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Tree-line

Tree-line

Tree-line or timberline is the edge of the habitat at which trees are capable of growing. Beyond the tree-line, they are unable to grow due to inappropriate environmental conditions. There are several types:
- Arctic tree-line The furthest north in the Northern Hemisphere that trees can grow; further north, it is too cold to sustain trees.
- Antarctic tree-line The furthest south in the Southern Hemisphere that trees can grow; further south, it is too cold to sustain trees.
- Alpine tree-line The highest elevation that trees can grow on mountains; higher up, it is too cold to sustain trees.
- Exposure tree-line On coasts, and on isolated mountains, the tree-line is often much lower than in corresponding altitudes inland and in larger, more complex mountain systems, because high wind speeds adversely affect tree growth.
- Desert tree-line The driest places that trees can grow; drier desert areas having insufficient rainfall to sustain trees.
- Wetland tree-line The wettest ground on the margins of muskegs and bogs that trees can grow in, below which the ground is too saturated with water, excluding oxygen from the soil that tree roots need to grow. However no such line exists for swamps, where trees, such as Bald cypress and the many mangrove species, are adapted to growing in permanently water-logged soil. At tree-line, tree growth is often very stunted, affected by wind, with the last trees forming low, dense matted bushes. These are known as krummholz, from the German for 'twisted wood'. The tree line, like many other natural lines (lake boundaries, for example), looks sharp from a distance, but upon sufficiently close inspection, it becomes a more gradual transition. Trees grow shorter towards the inhospitable climate until they simply stop growing. The climate above the tree-line is called an alpine climate. Please see that article for more details on what climatic factors cause trees to fail to grow.

Typical tree-line species

Some typical tree-line tree species (note the predominance of conifers):
- Foxtail Pine (Pinus balfouriana)
- Great Basin Bristlecone Pine (Pinus longaeva)
- Rocky Mountains Bristlecone Pine (Pinus aristata)
- Potosi Pinyon (Pinus culminicola)
- Macedonian Pine (Pinus peuce)
- Whitebark Pine (Pinus albicaulis)
- Swiss Pine (Pinus cembra)
- Mountain Pine (Pinus mugo)
- Hartweg's Pine (Pinus hartwegii)
- Subalpine Larch (Larix lyallii)
- Arctic White Birch (Betula pubescens subsp. tortuosa)
- Snow Gum (Eucalyptus pauciflora)
- Antarctic Beech (Nothofagus antarctica)

Table of alpine tree-lines

The alpine tree-line at a location is dependent on local variables, such as aspect of slope, rain shadow and proximity to either geographical pole. Given this caveat, here is a list of average tree-lines from locations around the globe:

Table of arctic and antarctic tree-lines

Like alpine tree-lines shown above, polar tree-lines are heavily influenced by local variables such as such as aspect of slope and degree of shelter; trees can often grow in river valleys at latitudes where they could not grow on a more exposed site. Maritime influences such as ocean currents also play a major role in determining how far from the equator trees can grow. Here are some typical polar treelines: ocean currents

Reference

- Arno, S. F. & Hammerly, R. P. 1984. Timberline. Mountain and Arctic Forest Frontiers. The Mountaineers, Seattle. ISBN 0-89886-085-7
- Ødum, S. 1979. Actual and potential tree-line in the North Atlantic region, especially in Greenland and the Faroes. Holarctic Ecology 2: 222-227.
- Ødum, S. 1991. Choice of species and origins for arboriculture in Greenland and the Faroe Islands. Dansk Dendrologisk Årsskrift 9: 3-78.
- Beringer, J., Tapper, N. J., McHugh, I., Lynch, A. H., Serreze, M. C., & Slater, A. 2001. Impact of Arctic treeline on synoptic climate. Geophysical Research Letters 28 (22): 4247-4250. Category:Trees Category:Conifers Category:Forestry Category:Climate

North

: This article concerns north as a direction per se. For other senses see North (disambiguation). North (disambiguation) North is one of the four cardinal directions, specifically the direction that, in Western culture, is treated as the primary direction: north is used (explicitly or implicitly) to define all other directions; the (visual) top edges of maps usually correspond to the northern edge of the territory represented, unless explicitly stated otherwise or landmarks are considered more useful for that territory than specific directions.
Definitions
North can mean: # true north, the direction along the earth's surface toward one pole of the earth's rotation, namely the pole that is clearly on one's left when standing at the Equator while facing the rising sun. # magnetic north, the direction along the earth's surface in which horizontal magnetic field strength has its most positive value (but see Flipping of planetary magnetic poles for an eventual event, so rare as to make unlikely any advance agreement on whether one or two retronyms would be involved in the replacement terminology) # a loosely specified direction, usually within half a right angle of true north, especially when stating travel instructions in an area where directions of travel are constrained by an approximately rectangular grid of streets, hallways, etc.; this is often called 'grid north' or 'plan north'. # the orientation of a traveller with respect to a visible or otherwise definite continuous two-way route, such that sustained travel over the whole of the route produces a change of position to a location further north, even if that involves travelling a part of the route in another direction, even straight south; often termed "northbound". # pertaining to the part of a route mainly or exclusively used by northbound traffic, where southbound traffic is separated by barriers, or where both are encouraged to stay mostly in one portion by rules of the road; often termed "northbound".
Etymology
The word north is traced to the Old High German nord, and the Proto-Indo-European unit ner-, meaning "left" (or "under"). (Presumably a natural primitive description of its concept is "to the left of the rising sun".)
Magnetic north and declination
Magnetic north is of interest because it is the direction indicated as north on a properly functioning (but uncorrected) magnetic compass. The difference between it and true north is called the magnetic declination (or simply the declination where the context is clear). For many purposes and physical circumstances, the error in direction that results from ignoring the distinction is tolerable; in others a mental or instrument compensation, based on assumed knowledge of the applicable declination, can solve all the problems. But simple generalizations on the subject should be treated as unsound, and as likely to reflect popular misconceptions about terrestrial magnetism.
Roles of north as prime direction
The visible rotation of the night sky about the visible celestial pole provides a vivid metaphor of that direction corresponding to up. Thus the choice of the north as corresponding to up in the northern hemisphere, or of south in that role in the southern, is, prior to world-wide communication, anything but an arbitrary one. On the contrary, it is of interest that Chinese culture ever considered south as the proper top end for maps.
In Western culture (unless making a point about harmful effects, or the arbitrary nature, of boreocentrism):
- Up is a metaphor for north
- Maps tend to be drawn for viewing with either true north or magnetic north at the top (page layout)
- Globes of the earth have the North Pole at the top, or if the earth's axis is represented as inclined from vertical (normally by the angle it has relative to the axis of the earth's orbit), in the top half.
- Maps are usually labelled to indicate which direction on the map corresponds to a direction on the earth,
- usually with a single arrow oriented to the map's representation of true north,
- occasionally with a single arrow oriented to the map's representation of magnetic north, or two arrows oriented to true and magnetic north respectively,
- occasionally with a compass rose, but if so, usually on a map with north at the top and usually with north decorated more prominently than any other compass point.
Roles of east and west as inherently subsidiary directions
It is worth noting that while the choice of north over south as prime direction reflects quite arbitrary historical factors, east and west are not nearly as natural alternatives as first glance might suggest. Their folk definitions are, respectively, "where the sun rises" and "where it sets". Except on the Equator, however, these definitions, taken together, would imply that
- east and west would not be 180 degrees apart, but instead would differ from that by up to twice the degrees of latitude of the location in question, and
- they would each move slightly from day to day and, in the temperate zones, markedly over the course of the year. Reasonably accurate folk astronomy, such as is usually attributed to Stone Age Celts, would arrive at east and west by noting the directions of rising and setting (preferably more than once each) and choosing as prime direction one of the two mutually opposite directions that lie halfway between those two. The true folk-astronomical definitions of east and west are "the directions, a right angle from the prime direction, that are closest to the rising and setting, respectively, of the sun (or moon).
See also

