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] Flames, the most visible portion of the fire, are produced in the combustion reaction when the fuel reaches its ignition point.

[5] Additionally, the burning of vegetation releases nitrogen into the atmosphere, unlike other plant nutrients such as potassium and phosphorus which remain in the ash and are quickly recycled into the soil.

[6][7] This loss of nitrogen produces a long-term reduction in the fertility of the soil, though it can be recovered by nitrogen-fixing plants such as clover, peas, and beans, by decomposition of animal waste and corpses, and by natural phenomena such as lightning.

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.

[9] The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago,[10] which contributed large amounts of oxygen to the atmosphere, as the new hordes of land plants pumped it out as a waste product.

[14] This widespread emergence of wildfire may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.

For small farmers, controlled fires are a convenient way to clear overgrown areas and release nutrients from standing vegetation back into the soil.

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.

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.

Hand-thrown incendiary bombs improvised from glass bottles, later known as Molotov cocktails, were deployed during the Spanish Civil War in the 1930s.

Also during that war, incendiary bombs were deployed against Guernica by Fascist Italian and Nazi German air forces that had been created specifically to support Franco's Nationalists.

Incendiary bombs were dropped by Axis and Allies during the Second World War, notably on Coventry, Tokyo, Rotterdam, London, Hamburg and 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.

[26] 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.

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, or 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.

[28] The International Energy Agency states that nearly 80% of the world's power has consistently come from fossil fuels such as petroleum, natural gas, and coal in the past decades.

[30] Fire is also used to provide mechanical work directly by thermal expansion, in both external and internal combustion engines.

Fire is a chemical process in which a fuel and an oxidizing agent react, yielding carbon dioxide and water.

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".

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.

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.