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149-712: Wetlands exist on every continent , except Antarctica . The water in wetlands is either freshwater , brackish or saltwater . The main types of wetland are defined based on the dominant plants and the source of the water. For example, marshes are wetlands dominated by emergent herbaceous vegetation such as reeds , cattails and sedges . Swamps are dominated by woody vegetation such as trees and shrubs (although reed swamps in Europe are dominated by reeds, not trees). Mangrove forest are wetlands with mangroves , halophytic woody plants that have evolved to tolerate salty water . Examples of wetlands classified by

298-707: A sink or a source of carbon, depending on the specific wetland. If they function as a carbon sink, they can help with climate change mitigation . However, wetlands can also be a significant source of methane emissions due to anaerobic decomposition of soaked detritus , and some are also emitters of nitrous oxide . Humans are disturbing and damaging wetlands in many ways, including oil and gas extraction , building infrastructure, overgrazing of livestock , overfishing , alteration of wetlands including dredging and draining, nutrient pollution , and water pollution . Wetlands are more threatened by environmental degradation than any other ecosystem on Earth, according to

447-574: A continent as "a large space of dry land comprehending many countries all joined together, without any separation by water. Thus Europe, Asia, and Africa is one great continent, as America is another." However, the old idea of Europe, Asia and Africa as "parts" of the world ultimately persisted with these being regarded as separate continents. By convention, continents "are understood to be large, continuous, discrete masses of land, ideally separated by expanses of water". In modern schemes with five or more recognized continents, at least one pair of continents

596-520: A continent. This divides the entire land surface of Earth into continents, regions, or quasi-continents. The criterion that each continent is a discrete landmass is commonly relaxed due to historical conventions and practical use. Of the seven most globally recognized continents, only Antarctica and Australia are completely separated from other continents by the ocean. Several continents are defined not as absolutely distinct bodies but as " more or less discrete masses of land". Africa and Asia are joined by

745-615: A discrete landmass is completely disregarded if the continuous landmass of Eurasia is classified as two separate continents (Asia and Europe). Physiographically , Europe and the Indian subcontinent are large peninsulas of the Eurasian landmass. However, Europe is considered a continent with its comparatively large land area of 10,180,000 square kilometres (3,930,000 sq mi), while the Indian subcontinent, with less than half that area,

894-529: A field study published in 2021 by researchers at Stockholm University found that they are often transferred from water to air when waves reach land, are a significant source of air pollution , and eventually get into rain. The researchers concluded that pollution may impact large areas. In 2024, a worldwide study of 45,000 groundwater samples found that 31% of samples contained levels of PFAS that were harmful to human health; these samples were taken from areas not near any obvious source of contamination. Rain

1043-401: A frequency and duration to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally included swamps, marshes, bogs, and similar areas.' For each of these definitions and others, regardless of the purpose, hydrology is emphasized (shallow waters, water-logged soils). The soil characteristics and

1192-425: A geological continent has been disputed by some geologists. However, a study conducted in 2021 found that part of the submerged continent is indeed Precambrian , twice as old as geologists had previously thought, which is further evidence that supports the idea of Zealandia being a geological continent. All seven geological continents are spatially isolated by geologic features. The term "continent" translates

1341-562: A large portion of the year. Temperatures for wetlands on the Arabian Peninsula can exceed 50 °C (122 °F) and these habitats would therefore be subject to rapid evaporation. In northeastern Siberia , which has a polar climate, wetland temperatures can be as low as −50 °C (−58 °F). Peatlands in arctic and subarctic regions insulate the permafrost , thus delaying or preventing its thawing during summer, as well as inducing its formation. The amount of precipitation

1490-483: A long duration. The final droplet size distribution is an exponential distribution . The number of droplets with diameter between d {\displaystyle d} and D + d D {\displaystyle D+dD} per unit volume of space is n ( d ) = n 0 e − d / ⟨ d ⟩ d D {\displaystyle n(d)=n_{0}e^{-d/\langle d\rangle }dD} . This

1639-550: A nearby continent to divide all the world's land into geographical regions. Under this scheme, most of the island countries and territories in the Pacific Ocean are grouped together with the continent of Australia to form the geographical region Oceania . In geology , a continent is defined as "one of Earth's major landmasses, including both dry land and continental shelves ". The geological continents correspond to seven large areas of continental crust that are found on

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1788-468: A part of South America. Apart from the current continents, the scope and meaning of the term continent includes past geological ones. Supercontinents , largely in evidence earlier in the geological record, are landmasses that comprise most of the world's cratons or continental cores. These have included Vaalbara , Kenorland , Columbia , Rodinia , Pannotia , and Pangaea . Over time, these supercontinents broke apart into large landmasses which formed

1937-460: A particular air temperature. How much water vapor a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a cloud (a group of visible tiny water or ice particles suspended above the Earth's surface) depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air

2086-410: A physical barrier such as a mountain ( orographic lift ). Conductive cooling occurs when the air comes into contact with a colder surface, usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation , either by the air or by the surface underneath. Evaporative cooling occurs when moisture

2235-543: A point are estimated by using the value of reflectivity data at individual grid points. A radar equation is then used, which is Z = A R b , {\displaystyle Z=AR^{b},} where Z represents the radar reflectivity, R represents the rainfall rate, and A and b are constants. Satellite-derived rainfall estimates use passive microwave instruments aboard polar orbiting as well as geostationary weather satellites to indirectly measure rainfall rates. If one wants an accumulated rainfall over

2384-539: A portion of the overall water cycle, which also includes atmospheric water (precipitation) and groundwater . Many wetlands are directly linked to groundwater and they can be a crucial regulator of both the quantity and quality of water found below the ground. Wetlands that have permeable substrates like limestone or occur in areas with highly variable and fluctuating water tables have especially important roles in groundwater replenishment or water recharge. Substrates that are porous allow water to filter down through

2533-530: A protective barrier to coastal shoreline. Mangroves stabilize the coastal zone from the interior and will migrate with the shoreline to remain adjacent to the boundary of the water. The main conservation benefit these systems have against storms and storm surges is the ability to reduce the speed and height of waves and floodwaters. Continent A continent is any of several large geographical regions. Continents are generally identified by convention rather than any strict criteria. A continent could be

2682-406: A rare rainfall event occurring on average once every 10 years. The rainfall will be greater and the flooding will be worse than the worst storm expected in any single year. A 100-year storm describes an extremely rare rainfall event occurring on average once in a century. The rainfall will be extreme and flooding worse than a 10-year event. The probability of an event in any year is the inverse of