- "The North", a sense of the term that refers to the wealthy and powerful "North" of the world, as contrasted to the poorer "South"
- Nordicity
- The world's most northern bagpipe orchestra, city, capital, zoo etc. Category:Orientation ja:北 simple:North

South

South is most commonly a noun, adjective, or adverb indicating direction or geography. South is one of the four cardinal directions or compass points. It is the opposite of North and at right angles to East and West. By convention, the bottom side of a map is South. To go south using a compass for navigation, set a bearing or azimuth of 180°. True south is the direction towards the southern end of the axis about which the earth rotates, called the South Pole. The South Pole is located in Antarctica. Magnetic south is the direction towards the south magnetic pole, some distance away from the south geographic pole. It is the direction to the right of an observer facing east. The etymology of South can be traced back to the Old English word suth, derived from the Old High German word sund, and perhaps sunne in Old English with sense of "the region of the sun." Category:Orientation ja:南 simple:South

Elevation

:For other senses of this word, see elevation (disambiguation).

Basic Definition

In geography, the elevation of a geographic location is its height above mean sea level (or some other fixed point). Elevation is mainly used when referring to points on the Earth itself, while altitude is used for points in the air, such as an aircraft.

Determining Elevation

If you are reading a map from home, it is possible you may need to determine the elevation of some place. The main sort of map to use for this purpose is a topographical map. Learning to read a topographic map is relatively easy although assistance may be required for beginners. Image:HaleakalaMap.jpg Example of a topographical map. Haleakala, Hawaii. If you are somewhere and want to find its elevation, you may also survey it. Questions often arise about where to measure elevation from. The elevation of a mountain usually refers to its summit. The elevation of a hill also refers to the summit. A valley's elevation is usually taken from the lowest point but is often taken all over the valley.

Links

- Altitude
- Topographical map
- [http://www.usgs.gov/ US Geographical Survey]
- [http://www.gsi.go.jp/ENGLISH/ Geographical Survey Institute]
- [http://www.thefreedictionary.com/elevation The Free Dictionary, Elevation]
- Category:Physical geography Category:Length

Mountain

has one of the largest visible base-to-summit elevation differences anywhere]] A mountain is a landform that extends above the surrounding terrain in a limited area. A mountain is generally much higher and steeper than a hill, but there is considerable overlap, and usage often depends on local custom. Some authorities define a mountain as a peak with a topographic prominence over an arbitrary value: for example, the Encyclopædia Britannica requires a prominence of 2,000 feet (610 m). 24% of the Earth's land mass is mountainous; 10% of the world's 6 billion people live in mountainous regions. All the world's major rivers are fed from mountain sources, and more than half of humanity depends on mountains for water [http://www.animana.org/tab2/22troubleattop.shtml]. The adjective montane is used to describe mountainous areas and the things associated with them.

Heights

Heights of mountains are generally given as heights above mean sea level. The Himalayas average 5km above sea level, whilst the Andes average 4km. Most other mountain ranges average 2-2.5km. The highest mountain on Earth is Everest, 8850 m, set in the world's most significant mountain range, the Himalaya. Other definitions of height are possible. The peak that is farthest from the centre of the Earth is Chimborazo in Ecuador. At 6,272 m above sea level it is not even the tallest peak in the Andes, but because the Earth bulges at the equator and Chimborazo is very close to the equator, it is 2,150 m further away from the Earth's centre than Everest. The peak that rises farthest from its base is Mauna Kea on Hawaii, whose peak is over 9,000 m above its base on the floor of the Pacific Ocean. The tallest known mountain in the solar system is Olympus Mons, located on Mars.

Characteristics

The altitude of mountains means that the tops exist in higher cold layers of the atmosphere. They are consequently often subject to glaciation and erosion through frost action. This produces the classic mountain peak shape. Some mountains have glacial lakes, created by melting glaciers; for example, there are an estimated 3000 in Bhutan. Sufficiently tall mountains have very different climatic conditions at the top than at the base, and will thus have different life zones at different altitudes on their slopes. The plants and animals of a zone are somewhat isolated when the zones above and below are inhospitable, and many unique species occur on mountainsides as a result. Extreme cases are known as sky islands. Cloud forests are forests on mountain sides which attract moisture from the air, creating a unique ecosystem. Mountains are not generally favored for human habitation; the weather is harsher, less food is available, and there is little level ground suitable for farming. At very high altitudes, there is less oxygen in the air, and less protection against solar radiation (UV). Acute mountain sickness (caused by hypoxia - a lack of oxygen in the blood) affects over half of lowlanders who spend more than a few hours above 3500 metres. Despite some biological adaptation by peoples who have lived on mountains for hundreds or thousands of years, babies' average birthweight is reduced by 100 grams for every 1000-metre gain in altitude. Most mountains of the world have been left in their natural state, and are today primarily used for recreation. Some mountains are very difficult to climb, and offer spectacular views. Some people therefore enjoy the sport of mountaineering. Mountains are also the site for the sport of downhill skiing. People engaging in these activities often stay at mountain resorts built for the purpose.

Geology

mountain resort.]] A mountain is usually produced by the movement of lithospheric plates, either orogenic movement or epeirogenic movement. The compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upwards, creating a landform higher than the surrounding features. The height of the feature makes it either a hill or, if higher and steeper, a mountain. The absolute heights of features termed mountains and hills vary greatly according to an area's topography. The major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity. Mountain creation tends to occur in discrete periods, each referred to as an orogeny. The orogeny may last millions of years, and the uplifted region is being eroded away, producing valley-and-peak topography, even while the uplift is taking place. Two types of mountain are formed depending on how the rock reacts to the tectonic forces – block mountains or fold mountains. The compressional forces in continental collisions may cause the compressed region to thicken, so the upper surface is forced upwards. In order to balance the weight, much of the compressed rock is forced downwards, producing deep "mountain roots". Mountains therefore form downwards as well as upwards (see isostasy). However, in some continental collisions part of one continent may simply override part of the other, crumpling in the process. Some isolated mountains were produced by volcanoes, including many apparently small islands that reach a great height above the ocean floor. Block mountains are created when large areas are widely broken up by faults creating large vertical displacements. The uplifted blocks are block mountains or horsts. The intervening dropped blocks are termed graben: these can be small or form extensive rift valley systems. This form of landscape can be seen in East Africa, the Vosges, the Basin and Range province of Western North America and the Rhine valley. Where rock does not fault it folds, either symmetrically or asymmetrically. The upfolds are anticlines and the downfolds are synclines; in asymmetric folding there may also be recumbent and overturned folds. The Jura mountains are an example of folding. Over time, erosion can bring about an inversion of relief: the soft upthrust rock is worn away so the anticlines are actually lower than the tougher, more compressed rock of the synclines.

