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
North America is a continent within the Northern Hemisphere and all within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the west and south by the Pacific Ocean, to the southeast by South America and the Caribbean Sea. North America covers an area of about 24,709,000 square kilometers, about 16.5% of the earth's land area and about 4.8% of its total surface. North America is the third largest continent by area, following Asia and Africa, the fourth by population after Asia and Europe. In 2013, its population was estimated at nearly 579 million people in 23 independent states, or about 7.5% of the world's population, if nearby islands are included. North America was reached by its first human populations during the last glacial period, via crossing the Bering land bridge 40,000 to 17,000 years ago; the so-called Paleo-Indian period is taken to have lasted until about 10,000 years ago. The Classic stage spans the 6th to 13th centuries.
The Pre-Columbian era ended in 1492, the transatlantic migrations—the arrival of European settlers during the Age of Discovery and the Early Modern period. Present-day cultural and ethnic patterns reflect interactions between European colonists, indigenous peoples, African slaves and their descendants. Owing to the European colonization of the Americas, most North Americans speak English, Spanish or French, their culture reflects Western traditions; the Americas are accepted as having been named after the Italian explorer Amerigo Vespucci by the German cartographers Martin Waldseemüller and Matthias Ringmann. Vespucci, who explored South America between 1497 and 1502, was the first European to suggest that the Americas were not the East Indies, but a different landmass unknown by Europeans. In 1507, Waldseemüller produced a world map, in which he placed the word "America" on the continent of South America, in the middle of what is today Brazil, he explained the rationale for the name in the accompanying book Cosmographiae Introductio:... ab Americo inventore... quasi Americi terram sive Americam.
For Waldseemüller, no one should object to the naming of the land after its discoverer. He used the Latinized version of Vespucci's name, but in its feminine form "America", following the examples of "Europa", "Asia" and "Africa". Other mapmakers extended the name America to the northern continent, In 1538, Gerard Mercator used America on his map of the world for all the Western Hemisphere; some argue that because the convention is to use the surname for naming discoveries, the derivation from "Amerigo Vespucci" could be put in question. In 1874, Thomas Belt proposed a derivation from the Amerrique mountains of Central America. Marcou corresponded with Augustus Le Plongeon, who wrote: "The name AMERICA or AMERRIQUE in the Mayan language means, a country of perpetually strong wind, or the Land of the Wind, and... the can mean... a spirit that breathes, life itself." The United Nations formally recognizes "North America" as comprising three areas: Northern America, Central America, The Caribbean.
This has been formally defined by the UN Statistics Division. The term North America maintains various definitions in accordance with context. In Canadian English, North America refers to the land mass as a whole consisting of Mexico, the United States, Canada, although it is ambiguous which other countries are included, is defined by context. In the United States of America, usage of the term may refer only to Canada and the US, sometimes includes Greenland and Mexico, as well as offshore islands. In France, Portugal, Romania and the countries of Latin America, the cognates of North America designate a subcontinent of the Americas comprising Canada, the United States, Mexico, Greenland, Saint Pierre et Miquelon, Bermuda. North America has been referred to by other names. Spanish North America was referred to as Northern America, this was the first official name given to Mexico. Geographically the North American continent has many subregions; these include cultural and geographic regions. Economic regions included those formed by trade blocs, such as the North American Trade Agreement bloc and Central American Trade Agreement.
