Iron
Iron is a chemical element with symbol Fe and atomic number 26. It is a metal, that belongs to group 8 of the periodic table, it is by mass the most common element on Earth, forming much of Earth's inner core. It is the fourth most common element in the Earth's crust. Pure iron is rare on the Earth's crust being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE; that event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost. Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts with oxygen and water to give brown to black hydrated iron oxides known as rust.
Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion. The body of an adult human contains about 3 to 5 grams of elemental iron in hemoglobin and myoglobin; these two proteins play essential roles in vertebrate metabolism oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. Chemically, the most common oxidation states of iron are +2 and +3. Iron shares many properties of other transition metals, including the other group 8 elements and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron forms many coordination compounds. At least four allotropes of iron are known, conventionally denoted α, γ, δ, ε; the first three forms are observed at ordinary pressures.
As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope; the physical properties of iron at high pressures and temperatures have been studied extensively, because of their relevance to theories about the cores of the Earth and other planets. Above 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed structure, known as ε-iron; the higher-temperature γ-phase changes into ε-iron, but does so at higher pressure. Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K, it is supposed to have a double hcp structure. The inner core of the Earth is presumed to consist of an iron-nickel alloy with ε structure.
The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus. This same trend appears for ruthenium but not osmium; the melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data still varies by tens of gigapascals and over a thousand kelvin. Below its Curie point of 770 °C, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom align with the spins of its neighbors, creating an overall magnetic field; this happens because the orbitals of those two electrons do not point toward neighboring atoms in the lattice, therefore are not involved in metallic bonding. In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometres across, such that the atoms in each domain have parallel spins, but different domains have other orientations.
Thus a macroscopic piece of iron will have a nearly zero overall magnetic field. Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field; this effect is exploited in devices that needs to channel magnetic fields, such as electrical transformers, magnetic recording heads, electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists after the external field is removed -- thus turning the iron object into a magnet. Similar behavior is exhibited by some iron compounds, such as the fer
Native Americans in the United States
Native Americans known as American Indians, Indigenous Americans and other terms, are the indigenous peoples of the United States, except Hawaii. There are over 500 federally recognized tribes within the US, about half of which are associated with Indian reservations; the term "American Indian" excludes Native Hawaiians and some Alaska Natives, while Native Americans are American Indians, plus Alaska Natives of all ethnicities. Native Hawaiians are not counted as Native Americans by the US Census, instead being included in the Census grouping of "Native Hawaiian and other Pacific Islander"; the ancestors of modern Native Americans arrived in what is now the United States at least 15,000 years ago much earlier, from Asia via Beringia. A vast variety of peoples and cultures subsequently developed. Native Americans were affected by the European colonization of the Americas, which began in 1492, their population declined precipitously due to introduced diseases as well as warfare, territorial confiscation and slavery.
After the founding of the United States, many Native American peoples were subjected to warfare and one-sided treaties, they continued to suffer from discriminatory government policies into the 20th century. Since the 1960s, Native American self-determination movements have resulted in changes to the lives of Native Americans, though there are still many contemporary issues faced by Native Americans. Today, there are over five million Native Americans in the United States, 78% of whom live outside reservations; when the United States was created, established Native American tribes were considered semi-independent nations, as they lived in communities separate from British settlers. The federal government signed treaties at a government-to-government level until the Indian Appropriations Act of 1871 ended recognition of independent native nations, started treating them as "domestic dependent nations" subject to federal law; this law did preserve the rights and privileges agreed to under the treaties, including a large degree of tribal sovereignty.
For this reason, many Native American reservations are still independent of state law and actions of tribal citizens on these reservations are subject only to tribal courts and federal law. The Indian Citizenship Act of 1924 granted U. S. citizenship to all Native Americans born in the United States. This emptied the "Indians not taxed" category established by the United States Constitution, allowed natives to vote in state and federal elections, extended the Fourteenth Amendment protections granted to people "subject to the jurisdiction" of the United States. However, some states continued to deny Native Americans voting rights for several decades. Bill of Rights protections do not apply to tribal governments, except for those mandated by the Indian Civil Rights Act of 1968. Since the end of the 15th century, the migration of Europeans to the Americas has led to centuries of population and agricultural transfer and adjustment between Old and New World societies, a process known as the Columbian exchange.