2831-472: A region such as the prairie potholes of North America's northern plain, pocosins , Carolina bays and baygalls of the Southeastern US, mallines of Argentina, Mediterranean seasonal ponds of Europe and California, turloughs of Ireland, billabongs of Australia, among many others. Wetlands are found throughout the world in different climates. Temperatures vary greatly depending on the location of

2980-407: A result of this warming, monthly rainfall is about 28% greater between 32 and 64 km (20 and 40 mi) downwind of cities, compared with upwind. Some cities induce a total precipitation increase of 51%. Increasing temperatures tend to increase evaporation which can lead to more precipitation. Precipitation generally increased over land north of 30°N from 1900 through 2005 but has declined over

3129-523: A separate land. In 1507 Martin Waldseemüller published a world map, Universalis Cosmographia , which was the first to show North and South America as separate from Asia and surrounded by water. A small inset map above the main map explicitly showed for the first time the Americas being east of Asia and separated from Asia by an ocean, as opposed to just placing the Americas on the left end of

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3278-738: A single landmass or a part of a very large landmass, as in the case of Asia or Europe. Due to this, the number of continents varies; up to seven or as few as four geographical regions are commonly regarded as continents. Most English-speaking countries recognize seven regions as continents. In order from largest to smallest in area, these seven regions are Asia , Africa , North America , South America , Antarctica , Europe , and Australia . Different variations with fewer continents merge some of these regions; examples of this are merging North America and South America into America , Asia and Europe into Eurasia , and Africa, Asia, and Europe into Afro-Eurasia . Oceanic islands are occasionally grouped with

3427-419: A strong influence on wetland water chemistry, particularly in coastal wetlands and in arid and semiarid regions with large precipitation deficits. Natural salinity is regulated by interactions between ground and surface water, which may be influenced by human activity. Carbon is the major nutrient cycled within wetlands. Most nutrients, such as sulfur , phosphorus , carbon , and nitrogen are found within

3576-649: A teardrop. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 mm (0.39 in). The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water. Raindrops associated with melting hail tend to be larger than other raindrops. Intensity and duration of rainfall are usually inversely related, i.e., high-intensity storms are likely to be of short duration and low-intensity storms can have

3725-425: A tendency to break up at larger sizes. Smaller drops are called cloud droplets, and their shape is spherical. As a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow. Large rain drops become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes . Contrary to popular belief, their shape does not resemble

3874-821: A third part of the world. Division into three parts eventually came to predominate. From the Greek viewpoint, the Aegean Sea was the center of the world; Asia lay to the east, Europe to the north and west, and Africa to the south. The boundaries between the continents were not fixed. Early on, the Europe–Asia boundary was taken to run from the Black Sea along the Rioni River (known then as the Phasis ) in Georgia . Later it

4023-470: A time period, one has to add up all the accumulations from each grid box within the images during that time. Rainfall intensity is classified according to the rate of precipitation, which depends on the considered time. The following categories are used to classify rainfall intensity: Terms used for a heavy or violent rain include gully washer, trash-mover and toad-strangler. The intensity can also be expressed by rainfall erosivity R-factor or in terms of

4172-502: A very large peninsula on the fringe of a continent. Where the Americas are viewed as a single continent (America), it is divided into two subcontinents ( North America and South America ) or three ( Central America being the third). When Eurasia is regarded as a single continent, Asia and Europe are treated as subcontinents. Some areas of continental crust are largely covered by the ocean and may be considered submerged continents . Notable examples are Zealandia , emerging from

4321-514: A wetland is "an ecosystem that arises when inundation by water produces soils dominated by anaerobic and aerobic processes, which, in turn, forces the biota, particularly rooted plants, to adapt to flooding". Sometimes a precise legal definition of a wetland is required. The definition used for regulation by the United States government is: 'The term "wetlands" means those areas that are inundated or saturated by surface or ground water at

4470-980: A wetland receives varies widely according to its area. Wetlands in Wales , Scotland , and western Ireland typically receive about 1,500 mm (59 in) per year. In some places in Southeast Asia , where heavy rains occur, they can receive up to 10,000 mm (390 in). In some drier regions, wetlands exist where as little as 180 mm (7.1 in) precipitation occurs each year. Temporal variation: Surface flow may occur in some segments, with subsurface flow in other segments. Wetlands vary widely due to local and regional differences in topography , hydrology , vegetation , and other factors, including human involvement. Other important factors include fertility, natural disturbance, competition, herbivory , burial and salinity. When peat accumulates, bogs and fens arise. The most important factor producing wetlands

4619-420: A wetland system includes its plants ( flora ) and animals ( fauna ) and microbes (bacteria, fungi). The most important factor is the wetland's duration of flooding. Other important factors include fertility and salinity of the water or soils. The chemistry of water flowing into wetlands depends on the source of water, the geological material that it flows through and the nutrients discharged from organic matter in

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4768-402: A wetland) affects hydro-periods (temporal fluctuations in water levels) by controlling the water balance and water storage within a wetland. Landscape characteristics control wetland hydrology and water chemistry. The O 2 and CO 2 concentrations of water depend upon temperature , atmospheric pressure and mixing with the air (from winds or water flows). Water chemistry within wetlands

4917-558: A whole. To replace these wetland ecosystem services , enormous amounts of money would need to be spent on water purification plants, dams, levees, and other hard infrastructure, and many of the services are impossible to replace. Floodplains and closed-depression wetlands can provide the functions of storage reservoirs and flood protection. The wetland system of floodplains is formed from major rivers downstream from their headwaters . "The floodplains of major rivers act as natural storage reservoirs, enabling excess water to spread out over

5066-593: A wide area, which reduces its depth and speed. Wetlands close to the headwaters of streams and rivers can slow down rainwater runoff and spring snowmelt so that it does not run straight off the land into water courses. This can help prevent sudden, damaging floods downstream." Notable river systems that produce wide floodplains include the Nile River , the Niger river inland delta, the Zambezi River flood plain,

5215-541: Is water droplets that have condensed from atmospheric water vapor and then fall under gravity . Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants , crop irrigation , and suitable conditions for many types of ecosystems . The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts . If enough moisture and upward motion

5364-472: Is a grassland biome located in semi-arid to semi-humid climate regions of subtropical and tropical latitudes , with rainfall between 750 and 1,270 mm (30 and 50 in) a year. They are widespread on Africa, and are also found in India, the northern parts of South America, Malaysia , and Australia. The humid subtropical climate zone is where winter rainfall is associated with large storms that

