Fire


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]Fire is hot because the conversion of the weak double bond in molecular oxygen, O2, to the stronger bonds in the combustion products carbon dioxide and water releases energy (418 kJ per 32 g of O2); the bond energies of the fuel play only a minor role here.[2] 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 vapor, oxygen and nitrogen. If hot enough, the gases may become ionized to produce plasma.[3] Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.

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.[4] If fire removes protective vegetation, heavy rainfall may lead to an increase in soil erosion by water.[5] 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 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.

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.[6][7] This does not apply if oxygen is supplied to the fire by some process other than thermal convection.


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The ignition and extinguishing of a pile of wood shavings
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The fire maps show the locations of actively burning fires around the world on a monthly basis, based on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. The colors are based on a count of the number (not size) of fires observed within a 1,000-square-kilometer area. White pixels show the high end of the count—as many as 100 fires in a 1,000-square-kilometer area per day. Yellow pixels show as many as 10 fires, orange shows as many as five fires, and red areas as few as one fire per day.
Fire from a New Year's Eve BBQ.
Charcoal burning.
The fire tetrahedron
A candle's flame
Northwest Crown Fire Experiment, Canada
Photo of a fire taken with a 1/4000th of a second exposure
Fire is affected by gravity. Left: Flame on Earth; Right: Flame on the ISS
Bushman starting a fire in Namibia
Process of ignition of a match
Painting of the Cathedral and the Academy building after the Great Fire of Turku, by Gustaf Wilhelm Finnberg, 1827
Hamburg after four fire-bombing raids in July 1943, which killed an estimated 50,000 people[25]
A coal-fired power station in the People's Republic of China
Disability-adjusted life year for fires per 100,000 inhabitants in 2004[27]
  no data
  less than 50
  50–100
  100–150
  150–200
  200–250
  250–300
  300–350
  350–400
  400–450
  450–500
  500–600
  more than 600
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This visualization shows fires detected in the United States from July 2002 through July 2011. Look for fires that reliably burn each year in western states and across the Southeast.
Fire-damaged restaurant waiting for demolition