Firestorm

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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).

Image:FirestormHamburg.jpg

A firestorm is the mass movement of air resulting from fire, creating a fire of extreme intensity over a wide area. 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.

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Mechanism of firestorms

After an area catches fire, the overlying air becomes extremely hot and rises rapidly. The pyrolysis of materials away from the fire releases gas that are not immediately burnt, and that can create a bubble.

Cold air then rushes in at ground level and mixes with the hot gas, closing the fire triangle; the bubble instantly burns. This creates a self-sustaining firestorm with temperatures peaking at over 2,000 degrees Celsius fed by the influx of oxygen.

The mechanism is similar to that of a flashover, except that it is in open air.

Experiments with test fires have shown that firestorms can create fast-moving vortices which can spread the fire beyond the original area, with winds potentially reaching tornado strength, effectively creating a fire tornado and complicating the job of firefighters.

In a larger fire, the firestorm is just where the leading edge of the fire is very intense. The large amount of flame at the fire front is self-perpetuating as the radiated and convected heat from the fire front pre-heats the vegetation and causes flammable gases to emit from the vegetable matter some distance away from the flame front. The flame front can then readily ignite the standing and fallen vegetable matter.

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 bushfires 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 fire fighters resorting to saving buildings and lives when the hot dry days during summer encourage the occurrence of enormous fires.

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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 flashover. 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. Contrarilly, 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.
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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, and Stuttgart. (see also: firebombing of Dresden, Tokyo, Kassel, and Operation Gomorrah).

During the course of World War II, the technique of Fire-bombing was refined: 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 devices (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 the Second World War were often hot enough to cremate the corpses they created.

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.

Nuclear weapons are also very likely to create firestorms in urban areas. This was responsible for a large portion of the destruction at Hiroshima and Nagasaki.

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See also

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Reference

de:Feuersturm fr:embrasement généralisé éclair sl:Ognjeni vihar

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