External links

- [http://www.ga.com.pl/tatry21.htm Pics from the Tatra Mountains - Poland]
- [http://bivouac.com Canadian Mountain Encyclopedia] - an exhaustive index of North American peaks, including thousands of unnamed ones. Includes the United States and Mexico as well as Canada. Category:Landforms Category:Mountains Category:Mountaineering Category:Geomorphology ko:산 ms:Gunung ja:山 simple:Mountain

Coast

.]] The coast is defined as the part of the land adjoining or near the ocean. A coastline is properly, a line on a map indicating the disposition of a coast but the word is often used to refer to the coast itself. The adjective, coastal describes something as being on, near or having to do with a coast. Coast is a very specific term and is only applied to that part of an island or continent that borders an ocean or its saltwater tributaries. A pelagic coast refers to a coast which fronts the open ocean, as opposed to a more sheltered coast in a gulf or bay. A shore on the other hand, can refer to parts of the land which adjoin any large body of water, including oceans (sea shore) and lakes (lake shore). Similarly, the somewhat related term bank refers to the land alongside or sloping down to a river (river bank) or of a body of water smaller than a lake. Bank is also used in some parts of the world to refer to an artificial ridge of earth intended to retain the water of a river or pond. In other places this may be called a levee. While many scientific experts might agree on a common definition of the term "coast", the delineation of the inland extents of a coast differ according to jurisdiction, with many scientific and government authorities in various countries differing for economic and social policy reasons. This is usually because defining lands as part of a coast may be seen to have environmental implications which would prevent development or attach regulations to their use.

Environmental importance

The coast and its adjacent areas on and off shore is an important part of a local ecosystem as the mixture of fresh water and salt water in estuaries provides many nutrients for marine life. Salt marshes and beaches also support a diversity of plants, animals, and insects crucial to the food chain. Like the ocean which shapes them, coasts are a dynamic environment with constant change. The earth's natural processes, particularly sea level rise, waves and various weather phenomena, have resulted in the erosion accretion and reshaping of coasts as well as flooding and creation of continental shelves and drowned river valleys (rias).

Human impacts

Coasts also face many environmental challenges relating to human-induced impacts. The human influence on climate change is thought to be a contributing factor of an accelerated trend in sea level rise which threatens coastal habitat as natural systems struggle to adapt faster. Human development of coastal land, particularly for recreational or industrial uses are similarly threatened by sea level rise, but also contribute to aesthetic problems of land use and reduced natural coastal habitat. Pollution is an ongoing concern along coasts with garbage and industrial debris littering beaches and sometimes entire coasts. The transportation of petroleum in tankers is a major hazard both for the open ocean and along coasts, particularly when large oil spills occur. Another major hazard for coastal marine life is the large number of small oil spills created by large and small vessels powered by petroleum which flush bilge water directly into the ocean. Both the terms coast and coastal are often used to describe a geographic location or region. For example, New Zealand's West Coast, or the East and West Coasts of the United States. A large part of the global population inhabits areas near a coast, partly to take advantage of marine resources such as fish, but more importantly to participate in seaborne trade with other nations. Many of the world's major cities that have developed in recent centuries were built on or near good harbours and have large port facilities to take advantage of marine transportation. Jurisdictions which are landlocked and have no coast are often at an economic disadvantage with overseas trade being more difficult; sometimes being forced to go to extravagant measures such as building canals to permit ocean-going vessels to travel inland. Coasts, especially those with beaches and warm water are also an important draw for tourists. In many island nations such as those of the Mediterranean, South Pacific and Caribbean, tourism by those who come to enjoy the coast is central to the economy. Coasts are popular destinations because of recreational activities such as swimming, fishing, surfing, boating, and sun bathing. Many tourists and residents also enjoy the salt air by the sea coast which some consider to have health benefits. Coastal weather is heavily influenced by the ocean and while this can sometimes result in dangerous storms such as Nor'easters and hurricanes, the coastal climate is often cooler and more temperate than corresponding inland areas. Consequently tourists from areas experiencing extremely warm and humid weather seek coastal areas for these reasons. The coast, especially for isolated nations such as Japan, the United Kingdom, Australia, New Zealand, Canada or the United States is often a crucial defensive frontier, both for warding off military invaders but also smugglers and illegal migrants. Coastal defenses have thus long been erected in many nations. Most coastal countries also have a navy and some form of coast guard.

Types of coast

- emergent coastline - coast has risen or sea level has fallen from previous level.
- submergent coastline - coast has fallen or sea level has risen from previous level.
- concordant coastline - rock bands run parallel to shore.
- discordant coastline - rock bands run perpendicular to shore.

Meanings

The precise definition of drought is made complex due to political considerations, but there are generally three types of conditions that are referred to as drought.
- Meteorological drought is brought about when there is a prolonged period with below average precipitation.
- Agricultural drought is brought about when there is insufficient moisture for average crop or range production. This condition can arise, even in times of average precipitation, due to soil conditions or agricultural techniques.
- Hydrologic drought is brought about when the water reserves available in sources such as aquifers, lakes, and reservoirs falls below the statistical average. This condition can arise, even in times of average (or above average) precipitation, when increased usage of water diminishes the reserves. When the word "drought" is used commonly, the most often intended definition is meteorological drought. However, when the word is used by urban planners, it is more frequently in the sense of hydrologic drought.

Consequences

Periods of drought can have significant environmental, economic and social consequences. The most common consequences are:
- Wildfires
- Migration or relocation of those impacted
- Social unrest
- War
- Famine due to lack of water for irrigation
- Disease
- Thirst The causes of these problems are complex and relate to increased dependence on external resources (inability to grow crops locally) and lowered quality (and thus contamination) of remaining water sources among other factors. The quality of national infrastructure can increase or decrease the impact of drought, especially with respect to famine, dramatically.

Famous droughts

18th and 19th centuries, Cape Verde

Three droughts were responsible for over 100,000 starvation deaths. These droughts spurred the migration of much of the population to locations such as New England, to participate in the whaling industry.

1900, India

250,000 to 3.25 million die due to drought, starvation and disease.

1921-22, Soviet Union

In the Ukraine and Volga regions, 250,000 to 5 million perish from drought.

1928-30, northwest China

Famine results in over 3 million deaths.

1936, Sichuan Province, China

This was the worst drought in the modern history of the area. 34 million farmers were displaced and 5 million people starved to death.

1930-37, United States

Three waves of drought during this time are collectively refered to as "the dustbowl". Because of several factors including the coincidence of the dustbowl and the Great Depression, this drought had a severe impact on the U.S., resulting in entire districts of the American Great Plains being depopulated.

1941, Sichuan Province, China

This was less severe than the 1936 drought, and resulted in the deaths of only 2.5 million. However, because of the war with Japan at the time, the indirect damage caused was far more dramatic and more devastating.

1965-67, India

Drought results in over 1.5 million deaths.

1968-74, Sahel

The Sahelian drought that began in 1968 was responsible for the deaths of between 100,000 and 250,000 people, the disruption of millions of lives, and the collapse of the agricultural bases of five countries.

1968-72, Iran

A blizzard ended a four-year drought, but the week long cold and snow caused the deaths of approximately 4,000 people.

1973–74, Ethiopia

This drought (part of the overall Sahelian drought) resulted in a military coup, which overthrew the government of Haile Selassie.

1983, Bolivia

Severe and prolonged drought during 1983 that affected large areas of seven of Bolivia's nine departments, an estimated 80 percent of the normal fall harvest of staple crops was lost. Total agricultural losses were estimated at U.S. $417.2 million: $277.7 million in crop losses and $139.5 million in livestock losses. These losses included over 200,000 cattle, 3,000,000 sheep, and 660,000 llamas and alpacas.

