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Earth (pronounced /ɝːθ/ (help·info))^[10] is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Terra.^{[note 3]}

Home to millions of species,^[11] including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years ago,^[12]^[13]^[14]^[15] and life appeared on its surface within a billion years. Since then, Earth’s biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth’s magnetic field, blocks harmful radiation, permitting life on land.^[16] The physical properties of the Earth, as well as its geological history and orbit, allowed life to persist during this period. The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.^[17]

Earth’s outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet’s surface.^{[note 4]}^{[note 5]} Earth’s interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.^{[note 6]} The Earth’s axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,^[18] producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days). Earth’s only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet’s rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.^[19] Later, asteroid impacts caused significant changes to the surface environment.

Both the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. The inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a flat Earth, and a modern perspective of the world as an integrated environment that requires stewardship. Humans first left the planet in 1961, when Yuri Gagarin reached outer space.

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Chronology

Main article: History of Earth

See also: Geological history of Earth

Scientists have been able to reconstruct detailed information about the planet’s past. About 4.54 billion years ago (within an uncertainty of 1%),^[12]^[13]^[14]^[15] the Earth and the other planets in the Solar System formed out of the solar nebula—a disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was largely completed within 10–20 million years.^[20] Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterward, possibly as the result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth’s mass^[21] impacting the Earth in a glancing blow.^[22] Some of this object’s mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.

Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans.^[19] Beginning with almost no dry land, the total amount of surface lying above the oceans has steadily increased. During the past two billion years, for example, the total size of the continents has doubled.^[23]^[24] As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), one the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya.^[25]

Evolution of life

Main article: Evolutionary history of life

The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life existed.^[26] The development of photosynthesis allowed the Sun’s energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of ozone (a form of molecular oxygen [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.^[27] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.^[28]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed “Snowball Earth“, and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.^[29]

Following the Cambrian explosion, about 535 mya, there have been five mass extinctions.^[30] The last extinction event was 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.^[31] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,^[32] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 mya and then intensified during the Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.^[33]

Future

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun’s core, the star’s total luminosity will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 Gyr (1.1 billion years) and by 40% over the next 3.5 Gyr.^[34] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet’s oceans.^[35]

The Earth’s increasing surface temperature will accelerate the inorganic CO₂ cycle, reducing its concentration to the lethal levels for plants (10 ppm for C4 photosynthesis) in 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.^[23] But even if the Sun were eternal and stable, the continued internal cooling of the Earth would have resulted in a loss of much of its atmosphere and oceans (due to lower volcanism).^[36] After another billion years the surface water will have completely disappeared^[17] and the mean global temperature will reach 70°C.^[23] The Earth is expected to be effectively habitable for another 500 million years or so.^[37]

The Sun, as part of its evolution, will expand to a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present size, roughly 1 AU (150,000,000 km).^[34]^[38] Earth’s fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will be in an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. Therefore, the planet is expected to escape envelopment by the expanded Sun’s sparse outer atmosphere, though most, if not all, existing life will be destroyed because of the Sun’s increased luminosity.^[34] However, a more recent simulation indicates that Earth’s orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun’s atmosphere and be destroyed.^[38]

Composition and structure

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation.^[39] It also is the only terrestrial planet with active plate tectonics.^[40]

Shape

Size comparison of inner planets (left to right): Mercury, Venus, Earth, and Mars

The Earth’s shape is very close to an oblate spheroid—a rounded shape with a bulge around the equator—although the precise shape (the geoid) varies from this by up to 100 meters.^[41] The average diameter of the reference spheroid is about 12,742 km. More approximately the distance is 40,000 km/π because the meter was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.^[42]

The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km larger than the pole to pole diameter.^[43] The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in billiard balls.^[44] Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.^[45]

F. W. Clarke’s Table of Crust Oxides
Compound	Formula	Composition
silica	SiO₂	59.71%
alumina	Al₂O₃	15.41%
lime	CaO	4.90%
Magnesia	MgO	4.36%
sodium oxide	Na₂O	3.55%
iron(II) oxide	FeO	3.52%
potassium oxide	K₂O	2.80%
iron(III) oxide	Fe₂O₃	2.63%
water	H₂O	1.52%
titanium dioxide	TiO₂	0.60%
phosphorus pentoxide	P₂O₅	0.22%
Total		99.22%

Chemical composition

The mass of the Earth is approximately 5.98×10²⁴ kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminum (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.^[46]

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth’s crust consists of oxygen. The more common rock constituents of the Earth’s crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.^{[note 7]}

Internal structure

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or rheological properties. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents.^[47] The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.^[48]

Geologic layers of the Earth^[49]
Depth^[50] km	Component Layer	Density g/cm³
0–60	Lithosphere^[51]	—
0–35	… Crust^[52]	2.2–2.9
35–60	… Upper mantle	3.4–4.4
35–2890	Mantle	3.4–5.6
100–700	… Asthenosphere	—
2890–5100	Outer core	9.9–12.2
5100–6378	Inner core	12.8–13.1

The internal heat of the planet is probably produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes. All three have half-life decay periods of more than a billion years.^[53] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.^[54] A portion of the core’s thermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.^[55]

Tectonic plates

According to plate tectonics theory, the outermost part of the Earth’s interior is made up of two layers: the lithosphere, comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which forms the inner part of the upper mantle. The asthenosphere behaves like a superheated material that is in a semi-fluidic, plastic-like state.^[56]

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent, divergent and transform. The last occurs where two plates move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.^[57]

Earth’s main plates^[58]
A map illustrating the Earth’s major plates.	Plate name	Area 10⁶ km²
	African Plate	61.3
	Antarctic Plate	60.9
	Australian Plate	47.2
	Eurasian Plate	67.8
	North American Plate	75.9
	South American Plate	43.6
	Pacific Plate	103.3

Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate actually fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr^[59] and the Pacific Plate moving 52–69 mm/yr. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr.^[60]

Surface

The Earth’s terrain varies greatly from place to place. About 70.8%^[61] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,^[43] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts^[62] also act to reshape the landscape.

Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center’s TerrainBase Digital Terrain Model.

As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the oceanic crustal material. Most of the ocean floor is less than 100 million years in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.^[63]^[64]

The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.^[65] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.^[66] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth’s surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.^[67] Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.^[68]

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.^[6]Close to 40% of the Earth’s land surface is presently used for cropland and pasture, or an estimated 1.3×10⁷ km² of cropland and 3.4×10⁷ km² of pastureland.^[69]

The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.^[70]

Hydrosphere

Elevation histogram of the surface of the Earth—approximately 71% of the Earth’s surface is covered with water.

The abundance of water on Earth’s surface is a unique feature that distinguishes the “Blue Planet” from others in the solar system. The Earth’s hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m.^[71]^[72] The average depth of the oceans is 3,800 m, more than four times the average height of the continents.^[70]

The mass of the oceans is approximately 1.35×10¹⁸ metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386×10⁹ km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.^{[note 8]} About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.^[73]

About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.^[74] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.^[75] Sea water has an important influence on the world’s climate, with the oceans acting as a large heat reservoir.^[76] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.^[77]

Atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.^[9] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation due to weather and seasonal factors.^[78]

Earth’s biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere that exists today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth’s magnetic field, blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.^[79] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth’s atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist.^[61]

Weather and climate

The Earth’s atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere’s mass is contained within the first 11 km of the planet’s surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.^[80]

The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.^[81] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.^[82]

Source regions of global air masses.

Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.^[80] Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.^[83]

The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.^[84] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen’s student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.^[81]

Upper atmosphere

This view from orbit shows the full Moon partially obscured by the Earth’s atmosphere. NASA image.

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.^[79] Each of these layers has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere. This is where the Earth’s magnetic fields interact with the solar wind.^[85] An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The Kármán line, defined as 100 km above the Earth’s surface, is a working definition for the boundary between atmosphere and space.^[86]

Due to thermal energy, some of the molecules at the outer edge of the Earth’s atmosphere have their velocity increased to the point where they can escape from the planet’s gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses.^[87] The leakage of hydrogen into space is a contributing factor in pushing the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.^[88] Hence the ability of hydrogen to escape from the Earth’s atmosphere may have influenced the nature of life which developed on the planet.^[89] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.^[90]

Magnetic field

The Earth’s magnetic field, which approximates a dipole.

The Earth’s magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet’s geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth’s magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.^[91]^[92]

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth’s atmosphere at the magnetic poles, it forms the aurora.^[93]

Orbit and rotation

Rotation

Earth’s axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit.

Earth’s rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time. Each of these seconds is slightly longer than an SI second because Earth’s solar day is now slightly longer than it was during the 19th century due to tidal acceleration.^[94]

Earth’s rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86164.098903691 seconds of mean solar time (UT1), or 23^h 56^m 4.098903691^s. ^[95]^{[note 9]} Earth’s rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86164.09053083288 seconds of mean solar time (UT1) (23^h 56^m 4.09053083288^s).^[95] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.^[96] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005^[97] and 1962–2005.^[98]

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth’s sky is to the west at a rate of 15°/h = 15′/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes; the apparent sizes of the Sun and the Moon are approximately the same.^[99]^[100]

Orbit

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter every 12 hours. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet’s diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.^[9]

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system’s common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth’s axis is tilted some 23.5 degrees from the perpendicular to the Earth–Sun plane, and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.^[101]^[9]

The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius.^[102]^{[note 10]} This is maximum distance at which the Earth’s gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 light years from the center of the galaxy, and about 20 light years above the galaxy’s equatorial plane in the Orion spiral arm.^[103]

Axial tilt and seasons

Because of the axial tilt of the Earth, the amount of sunlight reaching the surface varies over the course of the year. This results in seasonal change in climate, with summer in the northern hemisphere occuring when the north pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the arctic circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the south pole oriented opposite the direction of the north pole.

Earth and Moon from Mars, imaged by Mars Global Surveyor. From space, the Earth can be seen to go through phases similar to the phases of the Moon.

