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A firestorm is a large and destructive fire, 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 and wildfires. Although the word has been used to describe certain large fires,[1] the phenomenon's determining characteristic is a fire with its own storm-force winds from every point of the compass.[2][3] The Black Saturday bushfires and the Great Peshtigo Fire are possible examples of forest fires with some portion of combustion due to a firestorm, as is the Great Hinckley Fire. Firestorms have also occurred in cities, usually as a deliberate effect of targeted explosives such as occurred as a result of the aerial firebombings of Hamburg, Dresden, and the atomic bombing of Hiroshima.

Mechanism

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See also: Thermal column
Firestorm: fire (1), updraft (2), strong gusty winds (3) (A) pyrocumulonimbus cloud.

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, it has nowhere to go but upwards creating buoyancy in the air. This draft can be quickly increased if a low-level jet stream exists over or near the fire. As the wind begins to updraft and mushroom, strong inwardly-directed gusty winds develop around the fire, supplying it with additional air. This would seem to prevent the firestorm from spreading by the wind, but the tremendous turbulence created may also cause the strong surface inflow winds to change direction erratically. A firestorm may also develop into a mesocyclone and induce true tornadoes/fire whirls. This occurred with the 2002 Durango fire,[4] and probably with the much greater Peshtigo Fire.[5][6] Violent, erratic wind drafts suck movables into the fire and as is observed with all intense conflagrations, radiated heat from the fire can melt asphalt, some metals, and glass, and turn street tarmac into flammable hot liquid. The high temperatures within the firestorm zone ignite most everything that might possibly burn, until a tipping point is reached, that is, upon running low on fuel, which occurs after the firestorm has consumed so much of the available fuel within the firestorm zone that the necessary fuel density required to keep the firestorm's wind system active drops below the threshold level, at which time the firestorm breaks up into isolated conflagrations.

A firestorm does not appreciably ignite material at a distance ahead of itself; more accurately, the heat desiccates those materials and makes them more vulnerable to ignition by embers or firebrands, increasing the rate of fire spotting. During the formation of a firestorm many fires merge to form a single convective column of hot gases rising from the burning area and strong, fire-induced, radial (inwardly directed) winds are associated with the convective column. Thus the fire front is essentially stationary and the outward spread of fire is prevented by the in-rushing wind.[7]

Characterization of a Firestorm

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A firestorm is characterized by strong to gale-force winds blowing toward the fire, everywhere around the fire perimeter, an effect which is caused by the buoyancy of the rising column of hot gases over the intense mass fire, drawing in cool air from the periphery. These winds from the perimeter blow the fire brands into the burning area and tend to cool the unignited fuel outside the fire area so that ignition of material outside the periphery by radiated heat and fire embers is more difficult, thus limiting fire spread.[8] At Hiroshima, this inrushing to feed the fire is said to have prevented the firestorm perimeter from expanding, and thus the firestorm was confined to the area of the city damaged by the blast.[9]

Picture of a pyro-cumulonimbus taken from a commercial airliner cruising at about 10 km. In 2002 various sensing instruments detected 17 distinct pyrocumulonimbus cloud events in North America alone.[10]

Large wildfire conflagrations are distinct from firestorms if they have moving fire fronts which are driven by the ambient wind and do not develop their own wind system like true firestorms. (This does not mean that a firestorm must be stationary; as with any other convective storm, the circulation may follow surrounding pressure gradients and winds, if those lead it onto fresh fuel sources.) Furthermore, non-firestorm conflagrations can develop from a single ignition, whereas firestorms have only been observed where large numbers of fires are burning simultaneously over a relatively large area.[11]

Weather and Climate Effects

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Firestorms will produce hot buoyant smoke clouds of primarily water vapor that will form condensation clouds as it enters the cooler upper atmosphere, generating what is known as pyrocumulus clouds ("fire clouds") or, if large enough, pyrocumulonimbus ("fire storm") clouds. For example, the black rain that began to fall at ~20 minutes after the atomic bombing of Hiroshima produced in total 5–10 cm of black soot-filled rain in a 1–3 hour period.[12] Moreover, if the conditions are right, a large pyrocumulus can grow into a pyrocumulonimbus and produce lightning, which could potentially set off further fires. Apart from city and forest fires, pyrocumulus clouds can also be produced by volcanic eruptions due to the comparable amounts of hot buoyant material formed.

