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Darkness

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For other uses, see Darkness (disambiguation) and Dark (disambiguation).
"Absence of light" redirects here. For other uses, see Absence of light (disambiguation).

The Creation of Light by Gustave Doré
Darkness, the direct opposite of lightness, is defined as a lack of illumination, an absence of visible light, or a surface that absorbs light, such as black.

Human vision is unable to distinguish colors in conditions of very low luminance because the hue sensitive photoreceptor cells on the retina are inactive when light levels are insufficient, in the range of visual perception referred to as scotopic vision.

The emotional response to darkness has generated metaphorical usages of the term in many cultures, often used to describe an unhappy or foreboding feeling.

Referring to a time of day, complete darkness occurs when the Sun is more than 18° below the horizon, without the effects of twilight on the night sky.

Scientific
Perception
The perception of darkness differs from the mere absence of light due to the effects of after images on perception. In perceiving, the eye is active, and the part of the retina that is unstimulated produces a complementary afterimage.[1]

Physics
See also: Light and Heat death of the universe
In terms of physics, an object is said to be dark when it absorbs photons, causing it to appear dim compared to other objects. For example, matte black paint does not reflect much visible light and appears dark, whereas white paint reflects much light and appears bright.[2] For more information, see color. An object may appear dark, but it may be bright at a frequency that humans cannot perceive.

A dark area has limited light sources, making things hard to see. Exposure to alternating light and darkness (night and day) has caused several evolutionary adaptations to darkness. When a vertebrate, like a human, enters a dark area, its pupils dilate, allowing more light to enter the eye and improving night vision. Also, the light detecting cells in the human eye (rods and cones) will regenerate more unbleached rhodopsin when adapting to darkness.

One scientific measure of darkness is the Bortle scale, which indicates the night sky's and stars' brightness at a particular location, and the observability of celestial objects at that location.[3]

The material known as Vantablack is one of the darkest substances known, absorbing up to 99.965% of visible light (at 663 nm if the light is perpendicular to the material), and was developed by Surrey NanoSystems in the United Kingdom.[4][5] The name is a compound of the acronym VANTA (vertically aligned nanotube arrays) and the color black.[6]

Technical
The color of a point, on a standard 24-bit computer display, is defined by three RGB (red, green, blue) values, each ranging from 0–255. When the red, green, and blue components of a pixel are fully illuminated (255,255,255), the pixel appears white; when all three components are unilluminated (0,0,0), the pixel appears black.[7]

Cultural
Artistic

Caravaggio's The Calling of St Matthew uses darkness for its chiaroscuro effects.
Main article: Tints and shades

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Artists use darkness to emphasize and contrast the presence of light. Darkness can be used as a counterpoint to areas of lightness to create leading lines and voids. Such shapes draw the eye around areas of the painting. Shadows add depth and perspective to a painting. See chiaroscuro for a discussion of the uses of such contrasts in visual media.

Color paints are mixed together to create darkness, because each color absorbs certain frequencies of light. Theoretically, mixing together the three primary colors, or the three secondary colors, will absorb all visible light and create black. In practice, it is difficult to prevent the mixture from taking on a brown tint.

Literature

Separation of light and darkness on the first day of creation, from the Sistine Chapel ceiling by Michelangelo
Further information: Light and darkness
As a poetic term in the Western world, darkness is used to connote the presence of shadows, evil, and foreboding,[8] or in modern parlance, to connote that a story is grim, heavy, and/or depressing.[9]

Religion
The first creation narrative in Judaism and Christianity begins with darkness, into which is introduced the creation of light, and the separation of this light from the darkness (as distinct from the creation of the Sun and Moon on the fourth day of creation). Thus, although both light and darkness are included in the comprehensive works of God, darkness was considered "the second to last plague" (Exodus 10:21), and the location of "weeping and gnashing of teeth" (Matthew 8:12).

Erebus was a primordial deity in Greek mythology, representing the personification of darkness.

Philosophy
In Chinese philosophy, yin is the complementary feminine part of the taijitu and is represented by a dark lobe.

