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.
According to experts, 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.
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. 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.
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.
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. 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.
The same underlying combustion physics can also apply to man-made structures such as cities during war or natural disaster.
Firestorms are thought to have been part of the mechanism of large urban fires, such as accompanied 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. 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.
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."
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). 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. 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 firestorm 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.
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. 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, nor are firestorms likely in areas where modern buildings have totally collapsed, with the exceptions of Tokyo and Hiroshima due to the nature of their densely packed "flimsy" wooden buildings in World War II. 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 U.S. or Canadian cities following a nuclear detonation, nor are they expected to be likely 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.
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" 2.5 square miles (6.5 km2) of the city being destroyed by fire and firestorm effects according to one authoritative source, or approximately 8 square miles (21 km2) by another. In total about 4.5 kilotons of conventional ordnance was dropped on the city over a number of months during 1945 and this resulted in approximately 15 square miles (39 km2) of the city being destroyed by blast and fire effects. 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 — 16 square miles (41 km2), a quarter of the city. In contrast to these raids, when a single 16-kiloton nuclear bomb was dropped on Hiroshima, 4.5 square miles (12 km2) of the city was destroyed by blast, fire, and firestorm effects. Similarly, Major Cortez F. Enloe, a surgeon in the USAAF who worked with the United States Strategic Bombing Survey (USSBS), said that the 22-kiloton nuclear bomb dropped on Nagasaki did not do as much fire damage as the extended conventional airstrikes on Hamburg.
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", [Secretary of War] Stimson told [President] 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."
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. First, the order of blast and thermal events during a nuclear explosion is not ideal for the creation of fires. In an 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", also known as "blockbusters", were dropped first and were intended to rupture water mains, as well as to 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, into holes created by the prior blast weapons, such into attic and roof spaces. 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.