On a more continental and global extent, away from the direct vicinity of the fire, wildfire firestorms which produce pyrocumulonimbus cloud events have been found to "surprisingly frequently" generate minor "nuclear winter" effects.[13][14][15][16] These are analogous to minor volcanic winters, with each mass addition of volcanic gases additive in increasing the depth of the "winter" cooling, from near-imperceptible to "year without a summer" levels.

Pyro-cumulonimbus and atmospheric effects (in wildfires)

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A very important but poorly understood aspect of wildfire behavior—pyrocumulonimbus (pyroCb) firestorm dynamics and atmospheric impact - these play hand in hand and affect each other as we see in the Black Saturday case study below. The “pyroCb” is a fire-started or fire-augmented thunderstorm that in its most extreme manifestation injects huge abundances of smoke and other biomass-burning emissions into the lower stratosphere. The observed hemispheric spread of smoke and other biomass-burning emissions has known important climate consequences. PyroCbs have been spawned naturally and through hominization, and they are hypothesized to be part of the theoretical “nuclear winter” scenario. However, direct attribution of the stratospheric aerosols to pyroCbs only occurred in the last decade. Such an extreme injection by thunderstorms was previously judged to be unlikely because the extratopical tropopause is considered to be a strong barrier to convection. Two recurring themes have developed as pyroCb research unfolds. First, puzzling stratospheric aerosol-layer observations— and other layers reported as volcanic aerosol can now be explained in terms of pyroconvection. Second, pyroCb events occur surprisingly frequently, and they are likely a relevant aspect of several historic wildfires.[17]

On an intraseasonal level it is established that pyroCbs occur with surprising frequency. In 2002, at least 17 pyroCbs erupted in North America alone. Still to be determined is how often this process occurred in the boreal forests of Asia in 2002. However, it is now established that this most extreme form of pyroconvection, along with more frequent pyrocumulus convection, was widespread and persisted for at least 2 months. The characteristic injection height of pyroCb emissions is the upper troposphere, and a subset of these storms pollutes the lower stratosphere. Thus, a new appreciation for the role of extreme wildfire behavior and its atmospheric ramifications are now coming into focus.[17]

Black Saturday firestorm (Wildfire case study)

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Background

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The Black Saturday bushfires are some of Australia’s most destructive and deadly fires that fall under the category of a “firestorm” due to the extreme fire behavior and relationship with atmospheric responses that occurred during the fires.  This major wildfire event leads to a number of distinct electrified Pyrocumulonimbus plume clusters ranging roughly 15 km high.  These plumes were proven susceptible to striking new spot fires ahead of the main fire front. The newly ignited fire by this pyrogenic lightning, further highlights the feedback loops of influence between the atmosphere and fire behavior on Black Saturday associated with these pyroconvective processes.[18]

Role that pyroCb's have on fire in case study

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The examinations presented here for Black Saturday demonstrate that fires ignited by lightning generated within the fire plume can occur at much larger distances ahead of the main fire front—of up to 100 km. In comparison to fires ignited by burning debris transported by the fire plume, these only go ahead of the fire front up to about 33 km, noting that this also has implications in relation to understanding the maximum rate of spread of a wildfire.  This finding is important for the understanding and modeling of future firestorms and the large scale areas that can be affected by this phenomena.[18] As the individual spot fires grow together, they will begin to interact. This interaction will increase the burning rates, heat release rates, and flame height until the distance between them reaches a critical level. At the critical separation distance, the flames will begin to merge together and burn with the maximum rate and flame height. As these spot fires continue to grow together, the burning and heat release rates will finally start to decrease but remain at a much elevated level compared to the independent spot fire. The flame height is not expected to change significantly. The more spot fires, the bigger the increase in burning rate and flame height.[19]