Poetry
The use of darkness as a rhetorical device has a long-standing tradition. William Shakespeare, working in the 16th and 17th centuries, made a character called the "prince of darkness" (King Lear: III, iv) and gave darkness jaws with which to devour love. (A Midsummer Night's Dream: I, i)[10] Geoffrey Chaucer, a 14th-century Middle English writer of The Canterbury Tales, wrote that knights must cast away the "workes of darkness".[11] In Divine Comedy, Dante described hell as "solid darkness stain'd".[12]

Language
In Old English there were three words that could mean darkness: heolstor, genip, and sceadu.[13] Heolstor also meant "hiding-place" and became holster. Genip meant "mist" and fell out of use like many strong verbs. It is however still used in the Dutch saying "in het geniep" which means secretly. Sceadu meant "shadow" and remained in use. The word dark eventually evolved from the word deorc.[14]

See also
Lightness
Shadow
Theory of colours
Nyctophobia
References
Horner, David T. (2000). Demonstrations of Color Perception and the Importance of Contours, Handbook for Teaching Introductory Psychology. Vol. 2. Texas: Psychology Press. p. 217. ISBN 9780805836547. Afterimages are the complementary hue of the adapting stimulus and trichromatic theory fails to account for this fact
Mantese, Lucymarie (March 2000). "Photon-Driven Localization: How Materials Really Absorb Light". American Physical Society, Annual March Meeting. American Physical Society: E2.001. Bibcode:2000APS..MAR.E2001M.
Mizon, Bob (2016-07-04). Finding a Million-Star Hotel: An Astro-Tourist's Guide to Dark Sky Places. Springer. pp. 9–16. ISBN 978-3-319-33855-2.
Coldewey, Devin (15 July 2014). "Vantablack: U.K. Firm Shows Off 'World's Darkest Material'". NBC News. Archived from the original on 19 July 2014. Retrieved 19 July 2014.
Guinness World Records: Darkest manmade substance, 19 October 2015
Rossing, Thomas D.; Chiaverina, Christopher J. (2020-01-03). Light Science: Physics and the Visual Arts. Springer Nature. p. 172. ISBN 978-3-030-27103-9.
Kruegle, Herman (2011-03-15). CCTV Surveillance: Video Practices and Technology. Elsevier. p. 259. ISBN 978-0-08-046818-1.
Heart of Darkness: Literary Touchstone Classic. Prestwick House Inc. p. 6. ISBN 978-1-58049-812-8.
"Darkness". MacMillan Dictionary. Archived from the original on Dec 9, 2016. Retrieved 19 December 2022.
Shakespeare, William. "The Complete Works". The Tech, MIT.
Chaucer, Geoffrey. The Canterbury Tales, and Other Poems. The Second Nun's Tale.
Alighieri, Dante; Francis, Henry (trans.). The Divine Comedy.
Mitchell, Bruce; Fred C. Robinson (2001). A Guide to Old English. Glossary: Blackwell Publishing. pp. 332, 349, 363, 369. ISBN 978-0-631-22636-9.
Harper, Douglass (November 2001). "Dark". Online Etymology Dictionary. Retrieved 2007-01-18.

Nature

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For other uses, see Nature (disambiguation). "Natural" redirects here. For other uses, see Natural (disambiguation).

Shaki Waterfall, Armenia

Bachalpsee in the Swiss Alps

A winter landscape in Lapland, Finland

Lightning strikes during the eruption of the Galunggung volcano, West Java, in 1982

Life in the abyssal oceans

South Penghu Marine National Park of Taiwan, showing the wonder of nature
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are part of nature, human activity is often understood as a separate category from other natural phenomena.[1]

The word nature is borrowed from the Old French nature and is derived from the Latin word natura, or "essential qualities, innate disposition", and in ancient times, literally meant "birth".[2] In ancient philosophy, natura is mostly used as the Latin translation of the Greek word physis (φύσις), which originally related to the intrinsic characteristics of plants, animals, and other features of the world to develop of their own accord.[3][4] The concept of nature as a whole, the physical universe, is one of several expansions of the original notion;[1] it began with certain core applications of the word φύσις by pre-Socratic philosophers (though this word had a dynamic dimension then, especially for Heraclitus), and has steadily gained currency ever since.

