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Firestorm
 

This is an article about a specific circumstance of combustion. For the comic book superhero, see Firestorm (comics); for the science fiction novel, see Firestorm (novel); for the Howie Long film see Firestorm (film).

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 forest fires, and some of the largest forest fires, such as the Great Peshtigo Fire, have been firestorms. A firestorm can also be a deliberate effect of targeted explosives such as occurred as a result of the aeriel bombings of Dresden and Tokyo during World War II.

Mechanism of firestorms

A firestorm is created as a result of the "chimney 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 vortices akin to dust devils or small tornadoes which can also dart about erratically and quickly spread the fire to areas outside the central area of the fire.

Besides the enormous ash cloud produced by a firestorm, under the right conditions, it can also induce condensation, forming a cloud called a pyrocumulus. A large pyrocumulus cloud can produce lightning, which can set off further fires, but no rain at the surface as it is quickly evaporated aloft by the intense heat of the firestorm.

The intense heat of a firestorm can also produce radiated heat (infrared radiation) which ignites flammable material at a distance ahead of the fire itself.

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. The trees are full of oil to survive dry conditions. The oil fuels the fire and the fires are very difficult to bring under control, with firefighters resorting to saving buildings and lives when the hot dry days during summer encourage the occurrence of enormous fires.

Firestorms in wildfires

The firestorms often appear in thalwegs, crests or on plateaus. The 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.

In the hot countries (like the Mediterranean forest), the phenomenon can be initiated by the natural heat. The plants protect themselves from the heat by two mechanisms: the 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 because 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 emits 55 times more terpene than at 50 °C. This 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 relief 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 open relief such as plain or plateau.

The other factors that influences 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.

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 Chicago Fire, Great Fire of Rome, the Great Fire of London, 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 Hamburg, Dresden, Tokyo, Kassel, Darmstadt, Pforzheim, Braunschweig, Hildesheim and Stuttgart. (see also: firebombing of Dresden, Tokyo, Kassel, and Operation Gomorrah).

City: Date of the firestorm:

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 were also used to create firestorms in urban areas. This was responsible for a large portion of the destruction at Hiroshima and Nagasaki.

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.

See also

Reference

  • John Fleck, "Firestorms Get New Spin", The Albuquerque Journal, May 14, 2000.*

Fire | Storms | 1906 San Francisco earthquake

Tormenta ígnea | Feuersturm | embrasement généralisé éclair | Ognjeni vihar | 火災旋風

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Firestorm".

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