Fire-storm


This article is about fires. For other uses, see Firestorm (disambiguation).

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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. The Black Saturday bushfires, the Great Peshtigo Fire and the Ash Wednesday fires are examples of firestorms, as is that following the 1906 San Francisco Earthquake. Firestorms can also be deliberate effects of targeted explosives such as occurred as a result of the aerial bombings of Hamburg, Dresden, the atomic bombing of Hiroshima, and as part of the March 9/10, 1945 Operation Meetinghouse raid on Tokyo.[1]




Mechanism

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 gusty winds develop around the fire, directed inward which supplies the fire 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.[2] A firestorm may also develop into a mesocyclone and induce true tornadoes.[3] Probably, this is true for the Peshtigo Fire.[4]

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.[5][6] 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.

According to experts, firestorms do not appreciably ignite material at a distance ahead of itself, 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]

As a fire storm 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.[2]

Large wildfire conflagrations are distinct from firestorms as, crucially, the former have moving fire fronts which are driven by the ambient wind and do not develop their own wind system like true firestorms. Furthermore 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.[8] With the important caveat that the density of these simultaneously burning fires in a firestorm needing to be above a critical threshold for a firestorm to form, as a notable example of a case of large numbers of fires burning simultaneously over a large area without a firestorm developing is the Kuwaiti oil fires of 1991 due to the distance between each individual fire being 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.

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". 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.[9] A large pyrocumulus can grow into a pyrocumulonimbus and produce lightning, which can set off further fires. Apart from forest fires, pyrocumulus clouds can also be produced by volcanic eruptions due to the comparable amounts of hot Volcanic ash formed.

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.[4][10]

City firestorms

The same underlying combustion physics can also apply to man-made structures such as cities during war or disaster.

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 Hamburg and Dresden.[11] Of the two nuclear bombing raids during the war only the atomic bombing of Hiroshima resulted in a firestorm.[12]

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.[13]

City / Event Date of the firestorm Notes
Bombing of Hamburg (Germany)[11] 27 July 1943 46,000 dead.[14] A firestorm area of

approximately 4.5 square miles (12 km2) was reported at Hamburg.[15]

Bombing of Dresden (Germany)[11] 13 February 1945 up to 25,000 dead.[16] A firestorm area of

approximately 8 square miles (21 km2) was reported at Dresden.[15]

Firebombing of Tokyo (Japan) 9–10 March 1945 Firestorm covering 16 square miles (41 km2). 267,171 buildings destroyed, 83,793 dead.[17] The most devastating air raid in history with destruction greater than the atomic bombing of Hiroshima, although with fewer casualties.[17] Despite Tokyo commonly being assumed to be a firestorm event, it is more accurately termed a line fire conflagration, and not a true firestorm, due to the high ambient surface wind speed at the time of the firebombing in Tokyo preventing a true firestorm from forming. High ambient winds prior to and during the Tokyo fire overrode the fires ability to create its own wind system, with winds from every point of the compass. Instead the fire was fanned solely by the strong unidirectional ambient wind during the fire, and the fire damage although large, burnt across the city in a linear direction, with a thin fire front, much like a candle. Strong surface winds prior to and during a fire cause flames to slant forward, and fire to spread largely only in the direction of the wind.

Since these fires are characterized by little outward radial spread, it is clear that, in the general case firestorms will not develop in the presence of strong ground winds, as these ambient winds largely prevent the fire from creating its own wind system, and therefore these fires, however large the eventual destruction, are not true firestorm events.[18]

