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What is a Firestorm?

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  • Written By: Michael Anissimov
  • Edited By: Bronwyn Harris
  • Images By: Paulo M.f. Pires, David Lloyd, Greg Goebel
  • Last Modified Date: 26 October 2016
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A firestorm is a huge fire created when flames are so intense that they create and sustain their own wind system. Depending on the stack effect, also known as the chimney effect, the heat of the fire creates such a strong updraft that adjacent air is strongly drawn in, creating fierce winds that blows towards the center of the fire. A firestorm is especially likely to occur where there are gulf stream winds feeding it, or the temperature inversion layer is pierced by hot air from the fire. Firestorms are likely to occur whenever there is a sufficiently large fire.

Well-known firestorms have occurred both in natural conditions, such as the Great Peshtigo Fire in Wisconsin, or the Ash Wednesday fires in southeast Australia, and artificial conditions, such as in the aerial bombings of Hamburg, Dresden, and Tokyo or the nuclear bombings of Hiroshima and Nagasaki. One might think that the wind traveling towards the center of the fire would prevent it from being spread outwards, but this is not the case. The extreme turbulence caused around the flame front makes it possible for the fire to spread. Often, fire tornadoes, known as flame whirls, form in the chaotic turbulence, darting around erratically and setting everything in their path on fire. During the firebombing of Dresden, a huge fire tornado incinerated over 30,000 people gathered in a city square within 15 minutes. Firestorms in Hiroshima and Nagasaki caused many deaths after the initial explosion.

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There are several warning signs that point to the genesis of a firestorm in wildfire conditions. These include decreased visibility, decreased sound conduction, difficulties breathing, and the instantaneous roasting (pyrolysis) of leaves at a distance from the main fire. There are several main types of firestorm in a wildfire context. These include 1) a thermal bubble, where dense foliage in a small valley catches fire and creates a bubble of hot gas that cannot merge with the air above it due to its great heat, 2) fire carpets, where the entire floor of a wide and open valley catches fire, 3) confinement by a layer of cold air, similar to a thermal bubble but it can happen anywhere, where cold air prevents pyrolysis-released gases from rising, creating a "powder keg" that eventually explodes, 4) pyrolysis of an opposing slope, where a fire on one slope initiates spontaneous combustion all across an opposing slope, despite being separated by hundreds of feet, and 5) a firestorm at the bottom of a small valley, where pyrolysis-released gases coalesce in a river bed and are spontaneously ignited when the fire reaches it.

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