Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At room temperature and pressure, another solid form of carbon known as graphite is the chemically stable form, but diamond never converts to it. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are utilized in major industrial applications such as cutting and polishing tools, they are the reason that diamond anvil cells can subject materials to pressures found deep in the Earth. Because the arrangement of atoms in diamond is rigid, few types of impurity can contaminate it. Small numbers of defects or impurities color diamond blue, brown, purple, orange or red. Diamond has high optical dispersion. Most natural diamonds have ages between 1 billion and 3.5 billion years. Most were formed at depths between 150 and 250 kilometers in the Earth's mantle, although a few have come from as deep as 800 kilometers. Under high pressure and temperature, carbon-containing fluids dissolved minerals and replaced them with diamonds.
Much more they were carried to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites. Synthetic diamonds can be grown from high-purity carbon under high pressures and temperatures or from hydrocarbon gas by chemical vapor deposition. Imitation diamonds can be made out of materials such as cubic zirconia and silicon carbide. Natural and imitation diamonds are most distinguished using optical techniques or thermal conductivity measurements. Diamond is a solid form of pure carbon with its atoms arranged in a crystal. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond; the two most common allotropes of pure carbon are graphite. In graphite the bonds are sp2 orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart. In diamond they are sp3 and the atoms form tetrahedra with each bound to four nearest neighbors. Tetrahedra are rigid, the bonds are strong, of all known substances diamond has the greatest number of atoms per unit volume, why it is both the hardest and the least compressible.
It has a high density, ranging from 3150 to 3530 kilograms per cubic metre in natural diamonds and 3520 kg/m³ in pure diamond. In graphite, the bonds between nearest neighbors are stronger but the bonds between planes are weak, so the planes can slip past each other. Thus, graphite is much softer than diamond. However, the stronger bonds make graphite less flammable. Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness and thermal conductivity, as well as wide bandgap and high optical dispersion. Diamond's ignition point is 720 -- 800 °C in 850 -- 1000 °C in air; the equilibrium pressure and temperature conditions for a transition between graphite and diamond is well established theoretically and experimentally. The pressure changes linearly between 1.7 GPa at 0 K and 12 GPa at 5000 K. However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20 °C and 1 standard atmosphere, the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible.
However, at temperatures above about 4500 K, diamond converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa is needed. Above the triple point, the melting point of diamond increases with increasing pressure. At high pressures and germanium have a BC8 body-centered cubic crystal structure, a similar structure is predicted for carbon at high pressures. At 0 K, the transition is predicted to occur at 1100 GPa; the most common crystal structure of diamond is called diamond cubic. It is formed of unit cells stacked together. Although there are 18 atoms in the figure, each corner atom is shared by eight unit cells and each atom in the center of a face is shared by two, so there are a total of eight atoms per unit cell; each side of the unit cell is 3.57 angstroms in length. A diamond cubic lattice can be thought of as two interpenetrating face-centered cubic lattices with one displaced by 1/4 of the diagonal along a cubic cell, or as one lattice with two atoms associated with each lattice point.
Looked at from a <1 1 1> crystallographic direction, it is formed of layers stacked in a repeating ABCABC... pattern. Diamonds can form an ABAB... structure, known as hexagonal diamond or lonsdaleite, but this is far less common and is formed under different conditions from cubic carbon. Diamonds occur most as euhedral or rounded octahedra and twinned octahedra known as macles; as diamond's crystal structure has a cubic arrangement of the atoms, they have many facets that belong to a cube, rhombicosidodecahedron, tetrakis hexahedron or disdyakis dodecahedron. The crystals can be elongated. Diamonds are found coated in nyf, an opaque gum-like skin; some diamonds have opaque fibers. They are referred to as opaque if the fibers
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation include drizzle, sleet, snow and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates", thus and mist are not precipitation but suspensions, because the water vapor does not condense sufficiently to precipitate. Two processes acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called "showers."Moisture, lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed into clouds and rain. This process is active when freezing rain occurs. A stationary front is present near the area of freezing rain and serves as the foci for forcing and rising air.
Provided necessary and sufficient atmospheric moisture content, the moisture within the rising air will condense into clouds, namely stratus and cumulonimbus. The cloud droplets will grow large enough to form raindrops and descend toward the Earth where they will freeze on contact with exposed objects. Where warm water bodies are present, for example due to water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones. Lake-effect snowfall can be locally heavy. Thundersnow is possible within lake effect precipitation bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. Most precipitation is caused by convection; the movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes.