By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23.^[104]

The angle of the Earth’s tilt is relatively stable over long periods of time. However, the tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years. The orientation (rather than the angle) of the Earth’s axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth’s equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.^[105]

In modern times, Earth’s perihelion occurs around January 3, and the aphelion around July 4. However, these dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance results in an increase of about 6.9%^[106] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.^[107]

Moon

Name	Diameter	Mass	Semi-major axis	Orbital period
Moon	3,474.8 km	7.349×10²² kg	384,400 km	27 days, 7 hours, 43.7 minutes
Moon	2,159.2 mi	8.1×10¹⁹ (short) tons	238,700 mi	27 days, 7 hours, 43.7 minutes

The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth’s. It is the largest moon in the solar system relative to the size of its planet. (Charon is larger relative to the dwarf planet Pluto.) The natural satellites orbiting other planets are called “moons” after Earth’s Moon.

The gravitational attraction between the Earth and Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.

Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications—and the lengthening of Earth’s day by about 23 µs a year—add up to significant changes.^[108] During the Devonian period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.^[109]

The Moon may have dramatically affected the development of life by moderating the planet’s climate. Paleontological evidence and computer simulations show that Earth’s axial tilt is stabilized by tidal interactions with the Moon.^[110] Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth’s equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.^[111] If Earth’s axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life.^[112] However, this is a controversial subject, and further studies of Mars—which has a similar rotation period and axial tilt as Earth, but not its large Moon or liquid core—may settle the matter.

Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun’s diameter is about 400 times as large as the Moon’s, it is also 400 times more distant.^[100] This allows total and annular eclipses to occur on Earth.

A scale representation of the relative sizes of, and distance between, Earth and Moon.

The most widely accepted theory of the Moon’s origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon’s relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth’s crust.^[113]

Earth has at least two co-orbital asteroids, 3753 Cruithne and 2002 AA₂₉.^[114]

Habitability

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain metabolism.^[115] The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.^[116]

Biosphere

The planet’s life forms are sometimes said to form a “biosphere“. This biosphere is generally believed to have begun evolving about 3.5 billion years ago. Earth is the only place in the universe where life is known to exist. Some scientists believe that Earth-like biospheres might be rare.^[117]

The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land primarily latitude and height above the sea level separates biomes. Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are relatively barren of plant and animal life, while the greatest latitudinal diversity of species is found at the Equator.^[118]

Natural resources and land use

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.

Large deposits of fossil fuels are obtained from the Earth’s crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth’s crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics.^[119] These bodies form concentrated sources for many metals and other useful elements.

The Earth’s biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.^[120] Humans also live on the land by using building materials to construct shelters. In 1993, human use of land is approximately:

Land use	Percentage
Arable land:	13.13%^[6]
Permanent crops:	4.71%^[6]
Permanent pastures:	26%
Forests and woodland:	32%
Urban areas:	1.5%
Other:	30%

The estimated amount of irrigated land in 1993 was 2,481,250 km².^[6]

Natural and environmental hazards

Large areas are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.

A scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather conditions and a global rise in average sea levels.^[121]

Human geography

Antarctica

Oceania

Africa

Asia

Europe

North
America

South
America

Pacific
Ocean

Atlantic
Ocean

Indian
Ocean

Southern Ocean

Arctic Ocean

The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.

Main article: Human geography

See also: World

Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Earth has approximately 6,707,000,000 human inhabitants as of July 2008.^[122] Projections indicate that the world’s human population will reach seven billion in 2013 and 9.2 billion^[123] in 2050. Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world’s population is expected to be living in urban, rather than rural, areas.^[124]

It is estimated that only one eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is either desert (14%),^[125] high mountains (27%),^[126] or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.^[127] (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)

Independent sovereign nations claim the planet’s entire land surface, with the exception of some parts of Antarctica. As of 2007 there are 201 sovereign states, including the 192 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities. Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.

The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. It is not, however, a world government. While the U.N. provides a mechanism for international law and, when the consensus of the membership permits, armed intervention,^[128] it serves primarily as a forum for international diplomacy.

In total, about 400 people have been outside the Earth’s atmosphere as of 2004, and, of these, twelve have walked on the Moon. Normally the only humans in space are those on the International Space Station. The station’s crew of three people is usually replaced every six months.

Cultural viewpoint

The first photograph ever taken of an “Earthrise,” on Apollo 8.

The name Earth was derived from the Anglo-Saxon word erda, which means ground or soil. It became eorthe in Old English, then erthe in Middle English.^[129] The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.^[130]

Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess, also called the Mother Earth, is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism^[131] or Islam,^[132] assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life.^[133] Such assertions are opposed by the scientific community^[134]^[135] and other religious groups.^[136]^[137]^[138] A prominent example is the creation-evolution controversy.

In the past there were varying levels of belief in a flat Earth,^[139] but this was displaced by the concept of a Spherical Earth due to observation and circumnavigation.^[140] The human perspective regarding the Earth has changed following the advent of spaceflight, and the biosphere is now widely viewed from a globally integrated perspective.^[141]^[142] This is reflected in a growing environmental movement that is concerned about humankind’s effects on the planet.^[143]

Notes

^ All astronomical quantities vary, both secularly and periodically. The quantities given are the values at the instant J2000.0 of the secular variation, ignoring all periodic variations.
^ Due to natural fluctuations, ambiguities surrounding ice shelves, and mapping conventions for vertical datums, exact values for land and ocean coverage are not meaningful. Based on data from the Vector Map and Global Landcover datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of the earth’s surface. Note that the ice shields of Antarctica and Greenland are counted as land, even though much of the rock which supports them lies below sea level.
^ Note that by International Astronomical Union convention, the term “Terra” is used for naming extensive land masses, rather than for the planet Earth. C.f.:
Blue, Jennifer (2007-07-05). “Descriptor Terms (Feature Types)“. Gazetteer of Planetary Nomenclature. USGS. Retrieved on 2007-07-05.
^ Other planets in the solar system are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See: Msnbc (2007-03-02). “Rover reveals Mars was once wet enough for life“, NASA. Retrieved on 28 August 2007. Staff (2005-11-07). “Simulations Show Liquid Water Could Exist on Mars“, University of Arkansas. Retrieved on 8 August 2007.
^ As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: Tinetti, G. et al (July 2007). “Water vapour in the atmosphere of a transiting extrasolar planet“. Nature 448: 169–171. doi:10.1038/nature06002, http://www.nature.com/nature/journal/v448/n7150/abs/nature06002.html.
^ The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis.
^ This article incorporates text from the Encyclopædia Britannica Eleventh Edition article “Petrology”, a publication now in the public domain.
^ The total volume of the Earth’s oceans is: 1.4×10⁹ km³. The total surface area of the Earth is 5.1×10⁸ km². So, to first approximation, the average depth would be the ratio of the two, or 2.7 km.
^ Aoki, the ultimate source of these figures, uses the term “seconds of UT1″ instead of “seconds of mean solar time”.—Aoki, S.; Kinoshita, H.; Guinot, B.; Kaplan, G. H.; McCarthy, D. D.; Seidelmann, P. K. (1982). “The new definition of universal time“. Astronomy and Astrophysics 105 (2): 359–361, http://adsabs.harvard.edu/abs/1982A&A…105..359A. Retrieved on 23 September 2008.
^ For the Earth, the Hill radius is

,

where m is the mass of the Earth, a is an Astronomical Unit, and M is the mass of the Sun. So the radius in A.U. is about: .

WATER

edwinsebastian — Sun, 12 Oct 2008 04:33:15 +0000

Water

This article is about the chemical substance. For its chemical and physical properties, see water (molecule). For other uses, see Water (disambiguation).

Water in three states: liquid, solid (ice), and (invisible) vapor in air. Clouds are droplets of liquid, condensed from water vapor.

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. About 1.460 petatonnes (Pt) (10²¹kilograms) of water covers 71% of the Earth’s surface, mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation.^[1] Saltwater oceans hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%. Some of the Earth’s water is contained within water towers, biological bodies, manufactured products, and food stores. Other water is trapped in ice caps, glaciers, aquifers, or in lakes, sometimes providing fresh water for life on land.

Water moves continually through a cycle of evaporation or transpiration (evapotranspiration), precipitation, and runoff, usually reaching the sea. Winds carry water vapor over land at the same rate as runoff into the sea, about 36 Tt per year. Over land, evaporation and transpiration contribute another 71 Tt per year to the precipitation of 107 Tt per year over land. Clean, fresh drinking water is essential to human and other life. However, in many parts of the world—especially developing countries—there is a water crisis, and it is estimated that by 2025 more than half of the world population will be facing water-based vulnerability.^[2] Water plays an important role in the world economy, as it functions as a solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of freshwater is consumed by agriculture.^[3]

Types of water

Liquid water in motion

Water can appear in three states. Water takes many different forms on Earth: water vapor and clouds in the sky; seawater and rarely icebergs in the ocean; glaciers and rivers in the mountains; and aquifers in the ground.

Water can dissolve many different substances, giving it different tastes and odors. In fact, humans and other animals have developed senses to be able to evaluate the potability of water, avoiding water that is too salty or putrid. Humans also tend to prefer cold water rather than lukewarm, as cold water is likely to contain fewer microbes. The taste advertised in spring water or mineral water derives from the minerals dissolved in it, as pure H₂O is tasteless. As such, purity in spring and mineral water refers to purity from toxins, pollutants, and microbes.

