains, the Nile Valley and Delta, the Mediterranean coast are the main sources of fertile farming land. A wide variety of valuable crops including ce
The flowering plants known as angiosperms, Angiospermae or Magnoliophyta, are the most diverse group of land plants, with 64 orders, 416 families 13,164 known genera and c. 369,000 known species. Like gymnosperms, angiosperms are seed-producing plants. However, they are distinguished from gymnosperms by characteristics including flowers, endosperm within the seeds, the production of fruits that contain the seeds. Etymologically, angiosperm means a plant; the term comes from the Greek words sperma. The ancestors of flowering plants diverged from gymnosperms in the Triassic Period, 245 to 202 million years ago, the first flowering plants are known from 160 mya, they diversified extensively during the Early Cretaceous, became widespread by 120 mya, replaced conifers as the dominant trees from 100 to 60 mya. Angiosperms differ from other seed plants in several ways, described in the table below; these distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans.
Angiosperm stems are made up of seven layers. The amount and complexity of tissue-formation in flowering plants exceeds that of gymnosperms; the vascular bundles of the stem are arranged such that the phloem form concentric rings. In the dicotyledons, the bundles in the young stem are arranged in an open ring, separating a central pith from an outer cortex. In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue known as cambium. By the formation of a layer of cambium between the bundles, a complete ring is formed, a regular periodical increase in thickness results from the development of xylem on the inside and phloem on the outside; the soft phloem becomes crushed, but the hard wood persists and forms the bulk of the stem and branches of the woody perennial. Owing to differences in the character of the elements produced at the beginning and end of the season, the wood is marked out in transverse section into concentric rings, one for each season of growth, called annual rings.
Among the monocotyledons, the bundles are more numerous in the young stem and are scattered through the ground tissue. They once formed the stem increases in diameter only in exceptional cases; the characteristic feature of angiosperms is the flower. Flowers show remarkable variation in form and elaboration, provide the most trustworthy external characteristics for establishing relationships among angiosperm species; the function of the flower is to ensure fertilization of the ovule and development of fruit containing seeds. The floral apparatus may arise terminally from the axil of a leaf; as in violets, a flower arises singly in the axil of an ordinary foliage-leaf. More the flower-bearing portion of the plant is distinguished from the foliage-bearing or vegetative portion, forms a more or less elaborate branch-system called an inflorescence. There are two kinds of reproductive cells produced by flowers. Microspores, which will divide to become pollen grains, are the "male" cells and are borne in the stamens.
The "female" cells called megaspores, which will divide to become the egg cell, are contained in the ovule and enclosed in the carpel. The flower may consist only of these parts, as in willow, where each flower comprises only a few stamens or two carpels. Other structures are present and serve to protect the sporophylls and to form an envelope attractive to pollinators; the individual members of these surrounding structures are known as petals. The outer series is green and leaf-like, functions to protect the rest of the flower the bud; the inner series is, in general, white or brightly colored, is more delicate in structure. It functions to attract bird pollinators. Attraction is effected by color and nectar, which may be secreted in some part of the flower; the characteristics that attract pollinators account for the popularity of flowers and flowering plants among humans. While the majority of flowers are perfect or hermaphrodite, flowering plants have developed numerous morphological and physiological mechanisms to reduce or prevent self-fertilization.
Heteromorphic flowers have short carpels and long stamens, or vice versa, so animal pollinators cannot transfer pollen to the pistil. Homomorphic flowers may employ a biochemical mechanism called self-incompatibility to discriminate between self and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers; the botanical term "Angiosperm", from the Ancient Greek αγγείον, angeíon and σπέρμα, was coined in the form Angiospermae by Paul Hermann in 1690, as the name of one of his primary divisions of the plant kingdom. This included flowering plants possessing seeds enclosed in capsules, distinguished from his Gymnospermae, or flowering plants with achenial or schizo-carpic fruits, the whole fruit or each of its pieces being here regarded as a seed and naked; the term and its antonym were maintained by Carl Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia. Its use with any
In botany, a tree is a perennial plant with an elongated stem, or trunk, supporting branches and leaves in most species. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are usable as lumber or plants above a specified height. Trees are not a taxonomic group but include a variety of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants to compete for sunlight. Trees tend to be long-lived, some reaching several thousand years old. In wider definitions, the taller palms, tree ferns and bamboos are trees. Trees have been in existence for 370 million years, it is estimated. A tree has many secondary branches supported clear of the ground by the trunk; this trunk contains woody tissue for strength, vascular tissue to carry materials from one part of the tree to another. For most trees it is surrounded by a layer of bark. Below the ground, the roots spread out widely. Above ground, the branches divide into smaller shoots.