Precipitation is a major component of the water cycle, is responsible for depositing the fresh water on the planet. 505,000 cubic kilometres of water falls as precipitation each year. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres, but over land it is only 715 millimetres. Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes. Precipitation may occur on other celestial bodies, e.g. when it gets cold, Mars has precipitation which most takes the form of frost, rather than rain or snow. Precipitation is a major component of the water cycle, is responsible for depositing most of the fresh water on the planet. 505,000 km3 of water falls as precipitation each year, 398,000 km3 of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres. Mechanisms of producing precipitation include convective and orographic rainfall.
Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation, while stratiform processes involve weaker upward motions and less intense precipitation. Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice. Mixtures of different types of precipitation, including types in different categories, can fall simultaneously. Liquid forms of precipitation include drizzle. Rain or drizzle that freezes on contact within a subfreezing air mass is called "freezing rain" or "freezing drizzle". Frozen forms of precipitation include snow, ice needles, ice pellets and graupel; the dew point is the temperature to which a parcel must be cooled in order to become saturated, condenses to water. Water vapor begins to condense on condensation nuclei such as dust and salt in order to form clouds. An elevated portion of a frontal zone forces broad areas of lift, 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 form due to the lifting of advection fog during breezy conditions. There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, evaporative cooling. Adiabatic cooling occurs when air expands; the air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain. Conductive cooling occurs when the air comes into contact with a colder surface 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 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 lan
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics, including several countries. The term southern Africa or Southern Africa includes Angola, Eswatini, Malawi, Namibia, South Africa and Zimbabwe, though Angola may be included in Central Africa and Malawi, Mozambique and Zimbabwe in East Africa. From a political perspective the region is said to be unipolar with South Africa as a first regional power. Another geographic delineation for the region is the portion of Africa south of the Cunene and Zambezi Rivers – that is: South Africa, Eswatini, Botswana and the part of Mozambique which lies south of the Zambezi River; this definition is most used in South Africa for natural sciences and in guide books such as Roberts' Birds of Southern Africa, the Southern African Bird Atlas Project and Mammals of the Southern African Subregion. It is not used in political, economic or human geography contexts because this definition cuts Mozambique in two.
In the United Nations scheme of geographic regions, five states constitute Southern Africa: Botswana Eswatini Lesotho Namibia South AfricaThe Southern African Customs Union, created in 1969 comprises the five states in the UN subregion of Southern Africa. The Southern African Development Community was established in 1980 to facilitate co-operation in the region, it includes: Angola Botswana Comoros Democratic Republic of the Congo Eswatini Lesotho Madagascar Malawi Mauritius Mozambique Namibia Seychelles South Africa Tanzania Zambia Zimbabwe The region is sometimes reckoned to include other territories: Angola – part of Central Africa in the UN scheme. Comoros, Malawi, Mauritius, Mozambique, Réunion, Zambia, Zimbabwe – part of Eastern Africa in the UN scheme; the Democratic Republic of the Congo and Tanzania, though more reckoned in Central and Eastern Africa are included in Southern Africa as they are SADC members. The terrain of Southern Africa is varied; the region has both low-lying coastal areas, mountains.
In terms of natural resources, the region has the world's largest resources of platinum and the platinum group elements, chromium and cobalt, as well as uranium, copper, titanium and diamonds. The region is distinct from the rest of Africa, with some of its main exports including platinum, gold, copper and uranium, but it is similar in that it shares some of the problems of the rest of the continent. While colonialism has left its mark on the development over the course of history, today poverty, HIV/AIDS are some of the biggest factors impeding economic growth; the pursuit of economic and political stability is an important part of the region's goals, as demonstrated by the SADC. In terms of economic strength, South Africa is by far the dominant power of the region. South Africa's GDP alone is many times greater than the GDP's of all other countries in the region. Mining and tourism sectors dominate the economies of Southern African countries, apart from South Africa which has a mature and flourishing financial sector, retail sector, construction sector.