Different names are given to water’s various forms:

Snowflakes by Wilson Bentley, 1902

according to state
- solid – ice
- liquid – water
- gaseous – water vapor

according to meteorology:
- hydrometeor
  - precipitation

	precipitation according to moves		precipitation according to state
	vertical (falling) precipitation rain freezing rain drizzle freezing drizzle snow snow pellets snow grains ice pellets frozen rain hail ice crystals horizontal (seated) precipitation dew hoarfrost atmospheric icing glaze ice		liquid precipitation rain freezing rain drizzle freezing drizzle dew solid precipitation snow snow pellets snow grains ice pellets frozen rain hail ice crystals hoarfrost atmospheric icing glaze ice mixed precipitation in temperatures around 0 °C

- levitating particles
  - clouds
  - fog
  - BR (according to METAR)
- ascending particles (drifted by wind)
  - spindrift
  - stirred snow
according to occurrence
- groundwater
- meltwater
- meteoric water
- connate water
- fresh water
- surface water
- mineral water – contains much minerals
- brackish water
- dead water – strange phenomenon which can occur when a layer of fresh or brackish water rests on top of more dense salt water, without the two layers mixing. It is dangerous for ship traveling.
- seawater
- brine
according to uses
- tap water
- bottled water
- drinking water or potable water – useful for everyday drinking, without fouling, it contains balanced minerals that are not harmful to health (see below)
- purified water, laboratory-grade, analytical-grade or reagent-grade water – water which has been highly purified for specific uses in science or engineering. Often broadly classified as Type I, Type II, or Type III, this category of water includes, but is not limited to the following:

according to other features
- soft water – contains less minerals
- hard water – from underground, contains more minerals
- distilled water, double distilled water, deionized water – contains no minerals
- Water of crystallization — water incorporated into crystalline structures
- Hydrates — water bound into other chemical substances
- heavy water – made from heavy atoms of hydrogen – deuterium. It is in nature in normal water in very low concentration. It was used in construction of first nuclear reactors.
- tritiated water

according to microbiology

according to religion
- holy water

Chemical and physical properties

Water
Water is a necessary solvent for all known life, and an abundant compound on the earth’s surface.
Information and properties
Common name	water
IUPAC name	oxidane
Alternative names	aqua, dihydrogen monoxide, hydrogen hydroxide, (more)
Molecular formula	H₂O
CAS number	7732-18-5
InChI	InChI=1/H2O/h1H2
Molar mass	18.0153 g/mol
Density and phase	0.998 g/cm³ (liquid at 20 °C, 1 atm) 0.917 g/cm³ (solid at 0 °C, 1 atm)
Melting point	0 °C (273.15 K) (32 °F)
Boiling point	99.974 °C (373.124 K) (211.95 °F)
Specific heat capacity	4.184 J/(g·K) (liquid at 20 °C) 74.539 J/ (mol·K) (liquid at 25 °C)
Supplementary data page
Disclaimer and references

Main article: Water (molecule)

model of hydrogen bonds between molecules of water

Impact from a water drop causes an upward “rebound” jet surrounded by circular capillary waves.

Dew drops adhering to a spider web

capillary action of water compared to mercury

Water is the chemical substance with chemical formula H₂O: one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom.

The major chemical and physical properties of water are:

Water is a tasteless, odorless liquid at ambient temperature and pressure. The color of water and ice are, intrinsically, a very light blue hue, although water appears colorless in small quantities. Ice also appears colorless, and water vapor is essentially invisible as a gas.^[4]

Water is transparent, and thus aquatic plants can live within the water because sunlight can reach them. Only strong UV light is slightly absorbed.

Since oxygen has a higher electronegativity than hydrogen, water is a polar molecule. The oxygen has a slight negative charge while the hydrogens have a slight positive charge giving the article a strong effective dipole moment. The interactions between the different dipoles of each molecule cause a net attraction force associated with water’s high amount of surface tension.

Another very important force that causes the water molecules to stick to one another is the hydrogen bond.

The boiling point of water (and all other liquids) is directly related to the barometric pressure. For example, on the top of Mt. Everest water boils at about 68 °C (154 °F), compared to 100 °C (212 °F) at sea level. Conversely, water deep in the ocean near geothermal vents can reach temperatures of hundreds of degrees and remain liquid.

Water sticks to itself. Water has a high surface tension caused by the strong cohesion between water molecules because it is polar. The apparent elasticity caused by surface tension drives the capillary waves.

Water also has high adhesion properties because of its polar nature.

Capillary action refers to the tendency of water to move up a narrow tube against the force of gravity. This property is relied upon by all vascular plants, such as trees.

Water is a very strong solvent, referred to as the universal solvent, dissolving many types of substances. Substances that will mix well and dissolve in water, e.g. salts, sugars, acids, alkalis, and some gases: especially oxygen, carbon dioxide (carbonation), are known as “hydrophilic” (water-loving) substances, while those that do not mix well with water (e.g. fats and oils), are known as “hydrophobic” (water-fearing) substances.

All the major components in cells (proteins, DNA and polysaccharides) are also dissolved in water.

Pure water has a low electrical conductivity, but this increases significantly upon solvation of a small amount of ionic material such as sodium chloride.

Water has the second highest specific heat capacity of any known chemical compound, after ammonia, as well as a high heat of vaporization (40.65 kJ mol⁻¹), both of which are a result of the extensive hydrogen bonding between its molecules. These two unusual properties allow water to moderate Earth’s climate by buffering large fluctuations in temperature.

The maximum density of water is at 3.98 °C (39.16 °F).^[5] Water becomes even less dense upon freezing, expanding 9%. This causes an unusual phenomenon: ice floats upon water, and so water organisms can live inside a partly frozen pond because the water on the bottom has a temperature of around 4 °C (39 °F).

ADR label for transporting goods dangerously reactive with water

Water is miscible with many liquids, for example ethanol, in all proportions, forming a single homogeneous liquid. On the other hand, water and most oils are immiscible usually forming layers according to increasing density from the top. As a gas, water vapor is completely miscible with air.

Water forms an azeotrope with many other solvents.

Water can be split by electrolysis into hydrogen and oxygen.

As an oxide of hydrogen, water is formed when hydrogen or hydrogen-containing compounds burn or react with oxygen or oxygen-containing compounds. Water is not a fuel, it is an end-product of the combustion of hydrogen. The energy required to split water into hydrogen and oxygen by electrolysis or any other means is greater than the energy released when the hydrogen and oxygen recombine.^[6]

Elements which are more electropositive than hydrogen such as lithium, sodium, calcium, potassium and caesium displace hydrogen from water, forming hydroxides. Being a flammable gas, the hydrogen given off is dangerous and the reaction of water with the more electropositive of these elements is violently explosive.

Distribution of water in nature

Water in the Universe

Much of the universe’s water may be produced as a byproduct of star formation. When stars are born, their birth is accompanied by a strong outward wind of gas and dust. When this outflow of material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water observed is quickly produced in this warm dense gas.^[7]

Water has been detected in interstellar clouds within our galaxy, the Milky Way. It is believed that water exists in abundance in other galaxies too, because its components, hydrogen and oxygen, are among the most abundant elements in the universe. Interstellar clouds eventually condense into solar nebulae and solar systems, such as ours.

Water vapor is present on:

Mercury – 3.4% in the atmosphere, and large amounts of water in Mercury’s exosphere^[8]
Venus – 0.002% in the atmosphere
Earth – trace in the atmosphere (varies with climate)
Mars – 0.03% in the atmosphere
Jupiter – 0.0004% in the atmosphere
Saturn – in ices only
Enceladus (moon of Saturn) – 91% in the atmosphere
exoplanets known as HD 189733 b^[9] and HD 209458 b.^[10]

Liquid water is present on:

Earth – 71% of surface
Moon – small amounts of water have been found (in 2008) in the inside of volcanic pearls brought from Moon to Earth by the Apollo 15 crew in 1971.^[11]

Strong evidence suggests that liquid water is present just under the surface of Saturn’s moon Enceladus. Probably some liquid water is on Europa.

Water ice is present on:

Earth – mainly on ice sheets
polar ice caps on Mars
Titan
Europa
Enceladus

Probability or possibility of distribution of water ice is at: lunar ice on the Moon, Ceres (dwarf planet), Tethys (moon). Ice is probably in internal structure of Uranus, Neptune, and Pluto and on comets.

Water and habitable zone

The Solar System along center row range of possible habitable zones of varying size stars.

The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth is vital to the existence of life on Earth as we know it. The Earth is located in the habitable zone of the solar system; if it were slightly closer to or further from the Sun (about 5%, or about 8 million kilometres), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.^[12]

Earth’s mass allows gravity to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide a greenhouse effect which helps maintain a relatively steady surface temperature. If Earth were smaller, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars).

It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature of Earth has been relatively constant through geologic time despite varying levels of incoming solar radiation (insolation), indicating that a dynamic process governs Earth’s temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.

The state of water also depends on a planet’s gravity. If a planet is sufficiently massive, the water on it may be solid even at high temperatures, because of the high pressure caused by gravity.

There are various theories about origin of water on Earth.

Water on Earth

Main articles: Hydrology and Water distribution on Earth

Water covers 71% of the Earth’s surface; the oceans contain 97.2% of the Earth’s water. The Antarctic ice sheet, which contains 90% of all fresh water on Earth, is visible at the bottom. Condensed atmospheric water can be seen as clouds, contributing to the Earth’s albedo.

Hydrology is the study of the movement, distribution, and quality of water throughout the Earth. The study of the distribution of water is hydrography. The study of the distribution and movement of groundwater is hydrogeology, of glaciers is glaciology, of inland waters is limnology and distribution of oceans is oceanography. Ecological processes with hydrology are in focus of ecohydrology.

The collective mass of water found on, under, and over the surface of a planet is called hydrosphere. Earth’s approximate water volume (the total water supply of the world) is 1 360 000 000 km³ (326 000 000 mi³). Of this volume:

1 320 000 000 km³ (316 900 000 mi³ or 97.2%) is in the oceans.
25 000 000 km³ (6 000 000 mi³ or 1.8%) is in glaciers, ice caps and ice sheets.
13 000 000 km³ (3,000,000 mi³ or 0.9%) is groundwater.
250 000 km³ (60,000 mi³ or 0.02%) is fresh water in lakes, inland seas, and rivers.
13 000 km³ (3,100 mi³ or 0.001%) is atmospheric water vapor at any given time.

Groundwater and fresh water are useful or potentially useful to humans as water resources.

Liquid water is found in bodies of water, such as an ocean, sea, lake, river, stream, canal, pond, or puddle. The majority of water on Earth is sea water. Water is also present in the atmosphere in solid, liquid, and vapor states. It also exists as groundwater in aquifers.

The most important geological processes caused by water are: chemical weathering, water erosion, water sediment transport and sedimentation, mudflows, ice erosion and sedimentation by glacier.

Water cycle

Main article: Water cycle

water cycle

The water cycle (known scientifically as the hydrologic cycle) refers to the continuous exchange of water within the hydrosphere, between the atmosphere, soil water, surface water, groundwater, and plants.

Water moves perpetually through each of these regions in the water cycle consisting of following transfer processes:

evaporation from oceans and other water bodies into the air and transpiration from land plants and animals into air.
precipitation, from water vapor condensing from the air and falling to earth or ocean.
runoff from the land usually reaching the sea.