1984-85, Ethiopia

The 1984 - 1985 famine in Ethiopia was a direct result of this drought, and lead to another regime change–this time to a Marxist government.

External links

- [http://www.worldwaterforum.org/ Worldwaterforum]
- [http://www.drought.unl.edu/dm/monitor.html U.S. drought monitor].
- [http://www.usatoday.com/weather/whydro00.htm USAtoday´s resources: floods and droughts].
- [http://www.bom.gov.au/climate/drought/livedrought.shtml Managing drought].
- [http://www.drought.noaa.gov/ NOAA drought information center].
- [http://www.ngdc.noaa.gov/paleo/drought/drght_home.html Drought, a paleo perspective].
- [http://www.drought.unl.edu/index.htm US National Drought Mitigation Center].
- [http://www.droughtoutlook.com/ Droughtoutlook].
- [http://www.agric.nsw.gov.au/drought Drought and agriculture].
- [http://www.nrcs.usda.gov/feature/highlights/drought.html US DA].
- [http://www.disastercenter.com/drought.htm Drought disaster center].
- [http://www.floridadisaster.org/bpr/EMTOOLS/florida_drought_center.htm Florida Drought Center].
- [http://watersupplyconditions.water.ca.gov/ Drought preparedness].
- [http://www.state.nj.us/drbc/drought/kids_droughtinfo.htm Drought for kids].
- [http://www.westgov.org/wga/initiatives/drought2.htm Drought Act Initiative].
- [http://www.pbs.org/wgbh/amex/dustbowl/peopleevents/pandeAMEX06.html PBS The drought].
- [http://msnbc.msn.com/id/5262324 Cuba suffers through worst drought in history].
- [http://news.bbc.co.uk/2/hi/science/nature/3107893.stm Drought leaves Europe's farmers helpless].
- [http://www.cbc.ca/news/features/drought2002_timeline.html The CA drought of 2002].
- [http://dmc.engr.wisc.edu/courses/hazards/BB02-07.html Natural Hazards Causes and Effects Drought]. Category:Hydrology
- Category:Weather hazards

Desert

In geography, a desert is a landscape form or region that receives little precipitation - less than 250 mm per year. Deserts have a reputation for supporting very little life. Compared to wetter regions this may be true, although upon closer examination, deserts often harbor a wealth of life that usually remains hidden (especially during the daylight) to preserve moisture. Approximately one-third of Earth's land surface is desert. (See a map of the world's non-polar deserts, http://pubs.usgs.gov/gip/deserts/what/world.html.) Desert landscapes have certain common features. Desert soil is often composed mostly of rocky surfaces called regs. Sand dunes called ergs and stony or hamada surfaces compose the minority of desert surfaces. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation. Bottom lands may be salt-covered flats. Eolian (wind-driven) processes are major factors in shaping desert landscapes. Deserts sometimes contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Because deserts are dry, they are ideal places for human artifacts and fossils to be preserved. In the Köppen climate classification system, they are classed as (BW).

Types of desert

Most classifications rely on some combination of the number of days of rainfall, the total amount of annual rainfall, temperature, humidity, or other factors. In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least 12 consecutive months without rainfall, arid lands have less than 250 millimeters of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 millimeters. Arid and extremely arid land are deserts, and semiarid grasslands generally are referred to as steppes. However, lack of rainfall alone can't provide an accurate description of what a desert is. For example, Phoenix, Arizona receives less than 250 millimeters, (10 inches), of precipitation per year, and is immediately recognized as being located in a desert. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters of precipitation per year, but is not generally recognized as a desert region. The difference lies in something termed "potential evapotranspiration." The water budget of an area can be calculated using the formula P-PE+/-S, whereby P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation, coupled with the evaporative loss of water through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. Tucson, Arizona receives about 300 millimeters, (12 inches), of rain per year, however about 2500 millimeters, (100 inches), of water could evaporate over the course of a year. In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in other regions such as Alaska are much lower, so while these regions receive minimal precipitation, they should be designated as specifically different from the simple definition of a desert: a place where evaporation exceeds precipitation. That said, there are different forms of deserts. Cold deserts can be covered in snow; such locations don't receive much precipitation, and what does fall remains frozen as snow pack; these are more commonly referred to as tundra if a short season of above-freezing temperatures is experienced, or as an ice cap if the temperature remains below freezing year-round, rendering the land almost completely lifeless. Most non-polar deserts are hot because they have little water. Water tends to have a cooling, or at least a moderating, effect in environments where it is plentiful. In some parts of the world deserts are created by a rain shadow effect in which air masses lose much of their moisture as they move over a mountain range; other areas are arid by virtue of being very far from the nearest available sources of moisture (this is true in some middle-latitude landmass interior locations, particularly in Asia). Deserts are also classified by their geographical location and dominant weather pattern as trade wind, midlatitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in nonarid environments are paleodeserts, and extraterrestrial deserts exist on other planets.

Montane deserts

Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalaya, in parts of the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 meters (9,843 feet) and the thermal regime can be hemiboreal. These places owe their profound aridity (the average annual precipitation is often less than 40mm) to being very far from the nearest available sources of moisture.

Desert features

hemiboreal] Sand covers only about 20 percent of Earth's deserts. Most of the sand is in sand sheets and sand seas—vast regions of undulating dunes resembling ocean waves "frozen" in an instant of time. In general, there are 6 forms of deserts: i.Mountain and basin deserts, ii. Hamada deserts, which comprise of a plateaux landforms, iii. Regs which consist of rock pavements, iv. Ergs which are formed by sand seas, v. Intermontane Basins, and vi. Badlands which are located at the margins of arid lands comprising of clay-rich soil. Nearly 100 percent of desert surfaces are plains where eolian deflation—removal of fine-grained material by the wind—has exposed loose gravels consisting predominantly of pebbles but with occasional cobbles. The remaining surfaces of arid lands are composed of exposed bedrock outcrops, desert soils, and fluvial deposits including alluvial fans, playas, desert lakes, and oases/oasis. Bedrock outcrops commonly occur as small mountains surrounded by extensive erosional plains. There are several different types of dunes. Barchan dunes are produced by strong winds blowing across a level surface and are crescent shaped. Longitudinal or seif dunes are dunes that are parallel to a strong wind that blows in one general direction. Transverse dunes run are a right angle to the constant wind direction. Star dunes are star-shaped and have several ridges that spread out around a point. Oases are vegetated areas moistened by springs, wells, or by irrigation. Many are artificial. Oases are often the only places in deserts that support crops and permanent habitation.

Soils

irrigation Soils that form in arid climates are predominantly mineral soils (classified as Aridisols) with low organic content such as salt. The repeated accumulation of water in some soils causes distinct salt layers to form. Calcium carbonate precipitated from solution may cement sand and gravel into hard layers called "calcrete" that form layers up to 50 meters thick. Caliche is a reddish-brown to white layer found in many desert soils. Caliche commonly occurs as nodules or as coatings on mineral grains formed by the complicated interaction between water and carbon dioxide released by plant roots or by decaying organic material.