During the advent of modern scientific method in the last several centuries, nature became the passive reality, organized and moved by divine laws.[5][6] With the Industrial revolution, nature increasingly became seen as the part of reality deprived from intentional intervention: it was hence considered as sacred by some traditions (Rousseau, American transcendentalism) or a mere decorum for divine providence or human history (Hegel, Marx). However, a vitalist vision of nature, closer to the presocratic one, got reborn at the same time, especially after Charles Darwin.[1]

Within the various uses of the word today, "nature" often refers to geology and wildlife. Nature can refer to the general realm of living plants and animals, and in some cases to the processes associated with inanimate objects—the way that particular types of things exist and change of their own accord, such as the weather and geology of the Earth. It is often taken to mean the "natural environment" or wilderness—wild animals, rocks, forest, and in general those things that have not been substantially altered by human intervention, or which persist despite human intervention. For example, manufactured objects and human interaction generally are not considered part of nature, unless qualified as, for example, "human nature" or "the whole of nature". This more traditional concept of natural things that can still be found today implies a distinction between the natural and the artificial, with the artificial being understood as that which has been brought into being by a human consciousness or a human mind. Depending on the particular context, the term "natural" might also be distinguished from the unnatural or the supernatural.[1]

Earth
Nature timeline
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Main articles: Earth and Earth science

The Blue Marble, which is a famous view of the Earth, taken in 1972 by the crew of Apollo 17
Earth is the only planet known to support life, and its natural features are the subject of many fields of scientific research. Within the Solar System, it is third closest to the Sun; it is the largest terrestrial planet and the fifth largest overall. Its most prominent climatic features are its two large polar regions, two relatively narrow temperate zones, and a wide equatorial tropical to subtropical region.[7] Precipitation varies widely with location, from several metres of water per year to less than a millimetre. 71 percent of the Earth's surface is covered by salt-water oceans. The remainder consists of continents and islands, with most of the inhabited land in the Northern Hemisphere.

Earth has evolved through geological and biological processes that have left traces of the original conditions. The outer surface is divided into several gradually migrating tectonic plates. The interior remains active, with a thick layer of plastic mantle and an iron-filled core that generates a magnetic field. This iron core is composed of a solid inner phase, and a fluid outer phase. Convective motion in the core generates electric currents through dynamo action, and these, in turn, generate the geomagnetic field.

The atmospheric conditions have been significantly altered from the original conditions by the presence of life-forms,[8] which create an ecological balance that stabilizes the surface conditions. Despite the wide regional variations in climate by latitude and other geographic factors, the long-term average global climate is quite stable during interglacial periods,[9] and variations of a degree or two of average global temperature have historically had major effects on the ecological balance, and on the actual geography of the Earth.[10][11]

Geology
Main article: Geology
Geology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed. The field is a major academic discipline, and is also important for mineral and hydrocarbon extraction, knowledge about and mitigation of natural hazards, some Geotechnical engineering fields, and understanding past climates and environments.

Geological evolution

Three types of geological plate tectonic boundaries
The geology of an area evolves through time as rock units are deposited and inserted and deformational processes change their shapes and locations.

Rock units are first emplaced either by deposition onto the surface or intrude into the overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and later lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows, blanket the surface. Igneous intrusions such as batholiths, laccoliths, dikes, and sills, push upwards into the overlying rock, and crystallize as they intrude.

After the initial sequence of rocks has been deposited, the rock units can be deformed and/or metamorphosed. Deformation typically occurs as a result of horizontal shortening, horizontal extension, or side-to-side (strike-slip) motion. These structural regimes broadly relate to convergent boundaries, divergent boundaries, and transform boundaries, respectively, between tectonic plates.

