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A 'firestorm' is a conflagration which attains such intensity that it creates and sustains its own wind system. It is most commonly a natural phenomenon, created during some of the largest bushfires, forest fires, and wildfires. The Great Peshtigo Fire is one example of a firestorm. Firestorms can also be deliberate effects of targeted explosives such as occurred as a result of the aerial bombings of Dresden and Tokyo during World War II.

Contents

Mechanism of firestorms

Firestorms in wildfires

Firestorms in cities

Reference

See also

Mechanism of firestorms

A firestorm is created as a result of stack effect as the heat of the original fire draws in more and more of the surrounding air. This draft can be quickly increased if a low level jet stream exists over or near the fire, or when an atmospheric temperature inversion cap is pierced by it. As the updraft mushrooms, strong gusty winds develop around the fire, directed inward. This would seem to prevent the firestorm from spreading on the wind, but for the fact that tremendous turbulence is also created by the strong updraft which causes the strong surface inflow winds to change direction erratically. This wind shear is capable of producing small tornado or dust devil like circulations called fire whirls which can also dart around erratically, damage or destroy houses and buildings, and quickly spread the fire to areas outside the central area of the fire.
The greater draft of a firestorm draws in greater quantities of oxygen which significantly increases combustion, thereby also substantially increasing the production of heat. The intense heat of a firestorm manifests largely as radiated heat (infrared radiation) which ignites flammable material at a distance ahead of the fire itself.
Besides the enormous ash cloud produced by a firestorm, under the right conditions, it can also induce condensation, forming a cloud called a pyrocumulus or "fire cloud". A large pyrocumulus can produce lightning, which can set off further fires. Apart from forest fires, pyrocumuluses can also be produced by volcanic eruptions.
In Australia, the prevalence of eucalyptus trees that have oil in their leaves results in forest fires that are noted for their extremely tall and intense flame front. Hence the bush fires appear more as a fire-storm than a simple forest fire.

Firestorms in wildfires

Firestorms often appear in thalwegs or crests or on plateaus. Warning signs include:

★ Decreased visibility;

★ Decreased sound conduction;

★ Breathing difficulties (firefighters do not use SCBA on wildfires);

★ Roasting (pyrolysis) of the leaves by the radiated heat.
Some plants protect themselves from the heat of fire by two mechanisms: evapotranspiration, and the emission of volatile organic compounds (VOC). In case of drought, especially when the humidity is less than 30%, the emission of VOC is more important as evapotranspiration is drastically reduced.
When a fire comes nearer, the emission of VOC is increased to fight the rise of temperature; at 170 °C, the rosemary plant emits 55 times more terpene than at 50 °C. A temperature of 170 °C is considered a critical temperature, at which the emission of VOC can lead to an explosive mix with the air and thus to a flash over. Additionally, the fire itself emits pyrolysis gases that are not burnt, and that mix with the VOC; the explosive mix can be reached faster.
The topography has a complex influence. A closed relief, such as a small valley or a dry river, concentrates the heat and thus the emission of VOC, especially for rosemary, rockrose or Aleppo Pine. Contrarily, the kermes oak emits more VOC on an open relief such as plain or plateau.
Other factors that influence the occurrence of a firestorm are the natural heat, especially above 35 °C in the shadow, a humidity less than 30% and no strong wind. These conditions are met in climates such as the Mediterranean forest.
The firestorms can be classified in several types:

★ Thermal bubble: at the bottom of a small valley rich in combustible materials (plants), the combustible gas forms a bubble that cannot mix with the air because its temperature is too high; this bubble moves randomly, pushed by the wind.

★ Fire carpet: in a deep and opened small valley, the whole valley catches fire.

★ Confinement by a layer of cold air: a strong and cold wind prevents the pyrolysis gas from rising, which leads to the explosive situation.

★ Pyrolysis of the opposite slope: the fire progresses down a slope, but the radiated heat pyrolyses the plants on the facing slope, which catches fire seemingly spontaneously.

★ Bottom of a small valley: the gases accumulate in the bed of a dry river; when the fire comes, it completes the fire triangle and the bottom of the valley catches fire.

Firestorms in cities

The same underlying combustion physics can also apply to man-made structures such as cities.
Firestorms are thought to have been part of the mechanism of large urban fires such as the Great Fire of Rome, the Great Fire of London, the Great Fire of Chicago, and the fires resulting from the 1906 San Francisco Earthquake and the Great Kanto Earthquake. Firestorms were also created by the firebombing raids of World War II in Tokyo, Japan; and in the German cities of Hamburg, Dresden, Kassel, Darmstadt, Pforzheim, Braunschweig, Hildesheim and Stuttgart. (''see also: firebombing of Dresden, Tokyo, Kassel, and Operation Gomorrah).''

City / Event	Date of the firestorm	Notes
Great Fire of London	2 September 1666 - 5 September 1666	Most of the city of London burned to the ground; only 5 known deaths as a result.
Great Chicago Fire Peshtigo Fire Port Huron Fire	8 October 1871	Hundreds killed in Chicago from 8 October to 10 October; up to 2,500 killed in Peshtigo, Wisconsin; others killed in similar fires in Holland and Manistee, Michigan.
Great Kantō earthquake	1 September 1923	140 000 dead, most of them in firestorms in Tokyo and the port city of Yokohama. Total damages amounted to 40% of the GNP of that year.
Wuppertal (Germany)	10 May 1943
Hamburg (Germany)	24 July 1943	45,000 dead
Remscheid (Germany)	31 July 1943
Kassel (Germany)	23 October 1943	10,000 dead
Kaiserslautern (Germany)	14 July 1944
Braunschweig (Germany)	15 October 1944	2,600 dead
Saarbrücken (Germany)	5 August 1944
Darmstadt (Germany)	11 September 1944	12,300 dead
Stuttgart (Germany)	12 September 1944
Heilbronn (Germany)	6 December 1944	6,500 dead
Ulm (Germany)	17 December 1944
Dresden (Germany)	13 February 1945	35,000 dead
Pforzheim (Germany)	23 February 1945
Mainz (Germany)	27 February 1945
Tokyo (Japan)	9 March 1945	120,000 dead
Würzburg (Germany)	16 March 1945	5,000 dead
Kobe (Japan)	17 March 1945
Hildesheim (Germany)	23 March 1945
Oakland Hills Firestorm	20 October 1991	25 dead, $1.5 billion in damages
2003 Canberra bushfires	16 January 2003	4 dead, 500 houses destroyed

During the course of World War II, the Allies refined the technique of fire-bombing: the first wave of bombers would drop high explosives to expose the timbers within buildings and to rupture water mains. This was followed immediately by a wave dropping incendiary cluster bombs (early in the war phosphorus was used, though napalm came into usage by the end of the war) to start a conflagration. A third wave then followed after an interval of fifteen minutes or so, dropping fragmentation bombs; the slight delay allowing time for firefighters and their equipment to be caught in the open and destroyed, thus preventing efforts to hamper the spreading fires. The furnace-like conditions created in those firestorms resulting from the strategic bombing campaigns of World War II were often hot enough to cremate the corpses they created. Nuclear weapons can also create firestorms in urban areas. This was responsible for a large portion of the destruction at Hiroshima.
The author, Kurt Vonnegut, who was a prisoner of war in Dresden at the time of its fire-bombing, described some of the carnage of this incident in his novel ''Slaughterhouse-Five''.

Reference

★ John Fleck, "Firestorms Get New Spin", ''The Albuquerque Journal'', May 14, 2000.[1]

See also

★ Wildfire