Linguistically and culturally, the continent could be divided into Latin America. Anglo-America includes most of Northern America and Caribbean islands with English-speaking populations; the southern North American continent is composed of two regions. These are the Caribbean; the north of the continent maintains recognized regions as well. In contrast to the common definition of "North America", which encompasses the whole continent, the term "North America" is sometimes used to refer only to Mexico, the United States, Greenland; the term Northern America refers to the northern-most countries and territories of North America: the United States, Bermuda, St. Pierre and Miquelon and Greenland. Although the term does not refer to a unifie
The Mojave Desert is an arid rain-shadow desert and the driest desert in North America. It is in the southwestern United States within southeastern California and southern Nevada, it occupies 47,877 sq mi. Small areas extend into Utah and Arizona, its boundaries are noted by the presence of Joshua trees, which are native only to the Mojave Desert and are considered an indicator species, it is believed to support an additional 1,750 to 2,000 species of plants. The central part of the desert is sparsely populated, while its peripheries support large communities such as Las Vegas, Lancaster, Victorville, St. George; the Mojave Desert is bordered by the Great Basin Desert to its north and the Sonoran Desert to its south and east. Topographical boundaries include the Tehachapi Mountains to the west, the San Gabriel Mountains and San Bernardino Mountains to the south; the mountain boundaries are distinct because they are outlined by the two largest faults in California – the San Andreas and Garlock faults.
The Mojave Desert displays typical range topography. Higher elevations above 2,000 ft in the Mojave are referred to as the High Desert; the Mojave Desert occupies less than 50,000 sq mi, making it the smallest of the North American deserts. The Mojave Desert is referred to as the "high desert", in contrast to the "low desert", the Sonoran Desert to the south; the Mojave Desert, however, is lower than the Great Basin Desert to the north. The spelling Mojave originates from the Spanish language while the spelling Mohave comes from modern English. Both are used today, although the Mojave Tribal Nation uses the spelling Mojave; the Mojave Desert receives less than 2 inches of rain a year and is between 2,000 and 5,000 feet in elevation. The Mojave Desert contains the Mojave National Preserve, as well as the lowest and hottest place in North America: Death Valley at 282 ft below sea level, where the temperature surpasses 120 °F from late June to early August. Zion National Park in Utah lies at the junction of the Mojave, the Great Basin Desert, the Colorado Plateau.
Despite its aridity, the Mojave has long been a center of alfalfa production, fed by irrigation coming from groundwater and from the California Aqueduct. The Mojave is a desert of two distinct seasons. Winter months bring comfortable daytime temperatures, which drop to around 25 °F on valley floors, below 0 °F at the highest elevations. Storms moving from the Pacific Northwest can bring rain and in some places snow. More the rain shadow created by the Sierra Nevada as well as mountain ranges within the desert such as the Spring Mountains, bring only clouds and wind. In longer periods between storm systems, winter temperatures in valleys can approach 80 °F. Spring weather continues to be influenced by Pacific storms, but rainfall is more widespread and occurs less after April. By early June, it is rare for another Pacific storm to have a significant impact on the region's weather. Summer weather is dominated by heat. Temperatures on valley floors can soar above 130 °F at the lowest elevations. Low humidity, high temperatures, low pressure, draw in moisture from the Gulf of Mexico creating thunderstorms across the desert southwest known as the North American monsoon.
While the Mojave does not get nearly the amount of rainfall the Sonoran desert to the south receives, monsoonal moisture will create thunderstorms as far west as California's Central Valley from mid-June through early September. Autumn is pleasant, with one to two Pacific storm systems creating regional rain events. October is one of the sunniest months in the Mojave. After temperature, wind is the most significant weather phenomenon in the Mojave. Across the region windy days are common. During the June Gloom, cooler air can be pushed into the desert from Southern California. In Santa Ana wind events, hot air from the desert blows into the Los Angeles basin and other coastal areas. Wind farms in these areas generate power from these winds; the other major weather factor in the region is elevation. The highest peak within the Mojave is Charleston Peak at 11,918 feet, while the Badwater Basin in Death Valley is 279 feet below sea level. Accordingly and precipitation ranges wildly in all seasons across the region.