Historical perspective
Main articles: History of the Earth and Evolution

An animation showing the movement of the continents from the separation of Pangaea until the present day
Earth is estimated to have formed 4.54 billion years ago from the solar nebula, along with the Sun and other planets.[12] The Moon formed roughly 20 million years later. Initially molten, the outer layer of the Earth cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water v***r, most or all of which came from ice delivered by comets, produced the oceans and other water sources.[13] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago.[14]

Plankton inhabit oceans, seas and lakes, and have existed in various forms for at least 2 billion years[15]
Continents formed, then broke up and reformed as the surface of Earth reshaped over hundreds of millions of years, occasionally combining to make a supercontinent. Roughly 750 million years ago, the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia which broke apart about 540 million years ago, then finally Pangaea, which broke apart about 180 million years ago.[16]

During the Neoproterozoic era, freezing temperatures covered much of the Earth in glaciers and ice sheets. This hypothesis has been termed the "Snowball Earth", and it is of particular interest as it precedes the Cambrian explosion in which multicellular life forms began to proliferate about 530–540 million years ago.[17]

Since the Cambrian explosion there have been five distinctly identifiable mass extinctions.[18] The last mass extinction occurred some 66 million years ago, when a meteorite collision probably triggered the extinction of the non-avian dinosaurs and other large reptiles, but spared small animals such as mammals. Over the past 66 million years, mammalian life diversified.[19]

Several million years ago, a species of small African ape gained the ability to stand upright.[15] The subsequent advent of human life, and the development of agriculture and further civilization allowed humans to affect the Earth more rapidly than any previous life form, affecting both the nature and quantity of other organisms as well as global climate. By comparison, the Great Oxygenation Event, produced by the proliferation of algae during the Siderian period, required about 300 million years to culminate.

The present era is classified as part of a mass extinction event, the Holocene extinction event, the fastest ever to have occurred.[20][21] Some, such as E. O. Wilson of Harvard University, predict that human destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years.[22] The extent of the current extinction event is still being researched, debated and calculated by biologists.[23][24][25]

Atmosphere, climate, and weather

Blue light is scattered more than other wavelengths by the gases in the atmosphere, giving the Earth a blue halo when seen from space
Main articles: Atmosphere of Earth, Climate, and Weather
The Earth's atmosphere is a key factor in sustaining the ecosystem. The thin layer of gases that envelops the Earth is held in place by gravity. Air is mostly nitrogen, oxygen, water v***r, with much smaller amounts of carbon dioxide, argon, etc. The atmospheric pressure declines steadily with altitude. The ozone layer plays an important role in depleting the amount of ultraviolet (UV) radiation that reaches the surface. As DNA is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes.

Terrestrial weather occurs almost exclusively in the lower part of the atmosphere, and serves as a convective system for redistributing heat.[26] Ocean currents are another important factor in determining climate, particularly the major underwater thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions. These currents help to moderate the differences in temperature between winter and summer in the temperate zones. Also, without the redistributions of heat energy by the ocean currents and atmosphere, the tropics would be much hotter, and the polar regions much colder.

Lightning
Weather can have both beneficial and harmful effects. Extremes in weather, such as tornadoes or hurricanes and cyclones, can expend large amounts of energy along their paths, and produce devastation. Surface vegetation has evolved a dependence on the seasonal variation of the weather, and sudden changes lasting only a few years can have a dramatic effect, both on the vegetation and on the animals which depend on its growth for their food.

Climate is a measure of the long-term trends in the weather. Various factors are known to influence the climate, including ocean currents, surface albedo, greenhouse gases, variations in the solar luminosity, and changes to the Earth's orbit. Based on historical records, the Earth is known to have undergone drastic climate changes in the past, including ice ages.

A tornado in central Oklahoma
The climate of a region depends on a number of factors, especially latitude. A latitudinal band of the surface with similar climatic attributes forms a climate region. There are a number of such regions, ranging from the tropical climate at the equator to the polar climate in the northern and southern extremes. Weather is also influenced by the seasons, which result from the Earth's axis being tilted relative to its orbital plane. Thus, at any given time during the summer or winter, one part of the Earth is more directly exposed to the rays of the sun. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the Northern and Southern Hemispheres experience opposite seasons.