Vegetation

Most desert plants are drought- or salt-tolerant, such as xerophytes. Some store water in their leaves, roots, and stems. Other desert plants have long tap roots that penetrate the water table, anchor the soil, and control erosion. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Deserts typically have a plant cover that is sparse but enormously diverse. The Sonoran Desert of the American Southwest has the most complex desert vegetation on Earth. The giant saguaro cacti provide nests for desert birds and serve as "trees" of the desert. Saguaro grow slowly but may live 200 years. When 9 years old, they are about 15 centimeters high. After about 75 years, the cacti develop their first branches. When fully grown, saguaro are 15 meters tall and weigh as much as 10 tons. They dot the Sonoran and reinforce the general impression of deserts as cacti-rich land. Although cacti are often thought of as characteristic desert plants, other types of plants have adapted well to the arid environment. They include the pea family and sunflower family. Cold deserts have grasses and shrubs as dominant vegetation.

Water

sunflower Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 millimeters of rain once fell within 3 hours in the Sahara. Large Saharan storms may deliver up to 1 millimeter per minute. Normally dry stream channels, called arroyos or wadis, can quickly fill after heavy rains, and flash floods make these channels dangerous. Though little rain falls in deserts, deserts receive runoff from ephemeral, or short-lived, streams fed by rain and snow from adjacent highlands. These streams fill the channel with a slurry of mud and commonly transport considerable quantities of sediment for a day or two. Although most deserts are in basins with closed, or interior drainage, a few deserts are crossed by 'exotic' rivers that derive their water from outside the desert. Such rivers infiltrate soils and evaporate large amounts of water on their journeys through the deserts, but their volumes are such that they maintain their continuity. The Nile River, the Colorado River, and the Yellow River are exotic rivers that flow through deserts to deliver their sediments to the sea. Lakes form where rainfall or meltwater in interior drainage basins is sufficient. Desert lakes are generally shallow, temporary, and salty. Because these lakes are shallow and have a low bottom gradient, wind stress may cause the lake waters to move over many square kilometers. When small lakes dry up, they leave a salt crust or hardpan. The flat area of clay, silt, or sand encrusted with salt that forms is known as a playa. There are more than a hundred playas in North American deserts. Most are relics of large lakes that existed during the last ice age about 12,000 years ago. Lake Bonneville was a 52,000-square-kilometer lake almost 300 meters deep in Utah, Nevada, and Idaho during the Ice Age. Today the remnants of Lake Bonneville include Utah's Great Salt Lake, Utah Lake, and Sevier Lake. Because playas are arid land forms from a wetter past, they contain useful clues to climatic change. When the occassional preticipation does occur, it erodes the desert rocks quickly and powerfully. Wind is the other factor that erodes deserts- they are constant yet slow. The flat terrains of hardpans and playas make them excellent race tracks and natural runways for airplanes and spacecraft. Ground-vehicle speed records are commonly established on Bonneville Speedway, a race track on the Great Salt Lake hardpan. Space shuttles land on Rogers Lake Playa at Edwards Air Force Base, California.

Mineral resources

Some mineral deposits are formed, improved, or preserved by geologic processes that occur in arid lands as a consequence of climate. Ground water leaches ore minerals and redeposits them in zones near the water table. This leaching process concentrates these minerals as ore that can be mined. Evaporation in arid lands enriches mineral accumulation in their lakes. Playas may be sources of mineral deposits formed by evaporation. Water evaporating in closed basins precipitates minerals such as gypsum, salts (including sodium nitrate and sodium chloride), and borates. The minerals formed in these evaporite deposits depend on the composition and temperature of the saline waters at the time of deposition. Significant evaporite resources occur in the Great Basin Desert of the United States, mineral deposits made forever famous by the "20-mule teams" that once hauled borax-laden wagons from Death Valley to the railroad. Boron, from borax and borate evaporites, is an essential ingredient in the manufacture of glass, ceramics, enamel, agricultural chemicals, water softeners, and pharmaceuticals. Borates are mined from evaporite deposits at Searles Lake, California, and other desert locations. The total value of chemicals that have been produced from Searles Lake substantially exceeds US$1 billion. The Atacama Desert of South America is unique among the deserts of the world in its great abundance of saline minerals. Sodium nitrate has been mined for explosives and fertilizer in the Atacama since the middle of the 19th century. Nearly 3 million tonnes were mined during World War I. Valuable minerals located in arid lands include copper in the United States, Chile, Peru, and Iran; iron and lead-zinc ore in Australia; chromite in Turkey; and gold, silver, and uranium deposits in Australia and the United States. Nonmetallic mineral resources and rocks such as beryllium, mica, lithium, clays, pumice, and scoria also occur in arid regions. Sodium carbonate, sulfate, borate, nitrate, lithium, bromine, iodine, calcium, and strontium compounds come from sediments and near-surface brines formed by evaporation of inland bodies of water, often during geologically recent times. The Green River Formation of Colorado, Wyoming, and Utah contains alluvial fan deposits and playa evaporites created in a huge lake whose level fluctuated for millions of years. Economically significant deposits of trona, a major source of sodium compounds, and thick layers of oil shale were created in the arid environment. Some of the more productive petroleum areas on Earth are found in arid and semiarid regions of Africa and the Mideast, although the oil fields were originally formed in shallow marine environments. Recent climate change has placed these reservoirs in an arid environment. Other oil reservoirs, however, are presumed to be eolian in origin and are presently found in humid environments. The Rotliegendes, a hydrocarbon reservoir in the North Sea, is associated with extensive evaporite deposits. Many of the major U.S. hydrocarbon resources may come from eolian sands. Ancient alluvial fan sequences may also be hydrocarbon reservoirs.

List of deserts

Americas

- Atacama desert in Chile
- Mojave, Great Basin, Sonoran, and Chihuahuan See also: List of North American deserts

Africa

- Libyan
- Kalahari
- Sahara
- Namib

Asia-Pacific

- Dasht-e Kavir, central Iran.
- Gobi desert of Mongolia; Taklamakan desert in China.
- Kara Kum deserts in Central Asia.
- Thar-Cholistan desert in India and Pakistan.
- Kavir-e Lut, souteast Iran.
- Kyzyl Kum - Kazakhstan and Uzbekistan
- Negev - southern Israel
- Judean Desert - eastern Israel and Palestine
- Simpson Desert, Great Sandy Desert, Sturt's Stony Desert, Tanami Desert, Great Victoria Desert, Big Desert, Little Desert (all in Australia)
- Taklamakan - Xinjiang Uighur Autonomous Region of the People's Republic of China
- Rangipo Desert - New Zealand

Rainfall

In meteorology, precipitation is any kind of water that falls from the sky as part of the weather. This includes snow, rain, sleet, freezing rain, hail, and virga. Precipitation is a major part of the hydrologic cycle, and is responsible for depositing most of the fresh water on the planet. Precipitation is generated in clouds, which reach a point of saturation; at this point larger and larger droplets (or pieces of ice) form, which then fall to the earth under gravity. It is possible to 'seed' clouds to induce precipitation by releasing a fine dust or appropriate chemical (commonly silver nitrate) into a cloud, encouraging droplets to form, and increasing the probability of precipitation.

Orographic precipitation

Orographic precipitation, also known as relief precipitation, is precipitation generated by a forced upward movement of air upon encountering a physiographic upland (see anabatic wind). This upwards movement cools the air, resulting in a cloud formation and rainfall. In parts of the world subjected to relatively consistent winds (for example the trade winds), a wetter climate prevails on the windward side of a mountain than on the leeward (downwind) side as moisture is removed by orographic precipitation, leaving drier air (see katabatic wind) on the descending (generally warming), leeward side where a rain shadow is formed. Orographic precipitation is well known on oceanic islands, such as the Hawaiian Islands, where much of the rainfall received on an island is on the windward side, and the leeward side tends to be quite dry, almost desert-like, by comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 20 to 30 inches (500 to 750 mm) per year, and interior uplands receiving over 100 inches (2.5 m) per year. Leeward coastal areas are especially dry (<20 in (500 mm) per year at Waik&#299;k&#299;), and the tops of moderately high uplands are especially wet (~475 in (12 m) per year at Wai'ale'ale on Kaua'i).