The Mojave Desert has not supported a fire regime because of low fuel loads and connectivity. However, in the last few decades, invasive annual plants such as some within the genera Bromus and Brassica have facilitated fire; this has altered many areas of the desert. At higher elevations, fire regimes are infrequent; the Mojave Desert is defined by numerous mountain ranges creating its xeric conditions. These ranges create valleys, endorheic basins, salt pans, seasonal saline lakes when precipitation is high enough. These
Autumn known as fall in American English and sometimes in Canadian English, is one of the four temperate seasons. Autumn marks the transition from summer to winter, in September or March, when the duration of daylight becomes noticeably shorter and the temperature cools considerably. One of its main features in temperate climates is the shedding of leaves from deciduous trees; some cultures regard the autumnal equinox as "mid-autumn", while others with a longer temperature lag treat it as the start of autumn. Meteorologists use a definition based on Gregorian calendar months, with autumn being September and November in the northern hemisphere, March and May in the southern hemisphere. In North America, autumn traditionally starts on September 21 and ends on December 21, it is considered to end with the winter solstice. Popular culture in the United States associates Labor Day, the first Monday in September, as the end of summer and the start of autumn; as daytime and nighttime temperatures decrease, trees shed their leaves.
In traditional East Asian solar term, autumn starts on or around 8 August and ends on or about 7 November. In Ireland, the autumn months according to the national meteorological service, Met Éireann, are September and November. However, according to the Irish Calendar, based on ancient Gaelic traditions, autumn lasts throughout the months of August and October, or a few days depending on tradition; the names of the months in Manx Gaelic are based on autumn covering August and October. In Argentina and New Zealand, autumn begins on 1 March and ends on 31 May; the word autumn comes from the ancient Etruscan root autu- and has within it connotations of the passing of the year. It was borrowed by the neighbouring Romans, became the Latin word autumnus. After the Roman era, the word continued to be used as the Old French word autompne or autumpne in Middle English, was normalised to the original Latin. In the Medieval period, there are rare examples of its use as early as the 12th century, but by the 16th century, it was in common use.
Before the 16th century, harvest was the term used to refer to the season, as it is common in other West Germanic languages to this day. However, as more people moved from working the land to living in towns, the word harvest lost its reference to the time of year and came to refer only to the actual activity of reaping, autumn, as well as fall, began to replace it as a reference to the season; the alternative word fall for the season traces its origins to old Germanic languages. The exact derivation is unclear, with the Old English fiæll or feallan and the Old Norse fall all being possible candidates. However, these words all have the meaning "to fall from a height" and are derived either from a common root or from each other; the term came to denote the season in 16th-century England, a contraction of Middle English expressions like "fall of the leaf" and "fall of the year". During the 17th century, English emigration to the British colonies in North America was at its peak, the new settlers took the English language with them.
While the term fall became obsolete in Britain, it became the more common term in North America. The name backend, a once common name for the season in Northern England, has today been replaced by the name autumn. Association with the transition from warm to cold weather, its related status as the season of the primary harvest, has dominated its themes and popular images. In Western cultures, personifications of autumn are pretty, well-fed females adorned with fruits and grains that ripen at this time. Many cultures feature autumnal harvest festivals the most important on their calendars. Still extant echoes of these celebrations are found in the autumn Thanksgiving holiday of the United States and Canada, the Jewish Sukkot holiday with its roots as a full-moon harvest festival of "tabernacles". There are the many North American Indian festivals tied to harvest of ripe foods gathered in the wild, the Chinese Mid-Autumn or Moon festival, many others; the predominant mood of these autumnal celebrations is a gladness for the fruits of the earth mixed with a certain melancholy linked to the imminent arrival of harsh weather.