Most water vapor over the oceans returns to the oceans, but winds carry water vapor over land at the same rate as runoff into the sea, about 36 Tt per year. Over land, evaporation and transpiration contribute another 71 Tt per year. Precipitation, at a rate of 107 Tt per year over land, has several forms: most commonly rain, snow, and hail, with some contribution from fog and dew. Condensed water in the air may also refract sunlight to produce rainbows.

Water runoff often collects over watersheds flowing into rivers. A mathematical model used to simulate river or stream flow and calculate water quality parameters is hydrological transport model. Some of water is diverted to irrigation for agriculture. Rivers and seas offer opportunity for travel and commerce. Through erosion, runoff shapes the environment creating river valleys and deltas which provide rich soil and level ground for the establishment of population centers. A flood occurs when an area of land, usually low-lying, is covered with water. It is when a river overflows its banks or flood from the sea. A drought is an extended period of months or years when a region notes a deficiency in its water supply. This occurs when a region receives consistently below average precipitation.

Fresh water storage

Main article: Water resources

Some runoff water is trapped for periods, for example in lakes. At high altitude, during winter, and in the far north and south, snow collects in ice caps, snow pack and glaciers. Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface in springs, or more spectacularly in hot springs and geysers. Groundwater is also extracted artificially in wells. This water storage is important, since clean, fresh water is essential to human and other land-based life. In many parts of the world, it is in short supply.

Tides

High tide (left) and low tide (right).

Main article: Tide

Tides are the cyclic rising and falling of Earth’s ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and estuarine water bodies and produce oscillating currents known as tidal streams. The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the local bathymetry. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is an important ecological product of ocean tides.

Effects on life

An oasis is an isolated water source with vegetation in desert

Some of the biodiversity of a coral reef

Water reflecting light in Crissy Field

From a biological standpoint, water has many distinct properties that are critical for the proliferation of life that set it apart from other substances. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water. Water is vital both as a solvent in which many of the body’s solutes dissolve and as an essential part of many metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g. starches, triglycerides and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g. glucose, fatty acids and amino acids to be used for fuels for energy use or other purposes). Water is thus essential and central to these metabolic processes. Therefore, without water, these metabolic processes would cease to exist, leaving us to muse about what processes would be in its place, such as gas absorption, dust collection, etc.

Water is also central to photosynthesis and respiration. Photosynthetic cells use the sun’s energy to split off water’s hydrogen from oxygen. Hydrogen is combined with CO₂ (absorbed from air or water) to form glucose and release oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun’s energy and reform water and CO₂ in the process (cellular respiration).

Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H⁺, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as hydroxide ion (OH⁻) to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH values less than 7 while bases have values greater than 7. Stomach acid (HCl) is useful to digestion. However, its corrosive effect on the esophagus during reflux can temporarily be neutralized by ingestion of a base such as aluminum hydroxide to produce the neutral molecules water and the salt aluminum chloride. Human biochemistry that involves enzymes usually performs optimally around a biologically neutral pH of 7.4.

For example a cell of Escherichia coli contains 70% of water, a human body 60–70%, plant body up to 90% and the body of an adult jellyfish is made up of 94–98% water.

Aquatic life forms

Main articles: Hydrobiology and Aquatic plant

Some marine diatoms – a key phytoplankton group

Earth’s waters are filled with life. The earliest life forms appeared in water; nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales that also live in the water. Some kinds of animals, such as amphibians, spend portions of their lives in water and portions on land. Plants such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton is generally the foundation of the ocean food chain.

Aquatic animals must obtain oxygen to survive, and they do so in various ways. Fish have gills instead of lungs, although some species of fish, such as the lungfish, have both. Marine mammals, such as dolphins, whales, otters, and seals need to surface periodically to breathe air. Smaller life forms are able to absorb oxygen through their skin.

Effects on human civilization

Water Fountain

Civilization has historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, was situated between the major rivers Tigris and Euphrates; the ancient society of the Egyptians depended entirely upon the Nile. Large metropolises like Rotterdam, London, Montreal, Paris, New York City, Shanghai, Tokyo, Chicago, and Hong Kong owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore, have flourished for the same reason. In places such as North Africa and the Middle East, where water is more scarce, access to clean drinking water was and is a major factor in human development.

Health and pollution

Environmental Scientist sampling water.

Water fit for human consumption is called drinking water or potable water. Water that is not potable can be made potable by filtration or distillation (heating it until it becomes water vapor, and then capturing the vapor without any of the impurities it leaves behind), or by other methods (chemical or heat treatment that kills bacteria). Sometimes the term safe water is applied to potable water of a lower quality threshold (i.e., it is used effectively for nutrition in humans that have weak access to water cleaning processes, and does more good than harm). Water that is not fit for drinking but is not harmful for humans when used for swimming or bathing is called by various names other than potable or drinking water, and is sometimes called safe water, or “safe for bathing”. Chlorine is a skin and mucous membrane irritant that is used to make water safe for bathing or drinking. Its use is highly technical and is usually monitored by government regulations (typically 1 part per million (ppm) for drinking water, and 1–2 ppm of chlorine not yet reacted with impurities for bathing water).

This natural resource is becoming scarcer in certain places, and its availability is a major social and economic concern. Currently, about 1 billion people around the world routinely drink unhealthy water. Most countries accepted the goal of halving by 2015 the number of people worldwide who do not have access to safe water and sanitation during the 2003 G8 Evian summit.^[13] Even if this difficult goal is met, it will still leave more than an estimated half a billion people without access to safe drinking water and over 1 billion without access to adequate sanitation. Poor water quality and bad sanitation are deadly; some 5 million deaths a year are caused by polluted drinking water. The World Health Organization estimates that safe water could prevent 1.4 million child deaths from diarrhea each year.^[14] Water, however, is not a finite resource, but rather re-circulated as potable water in precipitation in quantities many degrees of magnitude higher than human consumption. Therefore, it is the relatively small quantity of water in reserve in the earth (about 1% of our drinking water supply, which is replenished in aquifers around every 1 to 10 years), that is a non-renewable resource, and it is, rather, the distribution of potable and irrigation water which is scarce, rather than the actual amount of it that exists on the earth. Water-poor countries use importation of goods as the primary method of importing water (to leave enough for local human consumption), since the manufacturing process uses around 10 to 100 times products’ masses in water.

In the developing world, 90% of all wastewater still goes untreated into local rivers and streams.^[15] Some 50 countries, with roughly a third of the world’s population, also suffer from medium or high water stress, and 17 of these extract more water annually than is recharged through their natural water cycles.^[16] The strain not only affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.

Human uses

Agriculture

irrigation of field crops

The most important use of water in agriculture is for an irrigation and irrigation is key component to produce enough food. Irrigation takes up to 90% of water withdrawn in some developing countries.^[17]

As a scientific standard

On 7 April 1795, the gram was defined in France to be equal to “the absolute weight of a volume of pure water equal to a cube of one hundredth of a meter, and to the temperature of the melting ice.”^[18] For practical purposes though, a metallic reference standard was required, one thousand times more massive, the kilogram. Work was therefore commissioned to determine precisely how massive one liter of water was. In spite of the fact that the decreed definition of the gram specified water at 0 °C—a highly stable temperature point—the scientists chose to redefine the standard and to perform their measurements at the most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 °C.^[19]

The Kelvin temperature scale of the SI system is based on the triple point of water, defined as exactly 273.16 K or 0.01 °C. The scale is a more accurate development of the Celsius temperature scale, which is defined by the boiling point (100 °C) and melting point (0 °C) of water.

Natural water consists mainly of the isotopes hydrogen-1 and oxygen-16, but there is also small quantity of heavier isotopes such as hydrogen-2 (deuterium). The amount of deuterium oxides or heavy water is very small, but it still affects the properties of water. Water from rivers and lakes tends to contain less deuterium than seawater. Therefore, a standard water called Vienna Standard Mean Ocean Water is defined as the standard water.

For drinking

A young girl drinking bottled water.

Main article: Drinking water

The human body is anywhere from 55% to 78% water depending on body size.^[20] To function properly, the body requires between one and seven liters of water per day to avoid dehydration; the precise amount depends on the level of activity, temperature, humidity, and other factors. Most of this is ingested through foods or beverages other than drinking straight water. It is not clear how much water intake is needed by healthy people, though most advocates agree that 6–7 glasses of water (approximately 2 litres) daily is the minimum to maintain proper hydration.^[21] Medical literature favors a lower consumption, typically 1 liter of water for an average male, excluding extra requirements due to fluid loss from exercise or warm weather.^[22] For those who have healthy kidneys, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) it is dangerous to drink too little. People can drink far more water than necessary while exercising, however, putting them at risk of water intoxication (hyperhydration), which can be fatal. The “fact” that a person should consume eight glasses of water per day cannot be traced back to a scientific source.^[23] There are other myths such as the effect of water on weight loss and constipation that have been dispelled.^[24]

An original recommendation for water intake in 1945 by the Food and Nutrition Board of the National Research Council read: “An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods.”^[25] The latest dietary reference intake report by the United States National Research Council in general recommended (including food sources): 2.7 liters of water total for women and 3.7 liters for men.^[26] Specifically, pregnant and breastfeeding women need additional fluids to stay hydrated. According to the Institute of Medicine—who recommend that, on average, women consume 2.2 litres and men 3.0 litres—this is recommended to be 2.4 litres (approx. 9 cups) for pregnant women and 3 litres (approx. 12.5 cups) for breastfeeding women since an especially large amount of fluid is lost during nursing.^[27] Also noted is that normally, about 20 percent of water intake comes from food, while the rest comes from drinking water and beverages (caffeinated included). Water is excreted from the body in multiple forms; through urine and feces, through sweating, and by exhalation of water vapor in the breath. With physical exertion and heat exposure, water loss will increase and daily fluid needs may increase as well.

Hazard symbol for No drinking water

Humans require water that does not contain too many impurities. Common impurities include metal salts and/or harmful bacteria, such as Vibrio. Some solutes are acceptable and even desirable for taste enhancement and to provide needed electrolytes.^[28]

The single largest freshwater resource suitable for drinking is Lake Baikal in Siberia, which has a very low salt and calcium content and is very clean.

