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definition : fire-storm

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 and wildfires. Although the word has been used to describe certain large fires, 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.


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. As the updraft mushrooms, strong inwardly-directed gusty winds develop around the fire, supplying it with additional air. This would seem to prevent the firestorm from spreading on the wind, but the tremendous turbulence created may also cause the strong surface inflow winds to change direction erratically. Firestorms resulting from the bombardment of urban areas in the Second World War were generally confined to the areas initially seeded with incendiary devices, and the firestorm did not appreciably spread outward. A firestorm may also develop into a mesocyclone and induce true tornadoes/fire whirls. This occurred with the 2002 Durango fire, and probably with the much greater Peshtigo 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 may ignite flammable material at a distance ahead of the fire itself. This also serves to expand the area and the intensity of the firestorm. 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 very high temperatures ignite anything that might possibly burn, until the firestorm runs low on fuel.

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.

==Characterization of a firestorm==

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.

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, with the important caveat that the density of simultaneously burning fires needs to be above a critical threshold for a firestorm to form (a notable example of large numbers of fires burning simultaneously over a large area without a firestorm developing was the Kuwaiti oil fires of 1991, where the distance between individual fires was too large).

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.

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 firestorm than a simple forest fire. Sometimes, emission of combustible gases from swamps (e.g., methane) has a similar effect. For instance, methane explosions enforced the Peshtigo Fire. 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. 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) ==== A very important but poorly understood aspect of wildfire behavior are pyrocumulonimbus (pyroCb) firestorm dynamics and their atmospheric impact. These are well illustrated 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. 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.

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 is now coming into focus.

==== Role that pyroCb's have on fire in case study ==== 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 frontof 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 phenomenon.

==== Importance for continued study of these firestorms ==== 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). Firestorms were also created by the firebombing raids of World War II in cities like Hamburg and Dresden. Of the two nuclear weapons used in combat, only Hiroshima resulted in a firestorm.

In contrast, experts suggest that due to the nature of modern U.S. city design and construction, a firestorm is unlikely after a nuclear detonation.


Firebombing is a technique designed to damage a target, generally an urban area, 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 explosive destroys roofs, making it easier for the incendiary devices to penetrate the structures and cause fires. The high explosives also disrupt the ability of firefighters to douse the fires. |- !City !Population in 1939 !American tonnage !British tonnage !Total tonnage |- |Berlin |4,339,000 |22,090 |45,517 |67,607 |- |Hamburg |1,129,000 |17,104 |22,583 |39,687 |- |Munich |841,000 |11,471 |7,858 |19,329 |- |Cologne |772,000 |10,211 |34,712 |44,923 |- |Leipzig |707,000 |5,410 |6,206 |11,616 |- |Essen |667,000 |1,518 |36,420 |37,938 |- |Dresden |642,000 |4,441 |2,659 |7,100 |} Unlike the highly combustible World War II cities that firestormed from conventional and nuclear weapons, fire experts suggest that due to the nature of modern U.S. city design and construction, a firestorm is unlikely to occur even after a nuclear detonation

There is also a sizable difference between the fuel loading of World War II cities that firestormed and that of modern cities, where the quantity of combustibles per square meter in the fire area in the latter is below the necessary requirement for a firestorm to form (40 kg/m²). Therefore, firestorms are not to be expected in modern North American cities after a nuclear detonation, and are expected to be unlikely in modern European cities.

Similarly, one reason for the lack of success in creating a true firestorm in the bombing of Berlin in World War II was that the building density, or builtupness factor, in Berlin was too low to support easy fire spread from building to building. Another reason was that much of the building construction was newer and better than in most of the old German city centers. Modern building practices in the Berlin of World War II led to more effective firewalls and fire-resistant construction. Mass firestorms never proved to be possible in Berlin. No matter how heavy the raid or what kinds of firebombs were dropped, no true firestorm ever developed.

==Nuclear weapons in comparison to conventional weapons== The incendiary effects of a nuclear explosion do not present any especially characteristic features. In principle, the same overall result with respect to destruction of life and property can be achieved by the use of conventional incendiary and high-explosive bombs. It has been estimated, for example, that the same fire ferocity and damage produced at Hiroshima by one 16-kiloton nuclear bomb from a single B-29 could have instead been produced by about 1,200 tons/1.2 kilotons of incendiary bombs from 220 B-29s distributed over the city.