Importance for continued study of these firestorms

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Black Saturday is just one of many varieties of firestorms with these pyroconvective processes and they are still being widely studied and compared. In addition to indicating this strong coupling on Black Saturday between the atmosphere and the fire activity, the lightning observations also suggest considerable differences in pyroCb characteristics between Black Saturday and the Canberra fire event. Differences between pyroCb events, such as for the Black Saturday and Canberra cases, indicate considerable potential for improved understanding of pyroconvection based on combining different data sets as presented in the research of the Black Saturday pyroCb's (including in relation to lightning, radar, precipitation, and satellite observations).[18]

A greater understanding of pyroCb activity is important, given that fire-atmosphere feedback processes can exacerbate the conditions associated with dangerous fire behavior. Additionally, understanding the combined effects of heat, moisture, and aerosols on cloud microphysics is important for a range of weather and climate processes, including in relation to improved modeling and prediction capabilities. It is essential to fully explore events such as these to properly characterize the fire behavior, pyroCb dynamics, and resultant influence on conditions in the upper troposphere and lower stratosphere (UTLS). It is also important to accurately characterize this transport process so that cloud, chemistry, and climate models have a firm basis on which to evaluate the pyrogenic source term, pathway from the boundary layer through cumulus cloud, and exhaust from the convective column.[18]

Since the discovery of smoke in the stratosphere and the pyroCb, only a small number of individual case studies and modeling experiments have been performed. Hence, there is still much to be learned about the pyroCb and its importance. With this work scientists have attempted to reduce the unknowns by revealing several additional occasions when pyroCbs were either a significant or sole cause for the type of stratospheric pollution usually attributed to volcanic injections.[17]

Firebombing

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Firestorms were also created by the firebombing raids of World War II in cities like Hamburg and Dresden.[20] The same underlying combustion physics of a firestorm and a natural environment can also apply to man-made structures such as cities during war or natural disaster.

Firebombing is a designed technique most commonly used to damage urban areas, through the use of fire, caused by incendiary devices, rather than from the blast effect of large bombs. Such raids often employ both incendiary devices and high explosives. The high explosives destroy the roofs of homes, making it easier for the incendiary devices to penetrate the structures and cause fires. The high explosives also work in a way of disrupting the ability of firefighters to douse the fires, due to the lack of access to clear road ways.[21]

According to physicist David Hafemeister, firestorms occurred after about 5% of all fire-bombing raids during World War II (but he does not explain if this is a percentage based on both Allied and Axis raids, or combined Allied raids, or U.S. raids alone).[22] In 2005, the American National Fire Protection Association stated in a report that three major firestorms resulted from Allied conventional bombing campaigns during World War II: Hamburg, Dresden, and Tokyo.[23]

See also

Potential Firestorms

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Portions of the following fires are often described as firestorms, but no reliable references, as of yet, corroborate this.