As most Native American groups had preserved their histories by oral traditions and artwork, the first written sources of the conflict were written by Europeans. Ethnographers classify the indigenous peoples of North America into ten geographical regions with shared cultural traits, called cultural areas; some scholars combine the Plateau and Great Basin regions into the Intermontane West, some separate Prairie peoples from Great Plains peoples, while some separate Great Lakes tribes from the Northeastern Woodlands. The ten cultural areas are as follows: Arctic, including Aleut and Yupik peoples Subarctic Northeastern Woodlands Southeastern Woodlands Great Plains Great Basin Northwest Plateau Northwest Coast California Southwest At the time of the first contact, the indigenous cultures were quite different from those of the proto-industrial and Christian immigrants; some Northeastern and Southwestern cultures, in particular, were matrilineal and operated on a more collective basis than that with which Europeans were familiar.
The majority of Indigenous American tribes maintained their hunting grounds and agricultural lands for use of the entire tribe. Europeans at that time had patriarchal cultures and had developed concepts of individual property rights with respect to land that were different; the differences in cultures between the established Native Americans and immigrant Europeans, as well as shifting alliances among different nations in times of war, caused extensive political tension, ethnic violence, social disruption. Before the European settlement of what is now the United States, Native Americans suffered high fatalities from contact with new European diseases, to which they had not yet acquired immunity. Smallpox epidemics are thought to have caused the greatest loss of life for indigenous populations. William M Denevan, noted author and Professor Emeritus of Geography at the University of Wisconsin-Madison, said on this subject in his essay "The Pristine Myth: The Landscape of the Americas in 1492".
Old World diseases were the primary killer. In many regions the tropical lowlands, populations fell by 90 percent or more in the first century after the contact. "Estimates of the pre-Columbian population of what today constitutes the U. S. vary ranging from William M Denevan's 3.8 million in his 1992 w
Metamorphic rock
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock is subjected to pressure, causing profound physical or chemical change; the protolith may be igneous, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface, they are classified by chemical and mineral assemblage. They may be formed by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it, they can form from tectonic processes such as continental collisions, which cause horizontal pressure and distortion. They are formed when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior; the study of metamorphic rocks provides information about the temperatures and pressures that occur at great depths within the Earth's crust. Some examples of metamorphic rocks are gneiss, marble and quartzite.
Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as index minerals, include sillimanite, staurolite and some garnet. Other minerals, such as olivines, amphiboles, micas and quartz, may be found in metamorphic rocks, but are not the result of the process of metamorphism; these minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are stable only within certain limits, the presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they formed; the change in the particle size of the rock during the process of metamorphism is called recrystallization. For instance, the small calcite crystals in the sedimentary rock limestone and chalk change into larger crystals in the metamorphic rock marble. Both high temperatures and pressures contribute to recrystallization.
High temperatures allow the atoms and ions in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their point of contact. The layering within metamorphic rocks is called foliation, it occurs when a rock is being shortened along one axis during recrystallization; this causes the platy or elongated crystals of minerals, such as mica and chlorite, to become rotated such that their long axes are perpendicular to the orientation of shortening. This results in a banded, or foliated rock, with the bands showing the colors of the minerals that formed them. Textures are separated into non-foliated categories. Foliated rock is a product of differential stress that deforms the rock in one plane, sometimes creating a plane of cleavage. For example, slate is a foliated metamorphic rock. Non-foliated rock does not have planar patterns of strain. Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated.
Where a rock has been subject to differential stress, the type of foliation that develops depends on the metamorphic grade. For instance, starting with a mudstone, the following sequence develops with increasing temperature: slate is a fine-grained, foliated metamorphic rock, characteristic of low grade metamorphism, while phyllite is fine-grained and found in areas of low grade metamorphism, schist is medium to coarse-grained and found in areas of medium grade metamorphism, gneiss coarse to coarse-grained, found in areas of high-grade metamorphism. Marble is not foliated, which allows its use as a material for sculpture and architecture. Another important mechanism of metamorphism is that of chemical reactions that occur between minerals without them melting. In the process atoms are exchanged between the minerals, thus new minerals are formed. Many complex high-temperature reactions may take place, each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism.
Metasomatism is the drastic change in the bulk chemical composition of a rock that occurs during the processes of metamorphism. It is due to the introduction of chemicals from other surrounding rocks. Water may transport these chemicals over great distances; because of the role played by water, metamorphic rocks contain many elements absent from the original rock, lack some that were present. Still, the introduction of new chemicals is not necessary for recrystallization to occur. Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock; the changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole. Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed country rock some distance away.