The shoots bear leaves, which capture light energy and convert it into sugars by photosynthesis, providing the food for the tree's growth and development. Trees reproduce using seeds. Flowers and fruit may be present, but some trees, such as conifers, instead have pollen cones and seed cones. Palms and bamboos produce seeds, but tree ferns produce spores instead. Trees play a significant role in moderating the climate, they remove carbon dioxide from the atmosphere and store large quantities of carbon in their tissues. Trees and forests provide a habitat for many species of plants. Tropical rainforests are among the most biodiverse habitats in the world. Trees provide shade and shelter, timber for construction, fuel for cooking and heating, fruit for food as well as having many other uses. In parts of the world, forests are shrinking as trees are cleared to increase the amount of land available for agriculture; because of their longevity and usefulness, trees have always been revered, with sacred groves in various cultures, they play a role in many of the world's mythologies.
Although "tree" is a term of common parlance, there is no universally recognised precise definition of what a tree is, either botanically or in common language. In its broadest sense, a tree is any plant with the general form of an elongated stem, or trunk, which supports the photosynthetic leaves or branches at some distance above the ground. Trees are typically defined by height, with smaller plants from 0.5 to 10 m being called shrubs, so the minimum height of a tree is only loosely defined. Large herbaceous plants such as papaya and bananas are trees in this broad sense. A applied narrower definition is that a tree has a woody trunk formed by secondary growth, meaning that the trunk thickens each year by growing outwards, in addition to the primary upwards growth from the growing tip. Under such a definition, herbaceous plants such as palms and papayas are not considered trees regardless of their height, growth form or stem girth. Certain monocots may be considered trees under a looser definition.
Aside from structural definitions, trees are defined by use. The tree growth habit is an evolutionary adaptation found in different groups of plants: by growing taller, trees are able to compete better for sunlight. Trees tend some reaching several thousand years old. Several trees are among the oldest organisms now living. Trees have modified structures such as thicker stems composed of specialised cells that add structural strength and durability, allowing them to grow taller than many other plants and to spread out their foliage, they differ from shrubs, which have a similar growth form, by growing larger and having a single main stem. The tree form has evolved separately in unrelated classes of plants in response to similar environmental challenges, making it a classic example of parallel evolution. With an estimated 60,000-100,000 species, the number of trees worldwide might total twenty-five per cent of all living plant species; the greatest number of these grow in tropical regions and many of these areas have not yet been surveyed by botanists, making tree diversity and ranges poorly known.
The majority of tree species are angiosperms. There are about 1000 species of gymnosperm trees, including conifers, cycads and gnetales. Most angiosperm trees are eudicots, the "true dicotyledons", so named because the seeds contain two cotyledons or seed leaves. There are some trees among the old lineages of flowering plants called basal angiosperms or paleodicots. Wood gives structural strength to the trunk of most types of tree; the vascular system of trees allows water and other chemicals to be di
A hedge or hedgerow is a line of spaced shrubs and sometimes trees and trained to form a barrier or to mark the boundary of an area, such as between neighbouring properties. Hedges used to separate a road from adjoining fields or one field from another, of sufficient age to incorporate larger trees, are known as hedgerows, they serve as windbreaks to improve conditions for the adjacent crops, as in bocage country. When clipped and maintained, hedges are a simple form of topiary; the development of hedges over the centuries is preserved in their structure. The first hedges enclosed land for cereal crops during the Neolithic Age; the farms were with fields about 0.1 hectares for hand cultivation. Some hedges date from the Bronze and Iron Ages, 2000–4000 years ago, when traditional patterns of landscape became established. Others were built during the Medieval field rationalisations. Many hedgerows separating fields from lanes in the United Kingdom and the Low Countries are estimated to have been in existence for more than seven hundred years, originating in the medieval period.