Most global banks have their regional offices for Southern Africa based in Johannesburg, South Africa. Over the years, some the other Southern African nations have invested in economic diversification, invested public funds into rail and air transportation as part of a concerted effort through SADC to boost regional trade and improve communication and transportation; the countries in this region belong to the Southern Africa Power Pool, which facilitates the development of a competitive electricity market within the SADC region and ensures sustainable energy developments through sound economic and social practices. The main objective of the power pool is to develop a world class and safe interconnected electrical system across the Southern African Region. According to a report by Southern Africa Power Pool, the three largest producers of electricity in Southern Africa as at 2017, include Eskom in South Africa with an estimated 46,963MW, Zesco in Zambia with 2,877MW and SNL of Angola with 2,442MW.
Southern Africa has a wide diversity of ecoregions including grassland, karoo and riparian zones. Though considerable disturbance has occurred in some regions from habitat loss due to human overpopulation or export-focused development, there remain significant numbers of various wildlife species, including white rhino, African leopard, kudu, blue wildebeest, vervet monkey and elephant, it has complex Plateaus. There are numerous environmental issues in Southern Africa, including air pollution and desertification. Southern Africa is home to many people, it was populated by indigenous or native Africans San and Pygmies in dispersed concentrations. Due to the Bantu expansion which edged the previous native African peoples to the more remote areas of the region, the majority of African ethnic groups in this region, including the Xhosa, Tsonga, Northern Ndebele, Southern Ndebele, Tswana and Shona people, BaLunda, Ovimbundu, Shona and Sukuma, speak Bantu languages; the process of colonization and settling resulted in a significant population of native European and Asian descent in many southern African co
A landlocked state or landlocked country is a sovereign state enclosed by land, or whose only coastlines lie on closed seas. There are 49 such countries, including five recognised states; as a rule, being landlocked creates political and economic handicaps that access to the high seas avoids. For this reason, states large and small across history have striven to gain access to open waters at great expense in wealth and political capital; the economic disadvantages of being landlocked can be alleviated or aggravated depending on degree of development, language barriers, other considerations. Some landlocked countries are quite affluent, such as Switzerland, Liechtenstein and Austria, all of which employ neutrality to their political advantage; the majority, are classified as Landlocked Developing Countries. Nine of the twelve countries with the lowest Human Development Indices are landlocked. Being landlocked has been disadvantageous to a country's development, it cuts a nation off from important sea resources such as fishing, impedes or prevents direct access to seaborne trade, a crucial component of economic and social advance.
As such, coastal regions tended to be wealthier and more populated than inland ones. Paul Collier in his book The Bottom Billion argues that being landlocked in a poor geographic neighborhood is one of four major development "traps" by which a country can be held back. In general, he found that when a neighboring country experiences better growth, it tends to spill over into favorable development for the country itself. For landlocked countries, the effect is strong, as they are limited in their trading activity with the rest of the world, he states, ``, you serve the world. Others have argued that being landlocked may be a blessing as it creates a "natural tariff barrier" which protects the country from cheap imports. In some instances, this has led to more robust local food systems. Landlocked developing countries have higher costs of international cargo transportation compared to coastal developing countries. Countries thus have made particular efforts to avoid being landlocked, by acquiring land that reaches the sea: As result of a 2005 territorial exchange with Ukraine, Moldova received a 600 m-long bank of the Danube River, subsequently building its Port of Giurgiulești there.
The International Congo Society, which owned the territory now constituting the Democratic Republic of the Congo, was awarded a narrow piece of land cutting through Angola to connect it to the sea by the Conference of Berlin in 1885. The Republic of Ragusa once gave the town of Neum to the Ottoman Empire because it did not want to have a land border with Venice. Since Bosnia and Herzegovina is a new country and ports have not been built for its need. There is no freight port along its short coastline at Neum, making it landlocked, although there are plans to change this. Instead the port of Ploče in Croatia is used. After World War I, in the Treaty of Versailles, a part of Germany designated "the Polish corridor" was given to the new Second Polish Republic, for access to the Baltic Sea; this without a large harbour. This was the pretext for making Danzig with its harbour the Free City of Danzig, to which Poland was given free access. However, the Germans placed obstacles to this free access when it came to military material.
In response, the small fishing harbour of Gdynia was soon enlarged. Until the dissolution of Austria–Hungary in 1918 at the end of World War I, Austrians and that empire's other nationalities had served in that country's navy, but since Austria and Hungary have both been landlocked countries. Countries can make agreements on getting free transport of goods through neighbor countries: The Treaty of Versailles required Germany to offer Czechoslovakia a lease for 99 years of parts of the ports in Hamburg and Stettin, allowing Czechoslovakia sea trade via the Elbe and Oder rivers. Stettin was annexed by Poland after World War II, but Hamburg continued the contract so that part of the port may still be used for sea trade by a successor of Czechoslovakia, the Czech Republic; the Danube is an international waterway, thus landlocked Austria, Moldova and Slovakia have secure access to the Black Sea. However, oceangoing ships cannot use the Danube, so cargo must be transloaded anyway, many overseas imports into Austria and Hungary use land transport from Atlantic and Mediterranean ports.