The formation of important ore minerals may o
Microorganism
A microorganism, or microbe, is a microscopic organism, which may exist in its single-celled form or in a colony of cells. The possible existence of unseen microbial life was suspected from ancient times, such as in Jain scriptures from 6th century BC India and the 1st century BC book On Agriculture by Marcus Terentius Varro. Microbiology, the scientific study of microorganisms, began with their observation under the microscope in the 1670s by Antonie van Leeuwenhoek. In the 1850s, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In the 1880s, Robert Koch discovered that microorganisms caused the diseases tuberculosis and anthrax. Microorganisms include all unicellular organisms and so are diverse. Of the three domains of life identified by Carl Woese, all of the Archaea and Bacteria are microorganisms; these were grouped together in the two domain system as Prokaryotes, the other being the eukaryotes. The third domain Eukaryota includes all multicellular organisms and many unicellular protists and protozoans.
Some protists are related to some to green plants. Many of the multicellular organisms are microscopic, namely micro-animals, some fungi and some algae, but these are not discussed here, they live in every habitat from the poles to the equator, geysers and the deep sea. Some are adapted to extremes such as hot or cold conditions, others to high pressure and a few such as Deinococcus radiodurans to high radiation environments. Microorganisms make up the microbiota found in and on all multicellular organisms. A December 2017 report stated that 3.45-billion-year-old Australian rocks once contained microorganisms, the earliest direct evidence of life on Earth. Microbes are important in human culture and health in many ways, serving to ferment foods, treat sewage, produce fuel and other bioactive compounds, they are essential tools in biology as model organisms and have been put to use in biological warfare and bioterrorism. They are a vital component of fertile soils. In the human body microorganisms make up the human microbiota including the essential gut flora.
They are the pathogens responsible for many infectious diseases and as such are the target of hygiene measures. The possible existence of microorganisms was discussed for many centuries before their discovery in the 17th century. By the fifth century BC, the Jains of present-day India postulated the existence of tiny organisms called nigodas; these nigodas are said to be born in clusters. According to the Jain leader Mahavira, the humans destroy these nigodas on a massive scale, when they eat, breathe and move. Many modern Jains assert that Mahavira's teachings presage the existence of microorganisms as discovered by modern science; the earliest known idea to indicate the possibility of diseases spreading by yet unseen organisms was that of the Roman scholar Marcus Terentius Varro in a 1st-century BC book titled On Agriculture in which he called the unseen creatures animalcules, warns against locating a homestead near a swamp: … and because there are bred certain minute creatures that cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and they cause serious diseases.
In The Canon of Medicine, Avicenna suggested that tuberculosis and other diseases might be contagious. Akshamsaddin mentioned the microbe in his work Maddat ul-Hayat about two centuries prior to Antonie Van Leeuwenhoek's discovery through experimentation: It is incorrect to assume that diseases appear one by one in humans. Disease infects by spreading from one person to another; this infection occurs through seeds that are so small they are alive. In 1546, Girolamo Fracastoro proposed that epidemic diseases were caused by transferable seedlike entities that could transmit infection by direct or indirect contact, or without contact over long distances. Antonie Van Leeuwenhoek is considered to be the father of microbiology, he was the first in 1673 to discover, describe and conduct scientific experiments with microoorganisms, using simple single-lensed microscopes of his own design. Robert Hooke, a contemporary of Leeuwenhoek used microscopy to observe microbial life in the form of the fruiting bodies of moulds.
In his 1665 book Micrographia, he made drawings of studies, he coined the term cell. Louis Pasteur exposed boiled broths to the air, in vessels that contained a filter to prevent particles from passing through to the growth medium, in vessels without a filter, but with air allowed in via a curved tube so dust particles would settle and not come in contact with the broth. By boiling the broth beforehand, Pasteur ensured that no microorganisms survived within the broths at the beginning of his experiment. Nothing grew in the broths in the course of Pasteur's experiment; this meant that the living organisms that grew in such broths came from outside, as spores on dust, rather than spontaneously generated within the broth. Thus, Pasteur supported the germ theory of disease. In 1876, Robert Koch established, he found that the blood of cattle which were infected with anthrax always had large numbers of Bacillus anthracis. Koch found that he could transmit anthrax from one animal to another by taking a small sample of blood from the infected animal and injecting it into a healthy one, this caused the healthy animal to become sick.