This view is presented in English poet John Keats' poem To Autumn, where he describes the season as a time of bounteous fecundity, a time of'mellow fruitfulness'. In North America, while most foods are harvested during the autumn, foods associated with the season include pumpkins and apples, which are used to make the seasonal beverage apple cider. Autumn in poetry, has been associated with melancholia; the possibilities and opportunities of summer are gone, the chill of winter is on the horizon. Skies turn grey, the amount of usable daylight drops and many people turn inward, both physically and mentally, it has been referred to as an unhealthy season. Similar examples may be found in Irish poet William Butler Yeats' poem The Wild Swans at Coole where the maturing season that the poet observes symbolically represents his own ageing self. Like the natural world that he observes, he too has reached his prime and now must look forward to the inevitability of old age and death. French p
The subtropics are geographic and climate zones located between the tropics at latitude 23.5° and temperate zones north and south of the Equator. Subtropical climates are characterized by warm to hot summers and cool to mild winters with infrequent frost. Most subtropical climates fall into two basic types: humid subtropical, where rainfall is concentrated in the warmest months, dry summer climate or, where seasonal rainfall is concentrated in the cooler months. Subtropical climates can occur at high elevations within the tropics, such as in the southern end of the Mexican Plateau and in Vietnam and Taiwan. Six climate classifications use the term to help define the various temperature and precipitation regimes for the planet Earth. A great portion of the world's deserts are located within the subtropics, due to the development of the subtropical ridge. Within savanna regimes in the subtropics, a wet season is seen annually during the summer, when most of the yearly rainfall falls. Within Mediterranean climate regimes, the wet season occurs during the winter.
Areas bordering warm oceans are prone to locally heavy rainfall from tropical cyclones, which can contribute a significant percentage of the annual rainfall. Plants such as palms, mango, pistachio and avocado are grown within the subtropics; the tropics have been defined as lying between the Tropic of Cancer and Tropic of Capricorn, located at latitudes 23.45° north and south, respectively. According to the American Meteorological Society, the poleward fringe of the subtropics is located at latitudes 35° north and south, respectively. Several methods have been used to define the subtropical climate. In the Trewartha climate classification, a subtropical region should have at least eight months with a mean temperature greater than 10 °C and at least one month with a mean temperature under 18 °C. German climatologists Carl Troll and Karlheinz Paffen defined Warm temperate zones as plain and hilly lands having an average temperature of the coldest month between 2 °C and 13 °C in the Northern Hemisphere and between 6 °C and 13 °C in the Southern Hemisphere, excluding oceanic and continental climates.
According to the Troll-Paffen climate classification, there exists one large subtropical zone named the warm-temperate subtropical zone, subdivided into seven smaller areas. According to the E. Neef climate classification, the subtropical zone is divided into two parts: Rainy winters of the west sides and Eastern subtropical climate. According to the Wilhelm Lauer & Peter Frankenberg climate classification, the subtropical zone is divided into three parts: high-continental and maritime. According to the Siegmund/Frankenberg climate classification, subtropical is one of six climate zones in the world. Heating of the earth near the equator leads to large amounts of upward motion and convection along the monsoon trough or intertropical convergence zone; the upper-level divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As the air moves towards the mid-latitudes, it cools and sinks, which leads to subsidence near the 30th parallel of both hemispheres.
This circulation leads to the formation of the subtropical ridge. Many of the world's deserts are caused by these climatological high-pressure areas, located within the subtropics; this regime is known as an arid subtropical climate, located in areas adjacent to powerful cold ocean currents. Examples of this climate are the coastal areas of southern Africa, the south of the Canary Islands and the coasts of Peru and Chile; the humid subtropical climate is located on the western side of the subtropical high. Here, unstable tropical airmasses in summer bring convective overturning and frequent tropical downpours, summer is the season of peak annual rainfall. In the winter the monsoon retreats, the drier trade winds bring more stable airmass and dry weather, frequent sunny skies. Areas that have this type of subtropical climate include Australia, Southeast Asia, parts of South America, the deep south of the United States. In areas bounded by warm ocean like the southeastern United States and East Asia, tropical cyclones can contribute to local rainfall within the subtropics.
Japan receives over half of its rainfall from typhoons. The Mediterranean climate is a subtropical climate with a wet season in winter and a dry season in the summer. Regions with this type of climate include the rim lands of the Mediterranean Sea, southwestern Australia around the Perth area, parts of the west coast of South American around Santiago, the coastal areas of western Mexico, coastal California in the United States; these climates do not see hard frosts or snow, which allows plants such as palms and citrus to flourish. As one moves toward the tropical side the slight winter cool season disappears, while at the poleward threshold of the subtropics the winters become cooler; some crops which have been traditionally farmed in tropical climates, such as mango and avocado, are cultivated in the subtropics. Pest control of the crops is less difficult than within the tropics, due to the cooler winters. Tree ferns are grown within subtropical areas within the subtropics and within topography within the tropics.