As a dissolving agent or solvent

Dissolving (or suspending) is used to wash everyday items such as the human body, clothes, floors, cars, food, and pets. Also, human wastes are carried by water in the sewage system. Its use as a cleaning solvent consumes most of water in industrialized countries.

Water can facilitate the chemical processing of wastewater. An aqueous environment can be favourable to the breakdown of pollutants, due to the ability to gain an homogenous solution that is pumpable and flexible to treat. Aerobic treatment can be used by applying oxygen or air to a solution reduce the reactivity of substances within it.

Water also facilitates biological processing of waste that have been dissolved within it. Microorganisms that live within water can access dissolved wastes and can feed upon them breaking them down into less polluting substances. Reedbeds and anaerobic digesters are both examples of biological systems that are particularly suited to the treatment of effluents.

Typically from both chemical and biological treatment of wastes, there is often a solid residue or cake that is left over from the treatment process. Depending upon its constituent parts, this ‘cake’ may be dried and spread on land as a fertilizer if it has beneficial properties, or alternatively disposed of in landfill or incinerated.

Water is the most abundant molecule in organisms. Fruits shrink when they are dried because they consist primarily of water.

As a heat transfer fluid

Ice used for cooling.

Water and steam are used as heat transfer fluids in diverse heat exchange systems, due to its availability and high heat capacity, both as a coolant and for heating. Cool water may even be naturally available from a lake or the sea. Condensing steam is a particularly efficient heating fluid because of the large heat of vaporization. A disadvantage is that water and steam are somewhat corrosive. In almost all electric power plants, water is the coolant, which vaporizes and drives steam turbines to drive generators.

In the nuclear industry, water can also be used as a neutron moderator. In a pressurized water reactor, water is both a coolant and a moderator. This provides a passive safety measure, as removing the water from the reactor also slows the nuclear reaction down.

Extinguishing fires

Water is used for fighting wildfires.

Water has a high heat of vaporization and is relatively inert, which makes it a good fire extinguishing fluid. The evaporation of water carries heat away from the fire. However, water cannot be used to fight fires of electric equipment, because impure water is electrically conductive, or of oils and organic solvents, because they float on water and the explosive boiling of water tends to spread the burning liquid.

Use of water in fire fighting should also take into account the hazards of a steam explosion, which may occur when water is used on very hot fires in confined spaces, and of a hydrogen explosion, when substances which react with water, such as certain metals or hot graphite, decompose the water, producing hydrogen gas.

The power of such explosions was seen in the Chernobyl disaster, although the water involved did not come from fire-fighting at that time but the reactor’s own water cooling system. A steam explosion occurred when the extreme over-heating of the core caused water to flash into steam. A hydrogen explosion may have occurred as a result of reaction between steam and hot zirconium.

Chemical uses

Organic reactions are usually quenched with water or a water solution of a suitable acid, base or buffer. Water is generally effective in removing inorganic salts. In inorganic reactions, water is a common solvent. In organic reactions, it is usually not used as a reaction solvent, because it does not dissolve the reactants well and is amphoteric (acidic and basic) and nucleophilic. Nevertheless, these properties are sometimes desirable. Also, acceleration of Diels-Alder reactions by water has been observed. Supercritical water has recently been a topic of research. Oxygen-saturated supercritical water combusts organic pollutants efficiently.

Recreation

Main article: Water sport (recreation)

Humans use water for many recreational purposes, as well as for exercising and for sports. Some of these include swimming, waterskiing, boating, and diving. In addition, some sports, like ice hockey and ice skating, are played on ice. Lakesides, beaches and waterparks are popular places for people to go to relax and enjoy recreation. Many find the sound of flowing water to be calming, too. Some keep fish and other life in aquariums or ponds for show, fun, and companionship. Humans also use water for snow sports i.e. skiing or snowboarding, which requires the water to be frozen. People may also use water for play fighting such as with snowballs, water guns or water balloons. They may also make fountains and use water in their public or private decorations.

Water industry

Main articles: Water industry and :Category:Water supply and sanitation by country

A water-carrier in India,1882. In many places where running water is not available, water has to be transported by people.

The water industry provides drinking water and wastewater services (including sewage treatment) to households and industry.

A manual water pump in China

water purification facility

Water used in landscaping.

Water supply facilities includes for example water wells cisterns for rainwater harvesting, water supply network, water purification facilities, water tanks, water towers, water pipes including old aqueducts. Atmospheric water generator is in development.

Drinking water is often collected at springs, extracted from artificial borings in the ground, or wells. Building more wells in adequate places is thus a possible way to produce more water, assuming the aquifers can supply an adequate flow. Other water sources are rainwater and river or lake water. This surface water, however, must be purified for human consumption. This may involve removal of undissolved substances, dissolved substances and harmful microbes. Popular methods are filtering with sand which only removes undissolved material, while chlorination and boiling kill harmful microbes. Distillation does all three functions. More advanced techniques exist, such as reverse osmosis. Desalination of abundant ocean or seawater is a more expensive solution used in coastal arid climates.

The distribution of drinking water is done through municipal water systems or as bottled water. Governments in many countries have programs to distribute water to the needy at no charge. Others argue that the market mechanism and free enterprise are best to manage this rare resource and to finance the boring of wells or the construction of dams and reservoirs.

Reducing waste by using drinking water only for human consumption is another option. In some cities such as Hong Kong, sea water is extensively used for flushing toilets citywide in order to conserve fresh water resources.

Polluting water may be the biggest single misuse of water; to the extent that a pollutant limits other uses of the water, it becomes a waste of the resource, regardless of benefits to the polluter. Like other types of pollution, this does not enter standard accounting of market costs, being conceived as externalities for which the market cannot account. Thus other people pay the price of water pollution, while the private firms’ profits are not redistributed to the local population victim of this pollution. Pharmaceuticals consumed by humans often end up in the waterways and can have detrimental effects on aquatic life if they bioaccumulate and if they are not biodegradable.

Wastewater facilities are sewers and wastewater treatment plants. Another way to remove pollution from surface runoff water is bioswale.

Industrial applications

Three Gorges Dam is the largest hydro-electric power station

Water is used in power generation. Hydroelectricity is electricity obtained from hydropower. Hydroelectric power comes from water driving a water turbine connected to a generator. Hydroelectricity is a low-cost, non-polluting, renewable energy source. The energy is supplied by the sun. Heat from the sun evaporates water, which condenses as rain in higher altitudes, from where it flows down.

Pressurized water is used in water blasting and water jet cutters. Also, very high pressure water guns are used for precise cutting. It works very well, is relatively safe, and is not harmful to the environment. It is also used in the cooling of machinery to prevent over-heating, or prevent saw blades from over-heating.

Water is also used in many industrial processes and machines, such as the steam turbine and heat exchanger, in addition to its use as a chemical solvent. Discharge of untreated water from industrial uses is pollution. Pollution includes discharged solutes (chemical pollution) and discharged coolant water (thermal pollution). Industry requires pure water for many applications and utilizes a variety of purification techniques both in water supply and discharge.

Food processing

Water can be used to cook foods such as noodles.

Water plays many critical roles within the field of food science. It is important for a food scientist to understand the roles that water plays within food processing to ensure the success of their products.

Solutes such as salts and sugars found in water affect the physical properties of water. The boiling and freezing points of water is affected by solutes. One mole of sucrose (sugar) raises the boiling point of water by 0.52 °C, and one mole of salt raises the boiling point by 1.04 °C while lowering the freezing point of water in a similar way.^[29] Solutes in water also affect water activity which affects many chemical reactions and the growth of microbes in food.^[30] Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water.^[29] Solutes in water lower water activity. This is important to know because most bacterial growth ceases at low levels of water activity.^[30] Not only does microbial growth affect the safety of food but also the preservation and shelf life of food.

Water hardness is also a critical factor in food processing. It can dramatically affect the quality of a product as well as playing a role in sanitation. Water hardness is classified based on the amounts of removable calcium carbonate salt it contains per gallon. Water hardness is measured in grains; 0.064 g calcium carbonate is equivalent to one grain of hardness.^[29] Water is classified as soft if it contains 1 to 4 grains, medium if it contains 5 to 10 grains and hard if it contains 11 to 20 grains.^[vague] ^[29] The hardness of water may be altered or treated by using a chemical ion exchange system. The hardness of water also affects its pH balance which plays a critical role in food processing. For example, hard water prevents successful production of clear beverages. Water hardness also affects sanitation; with increasing hardness, there is a loss of effectiveness for its use as a sanitizer.^[29]

Boiling, steaming, and simmering are popular cooking methods that often require immersing food in water or its gaseous state, steam. While cooking water is used for dishwashing too.

Water politics and water crisis

Main articles: Water politics and Water crisis

See also: Water resources, Water law, and Water right

Water politics is politics affected by water and water resources. Because of overpopulation, mass consumption, misuse, and water pollution, the availability of drinking water per capita is inadequate and shrinking as of the year 2006. For this reason, water is a strategic resource in the globe and an important element in many political conflicts. It causes health impacts and damage to biodiversity. The serious worldwide water situation is called water crisis.

UNESCO’s World Water Development Report (WWDR, 2003) from its World Water Assessment Program indicates that, in the next 20 years, the quantity of water available to everyone is predicted to decrease by 30%. 40% of the world’s inhabitants currently have insufficient fresh water for minimal hygiene. More than 2.2 million people died in 2000 from waterborne diseases (related to the consumption of contaminated water) or drought. In 2004, the UK charity WaterAid reported that a child dies every 15 seconds from easily preventable water-related diseases; often this means lack of sewage disposal; see toilet.

To halve, by 2015, the proportion of people without sustainable access to safe drinking water is one of the Millennium Development Goals.

Fresh water — now more precious than ever in our history for its extensive use in agriculture, high-tech manufacturing, and energy production — is increasingly receiving attention as a resource requiring better water management and sustainable use.

Organizations concerned in water protection include International Water Association (IWA), WaterAid, Water 1st, American Water Resources Association. Water related conventions are United Nations Convention to Combat Desertification (UNCCD), International Convention for the Prevention of Pollution from Ships, United Nations Convention on the Law of the Sea and Ramsar Convention. World Day for Water takes place at March 22 and World Ocean Day at June 8.

Water used in the production of a good or service is virtual water.

