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Firestorm

From Academic Kids

This is an article about a specific circumstance of combustion. For the comic book superhero, see Firestorm (comics).

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.

Contents

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 kind of "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.

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.

An extremely large firestorm can even create its own weather system, drawing air inward and creating thunderstorm-like weather which tends to aid the spread of the flames.

When the wind is strong, the gases are pushed away and can not accumulate; thus, paradoxically, a strong wind can lower the progression of the fire by preventing firestorms.

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" of the leaves by the radiating heat.

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

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 fire resulting from the 1906 San Francisco 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).

The fire-bombing technique consisted of dropping large amounts of high explosives to expose the timbers within buildings, followed by incendiary devices (fire-sticks) to ignite them. More high explosives followed to hamper the efforts of firefighters. It is said that in the Dresden firebombing, furnace conditions raised the temperature until people melted and caught fire. Others, who hid in underground bunkers, were asphyxiated as the fire consumed all the oxygen in the area.

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.

See also

Reference

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