Convectional rainfall

Convectional rainfall occurs when the air is heated up, usually by the land below it (land tends to heat up faster than air or water bodies). As the air heats up it rises. Inevitably cooling will result, and water vapor will condense out of the air to form droplets, and eventually clouds, if there is enough vapour. This kind of precipitation is most commonly found in tropical areas.

Frontal Rainfall

Frontal rainfall occurs at both warm fronts and cold fronts. At a warm front the warm air, being lighter, rides up over the cold air. As it rises it also cools down (adiabatic process). Moisture in the air condenses to form clouds, and precipitation occurs. At a cold front the warm air is forced up by the cold and the same process occurs.

Rainfall Patterns

Western

Major elements are prevailing westerlies and ocean currents moving equatorward. At high lattitudes the current is warmer than land, westerlies pick up moisture and cool when moving over land. When the land is warmer than the ocean clouds don't drop precipitation, but pick up additional moisture; it rains in the mountains. When the land is cooler than the ocean, then westerlies cool as they move inland and rain occurs in the lowlands.

Eastern and Central

Polar air masses (above 50° latitudes) are distinct from lower latitude air masses. The westerlies are warm air masses that move poleward from 30°N. In Eastern US the westernlies are often laden with moisture from Gulf of Mexico and Atlantic. When polar and westernlie air masses meet, precipitation occurs.

Inland Continental Areas

Areas not proximate to large bodies of water, warm faster than surrounding areas. Hot air masses rise from the center of the continent forming a low pressure area. This low pressure draws water laden clouds from the coasts. This area is heated & rises, this causes cooling adiabatically and precipitation. Resulting in summer rain, and less winter precipitation.

Muskeg

Muskeg is a soil type (also a peatland or wetland type called a bog) common in arctic and boreal areas. Muskeg itself consists of dead plants in various states of decomposition (i.e., peat), ranging from fairly intact sphagnum moss, to sedge peat, to highly decomposed muck. Pieces of wood such as buried tree branches can make up 5 to 15 percent of the peat soil. Muskeg tends to have a water table very near the surface. As well, the sphagnum moss forming it can hold 15 to 30 times its own weight in water, allowing the spongy wet muskeg to form even on sloping ground. Muskeg is wet, acidic and relatively infertile, preventing large trees from growing, though stunted pines may be found. Muskegs need two conditions to develop: abundant rain and cool summers. A dead plant that falls on dry soil is attacked by bacteria and fungi and quickly rots. If that plant lands in water or on saturated soil, though, it faces a different fate. Air can't get to it, so the bacteria and fungi can't function well. The cool temperatures slow them down even more. All this slows decomposition, and the plant debris accumulates to form peat and, eventually, a muskeg. Depending on the underlying topology of the land, muskeg can reach depths of 30 m (100 feet) or more. Muskeg can be a significant impediment to transportation. During the 1870s muskeg in Northern Ontario was reported to have swallowed a railroad engine whole when a track was laid on muskeg instead of clearing down to bedrock. Many other instances have been reported of [http://thelink.concordia.ca/article.pl?sid=04/03/24/0247209 heavy construction equipment vanishing into muskeg]in the spring as the frozen muskeg it was parked on during winter thawed. Superficially, muskeg resembles a grassy plain covered with short grasses, with strands of trees growing where land protrudes above the water table. Travelling through muskeg is a very strange experience to those not used to it. It can be very dangerous and arduous. Muskeg can grow directly on top of bodies of water, especially small lakes. Thinner patches can allow animals to drop through, becoming trapped under the muskeg and drowning. Muskeg patches are ideal habitats for beavers, Indian pitcher plants, agaric mushrooms and a variety of other organisms. category:wetlands category:landforms

Bog

A bog is a wetland type that accumulates acidic peat, a deposit of dead plant material. The term peat bog in common usage is not entirely redundant, although it would be proper to call these sphagnum bogs if the peat is composed mostly of acidophilic moss (peat moss or Sphagnum spp.). Lichens are a principal component of peat in the far north. Moisture is provided entirely by precipitation and for this reason bog waters are acidic and termed ombrotrophic (or cloud-fed) which accounts for their low plant nutrient status. Excess rainfall outflows giving bog waters a distinctive tan colour. Bogs are widely distributed in cold, temperate climates, mostly in the northern hemisphere (Boreal). The world's largest wetlands are the bogs of the Western Siberian Lowlands in Russia which cover more than 600,000 square kilometres. Sphagnum bogs were widespread in northern Europe. Ireland was more than 15 per cent bog; Achill Island off Ireland is 87 per cent bog. There are extensive bogs in Canada (called muskegs), Scotland, Denmark, Estonia, Finland, and northern Germany. There are also bogs in the Falkland Islands. Ombrotrophic wetlands - i.e. bogs are also found in the tropics with notable areas documented in Kalimantan. These habitats are forested. Extensive bogs cover the northern areas of the U.S. states of Minnesota and Michigan, most notably on Isle Royale in Lake Superior.

World's largest wetlands

The world's largest wetlands are the bogs of the Western Siberian Lowlands in Russia which cover more than 600,000 square kilometres. "The world's largest frozen peat bog is melting. An area stretching for a million square kilometres across the permafrost of western Siberia is turning into a mass of shallow lakes as the ground melts, according to Russian researchers just back from the region. The sudden melting of a bog the size of France and Germany combined could unleash billions of tonnes of methane, a potent greenhouse gas, into the atmosphere." [http://www.newscientist.com/article.ns?id=mg18725124.500 newscientist]

Linguistic distinctions

The Term bog derives from the Irish language word for "soft". The Germanic word moor had the same meaning. However, with the two words now available in English use, their meanings are slowly becoming distinct. In England, the lowland moors are now nearly all drained. They are largely forgotten except in place-names like Morton. The word moor is still applied to acid peat-land on hill-tops, where given enough precipitation, the land need not be at all flat to form a bog, even on limestone. In some cases, though by no means all, drainage has now led to the oxidation of the upland peat. Nonetheless, the upland moors still largely retain their acid peat-land characteristics where the underlying soil is acidic. This is typically on acid sands, where the moor tends towards heathland, or on plutonic rocks like the granite of Dartmoor where suitable patches have been converted into grassland for pasture.

Bog habitats

pasture Bogs are challenging environments for plant life because they are low in nutrients and very acidic. Carnivorous plants have adapted to these conditions by using insects as a nutrient source. The high acidity of bogs and the absorption of water by sphagnum moss reduce the amount of water available for plants. Some bog plants, such as Leatherleaf, have waxy leaves to help retain moisture. Bogs also offer a unique environment for animals. For instance, English bogs give a home to the boghopper beetle and a yellow fly called the hairy canary. Some bogs have preserved ancient oak logs useful in dendrochronology and they have yielded extremely well-preserved bog bodies, with organs, skin, and hair intact, such as Tollund Man and Lindow man, buried there thousands of years ago after apparent Celtic human sacrifice.