Bombing of Kassel in World War II 22 October 1943 9,000 dead. 24,000 dwellings destroyed. Area burned 23 square miles (60 km2); the percentage of this area which was destroyed by conventional conflagration and that destroyed by firestorm is unspecified.[19] Although a much larger area was destroyed by fire in Kassel than even Tokyo and Hamburg, the city fire caused a smaller less extensive firestorm than that at Hamburg.[20]
Bombing of Darmstadt in World War II 11 September 1944 8,000 dead. Area destroyed by fire 4 square miles (10 km2), again the percentage of this which was done by firestorm remains unspecified. 20,000 dwellings destroyed.[19]
Bombing of Ube, Yamaguchi in World War II A momentary fire storm of about 0.5 square miles (1.3 km2) was reported at Ube, Japan.[15] The reports that the Ube bombing produced a firestorm, along with computer modelling, has set one of the four physical conditions which a fire must meet to develop into a true firestorm. The size of the Ube firestorm is regarded as the lower size limit of a firestorm. Glasstone and Dolan:
The minimum requirements for a fire storm to develop: no.4 A minimum burning area of about 0.5 square miles (1.3 km2).
—Glasstone and Dolan (1977).[21]
Atomic bombing of Hiroshima (Japan) 6 August 1945 Firestorm covering 4.4 square miles (11 km2).[22] No estimate can be given of the number of fire deaths, since the fire area was largely within the blast damage region.[23]

Firebombing


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.[11]

Although incendiary bombs have been used to destroy buildings since the start of gunpowder warfare, World War II saw the first use of strategic bombing from the air to destroy the ability of the enemy to wage war. London, Coventry, and many other British cities were firebombed during the Blitz. Most large German cities were extensively firebombed starting in 1942 and almost all large Japanese cities were firebombed during the last six months of World War II. As Sir Arthur Harris, the officer commanding RAF Bomber Command from 1942 through to the end of the war in Europe, pointed out in his post-war analysis, although many attempts were made to create deliberate man made firestorms during World War II, few attempts succeeded:

"The Germans again and again missed their chance, ...of setting our cities ablaze by a concentrated attack. Coventry was adequately concentrated in point of space, but all the same there was little concentration in point of time, and nothing like the fire tornadoes of Hamburg or Dresden ever occurred in this country. But they did do us enough damage to teach us the principle of concentration, the principle of starting so many fires at the same time that no fire fighting services, however efficiently and quickly they were reinforced by the fire brigades of other towns could get them under control."
Arthur Harris[11]

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).[24] In 2005, the American National Fire Protection Association stated in a report that there were three major fire storms resulting from Allied conventional bombing campaigns during World War II: Hamburg, Dresden, and Tokyo.[25] They do not include the comparatively minor firestorms at Kassel, Darmstadt or even Ube into their major firestorm category. Despite later quoting and corroborating Glasstone and Dolan and data collected from these smaller firestorms:

based on World War II experience with mass fires resulting from air raids on Germany and Japan, the minimum requirements for a fire storm to develop are considered by some authorities to be the following: (1) at least 8 pounds of combustibles per square foot of fire area/(40 kg per square meter), (2) at least half of the structures in the area on fire simultaneously, (3) a wind of less than 8 miles per hour at the time, and (4) a minimum burning area of about half a square mile.
—Glasstone and Dolan (1977).[26]

Modern cities in comparison to WWII cities

Unlike the highly combustible WWII 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 after a nuclear detonation.[13] The explanation for this is that highrise buildings do not lend themselves to the formation of firestorms due to the baffle effect of the structures,[27] nor are fire storms likely in areas where buildings have totally collapsed.[28] There is also a sizable difference between the fuel loading of WWII cities that firestormed, including Hiroshima, and that of modern cities, were the quantity of combustibles per square meter in the fire area in the latter is below the necessary requirement of 40 kg of combustibles per square meter for a firestorm to form.[29][30] Therefore, firestorms are not to be expected in modern US or Canadian cities, following a nuclear detonation, nor are they to be likely in modern European cities.[31]

A U.S. Air Force table showing the number of bombs dropped by the Allies on Germany's seven largest cities during the war.[32]
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

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 WWII 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.[33]