He found that he could grow the bacteria in a nutrient broth inject it into a heal
Desert
A desert is a barren area of landscape where little precipitation occurs and living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. About one-third of the land surface of the world is semi-arid; this includes much of the polar regions where little precipitation occurs and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location. Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks which break in pieces. Although rain occurs in deserts, there are occasional downpours that can result in flash floods. Rain falling on hot rocks can cause them to shatter and the resulting fragments and rubble strewn over the desert floor are further eroded by the wind; this wafts them aloft in sand or dust storms.
Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, the wind sorts sand into uniform deposits; the grains are piled high in billowing sand dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones; these areas are known as desert pavements and little further erosion takes place. Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water in the form of seepages from aquifers. Where these are found, oases can occur. Plants and animals living in the desert need special adaptations to survive in the harsh environment. Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles and spines to deter herbivory; some annual plants germinate and die in the course of a few weeks after rainfall while other long-lived plants survive for years and have deep root systems able to tap underground moisture.
Animals need to find enough food and water to survive. Many stay in the shade or underground during the heat of the day, they tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine. Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall, they reproduce while conditions are favorable before returning to dormancy. People have struggled to live in the surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available and oases have provided opportunities for a more settled way of life; the cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. Desert farming is possible with the aid of irrigation, the Imperial Valley in California provides an example of how barren land can be made productive by the import of water from an outside source. Many trade routes have been forged across deserts across the Sahara Desert, traditionally were used by caravans of camels carrying salt, gold and other goods.
Large numbers of slaves were taken northwards across the Sahara. Some mineral extraction takes place in deserts, the uninterrupted sunlight gives potential for the capture of large quantities of solar energy. English desert and its Romance cognates all come from the ecclesiastical Latin dēsertum, a participle of dēserere, "to abandon"; the correlation between aridity and sparse population is complex and dynamic, varying by culture and technologies. In English before the 20th century, desert was used in the sense of "unpopulated area", without specific reference to aridity. Phrases such as "desert island" and "Great American Desert", or Shakespeare's "deserts of Bohemia" in previous centuries did not imply sand or aridity. A desert is a region of land, dry because it receives low amounts of precipitation has little coverage by plants, in which streams dry up unless they are supplied by water from outside the area. Deserts receive less than 250 mm of precipitation each year; the potential evapotranspiration may be large but the actual evapotranspiration may be close to zero.
Semideserts are regions which receive between 250 and 500 mm and when clad in grass, these are known as steppes. Deserts have been defined and classified in a number of ways combining total precipitation, number of days on which this falls and humidity, sometimes additional factors. For example, Arizona, receives less than 250 mm of precipitation per year, is recognized as being located in a desert because of its aridity-adapted plants; the North Slope of Alaska's Brooks Range receives less than 250 mm of precipitation per year and is classified as a cold desert. Other regions of the world have cold deserts, including areas of the Himalayas and other high-altitude areas in other parts of the world. Polar deserts cover much of the ice-free
Aeolian processes
Aeolian processes spelled eolian or æolian, pertain to wind activity in the study of geology and weather and to the wind's ability to shape the surface of the Earth. Winds may erode and deposit materials and are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments. Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts; the term is derived from the name of the keeper of the winds. Wind erodes the Earth's surface by abrasion. Regions which experience intense and sustained erosion are called deflation zones. Most aeolian deflation zones are composed of desert pavement, a sheet-like surface of rock fragments that remains after wind and water have removed the fine particles. Half of Earth's desert surfaces are stony deflation zones; the rock mantle in desert pavements protects the underlying material from deflation. A dark, shiny stain, called desert varnish or rock varnish, is found on the surfaces of some desert rocks that have been exposed at the surface for a long period of time.
Manganese, iron oxides and clay minerals form most varnishes and provide the shine. Deflation basins, called blowouts, are hollows formed by the removal of particles by wind. Blowouts are small, but may be up to several kilometers in diameter. Wind-driven grains abrade landforms. In parts of Antarctica wind-blown snowflakes that are technically sediments have caused abrasion of exposed rocks. Grinding by particles carried in the wind creates grooves or small depressions. Ventifacts are rocks which have been cut, sometimes polished, by the abrasive action of wind. Sculpted landforms, called yardangs, are up to tens of meters high and kilometers long and are forms that have been streamlined by desert winds; the famous Great Sphinx of Giza in Egypt may be a modified yardang. Major global aeolian dust movements thought to influence and/or be influenced by weather and climate variation: From Sahara averaged 182 million tons of dust each year between 2007 and 2011 and carry it past the western edge of the Sahara at longitude 15W.