5513-511: Is a time when air quality improves, freshwater quality improves, and vegetation grows significantly. Tropical cyclones , a source of very heavy rainfall, consist of large air masses several hundred miles across with low pressure at the centre and with winds blowing inward towards the centre in either a clockwise direction (southern hemisphere) or counterclockwise (northern hemisphere). Although cyclones can take an enormous toll in lives and personal property, they may be important factors in

5662-654: Is accompanied by plentiful precipitation year-round. The Mediterranean climate regime resembles the climate of the lands in the Mediterranean Basin , parts of western North America, parts of Western and South Australia , in southwestern South Africa and in parts of central Chile . The climate is characterized by hot, dry summers and cool, wet winters. A steppe is a dry grassland . Subarctic climates are cold with continuous permafrost and little precipitation. In 2022, levels of at least four perfluoroalkyl acids (PFAAs) in rain water worldwide greatly exceeded

5811-718: Is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature , or until it reaches saturation. The main ways water vapor is added to the air are wind convergence into areas of upward motion, precipitation or virga falling from above, daytime heating evaporating water from the surface of oceans, water bodies or wet land, transpiration from plants, cool or dry air moving over warmer water, and lifting air over mountains. Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Elevated portions of weather fronts (which are three-dimensional in nature) force broad areas of upward motion within

5960-563: Is also causing changes in the precipitation pattern, including wetter conditions across eastern North America and drier conditions in the tropics. Antarctica is the driest continent. The globally averaged annual precipitation over land is 715 mm (28.1 in), but over the whole Earth, it is much higher at 990 mm (39 in). Climate classification systems such as the Köppen classification system use average annual rainfall to help differentiate between differing climate regimes. Rainfall

6109-842: Is commonly referred to as the Marshall–Palmer law after the researchers who first characterized it. The parameters are somewhat temperature-dependent, and the slope also scales with the rate of rainfall ⟨ d ⟩ − 1 = 41 R − 0.21 {\displaystyle \langle d\rangle ^{-1}=41R^{-0.21}} (d in centimeters and R in millimeters per hour). Deviations can occur for small droplets and during different rainfall conditions. The distribution tends to fit averaged rainfall, while instantaneous size spectra often deviate and have been modeled as gamma distributions . The distribution has an upper limit due to droplet fragmentation. Raindrops impact at their terminal velocity , which

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6258-451: Is considered a subcontinent. The alternative view—in geology and geography—that Eurasia is a single continent results in a six-continent view of the world. Some view the separation of Eurasia into Asia and Europe as a residue of Eurocentrism : "In physical, cultural and historical diversity, China and India are comparable to the entire European landmass, not to a single European country. [...]." However, for historical and cultural reasons,

6407-410: Is derived from natural sources such as volcanoes, and wetlands (sulfate-reducing bacteria); and anthropogenic sources such as the combustion of fossil fuels , and mining where H 2 S is present. Nitric acid is produced by natural sources such as lightning, soil bacteria, and natural fires; while also produced anthropogenically by the combustion of fossil fuels and from power plants. In the past 20 years,

6556-440: Is determined by the pH , salinity , nutrients, conductivity , soil composition, hardness , and the sources of water. Water chemistry varies across landscapes and climatic regions. Wetlands are generally minerotrophic (waters contain dissolved materials from soils) with the exception of ombrotrophic bogs that are fed only by water from precipitation. Because bogs receive most of their water from precipitation and humidity from

6705-435: Is divided into several principal oceanic components by the continents and various geographic criteria. The geological definition of a continent has four criteria: high elevation relative to the ocean floor; a wide range of igneous, metamorphic and sedimentary rocks rich in silica; a crust thicker than the surrounding oceanic crust; and well-defined limits around a large enough area. The most restricted meaning of continent

6854-487: Is equally distributed through the year. Some areas with pronounced rainy seasons will see a break in rainfall mid-season when the Intertropical Convergence Zone or monsoon trough move poleward of their location during the middle of the warm season. When the wet season occurs during the warm season, or summer , rain falls mainly during the late afternoon and early evening hours. The wet season

7003-498: Is greater for larger drops due to their larger mass-to-drag ratio. At sea level and without wind, 0.5 mm (0.020 in) drizzle impacts at 2 m/s (6.6 ft/s) or 7.2 km/h (4.5 mph), while large 5 mm (0.20 in) drops impact at around 9 m/s (30 ft/s) or 32 km/h (20 mph). Rain falling on loosely packed material such as newly fallen ash can produce dimples that can be fossilized, called raindrop impressions . The air density dependence of

7152-411: Is hydrology, or flooding . The duration of flooding or prolonged soil saturation by groundwater determines whether the resulting wetland has aquatic, marsh or swamp vegetation . Other important factors include soil fertility, natural disturbance, competition, herbivory , burial, and salinity. When peat from dead plants accumulates, bogs and fens develop. Wetland hydrology is associated with

7301-507: Is inhabited by more numerous people and animals than our Europe, or Asia or Africa", the first known explicit identification of part of the Americas as a continent like the other three. Within a few years, the name "New World" began appearing as a name for South America on world maps, such as the Oliveriana (Pesaro) map of around 1504–1505. Maps of this time, though, still showed North America connected to Asia and showed South America as

7450-430: Is joined by land in some fashion. The criterion "large" leads to arbitrary classification: Greenland , with a surface area of 2,166,086 square kilometres (836,330 sq mi), is only considered the world's largest island, while Australia , at 7,617,930 square kilometres (2,941,300 sq mi), is deemed the smallest continent. Earth's major landmasses all have coasts on a single, continuous World Ocean , which

7599-497: Is measured in units of length per unit time, typically in millimeters per hour, or in countries where imperial units are more common, inches per hour. The "length", or more accurately, "depth" being measured is the depth of rain water that would accumulate on a flat, horizontal and impermeable surface during a given amount of time, typically an hour. One millimeter of rainfall is the equivalent of one liter of water per square meter. The standard way of measuring rainfall or snowfall

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7748-408: Is measured using rain gauges . Rainfall amounts can be estimated by weather radar . Air contains water vapor, and the amount of water in a given mass of dry air, known as the mixing ratio , is measured in grams of water per kilogram of dry air (g/kg). The amount of moisture in the air is also commonly reported as relative humidity ; which is the percentage of the total water vapor air can hold at

7897-420: Is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands . In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On

8046-424: Is referred to as banded structure. Rainbands in advance of warm occluded fronts and warm fronts are associated with weak upward motion, and tend to be wide and stratiform in nature. Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes . Rainbands associated with cold fronts can be warped by mountain barriers perpendicular to the front's orientation due to