A similar situation exists for the Rhine river where Switzerland has boat access, but not oceangoing ships. Luxembourg has such through the Moselle, but Liechtenstein has no boat access though it is located along the Rhine, as the Rhine is not navigable that far upstream; the Mekong is an international waterway. However, it is not navigable above the Khone Phapheng Falls. Free ports allow transshipment to short-distance ships or river vessels; the TIR Treaty allows sealed road transport without customs checks and charges in Europe. Losing access to the sea is a great blow to a nation, politically and economically; the following ar
Snow refers to forms of ice crystals that precipitate from the atmosphere and undergo changes on the Earth's surface. It pertains to frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size and accumulate on surfaces metamorphose in place, melt, slide or sublimate away. Snowstorms develop by feeding on sources of atmospheric moisture and cold air. Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals. Snowflakes take on a variety of shapes, basic among these are platelets, needles and rime; as snow accumulates into a snowpack, it may blow into drifts. Over time, accumulated snow metamorphoses, by sintering and freeze-thaw. Where the climate is cold enough for year-to-year accumulation, a glacier may form. Otherwise, snow melts seasonally, causing runoff into streams and rivers and recharging groundwater. Major snow-prone areas include the polar regions, the upper half of the Northern Hemisphere and mountainous regions worldwide with sufficient moisture and cold temperatures.
In the Southern Hemisphere, snow is confined to mountainous areas, apart from Antarctica. Snow affects such human activities as transportation: creating the need for keeping roadways and windows clear. Snow affects ecosystems, as well, by providing an insulating layer during winter under which plants and animals are able to survive the cold. Snow develops in clouds; the physics of snow crystal development in clouds results from a complex set of variables that include moisture content and temperatures. The resulting shapes of the falling and fallen crystals can be classified into a number of basic shapes and combinations, thereof; some plate-like and stellar-shaped snowflakes can form under clear sky with a cold temperature inversion present. Snow clouds occur in the context of larger weather systems, the most important of, the low pressure area, which incorporate warm and cold fronts as part of their circulation. Two additional and locally productive sources of snow are lake-effect storms and elevation effects in mountains.
Mid-latitude cyclones are low pressure areas which are capable of producing anything from cloudiness and mild snow storms to heavy blizzards. During a hemisphere's fall and spring, the atmosphere over continents can be cold enough through the depth of the troposphere to cause snowfall. In the Northern Hemisphere, the northern side of the low pressure area produces the most snow. For the southern mid-latitudes, the side of a cyclone that produces the most snow is the southern side. A cold front, the leading edge of a cooler mass of air, can produce frontal snowsqualls—an intense frontal convective line, when temperature is near freezing at the surface; the strong convection that develops has enough moisture to produce whiteout conditions at places which line passes over as the wind causes intense blowing snow. This type of snowsquall lasts less than 30 minutes at any point along its path but the motion of the line can cover large distances. Frontal squalls may form a short distance ahead of the surface cold front or behind the cold front where there may be a deepening low pressure system or a series of trough lines which act similar to a traditional cold frontal passage.
In situations where squalls develop post-frontally it is not unusual to have two or three linear squall bands pass in rapid succession only separated by 25 miles with each passing the same point in 30 minutes apart. In cases where there is a large amount of vertical growth and mixing the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder, dubbed thundersnow. A warm front can produce snow for a period, as warm, moist air overrides below-freezing air and creates precipitation at the boundary. Snow transitions to rain in the warm sector behind the front. Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water, warming the lower layer of air which picks up water vapor from the lake, rises up through the colder air above, freezes and is deposited on the leeward shores; the same effect occurs over bodies of salt water, when it is termed ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores.