Religion, philosophy, and literature

A Hindu ablution as practiced in Tamil Nadu

Water is considered a purifier in most religions. Major faiths that incorporate ritual washing (ablution) include Christianity, Hinduism, Rastafarianism, Islam, Shinto, Taoism, and Judaism. Immersion (or aspersion or affusion) of a person in water is a central sacrament of Christianity (where it is called baptism); it is also a part of the practice of other religions, including Judaism (mikvah) and Sikhism (Amrit Sanskar). In addition, a ritual bath in pure water is performed for the dead in many religions including Judaism and Islam. In Islam, the five daily prayers can be done in most cases after completing washing certain parts of the body using clean water (wudu). In Shinto, water is used in almost all rituals to cleanse a person or an area (e.g., in the ritual of misogi). Water is mentioned in the Bible 442 times in the New International Version and 363 times in the King James Version: 2 Peter 3:5(b) states, “The earth was formed out of water and by water” (NIV).

Some faiths use water especially prepared for religious purposes (holy water in some Christian denominations, Amrita in Sikhism and Hinduism). Many religions also consider particular sources or bodies of water to be sacred or at least auspicious; examples include Lourdes in Roman Catholicism, the Jordan River (at least symbolically) in some Christian churches, the Zamzam Well in Islam and the River Ganges (among many others) in Hinduism.

Water is often believed to have spiritual powers. In Celtic mythology, Sulis is the local goddess of thermal springs; in Hinduism, the Ganges is also personified as a goddess, while Saraswati have been referred to as goddess in Vedas. Also water is one of the “panch-tatva”s (basic 5 elements, others including fire, earth, space, air). Alternatively, gods can be patrons of particular springs, rivers, or lakes: for example in Greek and Roman mythology, Peneus was a river god, one of the three thousand Oceanids. In Islam, not only does water give life, but every life is itself made of water: “We made from water every living thing”.^[31]

The Ancient Greek philosopher Empedocles held that water is one of the four classical elements along with fire, earth and air, and was regarded as the ylem, or basic substance of the universe. Water was considered cold and moist. In the theory of the four bodily humors, water was associated with phlegm. Water was also one of the five elements in traditional Chinese philosophy, along with earth, fire, wood, and metal.

Water also plays an important role in literature as a symbol of purification. Examples include the critical importance of a river in As I Lay Dying by William Faulkner and the drowning of Ophelia in Hamlet.

THUNDER

edwinsebastian — Sun, 12 Oct 2008 04:19:55 +0000

Thunder

Thunder is the sound made by lightning. Depending on the nature of the lightning and distance of the listener, it can range from a sharp, loud crack to a long, low rumble. The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave which produces the sound of thunder.

Thunder A short sample of a clap of thunder Problems listening to the file? See media help.

Cause

The cause of thunder has been the subject of centuries of speculation and scientific inquiry. The first recorded theory is attributed to the Greek philosopher Aristotle in the third century BC, and an early speculation was that it was caused by the collision of clouds. Subsequently, numerous other theories have been proposed. By the mid-19th century, the accepted theory was that lightning produced a vacuum. In the 20th century a consensus evolved that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel. In a fraction of a second the air is heated to a temperature approaching 28,000 °C (50,000 °F)^[1]. This heating causes it to expand outward, plowing into the surrounding cooler air at a speed faster than sound would travel in that cooler air. The outward-moving pulse that results is a shock wave, ^[2] similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft. More recently, this consensus has been eroded by the observation that measured overpressures in simulated lightning are greater than what could be achieved by the amount of heating found. Alternative proposals rely on electrodynamic effects of the massive current acting on the plasma in the bolt of lightning.^[3]

Etymology

The d in Modern English thunder (from earlier Old English þunor) is epenthetic, and is now found as well in Modern Dutch donder, , (cp Middle Dutch donre, and Old Norse þorr, Old Frisian þuner, Old High German donar descended from Proto-Germanic *þunraz). In Latin the term was tonare “to thunder” (see also tornado). The name of the Germanic god Thor comes from the Old Norse word for thunder.

The shared Proto-Indo-European root is *tón-r̥.

Calculating distance

A flash of lightning, followed after some seconds by a rumble of thunder is, for many people, the first illustration of the fact that sound travels more slowly than light. Using this difference, one can estimate how far away the bolt of lightning is by timing the interval between seeing the flash and hearing thunder. The speed of sound in dry air is approximately 343 m/s or 1,127 feet per second or 768 mph (1,236 km/h) at 68°F (20 °C). ^[4] The speed of light is high enough that it can be taken as infinite in this calculation. Therefore, the lightning is approximately one kilometer distant for every 2.9 seconds (or one mile for every 4.6 seconds). In the same five seconds the light could have circled the globe 37 times. Thunder is seldom heard at distances over 24 kilometers (15 miles). A flash of lightning and a simultaneous sharp “clap!” of thunder, a thunderclap, therefore indicates that the lightning strike was very near.

Fear of thunder

Fear of thunder is known as astraphobia.

ICE

edwinsebastian — Sun, 12 Oct 2008 04:09:58 +0000

Ice

A fountain in Boise, Idaho, February 2007

Snowflakes (ice crystals) by Wilson Bentley, 1902

Ice is a solid phase, usually crystalline, of a non-metalic substance that is liquid or gas at room temperature, such as ammonia ice or methane ice. However, the word “ice” normally means water ice, technically restricted to one of the 15 known crystalline phases of water. In non-scientific contexts, it usually describes ice I_h, which is known to be the most abundant of these phases. It can appear transparent or an opaque bluish-white colour, depending on the presence of impurities such as air. The addition of other materials such as soil may further alter the appearance.

The most common phase transition to ice I_h occurs when liquid water is cooled below 0 °C (273.15 K, 32 °F) at standard atmospheric pressure. It can also deposit from a vapour with no intervening liquid phase, such as in the formation of frost.

Ice appears in nature in forms as varied as snowflakes and hail, icicles, glaciers, pack ice, and entire polar ice caps. It is an important component of the global climate, particularly in regard to the water cycle. Furthermore, ice has numerous cultural applications, from the ice cooling one’s drink to winter sports and ice sculpture.

The word is from Old English ís, in turn derived from Proto-Germanic *isaz.

Characteristics

String of ice found in the Adirondack Region of New York State

As a naturally occurring crystalline solid, ice is considered a mineral consisting of hydrogen oxide.

An unusual property of ice frozen at a pressure of one atmosphere is that the solid is some 8% less dense than liquid water. Water is the only known non-metallic substance to expand when it freezes. Ice has a density of 0.9167 g/cm³ at 0 °C, whereas water has a density of 0.9998 g/cm³ at the same temperature. Liquid water is most dense, essentially 1.00 g/cm³, at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the temperature drops to 0 °C. (In fact, the word “crystal” derives from Greek word for frost.) This is due to hydrogen bonds forming between the water molecules, which line up molecules less efficiently (in terms of volume) when water is frozen. The result of this is that ice floats on liquid water, which is an important factor in Earth’s climate (if ice had sunk instead of floating, any body of water would have frozen from the bottom to the surface, killing any fish and other creatures not resistant to freezing temperatures). Density of ice increases slightly with decreasing temperature (density of ice at −180 °C (93 K) is 0.9340 g/cm³).^{[citation needed]}

When ice melts, it absorbs as much heat energy (the heat of fusion) as it would take to heat an equivalent mass of water by 80 °C, while its temperature remains a constant 0 °C.

It is also theoretically possible to superheat ice beyond its equilibrium melting point. Simulations of ultrafast laser pulses acting on ice show it can be heated up to room temperature for an extremely short period (250 ps), without melting. ^[1]

Light reflecting from ice can appear blue, because ice absorbs more of the red frequencies than the blue ones. Also, icebergs containing impurities (e.g. sediments, algae, air bubbles) can appear green.^[2]

Types

Ice coating the branches of a tree

Feather ice on the plateau near Alta, Norway. The crystals form at temperatures below −30 °C (i.e. −22 °F).

Everyday ice and snow have a hexagonal crystal structure (ice I_h). Subjected to higher pressures and varying temperatures, ice can form in roughly a dozen different phases. Only a little less stable (metastable) than I_h is the cubic structure (I_c).

At other temperatures and pressures, other forms of ice exist, including II, III, V, VI, VII, VIII, IX, and X. With care all these types can be recovered at ambient pressure. The types are differentiated by their crystalline structure, ordering and density. There are also two metastable phases of ice under pressure, both fully hydrogen-disordered; these are IV and XII. Ice XII was discovered in 1996. In 2006, XIII and XIV were discovered.^[3] Ices XI, XIII, and XIV are hydrogen-ordered forms of ices I_h, V, and XII respectively.

As well as crystalline forms, solid water can exist in amorphous states as amorphous solid water (ASW), low-density amorphous ice (LDA), high-density amorphous ice (HDA), very high-density amorphous ice (VHDA) and hyperquenched glassy water (HGW).

Rime is a type of ice formed on cold objects when drops of water crystallize on them. This can be observed in foggy weather, when the temperature drops during night. Soft rime contains a high proportion of trapped air, making it appear white rather than transparent, and giving it a density about one quarter of that of pure ice. Hard rime is comparatively denser.

Aufeis is layered ice that forms in Arctic and subarctic stream valleys. Ice, frozen in the stream bed, blocks normal groundwater discharge, and causes the local water table to rise, resulting in water discharge on top of the frozen layer. This water then freezes, causing the water table to rise further and repeat the cycle. The result is a stratified ice deposit, often several metres thick.

Ice can also form icicles, similar to stalactites in appearance, as water drips and re-freezes.

Clathrate hydrates are forms of ice that contain gas molecules trapped within its crystal lattice. Pancake ice is a formation of ice generally created in areas with less calm conditions.

Some other substances (particularly solid forms of those usually found as fluids) are also called “ice”: dry ice, for instance, is a popular term for solid carbon dioxide.