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, as regards destruction by fire and blast, can be achieved by the use of conventional incendiary and high-explosive bombs.[34] It has been estimated for example that the fire damage suffered at Hiroshima after the dropping of the ~16 kiloton Little Boy nuclear weapon could have been produced by about 1 kiloton/1000 tons of incendiary bombs distributed over the city.[34] It may appear counterintuitive that the same amount of fire damage caused by a nuclear weapon could have instead been produced by a smaller total yield of conventional incendiary bombs, however WWII experience supports this assertion. A greater area of fire damage resulted after the conventional Bombing of Dresden in World War II where in total ~ 4.5 kilotons of conventional American ordnance was dropped over many nights and this resulted in 15 square miles (39 km2) of the city being destroyed by fire and firestorm effects.[35] Whereas after the atomic bombing of Hiroshima where a single 16 kiloton nuclear weapon was dropped yet a relatively smaller area of only 4.5 square miles (12 km2) of the city was destroyed.[23] Similarly, Major Cortez F. Enloe, a surgeon in the USAAF who worked with the United States Strategic Bombing Survey (USSBS), said that the atomic bomb dropped on Nagasaki did not do as much fire damage as the extended conventional airstrikes on Hamburg.[36] American historian Gabriel Kolko also echoed this sentiment:

During November 1944 American B-29's began their first incendiary bomb raids on Tokyo, and on 9 March 1945, wave upon wave dropped masses of small incendiaries containing an early version of napalm on the city's population. Soon small fires spread, connected, grew into a vast firestorm that sucked the oxygen out of the lower atmosphere. The bomb raid was a 'success' for the Americans; they killed 125,000 Japanese in one attack. The Allies bombed Hamburg and Dresden in the same manner, and Nagoya, Osaka, Kobe, and Tokyo again on May 24. In fact the atomic bomb used against Hiroshima was less lethal than massive fire bombing. Only its technique was novel, nothing more. There was another difficulty posed by mass conventional bombing, and that was its very success, a success that made the two modes of human destruction qualitatively identical in fact and in the minds of the American military. "I was a little fearful," Stimson told Truman, "that before we could get ready the Air Force might have Japan so thoroughly bombed out that the new weapon would not have a fair background to show its strength." To this the President "laughed and said he understood."[40]

This break from the linear expectation of more fire damage to occur after greater explosive yield is dropped can be easily explained by two major factors, firstly by noticing that the order of blast and thermal events during a nuclear explosion are not ideal for the creation of fires. In a conventional incendiary bombing raid, incendiary weapons followed after high explosive blast weapons were dropped, in a manner designed to create the greatest probability of fires from a limited quantity of explosive and incendiary weapons. The so-called two-ton "cookies",[41] also known as "blockbusters," were dropped first and were intended to rupture water mains, and blow off roofs, doors, and windows, creating an air flow that would feed the fires caused by the incendiaries that would then follow and be dropped, ideally, finding their way into holes created by the prior blast weapons, that is, into attic and roof spaces etc.[42][43][44] On the other hand nuclear weapons produce effects that are in the reverse order, with thermal effects, 'flash' occurring first, which is 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 that this fact led the nuclear weapon effects experts 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/1000 tons of incendiary bombs.[34]

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 amongst the major factors at play. As a few strategically placed incendiary devices would be sufficient to start a firestorm in a city if the conditions for a firestorm, namely a high fuel loading, are already inherent to the city. With the Great Fire of London in 1666 serving as an example, that given a densely packed, and predominately wooden and thatch house construction, a mass fire is conceivable from the mere incendiary power of no more than a single domestic fireplace. With on the other hand, the largest nuclear weapon conceivable will be incapable of igniting a city into a firestorm if the cities properties, namely a low fuel density, are not conducive to one developing.

Despite nuclear weapons disadvantage in terms of effectiveness at starting fires when compared to conventional weapons of lower or comparable yield, 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 when it comes to creating fires by nuclear weapons over conventional weapons, is that, nuclear weapons do undoubtedly produce all their thermal and explosive effects in a very short space of time, that is, to use Arthur Harris' terminology, they are the epitome of an air raid guaranteed to be concentrated in 'point in time'. Whereas early in WWII, 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 anti-aircraft barrage fire from the air defensives of the cities below, nuclear weapons largely remove these uncertain variables. Therefore nuclear weapons reduce the question of if 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 in unison successfully, as a single unit.

See also

Notes

  1. Bomber Command Arthur Harris's report, "Extract from the official account of Bomber Command by Arthur Harris, 1945", National Archives, Catalogue ref: AIR 16/487, which states that more than 1,600 acres (6.5 km2) were destroyed.

References

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