This uplifting can produce narrow but intense bands of precipitation, which deposit at a rate of many inches of snow each hour resulting in a large amount of total snowfall. The areas affected by lake-effect snow are called snowbelts; these include areas east of the Great Lakes, the west coasts of northern Japan, the Kamchatka Peninsula in Russia, areas near the Great Salt Lake, Black Sea, Caspian Sea, Baltic Sea, parts of the northern Atlantic Ocean. Orographic or relief snowfall is caused when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains; the lifting of air up the side of a mountain or range results in adiabatic cooling, condensation and precipitation. Moisture is removed by orographic lift, leaving drier, warmer air on the leeward side; the resulting enhanced productivity of snow fall and the decrease in temperature with elevation means that snow depth
In physical geography, tundra is a type of biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sami word тӯндар meaning "uplands", "treeless mountain tract". Tundra vegetation is composed of dwarf shrubs and grasses, lichens. Scattered trees grow in some tundra regions; the ecotone between the tundra and the forest is known as timberline. There are three regions and associated types of tundra: Arctic tundra, alpine tundra, Antarctic tundra. Arctic tundra occurs in the far Northern Hemisphere, north of the taiga belt; the word "tundra" refers only to the areas where the subsoil is permafrost, or permanently frozen soil. Permafrost tundra includes vast areas of northern Canada; the polar tundra is home to several peoples who are nomadic reindeer herders, such as the Nganasan and Nenets in the permafrost area. Arctic tundra is frozen for much of the year; the soil there is frozen from 25 to 90 cm down. Instead and sometimes rocky land can only support certain kinds of Arctic vegetation, low growing plants such as moss and lichen.
There are two main seasons and summer, in the polar tundra areas. During the winter it is cold and dark, with the average temperature around −28 °C, sometimes dipping as low as −50 °C. However, extreme cold temperatures on the tundra do not drop as low as those experienced in taiga areas further south. During the summer, temperatures rise somewhat, the top layer of seasonally-frozen soil melts, leaving the ground soggy; the tundra is covered in marshes, lakes and streams during the warm months. Daytime temperatures during the summer rise to about 12 °C but can drop to 3 °C or below freezing. Arctic tundras are sometimes the subject of habitat conservation programs. In Canada and Russia, many of these areas are protected through a national Biodiversity Action Plan. Tundra tends to be windy, with winds blowing upwards of 50–100 km/h. However, in terms of precipitation, it is desert-like, with only about 15–25 cm falling per year. Although precipitation is light, evaporation is relatively minimal. During the summer, the permafrost thaws just enough to let plants grow and reproduce, but because the ground below this is frozen, the water cannot sink any lower, so the water forms the lakes and marshes found during the summer months.
There is a natural pattern of accumulation of fuel and wildfire which varies depending on the nature of vegetation and terrain. Research in Alaska has shown fire-event return intervals that vary from 150 to 200 years, with dryer lowland areas burning more than wetter highland areas; the biodiversity of tundra is low: 1,700 species of vascular plants and only 48 species of land mammals can be found, although millions of birds migrate there each year for the marshes. There are a few fish species. There are few species with large populations. Notable animals in the Arctic tundra include reindeer, musk ox, Arctic hare, Arctic fox, snowy owl and polar bears near the ocean. Tundra is devoid of poikilotherms such as frogs or lizards. Due to the harsh climate of Arctic tundra, regions of this kind have seen little human activity though they are sometimes rich in natural resources such as petroleum, natural gas and uranium. In recent times this has begun to change in Alaska and some other parts of the world: for example, the Yamalo-Nenets Autonomous Okrug produces 90% of Russia's natural gas.
A severe threat to tundra is global warming. The melting of the permafrost in a given area on human time scales could radically change which species can survive there. Another concern is that about one third of the world's soil-bound carbon is in taiga and tundra areas; when the permafrost melts, it releases carbon in the form of carbon dioxide and methane, both of which are greenhouse gases. The effect has been observed in Alaska. In the 1970s the tundra was a carbon sink. Methane is produced when vegetation decays in wetlands; the amount of greenhouse gases which will be released under projected scenarios for global warming have not been reliably quantified by scientific studies, although a few studies were reported to be underway in 2011. It is uncertain whether the impact of increased greenhouse gases from this source will be minimal or massive. In locations where dead vegetation and peat has accumulated, there is a risk of wildfire, such as the 1,039 km2 of tundra which burned in 2007 on the north slope of the Brooks Range in Alaska.