8195-612: Is that of a continuous area of land or mainland, with the coastline and any land boundaries forming the edge of the continent. In this sense, the term continental Europe (sometimes referred to in Britain as "the Continent") is used to refer to mainland Europe, excluding islands such as Great Britain , Iceland , Ireland , and Malta , while the term continent of Australia may refer to the mainland of Australia , excluding New Guinea , Tasmania , and other nearby islands. Similarly,

8344-561: Is the expected amount of liquid precipitation accumulated over a specified time period over a specified area. A QPF will be specified when a measurable precipitation type reaching a minimum threshold is forecast for any hour during a QPF valid period. Precipitation forecasts tend to be bound by synoptic hours such as 0000, 0600, 1200 and 1800  GMT . Terrain is considered in QPFs by use of topography or based upon climatological precipitation patterns from observations with fine detail. Starting in

8493-445: Is the only place in the world where both crocodiles and alligators coexist. The saltwater crocodile inhabits estuaries and mangroves. Snapping turtles also inhabit wetlands. Birds , particularly waterfowl and waders use wetlands extensively. Mammals of wetlands include numerous small and medium-sized species such as voles , bats , muskrats and platypus in addition to large herbivorous and apex predator species such as

8642-411: Is the presence of a thick layer of air aloft which is above the melting point of water, which melts the frozen precipitation well before it reaches the ground. If there is a shallow near-surface layer that is below freezing, freezing rain (rain which freezes on contact with surfaces in subfreezing environments) will result. Hail becomes an increasingly infrequent occurrence when the freezing level within

8791-440: Is the standard rain gauge, which can be found in 100-mm (4-in) plastic and 200-mm (8-in) metal varieties. The inner cylinder is filled by 25 mm (0.98 in) of rain, with overflow flowing into the outer cylinder. Plastic gauges have markings on the inner cylinder down to 0.25 mm (0.0098 in) resolution, while metal gauges require use of a stick designed with the appropriate 0.25 mm (0.0098 in) markings. After

8940-399: Is to cool it. The dew point is the temperature to which a parcel must be cooled in order to become saturated. There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands. The air can rise due to convection , large-scale atmospheric motions, or

9089-407: Is usually considered an island of Africa, but its divergent evolution has caused it to be referred to as "the eighth continent" from a biological perspective. Geologists use four key attributes to define a continent: With the addition of Zealandia in 2017, Earth currently has seven recognized geological continents: Due to a seeming lack of Precambrian cratonic rocks, Zealandia's status as

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9238-543: Is usually saturated with water". More precisely, wetlands are areas where "water covers the soil , or is present either at or near the surface of the soil all year or for varying periods of time during the year, including during the growing season". A patch of land that develops pools of water after a rain storm would not necessarily be considered a "wetland", even though the land is wet. Wetlands have unique characteristics: they are generally distinguished from other water bodies or landforms based on their water level and on

9387-458: The continental United States refers to "the 49 States (including Alaska but excluding Hawaii ) located on the continent of North America, and the District of Columbia ." From the perspective of geology or physical geography , continent may be extended beyond the confines of continuous dry land to include the shallow, submerged adjacent area (the continental shelf ) and the islands on

9536-666: The Black Sea . The names were first applied just to lands near the coast and only later extended to include the hinterlands . But the division was only carried through to the end of navigable waterways and "... beyond that point the Hellenic geographers never succeeded in laying their finger on any inland feature in the physical landscape that could offer any convincing line for partitioning an indivisible Eurasia ..." Ancient Greek thinkers subsequently debated whether Africa (then called Libya ) should be considered part of Asia or

9685-589: The Great Basin and Mojave Deserts . The wet, or rainy, season is the time of year, covering one or more months, when most of the average annual rainfall in a region falls. The term green season is also sometimes used as a euphemism by tourist authorities. Areas with wet seasons are dispersed across portions of the tropics and subtropics . Savanna climates and areas with monsoon regimes have wet summers and dry winters. Tropical rainforests technically do not have dry or wet seasons, since their rainfall

9834-613: The Isthmus of Suez , and North America and South America by the Isthmus of Panama . In both cases, there is no complete separation of these landmasses by water (disregarding the Suez Canal and the Panama Canal , which are both narrow and shallow, as well as human-made). Both of these isthmuses are very narrow compared to the bulk of the landmasses they unite. North America and South America are treated as separate continents in

9983-503: The Millennium Ecosystem Assessment from 2005. Methods exist for assessing wetland ecological health . These methods have contributed to wetland conservation by raising public awareness of the functions that wetlands can provide. Since 1971, work under an international treaty seeks to identify and protect " wetlands of international importance ." A simplified definition of wetland is "an area of land that

10132-456: The Pacific Ocean , as well as the continent of Australia . Zealandia (a submerged continent ) has been called the eighth continent. The following table provides areas given by Encyclopædia Britannica for each continent in accordance with the seven-continent model, including Australasia along with Melanesia , Micronesia , and Polynesia as parts of Oceania. It also provides populations of continents according to 2021 estimates by

10281-584: The Pleistocene ice ages , greater areas of the continental shelf were exposed as dry land, forming land bridges between Tasmania and the Australian mainland . At those times, Australia and New Guinea were a single, continuous continent known as Sahul . Likewise, Afro-Eurasia and the Americas were joined by the Bering Land Bridge . Other islands, such as Great Britain , were joined to

10430-531: The Sundarbans in the Ganges - Brahmaputra delta. Wetlands contribute many ecosystem services that benefit people. These include for example water purification , stabilization of shorelines, storm protection and flood control . In addition, wetlands also process and condense carbon (in processes called carbon fixation and sequestration ), and other nutrients and water pollutants . Wetlands can act as

10579-719: The United Nations Statistics Division based on the United Nations geoscheme , which includes all of Egypt (including the Isthmus of Suez and the Sinai Peninsula ) as a part of Africa, all of Armenia, Azerbaijan, Cyprus, Georgia, Indonesia, Kazakhstan, and Turkey (including East Thrace ) as parts of Asia, all of Russia (including Siberia ) as a part of Europe, all of Panama and the United States (including Hawaii ) as parts of North America, and all of Chile (including Easter Island ) as

10728-443: The atmosphere , their water usually has low mineral ionic composition. In contrast, wetlands fed by groundwater or tides have a higher concentration of dissolved nutrients and minerals. Fen peatlands receive water both from precipitation and ground water in varying amounts so their water chemistry ranges from acidic with low levels of dissolved minerals to alkaline with high accumulation of calcium and magnesium . Salinity has