In outer space, hexagonal crystalline ice (the predominant form found on Earth), is extremely rare. Amorphous ice is more common; however, hexagonal crystalline ice can be formed via volcanic action.^[4]

Uses

Ice harvesting

Harvesting ice on Lake Saint Clair in Michigan, circa 1905

Ice being transported by cart in Mumbai, India

Ice has long been valued as a means of cooling. Until recently, the Hungarian Parliament building used ice harvested in the winter from Lake Balaton for air conditioning. Icehouses were used to store ice formed in the winter, to make ice available all year long, and early refrigerators were known as iceboxes, because they had a block of ice in them. In many cities, it was not unusual to have a regular ice delivery service during the summer. For the first half of the 19th century, ice harvesting had become big business in America. Frederic Tudor, who became known as the “Ice King,” worked on developing better insulation products for the long distance shipment of ice, especially to the tropics. The advent of artificial refrigeration technology has since made delivery of ice obsolete.

In 400 BC Iran, Persian engineers had already mastered the technique of storing ice in the middle of summer in the desert. The ice was brought in during the winters from nearby mountains in bulk amounts, and stored in specially designed, naturally cooled refrigerators, called yakhchal (meaning ice storage). This was a large underground space (up to 5000 m³) that had thick walls (at least two meters at the base) made out of a special mortar called sārooj, composed of sand, clay, egg whites, lime, goat hair, and ash in specific proportions, and which was known to be resistant to heat transfer. This mixture was thought to be completely water impenetrable. The space often had access to a Qanat, and often contained a system of windcatchers which could easily bring temperatures inside the space down to frigid levels on summer days. The ice was then used to chill treats for royalty on such occasions.

Sports

Ice surfing on the Żnin Small Lake

Ice also plays a role in winter recreation, in many sports such as ice skating, tour skating, ice hockey, ice fishing, ice climbing, curling, broomball and sled racing on bobsled, luge and skeleton. Many of the different sports played on ice get international attention every four years during the Winter Olympic Games.

A sort of sailboat on blades gives rise to ice boating. The human quest for excitement has even led to ice racing, where drivers must speed on lake ice, while also controlling the skid of their vehicle (similar in some ways to dirt track racing). The sport has even been modified for ice rinks.

Transportation

U.S. Coast Guard icebreakers near McMurdo Station, February 2002

Ice can also be an obstacle; for harbours near the poles, being ice-free is an important advantage; ideally, all year long. Examples are Murmansk (Russia), Petsamo (Russia, formerly Finland) and Vardø (Norway). Harbours which aren’t ice-free are opened up using icebreakers.

Ice forming on roads is a dangerous winter hazard. Black ice is very difficult to see, because it lacks the expected frosty surface. Whenever there is freezing rain or snow which occurs at a temperature near the melting point, it is common for ice to build up on the windows of vehicles. Driving safely requires the removal of the ice build-up. Ice scrapers are tools designed to break the ice free and clear the windows, though removing the ice can be a long and laborious process.

Far enough below the freezing point, a thin layer of ice crystals can form on the inside surface of windows. This usually happens when a vehicle has been left alone after being driven for a while, but can happen while driving, if the outside temperature is low enough. Moisture from the driver’s breath is the source of water for the crystals. It is troublesome to remove this form of ice, so people often open their windows slightly when the vehicle is parked in order to let the moisture dissipate, and it is now common for cars to have rear-window defrosters to solve the problem. A similar problem can happen in homes, which is one reason why many colder regions require double-pane windows for insulation.

When the outdoor temperature stays below freezing for extended periods, very thick layers of ice can form on lakes and other bodies of water, although places with flowing water require much colder temperatures. The ice can become thick enough to drive onto with automobiles and trucks. Doing this safely requires a thickness of at least 30 centimetres (one foot).

For ships, ice presents two distinct hazards. Spray, and freezing rain, can produce an ice build-up on the superstructure of a vessel sufficient to make it unstable, and to require it to be hacked off or melted with steam hoses. And icebergs — large masses of ice floating in water (typically created when glaciers reach the sea) — can be dangerous if struck by a ship when underway. Icebergs have been responsible for the sinking of many ships, the most famous probably being the Titanic.

For aircraft, ice can cause a number of dangers. As an aircraft climbs, it passes through air layers of different temperature and humidity, some of which may be conducive to ice formation. If ice forms on the wings or control surfaces, this may adversely affect the flying qualities of the aircraft. During the first non-stop flight of the Atlantic, the British aviators Captain John Alcock and Lieutenant Arthur Whitten Brown encountered such icing conditions – Brown left the cockpit and climbed onto the wing several times to remove ice which was covering the engine air intakes of the Vickers Vimy aircraft they were flying.

A particular icing vulnerability associated with reciprocating internal combustion engines is the carburettor. As air is sucked through the carburettor into the engine, the local air pressure is lowered, which causes adiabatic cooling. So, in humid near-freezing conditions, the carburettor will be colder, and tend to ice up. This will block the supply of air to the engine, and cause it to fail. For this reason, aircraft reciprocating engines with carburettors are provided with carburettor air intake heaters. The increasing use of fuel injection—which does not require carburettors—has made “carb icing” less of an issue for reciprocating engines.

Jet engines do not experience carb icing, but recent evidence indicates that they can be slowed, stopped, or damaged by internal icing in certain types of atmospheric conditions much more easily than previously believed. In most cases, the engines can be quickly restarted and flights are not endangered, but research continues to determine the exact conditions which produce this type of icing, and find the best methods to prevent, or reverse it, in flight.

Other uses

Ice pier during 1983 cargo operations. McMurdo Station, Antarctica

Engineers used pack ice’s formidable strength when they constructed Antarctica’s first floating ice pier in 1973.^[5] Such ice piers are used during cargo operations to load and offload ships. Fleet operations personnel make the floating pier during the winter. They build upon naturally-occurring frozen seawater in McMurdo Sound until the dock reaches a depth of about 22 feet (6.7 m). Ice piers have a lifespan of three to five years.
The manufacture and use of ice cubes or crushed ice is common for drinks.
Pagophagia, a type of pica eating disorder, is the compulsive consumption of ice.
Structures and ice sculptures are built out of large chunks of ice. The structures are mostly ornamental (as in the case with ice castles), and not practical for long-term habitation. Ice hotels exist on a seasonal basis in a few cold areas. Igloos are another example of a temporary structure, made primarily from snow.
During World War II, Project Habbakuk was a British programme which investigated the use of pykrete (wood fibres mixed with ice) as a possible material for warships, especially aircraft carriers, due to the ease with which a large deck could be constructed, but the idea was given up when there were not enough funds for construction of a prototype.
Ice can be used to start a fire by carving it into a lens which will focus sunlight onto kindling. When one waits long enough, a fire will start.^[6]
In global warming, ice plays an important part because it reflects 90% of the sun’s rays. Furthermore, ice cores help provide historical climate information.
In January and February 1658, the straits between the islands of Denmark, Great Belt and Little Belt froze over, allowing a Swedish army to March across the Belts and defeat the Danish army. The resulting Treaty of Roskilde ceded large areas of Denmark to Sweden.

At different pressures

Most liquids freeze at a higher temperature under pressure, because the pressure helps to hold the molecules together. However, the strong hydrogen bonds in water make it different: water freezes at a temperature below 0 °C under a pressure higher than 1 atm. Consequently, water also remains frozen at a temperature above 0 °C under a pressure lower than 1 atm. The melting of ice under high pressures is thought to contribute to the movement of glaciers. Ice formed at high pressure has a different crystal structure and density to ordinary ice. Ice, water, and water vapour can coexist at the triple point, which is exactly 273.16 K (by definition) at a pressure of 611.73 Pa.

Phases

Phase	Characteristics
Amorphous ice	Amorphous ice is an ice lacking crystal structure. Amorphous ice exists in three forms: low-density (LDA) formed at atmospheric pressure, or below, high density (HDA) and very high density amorphous ice (VHDA), forming at higher pressures. LDA forms by extremely quick cooling of liquid water (“hyperquenched glassy water”, HGW), by depositing water vapour on very cold substrates (“amorphous solid water”, ASW) or by heating high density forms of ice at ambient pressure (“LDA”).
Ice I_h	Normal hexagonal crystalline ice. Virtually all ice in the biosphere is ice I_h, with the exception only of a small amount of ice I_c.
Ice I_c	A Metastable cubic crystalline variant of ice. The oxygen atoms are arranged in a diamond structure. It is produced at temperatures between 130-150 K, and is stable for up to 200 K, when it transforms into ice I_h. It is occasionally present in the upper atmosphere.
Ice II	A rhombohedral crystalline form with highly ordered structure. Formed from ice I_h by compressing it at temperature of 190-210 K. When heated, it undergoes transformation to ice III.
Ice III	A tetragonal crystalline ice, formed by cooling water down to 250 K at 300 MPa. Least dense of the high-pressure phases. More dense than water.
Ice IV	A Metastable rhombohedral phase. Doesn’t easily form without a nucleating agent.
Ice V	A monoclinic crystalline phase. Formed by cooling water to 253 K at 500 MPa. Most complicated structure of all the phases.
Ice VI	A tetragonal crystalline phase. Formed by cooling water to 270 K at 1.1 GPa. Exhibits Debye relaxation.
Ice VII	A cubic phase. The hydrogen atoms positions are disordered; the material shows Debye relaxation. The hydrogen bonds form two interpenetrating lattices.
Ice VIII	A more ordered version of ice VII, where the hydrogen atoms assume fixed positions. Formed from ice VII, by cooling it below 5 °C.
Ice IX	A tetragonal metastable phase. Formed gradually from ice III by cooling it from 208 K to 165 K, stable below 140 K and pressures between 200 and 400 MPa. It has density of 1.16 g/cm³, slightly higher than ordinary ice.
Ice X	Proton-ordered symmetric ice. Forms at about 70 GPa.
Ice XI	An orthorhombic low-temperature equilibrium form of hexagonal ice. It is ferroelectric.
Ice XII	A tetragonal metastable dense crystalline phase. It is observed in the phase space of ice V and ice VI. It can be prepared by heating high-density amorphous ice from 77 K to about 183 K at 810 MPa.
Ice XIII	A monoclinic crystalline phase. Formed by cooling water to below 130 K at 500 MPa. The proton-ordered form of ice V.
Ice XIV	An orthorhombic crystalline phase. Formed below 118 K at 1.2 GPa. The proton-ordered form of ice XII.
Ice XV	The predicted, but not yet proven, proton-ordered form of ice VI. Thought to be formed by cooling water to around 108-80 K at 1.1 GPa.