Weather is a chaotic system that is readily modified by small changes to the environment, so accurate weather forecasting is limited to only a few days.[27] Overall, two things are happening worldwide: (1) temperature is increasing on the average; and (2) regional climates have been undergoing noticeable changes.[28]

Water on the Earth

The Iguazu Falls on the border between Brazil and Argentina
Main article: Water
Water is a chemical substance that is composed of hydrogen and oxygen (H2O) and is vital for all known forms of life.[29] 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 v***r, or steam. Water covers 71% of the Earth's surface.[30] On Earth, it is found mostly in oceans and other large bodies of water, with 1.6% of water below ground in aquifers and 0.001% in the air as v***r, clouds, and precipitation.[31][32] 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%. Additionally, a minute amount of the Earth's water is contained within biological bodies and manufactured products.

Oceans

A view of the Atlantic Ocean from Leblon, Rio de Janeiro
Main article: Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (an area of some 361 million square kilometers) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. More than half of this area is over 3,000 meters (9,800 feet) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Though generally recognized as several 'separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean.[33][34] This concept of a global ocean as a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.[35]

The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria: these divisions are (in descending order of size) the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean, and the Arctic Ocean. Smaller regions of the oceans are called seas, gulfs, bays and other names. There are also salt lakes, which are smaller bodies of landlocked saltwater that are not interconnected with the World Ocean. Two notable examples of salt lakes are the Aral Sea and the Great Salt Lake.

Lakes

Lake Mapourika, New Zealand
Main article: Lake
A lake (from Latin word lacus) is a terrain feature (or physical feature), a body of liquid on the surface of a world that is localized to the bottom of basin (another type of landform or terrain feature; that is, it is not global) and moves slowly if it moves at all. On Earth, a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is fed by a river.[36][37] The only world other than Earth known to harbor lakes is Titan, Saturn's largest moon, which has lakes of ethane, most likely mixed with methane. It is not known if Titan's lakes are fed by rivers, though Titan's surface is carved by numerous river beds. Natural lakes on Earth are generally found in mountainous areas, rift zones, and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last ice age. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.

Ponds

The Westborough Reservoir (Mill Pond) in Westborough, Massachusetts
Main article: Pond
A pond is a body of standing water, either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens designed for aesthetic ornamentation, fish ponds designed for commercial fish breeding, and solar ponds designed to store thermal energy. Ponds and lakes are distinguished from streams via current speed. While currents in streams are easily observed, ponds and lakes possess thermally driven micro-currents and moderate wind driven currents. These features distinguish a pond from many other aquatic terrain features, such as stream pools and tide pools.

Fire

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For other uses, see Fire (disambiguation).

An outdoor wood fire
0:15
The ignition and extinguishing of a pile of wood shavings
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.[1][a] At a certain point in the combustion reaction, called the ignition point, flames are produced. The flame is the visible portion of the fire. Flames consist primarily of carbon dioxide, water v***r, oxygen and nitrogen. If hot enough, the gases may become ionized to produce plasma. Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.[2]

Fire in its most common form can result in conflagration, which has the potential to cause physical damage through burning. Fire is an important process that affects ecological systems around the globe. The positive effects of fire include stimulating growth and maintaining various ecological systems. Its negative effects include hazard to life and property, atmospheric pollution, and water contamination.[3] If fire removes protective vegetation, heavy rainfall may lead to an increase in soil erosion by water.[4] Also, when vegetation is burned, the nitrogen it contains is released into the atmosphere, unlike elements such as potassium and phosphorus which remain in the ash and are quickly recycled into the soil. This loss of nitrogen caused by a fire produces a long-term reduction in the fertility of the soil, but this fecundity can potentially be recovered as molecular nitrogen in the atmosphere is "fixed" and converted to ammonia by natural phenomena such as lightning and by leguminous plants that are "nitrogen-fixing" such as clover, peas, and green beans.