Dracaena and yucca can grow within the subtropics. Tre
A high-pressure area, high or anticyclone is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, lower weight of the air outside the center. However, because the planet is rotating underneath the atmosphere, frictional forces arise as the planetary surface drags some atmosphere with it, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect, or the apparent force that arise when the observer is in a rotating frame of reference. Viewed from above this twist in wind direction is in the same direction as the rotation of the planet.
The strongest high-pressure areas are associated with cold air masses which push away out of polar regions during the winter when there is less sun to warm neighboring regions. These Highs change character and weaken once they move further over warmer water bodies. Somewhat weaker but more common are high-pressure areas caused by atmospheric subsidence, that is, areas where large masses of cooler drier air descend from an elevation of 8 to 15 km after the lower temperatures have precipitated out the water vapor. Many of the features of Highs may be understood in context of middle- or meso-scale and enduring dynamics of a planet's atmospheric circulation. For example, massive atmospheric subsidences occur as part of the descending branches of Ferrel cells and Hadley cells. Hadley cells help form the subtropical ridge, steer tropical waves and tropical cyclones across the ocean and is strongest during the summer; the subtropical ridge helps form most of the world's deserts. On English-language weather maps, high-pressure centers are identified by the letter H. Weather maps in other languages may use different letters or symbols.
The direction of wind flow around an atmospheric high-pressure area and a low-pressure area, as seen from above, depends on the hemisphere. High-pressure systems rotate clockwise in the northern Hemisphere; the scientific terms in English used to describe the weather systems generated by highs and lows were introduced in the mid-1800s by the British. The scientific theories which explain the general phenomena originated about two centuries earlier; the term cyclone was coined by Henry Piddington of the British East India Company to describe the devastating storm of December 1789 in Coringa, India. A cyclone forms around a low-pressure area. Anticyclone, the term for the kind of weather around a high-pressure area, was coined in 1877 by Francis Galton to indicate an area whose winds revolved in the opposite direction of a cyclone. In British English, the opposite direction of clockwise is referred to as anticlockwise, making the label anticyclones a logical extension. A simple rule is that for high-pressure areas, where air flows from the center outward, the coriolis force given by the earth's rotation to the air circulation is in the opposite direction of earth's apparent rotation if viewed from above the hemisphere's pole.
So, both the earth and winds around a low-pressure area rotate counter-clockwise in the northern hemisphere, clockwise in the southern. The opposite to these two cases occurs in the case of a high; these results derive from the Coriolis effect. High-pressure areas form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds known as convergence, near or above the level of non-divergence, near the 500 hPa pressure surface about midway up through the troposphere, about half the atmospheric pressure at the surface. High-pressure systems are alternatively referred to as anticyclones. On English-language weather maps, high-pressure centers are identified by the letter H in English, within the isobar with the highest pressure value. On constant pressure upper level charts, it is located within the highest height line contour.
Highs are associated with light winds at the surface and subsidence through the lower portion of the troposphere. In general, subsidence will dry out an air mass by compressional, heating. Thus, high pressure brings clear skies. During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise. At night, the absence of clouds means that outgoing longwave radiation is not absorbed, giving cooler diurnal low temperatures in all seasons; when surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the ridge, leading to widespread haze. If the low level relative humidity rises towards 100 percent overnight, fog can form. Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses. Once arctic air moves over an unfrozen ocean, the air mass modifies over the warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system.