Such events may both contribute to global warming. Antarctic tundra occurs on Antarctica and on several Antarctic and subantarctic islands, including South Georgia and the South Sandwich Islands and the Kerguelen Islands. Most of Antarctica is too cold and dry to support vegetation, most of the continent is covered by ice fields. However, some portions of the continent the Antarctic Peninsula, have areas of rocky soil that support plant life; the flora presently consists of around 300–400 lichens, 100 mosses, 25 liverworts, aro
United States dollar
The United States dollar is the official currency of the United States and its territories per the United States Constitution since 1792. In practice, the dollar is divided into 100 smaller cent units, but is divided into 1000 mills for accounting; the circulating paper money consists of Federal Reserve Notes that are denominated in United States dollars. Since the suspension in 1971 of convertibility of paper U. S. currency into any precious metal, the U. S. dollar is, de facto, fiat money. As it is the most used in international transactions, the U. S. dollar is the world's primary reserve currency. Several countries use it as their official currency, in many others it is the de facto currency. Besides the United States, it is used as the sole currency in two British Overseas Territories in the Caribbean: the British Virgin Islands and Turks and Caicos Islands. A few countries use the Federal Reserve Notes for paper money, while still minting their own coins, or accept U. S. dollar coins. As of June 27, 2018, there are $1.67 trillion in circulation, of which $1.62 trillion is in Federal Reserve notes.
Article I, Section 8 of the U. S. Constitution provides that the Congress has the power "To coin money". Laws implementing this power are codified at 31 U. S. C. § 5112. Section 5112 prescribes the forms; these coins are both designated in Section 5112 as "legal tender" in payment of debts. The Sacagawea dollar is one example of the copper alloy dollar; the pure silver dollar is known as the American Silver Eagle. Section 5112 provides for the minting and issuance of other coins, which have values ranging from one cent to 100 dollars; these other coins are more described in Coins of the United States dollar. The Constitution provides that "a regular Statement and Account of the Receipts and Expenditures of all public Money shall be published from time to time"; that provision of the Constitution is made specific by Section 331 of Title 31 of the United States Code. The sums of money reported in the "Statements" are being expressed in U. S. dollars. The U. S. dollar may therefore be described as the unit of account of the United States.
The word "dollar" is one of the words in the first paragraph of Section 9 of Article I of the Constitution. There, "dollars" is a reference to the Spanish milled dollar, a coin that had a monetary value of 8 Spanish units of currency, or reales. In 1792 the U. S. Congress passed a Coinage Act. Section 9 of that act authorized the production of various coins, including "DOLLARS OR UNITS—each to be of the value of a Spanish milled dollar as the same is now current, to contain three hundred and seventy-one grains and four sixteenth parts of a grain of pure, or four hundred and sixteen grains of standard silver". Section 20 of the act provided, "That the money of account of the United States shall be expressed in dollars, or units... and that all accounts in the public offices and all proceedings in the courts of the United States shall be kept and had in conformity to this regulation". In other words, this act designated the United States dollar as the unit of currency of the United States. Unlike the Spanish milled dollar, the U.
S. dollar is based upon a decimal system of values. In addition to the dollar the coinage act established monetary units of mill or one-thousandth of a dollar, cent or one-hundredth of a dollar, dime or one-tenth of a dollar, eagle or ten dollars, with prescribed weights and composition of gold, silver, or copper for each, it was proposed in the mid-1800s that one hundred dollars be known as a union, but no union coins were struck and only patterns for the $50 half union exist. However, only cents are in everyday use as divisions of the dollar. XX9 per gallon, e.g. $3.599, more written as $3.599⁄10. When issued in circulating form, denominations equal to or less than a dollar are emitted as U. S. coins while denominations equal to or greater than a dollar are emitted as Federal Reserve notes. Both one-dollar coins and notes are produced today, although the note form is more common. In the past, "paper money" was issued in denominations less than a dollar and gold coins were issued for circulation up to the value of $20.
The term eagle was used in the Coinage Act of 1792 for the denomination of ten dollars, subsequently was used in naming gold coins. Paper currency less than one dollar in denomination, known as "fractional currency", was sometimes pejoratively referred to as "shinplasters". In 1854, James Guthrie Secretary of the Treasury, proposed creating $100, $50 and $25 gold coins, which were referred to as a "Union", "Half Union", "Quarter Union", thus implying a denomination of 1 Union = $100. Today, USD notes are made from cotton fiber paper, unlike most common paper, made of wood fiber. U. S. coins are produced by the United States Mint. U. S. dollar banknotes are printed by the Bureau of Engraving and Printing and, since 1914, have been issued by t