Fire is one of the four classical elements and has been used by humans in rituals, in agriculture for clearing land, for cooking, generating heat and light, for signaling, propulsion purposes, smelting, forging, incineration of waste, cremation, and as a weapon or mode of destruction.

Physical properties
Chemistry
Main article: Combustion

The balanced chemical equation for the combustion of methane, a hydrocarbon
Fire is a chemical process in which a fuel and an oxidizing agent react, yielding carbon dioxide and water.[5] This process, known as a combustion reaction, does not proceed directly and involves intermediates.[5] Although the oxidizing agent is typically oxygen, other compounds are able to fulfill the role. For instance, chlorine trifluoride is able to ignite sand.[6]

Fires start when a flammable or a combustible material, in combination with a sufficient quantity of an oxidizer such as oxygen gas or another oxygen-rich compound (though non-oxygen oxidizers exist), is exposed to a source of heat or ambient temperature above the flash point for the fuel/oxidizer mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction. This is commonly called the fire tetrahedron. Fire cannot exist without all of these elements in place and in the right proportions. For example, a flammable liquid will start burning only if the fuel and oxygen are in the right proportions. Some fuel-oxygen mixes may require a catalyst, a substance that is not consumed, when added, in any chemical reaction during combustion, but which enables the reactants to combust more readily.

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.

If the oxidizer is oxygen from the surrounding air, the presence of a force of gravity, or of some similar force caused by acceleration, is necessary to produce convection, which removes combustion products and brings a supply of oxygen to the fire. Without gravity, a fire rapidly surrounds itself with its own combustion products and non-oxidizing gases from the air, which exclude oxygen and extinguish the fire. Because of this, the risk of fire in a spacecraft is small when it is coasting in inertial flight.[7][8] This does not apply if oxygen is supplied to the fire by some process other than thermal convection.

The fire tetrahedron
Fire can be extinguished by removing any one of the elements of the fire tetrahedron. Consider a natural gas flame, such as from a stove-top burner. The fire can be extinguished by any of the following:

turning off the gas supply, which removes the fuel source;
covering the flame completely, which smothers the flame as the combustion both uses the available oxidizer (the oxygen in the air) and displaces it from the area around the flame with CO2;
application of water, which removes heat from the fire faster than the fire can produce it (similarly, blowing hard on a flame will displace the heat of the currently burning gas from its fuel source, to the same end), or
application of a retardant chemical such as Halon to the flame, which retards the chemical reaction itself until the rate of combustion is too slow to maintain the chain reaction.
In contrast, fire is intensified by increasing the overall rate of combustion. Methods to do this include balancing the input of fuel and oxidizer to stoichiometric proportions, increasing fuel and oxidizer input in this balanced mix, increasing the ambient temperature so the fire's own heat is better able to sustain combustion, or providing a catalyst, a non-reactant medium in which the fuel and oxidizer can more readily react.

Flame
Main article: Flame
See also: Flame test

A candle's flame
A flame is a mixture of reacting gases and solids emitting visible, infrared, and sometimes ultraviolet light, the frequency spectrum of which depends on the chemical composition of the burning material 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, are fluorine and hydrogen, and hydrazine and nitrogen tetroxide. Hydrogen and hydrazine/UDMH flames are similarly pale blue, while burning boron and its compounds, evaluated in mid-20th century as a high energy fuel for jet and rocket engines, emits intense green flame, leading to its informal nickname of "Green Dragon".

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 in Canada 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.

Fire is affected by gravity. Left: Flame on Earth; Right: Flame on the ISS
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 micro gravity or zero gravity,[9] 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 may go out if not moved steadily, as the CO2 from combustion does not disperse as readily in micro gravity, and tends to smother the flame). There are several possible explanations for this difference, of which the most likely is that the temperature is sufficiently evenly distributed that soot is not formed and complete combustion occurs.[10] Experiments by NASA reveal that diffusion flames in micro gravity 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 micro gravity when compared to normal gravity conditions.[11] 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 adiabatic temperatures
Main article: Adiabatic flame temperature
The adiabatic flame temperature of a given fuel and oxidizer pair is that at which the gases achieve stable combustion.