10877-572: The beavers , coypu , swamp rabbit , Florida panther , jaguar , and moose . Wetlands attract many mammals due to abundant seeds, berries, and other vegetation as food for herbivores, as well as abundant populations of invertebrates, small reptiles and amphibians as prey for predators. Invertebrates of wetlands include aquatic insects such as dragonflies , aquatic bugs and beetles , midges, mosquitos , crustaceans such as crabs, crayfish, shrimps, microcrustaceans, mollusks like clams, mussels, snails and worms. Invertebrates comprise more than half of

11026-404: The leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass . The movement of the monsoon trough , or Intertropical Convergence Zone , brings rainy seasons to savannah climes . The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming

11175-422: The tectonic plates , but exclude small continental fragments such as Madagascar that are generally referred to as microcontinents . Continental crust is only known to exist on Earth. The idea of continental drift gained recognition in the 20th century. It postulates that the current continents formed from the breaking up of a supercontinent ( Pangaea ) that formed hundreds of millions of years ago. From

11324-409: The tropics appears to be convective; however, it has been suggested that stratiform precipitation also occurs. Graupel and hail indicate convection. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts , squall lines , and warm fronts. Orographic precipitation occurs on the windward side of mountains and is caused by

11473-455: The westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and from occasional tropical cyclones. Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degrees away from the equator. An oceanic (or maritime) climate is typically found along the west coasts at the middle latitudes of all the world's continents, bordering cool oceans, as well as southeastern Australia, and

11622-492: The "subcontinents" concerned are on different tectonic plates from the rest of the continent, providing a geological justification for the terminology. Greenland , generally considered the world's largest island on the northeastern periphery of the North American Plate , is sometimes referred to as a subcontinent. This is a significant departure from the more conventional view of a subcontinent as comprising

11771-638: The 16th century the English noun continent was derived from the term continent land , meaning continuous or connected land and translated from the Latin terra continens . The noun was used to mean "a connected or continuous tract of land" or mainland . It was not applied only to very large areas of land—in the 17th century, references were made to the continents (or mainlands) of the Isle of Man , Ireland and Wales and in 1745 to Sumatra . The word continent

11920-533: The Americas, Columbus never believed he had reached a new continent—he always thought it was part of Asia. In 1501, Amerigo Vespucci and Gonçalo Coelho attempted to sail around what they considered the southern end of the Asian mainland into the Indian Ocean , passing through Fernando de Noronha . After reaching the coast of Brazil , they sailed along the coast of South America much farther south than Asia

12069-697: The Black Sea and the Caspian Sea , and the border between Asia and Africa at the isthmus between the Red Sea and the mouth of Lake Bardawil on the Mediterranean Sea . The Roman author Pliny the Elder , writing in the 1st century CE, stated that "The whole globe is divided into three parts, Europe, Asia, and Africa", adding: I shall first then speak of Europe, the foster-mother of that people which has conquered all other nations, and itself by far

12218-691: The Catholic church, began to associate itself with the concept of Europe. Through the Roman period and the Middle Ages , a few writers took the Isthmus of Suez as the boundary between Asia and Africa, but most writers continued to consider it the Nile or the western border of Egypt (Gibbon). In the Middle Ages, the world was usually portrayed on T and O maps , with the T representing the waters dividing

12367-461: The EPA's lifetime drinking water health advisories as well as comparable Danish, Dutch, and European Union safety standards, leading to the conclusion that "the global spread of these four PFAAs in the atmosphere has led to the planetary boundary for chemical pollution being exceeded". It had been thought that PFAAs would eventually end up in the oceans, where they would be diluted over decades, but

12516-404: The Earth's atmosphere which form clouds decks such as altostratus or cirrostratus . Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions. Coalescence occurs when water droplets fuse to create larger water droplets. Air resistance typically causes

12665-446: The Earth, the text claims that the Earth has seven continents and Lord Vishnu Measured the entire universe from his first foot from the land of Earth which has 7 continents. ato devā avantu no yato viṣṇurvicakrame | pṛthivyāḥ saptadhāmabhiḥ || idaṃ viṣṇurvi cakrame tredhā ni dadhe padam | samūḷhamasya pāṃsure || trīṇi padā vi cakrame viṣṇurghopā adābhyaḥ | ato dharmāṇi dhārayan || The Gods be gracious unto us even from

12814-619: The Greek word ἤπειρος , meaning "landmass, terra firma", the proper name of Epirus and later especially used for Asia (i.e. Asia Minor ). The first distinction between continents was made by ancient Greek mariners who gave the names Europe and Asia to the lands on either side of the waterways of the Aegean Sea , the Dardanelles strait, the Sea of Marmara , the Bosporus strait and

12963-678: The Internet, such as CoCoRAHS or GLOBE. If a network is not available in the area where one lives, the nearest local weather or met office will likely be interested in the measurement. One of the main uses of weather radar is to be able to assess the amount of precipitations fallen over large basins for hydrological purposes. For instance, river flood control , sewer management and dam construction are all areas where planners use rainfall accumulation data. Radar-derived rainfall estimates complement surface station data which can be used for calibration. To produce radar accumulations, rain rates over

13112-889: The Northeast and Midwest, which in the past decade, have seen 31 and 16 percent more heavy downpours compared to the 1950s. Rhode Island is the state with the largest increase, 104%. McAllen, Texas is the city with the largest increase, 700%. Heavy downpour in the analysis are the days where total precipitation exceeded the top one percent of all rain and snow days during the years 1950–2014. The most successful attempts at influencing weather involve cloud seeding , which include techniques used to increase winter precipitation over mountains and suppress hail . Rainbands are cloud and precipitation areas which are significantly elongated. Rainbands can be stratiform or convective , and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation

13261-639: The Okavango River inland delta, the Kafue River flood plain, the Lake Bangweulu flood plain (Africa), Mississippi River (US), Amazon River (South America), Yangtze River (China), Danube River (Central Europe) and Murray-Darling River (Australia). Groundwater replenishment can be achieved for example by marsh , swamp , and subterranean karst and cave hydrological systems. The surface water visibly seen in wetlands only represents

13410-791: The US, the best known classifications are the Cowardin classification system and the hydrogeomorphic (HGM) classification system. The Cowardin system includes five main types of wetlands: marine (ocean-associated), estuarine (mixed ocean- and river-associated), riverine (within river channels), lacustrine (lake-associated) and palustrine (inland nontidal habitats). Peatlands are a unique kind of wetland where lush plant growth and slow decay of dead plants (under anoxic conditions) results in organic peat accumulating; bogs, fens, and mires are different names for peatlands. Variations of names for wetland systems: Some wetlands have localized names unique to