Uses of bogs

Industrial uses

A bog is a very early stage in the formation of coal deposits. In fact, bogs can catch fire and often sustain long-lasting smouldering blazes, producing smoke and CO₂ causing health and environmental problems. After drying, peat is used as a fuel. More than 20 percent of home heat in Ireland comes from peat, and it is also used for fuel in Finland, Scotland, Germany, and Russia. Russia is the leading producer of peat for fuel at more than 90 million metric tons per year. Ireland's Bord na Móna (peat board) was one of the first companies to mechanically harvest peat. The other major use of dried peat is as a soil amendment (sold as moss peat or sphagnum) to increase the soil's capacity to retain [http://wiktionary.org/wiki/moisture moisture] and enrich the soil. It is also used as a mulch. Some distilleries, notably Laphroaig, use peat fires to smoke the barley used in making scotch whisky. These industrial uses of peat threaten the continued existence of bogs. More than 90 percent of the bogs in England have been destroyed.

Other uses

Crops of blueberries, cranberries and lingonberries are grown in bogs. Sphagnum bogs are also used for sport, but this can be damaging. Bog snorkelling is popular in England and Wales and has even produced the associated sport of [http://www.myra-simon.com/bike/bog-snork.html mountain bike bog snorkelling]. Llanwrtyd Wells, the smallest town in Wales, hosts the World Bog Snorkelling Championships. In this event, competitors with mask, snorkel, and SCUBA fins swim along a 60-meter trench cut through a peat bog. The last Sunday in July is [http://www.wildlifetrusts.org/index.php?section=places:events:bogday International Bog Day]. Bog is also a United Kingdom slang word for toilet.

Literature

Gothic Fiction is commonly set on the moor, an English bog. One example is "The Hound of the Baskervilles", a Sherlock Holmes story by Arthur Conan Doyle. "The Bog People: Iron-Age Man Preserved", by P.V. Glob, is a classic study of archaeology. The book is about the iron-age culture of Denmark, and the victims of ritual sacrifice by strangulation. The corpses were thrown into peat bogs where they were discovered after 2000 years, perfectly preserved, down to their facial expressions, although well-tanned by the acidic environment of the Danish bogs.

Oxygen

Oxygen is a chemical element in the periodic table. It has the symbol O and atomic number 8. The element is very common, found not only on Earth but throughout the universe, usually covalently bonded with other elements. Unbound oxygen (usually called molecular oxygen, O₂, a diatomic molecule) first appeared on Earth during the Paleoproterozoic era (between 2500 million years ago and 1600 million years ago) and as a product of the metabolic action of early anaerobes (archaea and bacteria). The presence of free oxygen drove most of the organisms then living to extinction. The atmospheric abundance of free oxygen in later geological epochs and up to the present has been largely driven by photosynthetic organisms, roughly three quarters by phytoplankton and algae in the oceans and one quarter from terrestrial plants.

Characteristics

At standard temperature and pressure, oxygen is mostly found as a gas consisting of a diatomic molecule with the chemical formula O₂. O₂ has two energetic forms:
- The low-energy predominant single-bonded diradical triplet oxygen. This native diradical quality of oxygen contributes to its destructive chemical nature. This form is stabilized by the degeneracy effect.
- The high-energy double-bonded molecule singlet oxygen. Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. The word oxygen derives from two words in Greek, οξυς (oxys) (acid, sharp) and γεινομαι (geinomai) (engender). The name "oxygen" was chosen because, at the time it was discovered in the late 18th century, it was believed that all acids contained oxygen. The definition of acid has since been revised to not require oxygen in the molecular structure. Liquid O₂ and solid O₂ have a light blue color and both are highly paramagnetic. Liquid O₂ is usually obtained by the fractional distillation of liquid air. Liquid and solid O₃ (ozone) have a deeper color of blue. A recently discovered allotrope of oxygen, tetraoxygen (O₄), is a deep red solid that is created by pressurizing O₂ to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O₂ or O₃.

Applications

Liquid oxygen finds use as an oxidizer in rocket propulsion. Oxygen is essential to respiration, so oxygen supplementation has found use in medicine (as oxygen therapy). People who climb mountains or fly in airplanes sometimes have supplemental oxygen supplies (as air). Oxygen is used in welding (such as the oxyacetylene torch), and in the making of steel and methanol. Oxygen presents two absorption bands centered in the wavelengths 687 and 760 nanometers. Some scientists have proposed to use the measurement of the radiance coming from vegetation canopies in those oxygen bands to characterize plant health status from a satellite platform. This is because in those bands, it is possible to discriminate the vegetation's reflectance from the vegetation's fluorescence, which is much weaker. The measurement presents several technical difficulties due to the low signal to noise ratio and due to the vegetation's architecture, but it has been proposed as possibility to monitor the carbon cycle from satellite, thus in a global scale. Oxygen, as a mild euphoric, has a history of recreational use that extends into modern times. Oxygen bars can be seen at parties to this day. In the 19th century, oxygen was often mixed with nitrous oxide to promote an analgesic effect; indeed, such a mixture (Entonox) is commonly used in medicine today.

History

Oxygen was first discovered by Michał Sędziwój, Polish alchemist and philosopher in late 16th century. Sędziwój assumed the existence of oxygen by warming nitre (saltpeter). He thought of the gas given off as "the elixir of life". Oxygen was again discovered by the Swedish pharmacist Carl Wilhelm Scheele sometime before 1773, but the discovery was not published until after the independent discovery by Joseph Priestley on August 1, 1774, who called the gas dephlogisticated air (see phlogiston theory). Priestley published his discoveries in 1775 and Scheele in 1777; consequently Priestley is usually given the credit. It was named by Antoine Laurent Lavoisier after Priestley's publication in 1775.

Occurrence

Oxygen is the second most common component of the earth's atmosphere (20.947% by volume).

Compounds

Due to its electronegativity, oxygen forms chemical bonds with almost all other elements (which is the origin of the original definition of oxidation). The only elements to escape the possibility of oxidation are a few of the noble gases. The most famous of these oxides is dihydrogen monoxide, or water (H₂O). Other well known examples include compounds of carbon and oxygen, such as carbon dioxide (CO₂), alcohols (R-OH), aldehydes, (R-CHO), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO₃⁻), perchlorates (ClO₄⁻), chromates (CrO₄²⁻), dichromates (Cr₂O₇²⁻), permanganates (MnO₄⁻), and nitrates (NO₃⁻) are strong oxidizing agents in and of themselves. Many metals such as iron bond with oxygen atoms, iron (III) oxide (Fe₂O₃). Ozone (O₃) is formed by electrostatic discharge in the presence of molecular oxygen. A double oxygen molecule (O₂)₂ is known and is found as a minor component of liquid oxygen. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.

Isotopes

Oxygen has fifteen known isotopes with atomic masses ranging from 12 to 26. Three of them are stable and twelve are radioactive. The radioisotopes all have half lives of less than three minutes. The stable isotopes have mass numbers of 16, 17 and 18, of which oxygen-16 is the most common (over 99%).

Precautions

Oxygen can be toxic at elevated partial pressures (i.e. high relative concentrations). This is important in some forms of scuba diving, such as with a rebreather. Certain derivatives of oxygen, such as ozone (O₃), singlet oxygen, hydrogen peroxide, hydroxyl radicals and superoxide, are also highly toxic. The body has developed mechanisms to protect against these toxic species. For instance, the naturally-occurring glutathione can act as an antioxidant, as can bilirubin which is normally a breakdown product of hemoglobin. Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. This is true as well of compounds of oxygen such as chlorates, perchlorates, dichromates, etc. Compounds with a high oxidative potential can often cause chemical burns. The fire that killed the Apollo 1 crew on a test launchpad spread so rapidly because the pure oxygen atmosphere was at normal atmospheric pressure instead of the one third pressure that would be used during an actual launch. (See partial pressure.) Oxygen derivatives are prone to form free radicals, especially in metabolic processes. Because they can cause severe damage to cells and their DNA, they are thought to be related to cancer and aging.