Variation: 86%. Destination: 132 mln tons cross the Atlantic, 27.7 mln tons fall in Amazon Basin, 43 mln make it to the Caribbean. Texas and Florida receive the dust. Events have become far more common in recent decades. Source: NASA's Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation data. Harmattan winter dust storms in West Africa occur blowing dust to the ocean. Gobi Desert to Korea, Japan and Western USA. See Asian dust. Thar Desert pre-monsoon towards Delhi, Uttar Pradesh, Indo-Gangetic Plain. See 2018 Indian dust storms. Shamal June–July winds blowing dust in north to south in Saudi Arabia, Iraq, UAE, parts of Pakistan. Haboob dust storms in Sudan, Arizona associated with monsoon. Khamsin dust from Libya and Levant in Spring associated with extratropical cyclones. Dust Bowl event in USA, carried sand eastward. 5500 tons were deposited in Chicago area. Sirocco sandy winds from Africa/Sahara blowing north into South Europe. Kalahari Desert blowing east to southern Indian Ocean and Australia.
Particles are transported by winds through suspension and creeping along the ground. Small particles may be held in the atmosphere in suspension. Upward currents of air support the weight of suspended particles and hold them indefinitely in the surrounding air. Typical winds near Earth's surface suspend particles less than 0.2 millimeters in diameter and scatter them aloft as dust or haze. Saltation is downwind movement of particles in a series of skips. Saltation lifts sand-size particles no more than one centimeter above the ground and proceeds at one-half to one-third the speed of the wind. A saltating grain may hit other grains; the grain may hit larger grains that are too heavy to hop, but that creep forward as they are pushed by saltating grains. Surface creep accounts for as much as 25 percent of grain movement in a desert. Aeolian turbidity currents are better known as dust storms. Air over deserts is cooled when rain passes through it; this cooler and denser air sinks toward the desert surface.
When it reaches the ground, the air is deflected forward and sweeps up surface debris in its turbulence as a dust storm. Crops, people and even climates are affected by dust storms; some dust storms are intercontinental, a few may circle the globe, they may engulf entire planets. When the Mariner 9 spacecraft entered its orbit around Mars in 1971, a dust storm lasting one month covered the entire planet, thus delaying the task of photo-mapping the planet's surface. Most of the dust carried by dust storms is in the form of silt-size particles. Deposits of this windblown silt are known as loess; the thickest known deposit of loess, 335 meters, is on the Loess Plateau in China. This same Asian dust is blown for thousands of miles, forming deep beds in places as far away as Hawaii. In Europe and in the Americas, accumulations of loess are from 20 to 30 meters thick; the soils developed on loess are highly productive for agriculture. Aeolian transport from deserts plays an important role in ecosystems globally, e.g. by transport of minerals from the Sahara to the Ama
Arid
A region is arid when it is characterized by a severe lack of available water, to the extent of hindering or preventing the growth and development of plant and animal life. Environments subject to arid climates are called xeric or desertic. Most "arid" climates straddle the Equator; the distribution of aridity observed at any one point in time is the result of the general circulation of the atmosphere. The latter does change over time through climate change. For example, temperature increase across the Nile Basin over the next 30–40 years could change the region from semi-arid to arid, resulting in a significant reduction in agricultural land. In addition, changes in land use can result in greater demands on soil water and induce a higher degree of aridity. Aridity index Arid Forest Research Institute Desert climate Desiccation tolerance Drought Relative humidity Saturation vapor pressure Griffiths, J. F.'Climatology', Chapter 2 in Handbook of Applied Meteorology, Edited by David D. Houghton, John Wiley and Sons, ISBN 0-471-08404-2.
Durrenberger, R. W.'Arid Climates', article in The Encyclopedia of Climatology, p. 92-101, Edited by J. E. Oliver and R. W. Fairbridge, Van Nostrand Reinhold Company, New York, ISBN 0-87933-009-0. Stadler, S. J'Aridity Indexes', article in The Encyclopedia of Climatology, p. 102-107, Edited by J. E. Oliver and R. W. Fairbridge, Van Nostrand Reinhold Company, New York, ISBN 0-87933-009-0. Blue Peace for the Nile Report, 2009, Strategic Foresight Group