13559-608: The United States' commercial fish and shellfish stocks depend solely on estuaries to survive. Amphibians such as frogs and salamanders need both terrestrial and aquatic habitats in which to reproduce and feed. Because amphibians often inhabit depressional wetlands like prairie potholes and Carolina bays, the connectivity among these isolated wetlands is an important control of regional populations. While tadpoles feed on algae, adult frogs forage on insects. Frogs are sometimes used as an indicator of ecosystem health because their thin skin permits absorption of nutrients and toxins from

13708-405: The atmosphere exceeds 3,400 m (11,000 ft) above ground level. Convective rain , or showery precipitation, occurs from convective clouds (e.g., cumulonimbus or cumulus congestus ). It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in

13857-517: The concentrations of nitric and sulfuric acid has decreased in presence of rainwater, which may be due to the significant increase in ammonium (most likely as ammonia from livestock production), which acts as a buffer in acid rain and raises the pH. The Köppen classification depends on average monthly values of temperature and precipitation. The most commonly used form of the Köppen classification has five primary types labeled A through E. Specifically,

14006-477: The concept of a continent may go beyond the continental shelf to include oceanic islands and continental fragments. In this way, Iceland is considered a part of Europe, and Madagascar a part of Africa. Extrapolating the concept to its extreme, some geographers group the Australian continental landmass with other islands in the Pacific Ocean into Oceania , which is usually considered a region rather than

14155-612: The contiguous United States, total annual precipitation increased at an average rate of 6.1 percent since 1900, with the greatest increases within the East North Central climate region (11.6 percent per century) and the South (11.1 percent). Hawaii was the only region to show a decrease (−9.25 percent). Analysis of 65 years of United States of America rainfall records show the lower 48 states have an increase in heavy downpours since 1950. The largest increases are in

14304-647: The course of the week, the likelihood of rain increases: it peaks by Saturday, after five days of weekday pollution has been built up. In heavily populated areas that are near the coast, such as the United States' Eastern Seaboard , the effect can be dramatic: there is a 22% higher chance of rain on Saturdays than on Mondays. The urban heat island effect warms cities 0.6 to 5.6 °C (33.1 to 42.1 °F) above surrounding suburbs and rural areas. This extra heat leads to greater upward motion, which can induce additional shower and thunderstorm activity. Rainfall rates downwind of cities are increased between 48% and 116%. Partly as

14453-412: The crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature difference between cloud and ground level, these ice crystals may melt as they fall and become rain. Raindrops have sizes ranging from 0.1 to 9 mm (0.0039 to 0.3543 in) mean diameter but develop

14602-559: The descending and generally warming, leeward side where a rain shadow is observed. In Hawaii , Mount Waiʻaleʻale , on the island of Kauai, is notable for its extreme rainfall, as it is amongst the places in the world with the highest levels of rainfall, with 9,500 mm (373 in). Systems known as Kona storms affect the state with heavy rains between October and April. Local climates vary considerably on each island due to their topography, divisible into windward ( Koʻolau ) and leeward ( Kona ) regions based upon location relative to

14751-405: The division into three of what is really a single landmass, a debate that continues nearly two and a half millennia later. Herodotus believed Europe to be larger (at least in width) than the other two continents: I wonder, then, at those who have mapped out and divided the world into Libya, Asia, and Europe; for the difference between them is great, seeing that in length Europe stretches along both

14900-573: The earth'. A. Glucklich adds that 'In the Matsya Purana , for instance, there is a seven-part map of the world ... [it has] one centre, where an immense mountain – Mount Meru (or Maha Meru, Great Meru) – stands ... The continents encircle the mountain in seven concentric circles ... It seems clear that the Himalayas were the approximate location of Mt. Meru and the text is clear that the earth has seven continents. Rain storm Rain

15049-502: The following areas: According to the Ramsar Convention: The economic worth of the ecosystem services provided to society by intact, naturally functioning wetlands is frequently much greater than the perceived benefits of converting them to 'more valuable' intensive land use – particularly as the profits from unsustainable use often go to relatively few individuals or corporations, rather than being shared by society as

15198-511: The formation of a low-level barrier jet . Bands of thunderstorms can form with sea breeze and land breeze boundaries if enough moisture is present. If sea breeze rainbands become active enough just ahead of a cold front, they can mask the location of the cold front itself. Once a cyclone occludes an occluded front (a trough of warm air aloft) will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also termed

15347-482: The front is unstable enough for convection. Banding within the comma head precipitation pattern of an extratropical cyclone can yield significant amounts of rain. Behind extratropical cyclones during fall and winter, rainbands can form downwind of relative warm bodies of water such as the Great Lakes . Downwind of islands, bands of showers and thunderstorms can develop due to low-level wind convergence downwind of

15496-400: The ground. This is termed virga and is more often seen in hot and dry climates. Stratiform (a broad shield of precipitation with a relatively similar intensity) and dynamic precipitation (convective precipitation which is showery in nature with large changes in intensity over short distances) occur as a consequence of slow ascent of air in synoptic systems (on the order of cm/s), such as in

15645-552: The higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desert-like climate just downwind across western Argentina. The Sierra Nevada range creates the same effect in North America forming

15794-438: The inner cylinder is filled, the amount inside it is discarded, then filled with the remaining rainfall in the outer cylinder until all the fluid in the outer cylinder is gone, adding to the overall total until the outer cylinder is empty. Other types of gauges include the popular wedge gauge (the cheapest rain gauge and most fragile), the tipping bucket rain gauge, and the weighing rain gauge. For those looking to measure rainfall

15943-466: The interface between truly terrestrial ecosystems and aquatic systems, making them inherently different from each other, yet highly dependent on both." In environmental decision-making, there are subsets of definitions that are agreed upon to make regulatory and policy decisions. Under the Ramsar international wetland conservation treaty , wetlands are defined as follows: An ecological definition of

16092-422: The island edges. Offshore California , this has been noted in the wake of cold fronts. Rainbands within tropical cyclones are curved in orientation. Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm . The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity. The phrase acid rain

16241-460: The known animal species in wetlands, and are considered the primary food web link between plants and higher animals (such as fish and birds). Depending on a wetland's geographic and topographic location, the functions it performs can support multiple ecosystem services , values, or benefits. United Nations Millennium Ecosystem Assessment and Ramsar Convention described wetlands as a whole to be of biosphere significance and societal importance in

16390-490: The mainlands of their continents. At that time, there were just three discrete landmasses in the world: Africa - Eurasia - America , Antarctica , and Australia - New Guinea ( Sahul ). There are several ways of distinguishing the continents: In the English-speaking countries, geographers often use the term Oceania to denote a geographical region which includes most of the island countries and territories in