The root word of'hedge' is much older: it appears in the Old English language, in German, Dutch to mean'enclosure', as in the name of the Dutch city The Hague, or more formally's Gravenhage, meaning The Count's hedge. Charles the Bald is recorded as complaining in 864, at a time when most official fortifications were constructed of wooden palisades, that some unauthorized men were constructing haies et fertés – interwoven hedges of hawthorns. In parts of Britain, early hedges were destroyed to make way for the manorial open-field system. Many were replaced after the Enclosure Acts removed again during modern agricultural intensification, now some are being replanted for wildlife. A hedge may consist of a single species or several mixed at random. In many newly planted British hedges, at least 60 per cent of the shrubs are hawthorn and hazel, alone or in combination; the first two are effective barriers to livestock. Other shrubs and trees used include holly, oak and willow. Of the hedgerows in the Normandy region of France, Martin Blumenson said, The hedgerow is a fence, half earth, half hedge.
The wall at the base is a dirt parapet that varies in thickness from one to four or more feet and in height from three to twelve feet. Growing out of the wall is a hedge of hawthorn, brambles and trees, in thickness from one to three feet. Property demarcations, hedgerows protect crops and cattle from the ocean winds that sweep across the land; the hedgerows of Normandy became barriers that slowed the advance of Allied troops following the D-Day invasion of WWII. Formal, or modern garden hedges are grown in many varieties, including the following species: Berberis thunbergii Buxus sempervirens Carpinus betulus Crataegus monogyna Fagus sylvatica Fagus sylvatica ‘Purpurea’ Ilex aquifolium Ligustrum ovalifolium Photinia fraseri Prunus laurocerasus Prunus lusitanica Quercus ilex Taxus baccata Thuja occidentalis Thuja plicata Hedgerow trees are trees that grow in hedgerows but have been allowed to reach their full height and width. There are thought to be around 1.8 million hedgerow trees in Britain with 98% of these being in England and Wales.
Hedgerow trees are both an important part of the English landscape and valuable habitats for wildlife. Many hedgerow trees are veteran trees and therefore of great wildlife interest; the most common species are oak and ash, though in the past elm would have been common. Around 20 million elm trees, most of them hedgerow trees, were felled or died through Dutch elm disease in the late 1960s. Many other species are used, notably including beech and various fruit trees; the age structure of British hedgerow trees is old because the number of new trees is not sufficient to replace the number of trees that are lost through age or disease. New trees can be established by planting but it is more successful to leave standard trees behind when laying hedges. Trees should be left at no closer than 10 metres apart and the distances should vary so as to create a more natural landscape; the distance allows the young trees to develop full crowns without competing or producing too much shade. It is suggested that hedgerow trees cause gaps in hedges but it has been found that cutting some lower branches off lets sufficient light through to the hedge below to allow it to grow.
Hedges are recognised as part of a cultural heritage and historical record and for their great value to wildlife and the landscape. They are valued too for the major role they have to play in preventing soil loss and reducing pollution, for their potential to regulate water supply and to reduce flooding. In addition to maintaining the health of the environment, hedgerows play a huge role in providing shelter for smaller animals like birds and insects. Recent study by Emma Coulthard mentioned the possibility that hedgerows may act as guides for moths, like A. rumicis, when flying from one location to another. As moths are nocturnal, it is unlikely that they use visual aids as guides, but rather are following sensory or olfactory markers on the hedgerows. Hedges were used as a source of firewood, for providing shelter from wind, rain an
Europe is a continent located in the Northern Hemisphere and in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west and the Mediterranean Sea to the south, it comprises the westernmost part of Eurasia. Since around 1850, Europe is most considered to be separated from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas and the waterways of the Turkish Straits. Although the term "continent" implies physical geography, the land border is somewhat arbitrary and has been redefined several times since its first conception in classical antiquity; the division of Eurasia into two continents reflects East-West cultural and ethnic differences which vary on a spectrum rather than with a sharp dividing line. The geographic border does not follow political boundaries, with Turkey and Kazakhstan being transcontinental countries. A strict application of the Caucasus Mountains boundary places two comparatively small countries and Georgia, in both continents.