FIRE

edwinsebastian — Sun, 12 Oct 2008 03:58:32 +0000

Fire

U.S. Air Force Airmen from the 20th Civil Engineer Squadron Fire Protection Flight neutralize a live fire during a field training exercise.

Fire is the heat and light energy released during a chemical reaction, in particular a combustion reaction. Depending on the substances alight, and any impurities outside, the color of the flame and the fire’s intensity might vary. Fire in its most common form can result in conflagration, and has the potential to cause physical damage through burning.Fire can be red, orange, yellow, blue, and even green.

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Chemistry

Chemical reaction

The fire tetrahedron

Fires start when a flammable and/or a combustible material with an adequate supply of oxygen or another oxidizer is subjected to enough heat and is able to sustain a chain reaction. This is commonly called the fire tetrahedron. No fire can exist without all of these elements being in place.

Once ignited, a chain reaction must take place whereby fires can sustain their own heat by the further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of an oxidizer and fuel.

Fire can be extinguished by removing any one of the elements of the fire tetrahedron. Fire extinguishing by the application of water acts by removing heat from the fuel faster than combustion generates it. Application of carbon dioxide is intended primarily to starve the fire of oxygen. A forest fire may be fought by starting smaller fires in advance of the main blaze, to deprive it of fuel. Other gaseous fire suppression agents, such as halon or HFC-227, interfere with the chemical reaction itself.

Flame

A log on fire

Main article: Flame

A flame is an exothermic, self-sustaining, oxidizing chemical reaction producing energy and glowing hot matter, of which a very small portion is plasma. It consists of reacting gases and solids emitting visible and infrared light, the frequency spectrum of which depends on the chemical composition of the burning elements and intermediate reaction products.

In many cases, such as the burning of organic matter, for example wood, or the incomplete combustion of gas, incandescent solid particles called soot produce the familiar red-orange glow of ‘fire’. This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single-wavelength radiation from various electron transitions in the excited molecules formed in the flame. Usually oxygen is involved, but hydrogen burning in chlorine also produces a flame, producing hydrogen chloride (HCl). Other possible combinations producing flames, amongst many more, are fluorine and hydrogen, and hydrazine and nitrogen tetroxide.

The glow of a flame is complex. Black-body radiation is emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also photon emission by de-excited atoms and molecules in the gases. Much of the radiation is emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation, and on chemical makeup for the emission spectra. The dominant color in a flame changes with temperature. The photo of the forest fire is an excellent example of this variation. Near the ground, where most burning is occurring, the fire is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke.

The National Aeronautics and Space Administration (NASA) of the United States has recently found that gravity plays a role. Modifying the gravity causes different flame types.^[1] The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient (although it will go out if not moved steadily, as the CO₂ from combustion does not disperse in microgravity, and tends to smother the flame). There are several possible explanations for this difference, of which the most likely is that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs.^[2] Experiments by NASA reveal that diffusion flames in microgravity allow more soot to be completely oxidized after they are produced than diffusion flames on Earth, because of a series of mechanisms that behave differently in microgravity when compared to normal gravity conditions.^[3] These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.

In combustion engines, various steps are taken to eliminate a flame. The method depends mainly on whether the fuel is oil, wood, or a high-energy fuel such as jet fuel.

Typical temperatures of fires and flames

Oxyhydrogen flame: 9000 or above (3645 °F) ^[4]
Bunsen burner flame: 1300 to 1600 °C (2372 to 2912 °F) ^[5]
Blowtorch flame: 1,300 °C (2372 °F) ^[6]
Candle flame: 1000 °C (1832 °F)
Smoldering cigarette:
- Temperature without drawing: side of the lit portion; 400 °C (750 °F); middle of the lit portion: 585 °C (1110 °F)
- Temperature during drawing: middle of the lit portion: 700 °C (1290 °F)
- Always hotter in the middle. Temperatures of flames by appearance

The temperature of flames with carbon particles emitting light can be assessed by their color:^[7]

Red
- Just visible: 525 °C (977 °F)
- Dull: 700 °C (1290 °F)
- Cherry, dull: 800 °C (1470 °F)
- Cherry, full: 900 °C (1650 °F)
- Cherry, clear: 1000 °C (1830 °F)
Orange
- Deep: 1100 °C (2010 °F)
- Clear: 1200 °C (2190 °F)
White
- Whitish: 1300 °C (2370 °F)
- Bright: 1400 °C (2550 °F)
- Dazzling: 1500 °C (2730 °F)

Controlling fire

A forest fire

The ability to control fire was a major change in the habits of early humans. Making fire to generate heat and light made it possible for people to cook food, increasing the variety and availability of nutrients. Fire also kept nocturnal predators at bay. Archaeology indicates that ancestors or relatives of modern humans might have controlled fire as early as 790,000 years ago. The Cradle of Humankind site has evidence for controlled fire from 1 to 1.8 million years ago.^[8]

By the Neolithic Revolution, during the introduction of grain based agriculture, people all over the world used fire as a tool in landscape management. These fires were typically controlled burns or “cool fires”, as opposed to uncontrolled “hot fires” that damage the soil. Hot fires destroy plants and animals, and endanger communities. This is especially a problem in the forests of today where traditional burning is prevented in order to encourage the growth of timber crops. Cool fires are generally conducted in the spring and fall. They clear undergrowth, burning up biomass that could trigger a hot fire should it get too dense. They provide a greater variety of environments, which encourages game and plant diversity. For humans, they make dense, impassable forests traversable.

The first technical application of the fire may have been the extracting and treating of metals. There are numerous modern applications of fire. In its broadest sense, fire is used by nearly every human being on earth in a controlled setting every day. Users of internal combustion vehicles employ fire every time they drive. Thermal power stations provide electricity for a large percentage of humanity.

The use of fire in warfare has a long history. Hunter-gatherer groups around the world have been noted as using grass and forest fires to injure their enemies and destroy their ability to find food, so it can be assumed that fire has been used in warfare for as long as humans have had the knowledge to control it. Homer detailed the use of fire by Greek commandos who hid in a wooden horse to burn Troy during the Trojan war. Later the Byzantine fleet used Greek fire to attack ships and men. In the First World War, the first modern flamethrowers were used by infantry, and were successfully mounted on armoured vehicles in the Second World War. In the latter war, incendiary bombs were used by Axis and Allies alike, notably on Rotterdam, London, Hamburg and, notoriously, at Dresden, in the latter two cases firestorms were deliberately caused in which a ring of fire surrounding each city was drawn inward by an updraft caused by a central cluster of fires. The United States Army Air Force also extensively used incendiaries against Japanese targets in the latter months of the war, devastating entire cities constructed primarily of wood and paper houses. In the Second World War, the use of napalm and molotov cocktails was popularized, though the former did not gain public attention until the Vietnam War. More recently many villages were burned during the Rwandan Genocide.

Fire fuel

A coal-fired power station in the People’s Republic of China

Setting fuel aflame releases usable energy. Wood was a prehistoric fuel, and is still viable today. The use of fossil fuels, such as petroleum, natural gas and coal, in power plants supplies the vast majority of the world’s electricity today; the International Energy Agency states that nearly 80% of the world’s power comes from these sources.^[9] The fire in a power station is used to heat water, creating steam that drives turbines. The turbines then spin an electric generator to produce power.

The unburnable solid remains of a combustible material left after a fire is called clinker if its melting point is below the flame temperature, so that it fuses and then solidifies as it cools, and ash if its melting point is above the flame temperature. Incomplete combustion of a carbonaceous fuel can result in the production of soot.

Fire protection and prevention

Main article: Fire protection

A structure fire

Fire fighting services are provided in most developed areas to extinguish or contain uncontrolled fires. Trained firefighters use Fire apparatus, water supply resources such as water mains and fire hydrants or they might use A and B class foam depending on what is feeding the fire. An array of other equipment to combat the spread of fires.

Fire prevention is intended to reduce sources of ignition, and is partially focused on programs to educate people from starting fires.^[10] Buildings, especially schools and tall buildings, often conduct fire drills to inform and prepare citizens on how to react to a building fire. Purposely starting destructive fires constitutes arson and is a criminal offense in most jurisdictions.

Model building codes require passive fire protection and active fire protection systems to minimize damage resulting from a fire. The most common form of active fire protection is fire sprinklers. To maximize passive fire protection of buildings, building materials and furnishings in most developed countries are tested for fire-resistance, combustibility and flammability. Upholstery, carpeting and plastics used in vehicles and vessels are also tested.

Practical uses

A blacksmith’s fire is used primarily for forging iron.

Fire is or has been used:

For light, heat (for cooking, survival and comfort), and protection
As a weapon of warfare, especially during ancient and medieval times.
For fire-stick farming
For cremation
For welding
For celebration (such as, birthday candles)
For back-burning in fighting fires
For controlled burn-offs for preventing wildfires
For burn-offs to clear land for agriculture
For recreational use as a campfire.

Hello WORLD!

edwinsebastian — Tue, 19 Aug 2008 01:16:27 +0000

Hello, everybody in this world! How are you today? I hope, all of you is fine everyday. Don’t forget to kept smile every time you can.

Edwinsebastian's Weblog

EARTH

Earth

Contents

Chronology

Evolution of life

Future

Composition and structure

Shape

Chemical composition

Internal structure

Tectonic plates

Surface

Hydrosphere

Atmosphere

Weather and climate

Upper atmosphere

Magnetic field

Orbit and rotation

Rotation

Orbit

Axial tilt and seasons

Moon

Habitability

Biosphere

Natural resources and land use

Natural and environmental hazards

Human geography

Cultural viewpoint

Notes

WATER

Water

Contents

Types of water

Chemical and physical properties

Distribution of water in nature

Water in the Universe

Water and habitable zone

Water on Earth

Water cycle

Fresh water storage

Tides

Effects on life

Aquatic life forms

Effects on human civilization

Health and pollution

Human uses

Agriculture

As a scientific standard

For drinking

As a dissolving agent or solvent

As a heat transfer fluid

Extinguishing fires

Chemical uses

Recreation

Water industry

Industrial applications

Food processing

Water politics and water crisis

Religion, philosophy, and literature

THUNDER

Thunder

Contents

Cause

Etymology

Calculating distance

Fear of thunder

ICE

Ice

Contents

Characteristics

Types

Uses

Ice harvesting

Sports

Transportation

Other uses

At different pressures

Phases

FIRE