16539-429: The map and Asia on the right end. In the accompanying book Cosmographiae Introductio , Waldseemüller noted that the earth is divided into four parts, Europe, Asia, Africa, and the fourth part, which he named "America" after Amerigo Vespucci's first name. On the map, the word "America" was placed on part of South America. The Sanskrit text Rig Veda often dated 1500 BCE has the earliest mention of seven continents in

16688-427: The maximum raindrop diameter together with fossil raindrop imprints has been used to constrain the density of the air 2.7 billion years ago. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater . The METAR code for rain is RA, while the coding for rain showers is SHRA. In certain conditions, precipitation may fall from a cloud but then evaporate or sublime before reaching

16837-549: The mid-17th century, Peter Heylin wrote in his Cosmographie that "A Continent is a great quantity of Land, not separated by any Sea from the rest of the World, as the whole Continent of Europe, Asia, Africa." In 1727, Ephraim Chambers wrote in his Cyclopædia, "The world is ordinarily divided into two grand continents: the Old and the New ." And in his 1752 atlas, Emanuel Bowen defined

16986-543: The most beauteous portion of the earth. Indeed, many persons have, not without reason, considered it, not as a third part only of the earth, but as equal to all the rest, looking upon the whole of our globe as divided into two parts only, by a line drawn from the river Tanais to the Straits of Gades. Following the fall of the Western Roman Empire , the culture that developed in its place , linked to Latin and

17135-428: The most inexpensively, a can that is cylindrical with straight sides will act as a rain gauge if left out in the open, but its accuracy will depend on what ruler is used to measure the rain with. Any of the above rain gauges can be made at home, with enough know-how. When a precipitation measurement is made, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through

17284-462: The number of heavy precipitation events over many areas during the past century, as well as an increase since the 1970s in the prevalence of droughts—especially in the tropics and subtropics. Changes in precipitation and evaporation over the oceans are suggested by the decreased salinity of mid- and high-latitude waters (implying more precipitation), along with increased salinity in lower latitudes (implying less precipitation and/or more evaporation). Over

17433-475: The ocean primarily in New Zealand and New Caledonia , and the almost completely submerged Kerguelen Plateau in the southern Indian Ocean . Some islands lie on sections of continental crust that have rifted and drifted apart from a main continental landmass. While not considered continents because of their relatively small size, they may be considered microcontinents . Madagascar, the largest example,

17582-455: The others together, and it appears to me to be wider beyond all comparison. Eratosthenes , in the 3rd century BCE, noted that some geographers divided the continents by rivers (the Nile and the Don), thus considering them "islands". Others divided the continents by isthmuses , calling the continents "peninsulas". These latter geographers set the border between Europe and Asia at the isthmus between

17731-479: The place whence Vishnu strode Through the seven regions of the earth! Through all this world strode Vishnu; thrice his foot he planted, and the whole Was gathered in his footstep's dust. Vishnu, the Guardian, he whom none deceiveth , made three steps; thenceforth Establishing his high decrees. In regard to the above-quoted verses, it is commonly accepted that there are Seven Continents or 'regions of

17880-830: The plants and animals controlled by the wetland hydrology are often additional components of the definitions. Wetlands can be tidal (inundated by tides) or non-tidal. The water in wetlands is either freshwater , brackish , saline , or alkaline . There are four main kinds of wetlands – marsh , swamp , bog , and fen (bogs and fens being types of peatlands or mires ). Some experts also recognize wet meadows and aquatic ecosystems as additional wetland types. Sub-types include mangrove forests , carrs , pocosins , floodplains , peatlands, vernal pools , sinks , and many others. The following three groups are used within Australia to classify wetland by type: Marine and coastal zone wetlands, inland wetlands and human-made wetlands. In

18029-478: The precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions. Areas in their path can receive a year's worth of rainfall from a tropical cyclone passage. The fine particulate matter produced by car exhaust and other human sources of pollution forms cloud condensation nuclei leads to the production of clouds and increases the likelihood of rain. As commuters and commercial traffic cause pollution to build up over

18178-495: The present continents. Certain parts of continents are recognized as subcontinents, especially the large peninsulas separated from the main continental landmass by geographical features. The most widely recognized example is the Indian subcontinent . The Arabian Peninsula , Southern Africa , the Southern Cone of South America, and Alaska in North America might be considered further examples. In many of these cases,

18327-579: The primary types are A, tropical; B, dry; C, mild mid-latitude; D, cold mid-latitude; and E, polar. The five primary classifications can be further divided into secondary classifications such as rain forest , monsoon , tropical savanna , humid subtropical , humid continental , oceanic climate , Mediterranean climate , steppe , subarctic climate , tundra , polar ice cap , and desert . Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1,750 and 2,000 mm (69 and 79 in). A tropical savanna

18476-399: The rainfall time-structure n-index . The average time between occurrences of an event with a specified intensity and duration is called the return period . The intensity of a storm can be predicted for any return period and storm duration, from charts based on historic data for the location. The return period is often expressed as an n -year event. For instance, a 10-year storm describes

18625-428: The return period (assuming the probability remains the same for each year). For instance, a 10-year storm has a probability of occurring of 10 percent in any given year, and a 100-year storm occurs with a 1 percent probability in a year. As with all probability events, it is possible, though improbable, to have multiple 100-year storms in a single year. The Quantitative Precipitation Forecast (abbreviated QPF)

18774-442: The rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds ), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind ) on

18923-552: The seven-continent model. However, they may also be viewed as a single continent known as America . This viewpoint was common in the United States until World War II , and remains prevalent in some Asian six-continent models. The single American continent model remains a common view in European countries like France, Greece, Hungary, Italy, Malta, Portugal, Spain, Latin American countries and some Asian countries. The criterion of

19072-415: The shelf ( continental islands ), as they are structurally part of the continent. From this perspective, the edge of the continental shelf is the true edge of the continent, as shorelines vary with changes in sea level. In this sense the islands of Great Britain and Ireland are part of Europe, while Australia and the island of New Guinea together form a continent. Taken to its limit, this view could support

19221-428: The soil and underlying rock into aquifers which are the source of much of the world's drinking water . Wetlands can also act as recharge areas when the surrounding water table is low and as a discharge zone when it is high. Mangroves , coral reefs , salt marsh can help with shoreline stabilization and storm protection. Tidal and inter-tidal wetland systems protect and stabilize coastal zones. Coral reefs provide

19370-533: The soil of wetlands. Anaerobic and aerobic respiration in the soil influences the nutrient cycling of carbon, hydrogen, oxygen, and nitrogen, and the solubility of phosphorus thus contributing to the chemical variations in its water. Wetlands with low pH and saline conductivity may reflect the presence of acid sulfates and wetlands with average salinity levels can be heavily influenced by calcium or magnesium. Biogeochemical processes in wetlands are determined by soils with low redox potential. The life forms of