Europe covers 2 % of the Earth's surface. Politically, Europe is divided into about fifty sovereign states of which the Russian Federation is the largest and most populous, spanning 39% of the continent and comprising 15% of its population. Europe had a total population of about 741 million as of 2016; the European climate is affected by warm Atlantic currents that temper winters and summers on much of the continent at latitudes along which the climate in Asia and North America is severe. Further from the sea, seasonal differences are more noticeable than close to the coast. Europe, in particular ancient Greece, was the birthplace of Western civilization; the fall of the Western Roman Empire in 476 AD and the subsequent Migration Period marked the end of ancient history and the beginning of the Middle Ages. Renaissance humanism, exploration and science led to the modern era. Since the Age of Discovery started by Portugal and Spain, Europe played a predominant role in global affairs. Between the 16th and 20th centuries, European powers controlled at various times the Americas all of Africa and Oceania and the majority of Asia.
The Age of Enlightenment, the subsequent French Revolution and the Napoleonic Wars shaped the continent culturally and economically from the end of the 17th century until the first half of the 19th century. The Industrial Revolution, which began in Great Britain at the end of the 18th century, gave rise to radical economic and social change in Western Europe and the wider world. Both world wars took place for the most part in Europe, contributing to a decline in Western European dominance in world affairs by the mid-20th century as the Soviet Union and the United States took prominence. During the Cold War, Europe was divided along the Iron Curtain between NATO in the West and the Warsaw Pact in the East, until the revolutions of 1989 and fall of the Berlin Wall. In 1949 the Council of Europe was founded, following a speech by Sir Winston Churchill, with the idea of unifying Europe to achieve common goals, it includes all European states except for Belarus and Vatican City. Further European integration by some states led to the formation of the European Union, a separate political entity that lies between a confederation and a federation.
The EU originated in Western Europe but has been expanding eastward since the fall of the Soviet Union in 1991. The currency of most countries of the European Union, the euro, is the most used among Europeans. In classical Greek mythology, Europa was a Phoenician princess; the word Europe is derived from her name. The name contains the elements εὐρύς, "wide, broad" and ὤψ "eye, countenance", hence their composite Eurṓpē would mean "wide-gazing" or "broad of aspect". Broad has been an epithet of Earth herself in the reconstructed Proto-Indo-European religion and the poetry devoted to it. There have been attempts to connect Eurṓpē to a Semitic term for "west", this being either Akkadian erebu meaning "to go down, set" or Phoenician'ereb "evening, west", at the origin of Arabic Maghreb and Hebrew ma'arav. Michael A. Barry, professor in Princeton University's Near Eastern Studies Department, finds the mention of the word Ereb on an Assyrian stele with the meaning of "night, sunset", in opposition to Asu " sunrise", i.e. Asia.
The same naming motive according to "cartographic convention" appears in Greek Ἀνατολή. Martin Litchfield West stated that "phonologically, the match between Europa's name and any form of the Semitic word is poor." Next to these hypotheses there is a Proto-Indo-European root *h1regʷos, meaning "darkness", which produced Greek Erebus. Most major world languages use words derived from Europa to refer to the continent. Chinese, for example, uses the word Ōuzhōu. In some Turkic languages the Persian name Frangistan is used casually in referring to much of Europe, besides official names such as Avrupa or Evropa; the prevalent definition of Europe as a geographical term has been in use since the mid-19th century. Europe is taken to be bounded by large bodies of water
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