When cold air moves
Asia is Earth's largest and most populous continent, located in the Eastern and Northern Hemispheres. It shares the continental landmass of Eurasia with the continent of Europe and the continental landmass of Afro-Eurasia with both Europe and Africa. Asia covers an area of 44,579,000 square kilometres, about 30% of Earth's total land area and 8.7% of the Earth's total surface area. The continent, which has long been home to the majority of the human population, was the site of many of the first civilizations. Asia is notable for not only its overall large size and population, but dense and large settlements, as well as vast populated regions, its 4.5 billion people constitute 60% of the world's population. In general terms, Asia is bounded on the east by the Pacific Ocean, on the south by the Indian Ocean, on the north by the Arctic Ocean; the border of Asia with Europe is a historical and cultural construct, as there is no clear physical and geographical separation between them. It has moved since its first conception in classical antiquity.
The division of Eurasia into two continents reflects East–West cultural and ethnic differences, some of which vary on a spectrum rather than with a sharp dividing line. The most accepted boundaries place Asia to the east of the Suez Canal separating it from Africa. China and India alternated in being the largest economies in the world from 1 to 1800 CE. China was a major economic power and attracted many to the east, for many the legendary wealth and prosperity of the ancient culture of India personified Asia, attracting European commerce and colonialism; the accidental discovery of a trans-Atlantic route from Europe to America by Columbus while in search for a route to India demonstrates this deep fascination. The Silk Road became the main east–west trading route in the Asian hinterlands while the Straits of Malacca stood as a major sea route. Asia has exhibited economic dynamism as well as robust population growth during the 20th century, but overall population growth has since fallen. Asia was the birthplace of most of the world's mainstream religions including Hinduism, Judaism, Buddhism, Taoism, Islam, Sikhism, as well as many other religions.
Given its size and diversity, the concept of Asia—a name dating back to classical antiquity—may have more to do with human geography than physical geography. Asia varies across and within its regions with regard to ethnic groups, environments, historical ties and government systems, it has a mix of many different climates ranging from the equatorial south via the hot desert in the Middle East, temperate areas in the east and the continental centre to vast subarctic and polar areas in Siberia. The boundary between Asia and Africa is the Red Sea, the Gulf of Suez, the Suez Canal; this makes Egypt a transcontinental country, with the Sinai peninsula in Asia and the remainder of the country in Africa. The border between Asia and Europe was defined by European academics; the Don River became unsatisfactory to northern Europeans when Peter the Great, king of the Tsardom of Russia, defeating rival claims of Sweden and the Ottoman Empire to the eastern lands, armed resistance by the tribes of Siberia, synthesized a new Russian Empire extending to the Ural Mountains and beyond, founded in 1721.
The major geographical theorist of the empire was a former Swedish prisoner-of-war, taken at the Battle of Poltava in 1709 and assigned to Tobolsk, where he associated with Peter's Siberian official, Vasily Tatishchev, was allowed freedom to conduct geographical and anthropological studies in preparation for a future book. In Sweden, five years after Peter's death, in 1730 Philip Johan von Strahlenberg published a new atlas proposing the Urals as the border of Asia. Tatishchev announced; the latter had suggested the Emba River as the lower boundary. Over the next century various proposals were made until the Ural River prevailed in the mid-19th century; the border had been moved perforce from the Black Sea to the Caspian Sea into which the Ural River projects. The border between the Black Sea and the Caspian is placed along the crest of the Caucasus Mountains, although it is sometimes placed further north; the border between Asia and the region of Oceania is placed somewhere in the Malay Archipelago.
The Maluku Islands in Indonesia are considered to lie on the border of southeast Asia, with New Guinea, to the east of the islands, being wholly part of Oceania. The terms Southeast Asia and Oceania, devised in the 19th century, have had several vastly different geographic meanings since their inception; the chief factor in determining which islands of the Malay Archipelago are Asian has been the location of the colonial possessions of the various empires there. Lewis and Wigen assert, "The narrowing of'Southeast Asia' to its present boundaries was thus a gradual process." Geographical Asia is a cultural artifact of European conceptions of the world, beginning with the Ancient Greeks, being imposed onto other cultures, an imprecise concept causing endemic contention about what it means. Asia does not correspond to the cultural borders of its various types of constituents. From the time of Herodotus a minority of geographers have rejected the three-continent system on the grounds that there is no substantial physical separation between