Definitions

Firestorm

Firestorm

[fahyuhr-stawrm]

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 and the Ash Wednesday fires are two examples of a firestorm. Firestorms can also be deliberate effects of targeted explosives such as occurred as a result of the aerial bombings of Dresden, Hamburg, Tokyo and the atomic bombing of Hiroshima during World War II.

Mechanism of firestorms

A firestorm is created as a result of the 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 the tremendous turbulence also created 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 pyrocumulus cloud or "fire cloud". A large pyrocumulus can produce lightning, which can set off further fires. Apart from forest fires, pyrocumulus clouds 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 Mediterranean forests.

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 1871 Great Chicago Fire, and the fires resulting from the 1906 San Francisco earthquake and the 1923 Great Kantō earthquake. Firestorms were also created by the firebombing raids of World War II in cities like London, Tokyo, Kobe, Hiroshima, Hamburg and Dresden.

City / Event Date of the firestorm Notes
Great Fire of London 2 September 1666 - 5 September 1666 Most of the City of London, (known as the "Square Mile"), this affected a much smaller area than that covered by modern London.
1871 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.
1923 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.
Hamburg (Germany) 27 July 1943 45,000 dead
Kassel (Germany) 23 October 1943 10,000 dead
Braunschweig (Germany) 15 October 1944 2,600 dead
Darmstadt (Germany) 11 September 1944 12,300 dead
Heilbronn (Germany) 6 December 1944 6,500 dead
Dresden (Germany) 13 February 1945 25,000-45,000 dead
Pforzheim (Germany) 23 February 1945 17,000 dead
Tokyo (Japan) 9 March 1945 120,000 dead
Würzburg (Germany) 16 March 1945 5,000 dead
Kobe (Japan) 17 March 1945 8,841 dead
Hiroshima (Japan) 6 August 1945 100,000+ dead
Oakland Firestorm of 1991 20 October 1991 25 dead, $1.5 billion in damages
2003 Canberra bushfires 16 January 2003 4 dead, 500 houses destroyed

Early in World War II many British cities were firebombed, a particularly notable raid was the Coventry Blitz on 14 November 1940. During the Coventry Blitz the Germans pioneered several innovations which were to influence all future strategic bomber raids during the war. These were: The use of pathfinder aircraft with electronic aids to navigate, to mark the targets before the main bomber raid; The use of high explosive bombs and air-mines (blockbuster bombs) coupled with thousands of incendiary bombs intended to set the city ablaze. The first wave of follow-up bombers dropped high explosive bombs, the intent of which was knock out the utilities (the water supply, electricity network and gas mains), and to crater the road - making it difficult for the fire engines to reach fires started by the follow-up waves of bombers. The follow-up waves dropped a combination of high explosive and incendiary bombs. There were two types of incendiary bombs: those made of magnesium and those made of petroleum. The high explosive bombs and the larger air-mines were not only designed to hamper the Coventry fire brigade, they were also intended to damage roofs, making it easier for the incendiary bombs to fall into buildings and ignite them. Arthur Travers Harris, commander of RAF Bomber Command, wrote after the war "Coventry was adequately concentrated in point of space [to start a firestorm], but all the same there was little concentration in point of time", so a firestorm was not ignited. They did not have the numbers or size of aircraft (they only had a twin-engine bomber).

It would not be until later in the war when "Bomber" Harris and the RAF managed sufficient concentration of bombers over one target close to simultaneously that a fire storm could be ignited. For example during the Dresden raid on February 13 1945, first attack was carried out entirely by No. 5 Group, using their own low-level marking methods and tactics. The pathfinders marked the Ostragehege stadium as the initial aiming point and each bomber fanned out from that point releasing their bombs at slightly different preassigned times on slightly different preassigned trajectories. The first bombs of No. 5 Group were released at 22:14 (CET) with all but one bomber releasing all their bombs within two minutes. The fan shaped area of destruction that the 244 Lancaster bombers created was one and a quarter miles long and at its extreme about one and three quarters miles wide. This raid by the RAF, with follow up raids by more RAF bombers and bombers of the USAAF, caused one of the most devastating and famous firestorms in history.

One of the most terrifying firestorms resulted from one bombing raid against Hamburg, on July 27, 1943, shortly before midnight. A number of factors combined to give the enormous destruction that followed; the unusually dry and warm weather, the concentration of the bombing in one area and that the city's firefighters were unable to reach the initial fires — the high explosive "Cookies" used in the early part of the raid had prevented them getting into the center of the city from the periphery where they were working on the results of the 24th. The bombings culminated in the spawning of the so-called "Feuersturm" (firestorm). Quite literally a tornado of fire, this phenomenon created a huge outdoor blast furnace, containing winds of up to 240 km/h (150 mph) and reaching temperatures of 800 °C (1,500 °F). It caused asphalt on the streets to burst into flame, cooked people to death in air-raid shelters, sucked pedestrians off the sidewalks like leaves into a vacuum cleaner and incinerated some eight square miles (21 km²) of the city. Most of the 40,000 casualties caused by Operation Gomorrah happened on this single night.

In 1945, Tokyo had an average 103,000 inhabitants per square mile and peak levels as high as 135,000 per square mile, the highest density of any industrial city in the world. With firefighting measures ludicrously inadequate to the task, of the city were destroyed on a night when fierce winds whipped the flames and walls of fire blocked tens of thousands fleeing for their lives. An estimated 1.5 million people lived in the burned out areas.

Nuclear weapons can also create firestorms in urban areas. This was responsible for a large portion of the destruction at Hiroshima.

See also

References

Further reading

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

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