19519-410: The soils and plants at higher elevations. Plants and animals may vary within a wetland seasonally or in response to flood regimes. There are four main groups of hydrophytes that are found in wetland systems throughout the world. Submerged wetland vegetation can grow in saline and fresh-water conditions. Some species have underwater flowers, while others have long stems to allow the flowers to reach

19668-683: The sources of water include tidal wetlands , where the water source is ocean tides ); estuaries , water source is mixed tidal and river waters; floodplains , water source is excess water from overflowed rivers or lakes; and bogs and vernal ponds , water source is rainfall or meltwater . The world's largest wetlands include the Amazon River basin , the West Siberian Plain , the Pantanal in South America, and

19817-406: The spatial and temporal dispersion, flow, and physio-chemical attributes of surface and ground waters. Sources of hydrological flows into wetlands are predominantly precipitation , surface water (saltwater or freshwater), and groundwater. Water flows out of wetlands by evapotranspiration , surface flows and tides , and subsurface water outflow. Hydrodynamics (the movement of water through and from

19966-654: The surface of the water. When trees and shrubs comprise much of the plant cover in saturated soils, those areas in most cases are called swamps . The upland boundary of swamps is determined partly by water levels. This can be affected by dams Some swamps can be dominated by a single species, such as silver maple swamps around the Great Lakes . Others, like those of the Amazon basin , have large numbers of different tree species. Other examples include cypress ( Taxodium ) and mangrove swamps. Many species of fish are highly dependent on wetland ecosystems. Seventy-five percent of

20115-495: The surface. Submerged species provide a food source for native fauna, habitat for invertebrates, and also possess filtration capabilities. Examples include seagrasses and eelgrass . Floating water plants or floating vegetation are usually small, like those in the Lemnoideae subfamily (duckweeds). Emergent vegetation like the cattails ( Typha spp.), sedges ( Carex spp.) and arrow arum ( Peltandra virginica ) rise above

20264-463: The surrounding environment resulting in increased extinction rates in unfavorable and polluted environmental conditions. Reptiles such as snakes , lizards , turtles , alligators and crocodiles are common in wetlands of some regions. In freshwater wetlands of the Southeastern US, alligators are common and a freshwater species of crocodile occurs in South Florida. The Florida Everglades

20413-651: The three continents. By the middle of the 18th century, "the fashion of dividing Asia and Africa at the Nile, or at the Great Catabathmus [the boundary between Egypt and Libya ] farther west, had even then scarcely passed away". Christopher Columbus sailed across the Atlantic Ocean to the Caribbean in 1492, sparking a period of European exploration of the Americas . But despite four voyages to

20562-492: The tropics since the 1970s. Globally there has been no statistically significant overall trend in precipitation over the past century, although trends have varied widely by region and over time. Eastern portions of North and South America, northern Europe, and northern and central Asia have become wetter. The Sahel, the Mediterranean, southern Africa and parts of southern Asia have become drier. There has been an increase in

20711-436: The types of plants that live within them. Specifically, wetlands are characterized as having a water table that stands at or near the land surface for a long enough period each year to support aquatic plants . A more concise definition is a community composed of hydric soil and hydrophytes . Wetlands have also been described as ecotones , providing a transition between dry land and water bodies. Wetlands exist "...at

20860-543: The vicinity of cold fronts and near and poleward of surface warm fronts . Similar ascent is seen around tropical cyclones outside the eyewall , and in comma-head precipitation patterns around mid-latitude cyclones . A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually, their passage is associated with a drying of the air mass. Occluded fronts usually form around mature low-pressure areas. What separates rainfall from other precipitation types, such as ice pellets and snow,

21009-479: The view of Europe as a separate continent continues in almost all categorizations. If continents are defined strictly as discrete landmasses, embracing all the contiguous land of a body, then Africa, Asia, and Europe form a single continent which may be referred to as Afro-Eurasia . Combined with the consolidation of the Americas, this would produce a four-continent model consisting of Afro-Eurasia, America, Antarctica, and Australia. When sea levels were lower during

21158-471: The view that there are only three continents: Antarctica, Australia-New Guinea, and a single mega-continent which joins Afro-Eurasia and America via the contiguous continental shelf in and around the Bering Sea. The vast size of the latter compared to the first two might even lead some to say it is the only continent, the others being more comparable to Greenland or New Zealand. As a cultural construct ,

21307-408: The warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The front creates the portion of an occluded cyclone known as its comma head , due to the comma -like shape of the mid-tropospheric cloudiness that accompanies the feature. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along

21456-555: The water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing (in their top) and is also known as the warm rain process. In clouds below freezing, when ice crystals gain enough mass they begin to fall. This generally requires more mass than coalescence when occurring between

21605-512: The wetland. Many of the world's wetlands are in the temperate zones , midway between the North or South Poles and the equator. In these zones, summers are warm and winters are cold, but temperatures are not extreme. In subtropical zone wetlands, such as along the Gulf of Mexico , average temperatures might be 11 °C (52 °F). Wetlands in the tropics are subjected to much higher temperatures for

21754-558: Was first used by Scottish chemist Robert Augus Smith in 1852. The pH of rain varies, especially due to its origin. On America's East Coast, rain that is derived from the Atlantic Ocean typically has a pH of 5.0–5.6; rain that comes across the continental from the west has a pH of 3.8–4.8; and local thunderstorms can have a pH as low as 2.0. Rain becomes acidic primarily due to the presence of two strong acids, sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ). Sulfuric acid

21903-438: Was known to extend, confirming that this was a land of continental proportions. On return to Europe, an account of the voyage, called Mundus Novus ("New World"), was published under Vespucci's name in 1502 or 1503, although it seems that it had additions or alterations by another writer. Regardless of who penned the words, Mundus Novus credited Vespucci with saying, "I have discovered a continent in those southern regions that

22052-417: Was used in translating Greek and Latin writings about the three "parts" of the world, although in the original languages no word of exactly the same meaning as continent was used. While continent was used on the one hand for relatively small areas of continuous land, on the other hand geographers again raised Herodotus's query about why a single large landmass should be divided into separate continents. In

22201-685: Was viewed as running from the Black Sea through Kerch Strait , the Sea of Azov and along the Don River (known then as the Tanais ) in Russia . The boundary between Asia and Africa was generally taken to be the Nile River. Herodotus in the 5th century BCE objected to the whole of Egypt being split between Asia and Africa ("Libya") and took the boundary to lie along the western border of Egypt, regarding Egypt as part of Asia. He also questioned

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