It may seem counterintuitive that the same amount of fire damage caused by a nuclear weapon could have instead been produced by smaller total yield of thousands of incendiary bombs; however, World War II experience supports this assertion. For example, although not a perfect clone of the city of Hiroshima in 1945, in the conventional bombing of Dresden, the combined Royal Air Force (RAF) and United States Army Air Forces (USAAF) dropped a total of 3441.3 tons (approximately 3.4 kilotons) of ordnance (about half of which was incendiary bombs) on the night of 13–14 February 1945, and this resulted in "more than" of the city being destroyed by fire and firestorm effects according to one authoritative source, or approximately by another. During the Operation MeetingHouse firebombing of Tokyo on 9–10 March 1945, 279 of the 334 B-29s dropped 1,665 tons of incendiary and high-explosive bombs on the city, resulting in the destruction of over 10,000 acres of buildings — , a quarter of the city. In contrast to these raids, when a single 16-kiloton nuclear bomb was dropped on Hiroshima, of the city was destroyed by blast, fire, and firestorm effects. File:AtomicEffects-Hiroshima.jpg|Hiroshima after the bombing and firestorm. No known aerial photography of the firestorm exists. Image:Firebombing of Tokyo.jpg|Note the ambient wind blowing the fire's smoke plume inland. The firebombing of Tokyo on the night of 9–10 March 1945 was the single deadliest air raid of World War II, with a greater total area of fire damage and loss of life than either nuclear bombing as a single event. Due largely to the greater population density and fire conditions. 279 B-29s dropped about 1,700 tons of ordnance on target. On the other hand, nuclear weapons produce effects that are in the reverse order, with thermal effects and "flash" occurring first, which are then followed by the slower blast wave. It is for this reason that conventional incendiary bombing raids are considered to be a great deal more efficient at causing mass fires than nuclear weapons of comparable yield. It is likely this led the nuclear weapon effects experts Franklin D'Olier, Samuel Glasstone and Philip J. Dolan to state that the same fire damage suffered at Hiroshima could have instead been produced by about 1 kiloton/1,000 tons of incendiary bombs.

The second factor explaining the non-intuitive break in the expected results of greater explosive yield producing greater city fire damage is that city fire damage is largely dependent not on the yield of the weapons used, but on the conditions in and around the city itself, with the fuel loading per square meter value of the city being one of the major factors. A few hundred strategically placed incendiary devices would be sufficient to start a firestorm in a city if the conditions for a firestorm, namely high fuel loading, are already inherent to the city (see Bat bomb). The Great Fire of London in 1666, although not forming a firestorm due to the single point of ignition, serves as an example that, given a densely packed and predominately wooden and thatch building construction in the urban area, a mass fire is conceivable from the mere incendiary power of no more than a domestic fireplace. On the other hand, the largest nuclear weapon conceivable will be incapable of igniting a city into a firestorm if the city's properties, namely its fuel density, are not conducive to one developing.

Despite the disadvantage of nuclear weapons when compared to conventional weapons of lower or comparable yield in terms of effectiveness at starting fires, for the reasons discussed above, nuclear weapons also do not add any fuel to a city, and fires are entirely dependent on what was contained in the city prior to bombing, in direct contrast to the incendiary device effect of conventional raids. One undeniable advantage of nuclear weapons over conventional weapons when it comes to creating fires is that nuclear weapons undoubtedly produce all their thermal and explosive effects in a very short period of time; that is, to use Arthur Harris's terminology, they are the epitome of an air raid guaranteed to be concentrated in "point in time". In contrast, early in World War II, the ability to achieve conventional air raids concentrated in "point of time" depended largely upon the skill of pilots to remain in formation, and their ability to hit the target whilst at times also being under heavy fire from anti-aircraft fire from the air defensives of the cities below. Nuclear weapons largely remove these uncertain variables. Therefore, nuclear weapons reduce the question of whether a city will firestorm or not to a smaller number of variables, to the point of becoming entirely reliant on the intrinsic properties of the city, such as fuel loading, and predictable atmospheric conditions, such as wind speed, in and around the city, and less reliant on the unpredictable possibility of hundreds of bomber crews acting together successfully as a single unit.

==See also== * Blackout (wartime) * Civilian casualties of strategic bombing * Fire whirl * Wildfire * Wildfire modeling

===Potential firestorms=== Portions of the following fires are often described as firestorms, but that has not been corroborated by any reliable references:

*Great Fire of Rome (64 AD) *Great Fire of London (1666) *Great Chicago Fire (1871) *San Francisco earthquake (1906) *Great Kantō earthquake (1923) *Tillamook Burn (1933–1951) *Second Great Fire of London (1940) *Ash Wednesday bushfires (1983) *Yellowstone fires (1988) *Canberra bushfires (2003) *Okanagan Mountain Park Fire (2003) *Black Saturday bushfires (2009) *Fort McMurray wildfire (2016) *Predrógâo Grande wildfire (2017)


===Further reading=== * * * * * * * * * * * *

Category:Fire Category:Wildfire ecology Category:Storm Category:1906 San Francisco earthquake Category:Firebombings Category:Types of fire Category:Wildfires

Texte soumis à la licence CC-BY-SA. Source : Article de Wikipédia

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