References

- [http://periodic.lanl.gov/elements/8.html Los Alamos National Laboratory – Oxygen]
- [http://physics.nist.gov/cgi-bin/AtData/main_asd Nist atomic spectra database]
- [http://chartofthenuclides.com/default.html Nuclides and Isotopes Fourteenth Edition]: Chart of the Nuclides, General Electric Company, 1989

External links

- [http://www.priestleysociety.net Priestley Society, Dedicated to Joseph Priestley the man who discovered oxygen]
- [http://www.best-home-remedies.com/minerals/oxygen.htm Oxygen - Benefits, Deficiency Symptoms And Food Sources]
- [http://www.josephpriestley.info Joseph Priestley Information Website, about the man who discovered oxygen]
- [http://periodic.lanl.gov/elements/8.html Los Alamos National Laboratory – Oxygen]
- [http://www.webelements.com/webelements/elements/text/O/index.html WebElements.com – Oxygen]
- [http://education.jlab.org/itselemental/ele008.html It's Elemental – Oxygen]
- [http://members.tripod.com/tjaartdb0/html/oxygen_toxicity.html Oxygen Toxicity]
- [http://www.uigi.com/oxygen.html Oxygen (O2) Properties, Uses, Applications]
- [http://www.compchemwiki.org/index.php?title=Oxygen Computational Chemistry Wiki]
- [http://koti.mbnet.fi/antitz/dime/en Tests with liquid oxygen :-)] Category:Nonmetals Category:Chalcogens als:Sauerstoff ko:산소 ms:Oksigen ja:酸素 simple:Oxygen th:ออกซิเจน

Swamp

:This article is about the wetland type (a landform). For other uses of the term "swamp", see swamp (disambiguation). A swamp is a wetland that features permanent inundation of large areas of land by shallow bodies of water, generally with a substantial number of hummocks, or dry-land protrusions. Swamps usually are regarded as including a large amount of woody vegetation. When a wetland area does not, it is usually termed a marsh. A mire (or quagmire) is a low-lying wetland of deep, soft soil or mud that sinks underfoot. Swamps are generally characterized by very slow-moving waters, often rich in tannins from decaying vegetation. They are usually associated with adjacent rivers or lakes. In some cases, rivers become swamps for a distance. Swamps are features of areas with very low topographic relief, although they may be surrounded by mountains. The most famous swamps in the United States are the Okefenokee Swamp (home to the cartoon characters of Pogo, by Walt Kelly) and the Great Dismal Swamp. The Okefenokee is located in extreme southeastern Georgia and extends slightly into northeastern Florida. The Great Dismal Swamp lies in extreme southeastern Virginia and extreme northeastern North Carolina. Both are National Wildlife Refuges. Another swamp area, Reelfoot Lake of extreme western Tennessee, was created by the New Madrid earthquake of 1812. Caddo Lake, the Great Dismal and Reelfoot are swamps that are centered at large lakes. Swamps are often called bayous in the southeastern United States. Swamps are characterized by rich biodiversity and specialized organisms. For instance, southeastern U.S. swamps, such as those mentioned above, feature trees such as the Bald cypress and Water tupelo, which are adapted to growing in standing water, and animals such as the American alligator. A common species name in biological nomenclature is the Latin palustris, meaning "of the swamp". Examples of this are Quercus palustris (pin oak) and Thelypteris palustris (marsh fern). marsh fern]]

List of Major Swamps

Africa

- Bangweulu Swamp, Zambia
- Okavango Swamp, Botswana
- Sudd, Sudan
- Niger Delta,Nigeria

Asia

- Asmat Swamp, Indonesia
- Vasyugan Swamp, Russia

North America

- Caddo Lake, United States
- Great Black Swamp, United States
- Great Dismal Swamp, United States
- Okefenokee Swamp, Georgia, United States
- Reelfoot Lake, United States
- Big Cypress National Preserve, Florida, United States
- Limberlost, Indiana, United States
- Honey Island Swamp, Louisiana, United States

South America

- Pantanal, Brazil

Taxodium

Taxodium ascendens - Pond Cypress
Taxodium distichum - Bald Cypress
Taxodium mucronatum - Montezuma Cypress Taxodium is a genus of one to three species (depending on taxonomic opinion) of extremely flood-tolerant conifers in the cypress family, Cupressaceae, one of several genera in the family commonly known as cypresses. Within the family, Taxodium is most closely related to Chinese Swamp Cypress (Glyptostrobus pensilis) and Sugi (Cryptomeria japonica). Species of Taxodium occur in the southern part of the North American continent and are deciduous in the north and semi-evergreen to evergreen in the south. They are large trees, reaching 30-45 m tall and 2-3 m (exceptionally 11 m) trunk diameter. The needle-like leaves, 0.5-2 cm long, are borne spirally on the shoots, twisted at the base so as to appear in two flat rows on either side of the shoot. The cones are globose, 2-3.5 cm diameter, with 10-25 scales, each scale with 1-2 seeds; they are mature in 7-9 months after pollination, when they disintegrate to release the seeds. The male (pollen) cones are produced in pendulous racemes, and shed their pollen in early spring.

Species

The three taxa of Taxodium are treated here as distinct species, though some botanists treat them in just one or two species, with the others considered as varieties of the first described. The three are distinct in ecology, growing in different environments, but hybridise where they meet.
- Taxodium distichum - Bald Cypress The most familiar species in the genus is the Bald Cypress, native to much of the southeastern United States, from Delaware to Texas and inland up the Mississippi River to southern Indiana. It occurs mainly along rivers with silt-rich flood deposits.
- Taxodium ascendens - Pond Cypress The Pond Cypress occurs within the range of Bald Cypress, but only on the southeastern coastal plain from North Carol

Tree-line

Typical tree-line species

Table of alpine tree-lines

Table of arctic and antarctic tree-lines

Reference

North

Definitions

Etymology

Magnetic north and declination

Roles of north as prime direction

Roles of east and west as inherently subsidiary directions

See also

South

Elevation

Basic Definition

Determining Elevation

Links

Mountain

Heights

Characteristics

Geology

See also

External links

Coast

Environmental importance

Human impacts

Types of coast

See also

Coastal landforms & features

Processes

Related topics & articles

Famous coasts

Drought

Meanings

Consequences

Famous droughts

18th and 19th centuries, Cape Verde

1900, India

1921-22, Soviet Union

1928-30, northwest China

1936, Sichuan Province, China

1930-37, United States

1941, Sichuan Province, China

1965-67, India

1968-74, Sahel

1968-72, Iran

1973–74, Ethiopia

1983, Bolivia

1984-85, Ethiopia

See also

External links

Desert

Types of desert

Montane deserts

Desert features

Soils

Vegetation

Water

Mineral resources

List of deserts

Americas

Africa

Asia-Pacific

See also

Rainfall

Orographic precipitation

Convectional rainfall

Frontal Rainfall

Rainfall Patterns

Western

Eastern and Central

Inland Continental Areas

See also

Muskeg

Bog

World's largest wetlands

Linguistic distinctions

Bog habitats

Uses of bogs

Industrial uses