Oxy–dicyanoacetylene 4,990 °C (9,000 °F)
Oxy–acetylene 3,480 °C (6,300 °F)
Oxyhydrogen 2,800 °C (5,100 °F)
Air–acetylene 2,534 °C (4,600 °F)
Blowtorch (air–MAPP gas) 2,200 °C (4,000 °F)
Bunsen burner (air–natural gas) 1,300 to 1,600 °C (2,400 to 2,900 °F)[12]
Candle (air–paraffin) 1,000 °C (1,800 °F)
Fire science & ecology
Main article: Fire ecology
Every natural ecosystem on land has its own fire regime, and the organisms in those ecosystems are adapted to or dependent upon that fire regime. Fire creates a mosaic of different habitat patches, each at a different stage of succession.[13] Different species of plants, animals, and microbes specialize in exploiting a particular stage, and by creating these different types of patches, fire allows a greater number of species to exist within a landscape.

Fire science is a branch of physical science which includes fire behavior, dynamics, and combustion. Applications of fire science include fire protection, fire investigation, and wildfire management.

Fossil record
Main article: Fossil record of fire
The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago,[14] permitting the accumulation of oxygen in the atmosphere as never before, as the new hordes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire.[15] Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago, by fossils of charcoalified plants.[16][17] Apart from a controversial gap in the Late Devonian, charcoal is present ever since.[17] The level of atmospheric oxygen is closely related to the prevalence of charcoal: clearly oxygen is the key factor in the abundance of wildfire.[18] Fire also became more abundant when grasses radiated and became the dominant component of many ecosystems, around 6 to 7 million years ago;[19] this kindling provided tinder which allowed for the more rapid spread of fire.[18] These widespread fires may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.[18]

Human control
Early human control
Main article: Control of fire by early humans

Bushman starting a fire in Namibia
The ability to control fire was a dramatic change in the habits of early humans. Making fire to generate heat and light made it possible for people to cook food, simultaneously increasing the variety and availability of nutrients and reducing disease by killing organisms in the food.[20] The heat produced would also help people stay warm in cold weather, enabling them to live in cooler climates. Fire also kept nocturnal predators at bay. Evidence of occasional cooked food is found from 1 million years ago.[21] Although this evidence shows that fire may have been used in a controlled fashion about 1 million years ago,[22][23] other sources put the date of regular use at 400,000 years ago.[24] Evidence becomes widespread around 50 to 100 thousand years ago, suggesting regular use from this time; interestingly, resistance to air pollution started to evolve in human populations at a similar point in time.[24] The use of fire became progressively more sophisticated, as it was used to create charcoal and to control wildlife from 'tens of thousands' of years ago.[24]

Fire has also been used for centuries as a method of torture and ex*****on, as evidenced by death by burning as well as torture devices such as the iron boot, which could be filled with water, oil, or even lead and then heated over an open fire to the agony of the wearer.

Here, food is cooked in a cauldron above fire in South Africa
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", which damage the soil. Hot fires destroy plants and animals, and endanger communities.[25] 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 autumn. 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. Another human use for fire in regards to landscape management is its use to clear land for agriculture. Slash-and-burn agriculture is still common across much of tropical Africa, Asia and South America. "For small farmers, it is a convenient way to clear overgrown areas and release nutrients from standing vegetation back into the soil", said Miguel Pinedo-Vasquez, an ecologist at the Earth Institute’s Center for Environmental Research and Conservation.[26] However, this useful strategy is also problematic. Growing population, fragmentation of forests and warming climate are making the earth's surface more prone to ever-larger escaped fires. These harm ecosystems and human infrastructure, cause health problems, and send up spirals of carbon and soot that may encourage even more warming of the atmosphere – and thus feed back into more fires. Globally today, as much as 5 million square kilometres – an area more than half the size of the United States – burns in a given year.[26]

Later human control

The Lyceum in 1861
The Great Fire of London (1666) and Hamburg after four fire-bombing raids in July 1943, which killed an estimated 50,000 people[27]
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 by igniting fuels such as coal, oil or natural gas, then using the resultant heat to boil water into steam, which then drives turbines.