  1. ^ American National Fire Protection Association (2005), Scawthorn, Charles; Eidinger, John M.; Schiff, Anshel J. (eds.), Fire Following Earthquake, Issue 26 of Monograph (American Society of Civil Engineers. Technical Council on Lifeline Earthquake Engineering), American Society of Civil Engineers Technical Council on Lifeline Earthquake Engineering (illustrated ed.), ASCE Publications, p. 68, ISBN 978-0-7844-0739-4
  2. ^ Alexander Mckee's Dresden 1945: The Devil's Tinderbox
  3. ^ "PROBLEMS OF FIRE IN NUCLEAR WARFARE (1961)" (PDF). Dtic.mil. Archived from the original (PDF) on 18 February 2013. Retrieved 11 May 2016. A fire storm is characterized by strong to gale force winds blowing toward the fire everywhere around the fire perimeter and results from the rising column of hot gases over an intense, mass fire drawing in the cool air from the periphery. These winds blow the fire brands into the burning area and tend to cool the unignited fuel outside so that ignition by radiated heat is more difficult, thus limiting fire spread.
  4. ^ Weaver & Biko.
  5. ^ Gess & Lutz 2003, p. 234
  6. ^ Hemphill, Stephanie (27 November 2002). "Peshtigo: A Tornado of Fire Revisited". Minnesota Public Radio. Retrieved 22 July 2015. The town was at the center of a tornado of flame. The fire was coming from all directions at once, and the winds were roaring at 100 mph.
  7. ^ Glasstone, Samuel; Dolan, Philip J., eds. (1977), ""Chapter VII — Thermal Radiation and Its Effects" (PDF), The Effects of Nuclear Weapons (Third ed.), United States Department of Defense and the Energy Research and Development Administration, pp. 229, 200, § "Mass Fires" ¶ 7.58
  8. ^ "Problems of fire in Nuclear Warfare 1961" (PDF). Dtic.mil. pp. 8 & 9. Archived from the original (PDF) on 18 February 2013. Retrieved 11 May 2016.
  9. ^ "Direct Effects of Nuclear Detonations" (PDF). Dge.standord.edu. Retrieved 2016-05-11.
  10. ^ "NASA – Fire-Breathing Storm Systems". Nasa.gov. 19 October 2010. Archived from the original on 24 August 2014. Retrieved 2016-05-11.
  11. ^ Glasstone, Samuel; Dolan, Philip J., eds. (1977), ""Chapter VII — Thermal Radiation and Its Effects" (PDF), The Effects of Nuclear Weapons (Third ed.), United States Department of Defense and the Energy Research and Development Administration, pp. 229, 200, § "Mass Fires" ¶ 7.59
  12. ^ "Atmospheric Processes : Chapter=4" (PDF). Globalecology.stanford.edu. Retrieved 2016-05-11.
  13. ^ Fromm, M.; Stocks, B.; Servranckx, R.; et al. (2006). "Smoke in the Stratosphere: What Wildfires have Taught Us About Nuclear Winter". Eos, Transactions, American Geophysical Union. 87 (52 Fall Meet. Suppl). Washington, D.C.: American Geophysical Union: Abstract U14A–04. Bibcode:2006AGUFM.U14A..04F. Archived from the original on 6 October 2014.
  14. ^ "NASA – Fire-Breathing Storm Systems". Archived from the original on 24 August 2014. Retrieved 2016-05-11.{{cite web}}: CS1 maint: unfit URL (link)
  15. ^ Fromm, M.; Tupper, A.; Rosenfeld, D.; Servranckx, R.; McRae, R. (2006). "Violent pyro-convective storm devastates Australia's capital and pollutes the stratosphere". Geophysical Research Letters. 33 (5). Bibcode:2006GeoRL..33.5815F. doi:10.1029/2005GL025161. S2CID 128709657.
  16. ^ Riebeek, Holli (2010-08-31). "Russian Firestorm: Finding a Fire Cloud from Space : Feature Articles". Earthobservatory.nasa.gov. Retrieved 2016-05-11.
  17. ^ a b c Fromm, Michael; Lindsey, Daniel T.; Servranckx, René; Yue, Glenn; Trickl, Thomas; Sica, Robert; Doucet, Paul; Godin-Beekmann, Sophie (2010). "The Untold Story of Pyrocumulonimbus". Bulletin of the American Meteorological Society. 91 (9): 1193–1210. doi:10.1175/2010bams3004.1.
  18. ^ a b c d Dowdy, Andrew J.; Fromm, Michael D.; McCarthy, Nicholas (2017-07-27). "Pyrocumulonimbus lightning and fire ignition on Black Saturday in southeast Australia". Journal of Geophysical Research: Atmospheres. 122 (14): 2017JD026577. doi:10.1002/2017jd026577. ISSN 2169-8996. S2CID 134053333.
  19. ^ Werth, Paul; et al. (March 2016). "Specific Effects of Fire Interaction" (PDF). Synthesis of Knowledge of Extreme Fire Behavior. 2: 88–97. {{cite journal}}: Explicit use of et al. in: |first= (help)
  20. ^ Harris 2005, p. 83
  21. ^ Harris 2005, p. 83
  22. ^ Hafemeister 1991, p. 24 (¶ 2nd to last).
  23. ^ American National Fire Protection Association 2005, p. 24.
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