The use of fire in warfare has a long history. Fire was the basis of all early thermal weapons. Homer detailed the use of fire by Greek soldiers 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 Tokyo, 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.[28] 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. The use of na**lm was employed in July 1944, towards the end of the Second World War; although its use did not gain public attention until the Vietnam War.[citation needed] Molotov cocktails were also used.

Productive use for energy

A coal-fired power station in China
Burning fuel converts chemical energy into heat energy; wood has been used as fuel since prehistory. As of 2002 fossil fuels such as petroleum, natural gas, and coal in power plants supply the vast majority of the world's electricity; the International Energy Agency states that nearly 80% of the world's power came from these sources in 2002.[29] 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 electricity. Fire is also used to provide mechanical work directly by thermal expansion, in both external and internal combustion engines.

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.

Fire management
Controlling a fire to optimize its size, shape, and intensity is generally called fire management, and the more advanced forms of it, as traditionally (and sometimes still) practiced by skilled cooks, blacksmiths, ironmasters, and others, are highly skilled activities. They include knowledge of which fuel to burn; how to arrange the fuel; how to stoke the fire both in early phases and in maintenance phases; how to modulate the heat, flame, and smoke as suited to the desired application; how best to bank a fire to be revived later; how to choose, design, or modify stoves, fireplaces, bakery ovens, industrial furnaces; and so on. Detailed expositions of fire management are available in various books about blacksmithing, about skilled camping or military scouting, and about domestic arts.

Protection and prevention
Main articles: Wildfire and Fire protection

An abandoned convent on fire in Quebec
Wildfire prevention programs around the world may employ techniques such as wildland fire use and prescribed or controlled burns.[30][31] Wildland fire use refers to any fire of natural causes that is monitored but allowed to burn. Controlled burns are fires ignited by government agencies under less dangerous weather conditions.[32]

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.

Fire prevention is intended to reduce sources of ignition. Fire prevention also includes education to teach people how to avoid causing fires.[33] 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 crime in most jurisdictions.[34]

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.

Where fire prevention and fire protection have failed to prevent damage, fire insurance can mitigate the financial impact.[35]

See also
Aodh (given name)
Bonfire
The Chemical History of a Candle
Colored fire
Control of fire by early humans
Deflagration
Fire (classical element)
Fire investigation
Fire lookout
Fire lookout tower
Fire making
Fire pit
Fire safety
Fire triangle
Fire whirl
Fire worship
Flame test
Life Safety Code
List of fires
List of light sources
Phlogiston theory
Piano burning
Prometheus, the Greek mythological figure who gave mankind fire
Pyrokinesis
Pyrolysis
Pyromania
Self-immolation
References
Notes
Slower oxidative processes like rusting or digestion are not included by this definition.
Citations
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Morris, S. E.; Moses, T. A. (1987). "Forest Fire and the Natural Soil Erosion Regime in the Colorado Front Range". Annals of the Association of American Geographers. 77 (2): 245–54. doi:10.1111/j.1467-8306.1987.tb00156.x.
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CFM-1 experiment results Archived 2007-09-12 at the Wayback Machine, National Aeronautics and Space Administration, April 2005.
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Bowman, D. M. J. S.; Balch, J. K.; Artaxo, P.; Bond, W. J.; Carlson, J. M.; Cochrane, M. A.; d'Antonio, C. M.; Defries, R. S.; Doyle, J. C.; Harrison, S. P.; Johnston, F. H.; Keeley, J. E.; Krawchuk, M. A.; Kull, C. A.; Marston, J. B.; Moritz, M. A.; Prentice, I. C.; Roos, C. I.; Scott, A. C.; Swetnam, T. W.; Van Der Werf, G. R.; Pyne, S. J. (2009). "Fire in the Earth system". Science. 324 (5926): 481–4. Bibcode:2009Sci...324..481B. doi:10.1126/science.1163886. PMID 19390038. S2CID 22389421.