Firestorm - Mechanism

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 supply 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. This wind shear is capable of producing small tornado- or dust devil-like circulations called fire whirls which can also dart around erratically, damage or destroy houses and buildings, and quickly spread the fire to areas outside the central area of the fire. However it was typical in WWII that the firestorms created in cities were mainly confined to the areas initially seeded with incendiary bombs and the firestorm did not appreciably spread outward. A firestorm may also develop into a mesocyclone and induce true tornadoes. Probably, this is true for the 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. Radiated heat from the fire can melt asphalt, metal, and glass, and turn street tarmac into flammable hot liquid. The very high temperatures ignite anything that might possibly burn, until the firestorm runs out of fuel.

However 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.

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 is more difficult, thus limiting fire spread.

Large conflagrations, are distinct from firestorms, as the former have moving fire fronts which are driven by the ambient wind and do not develop their own wind system. 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. With the caveat that the density of these simultaneously burning fires in a firestorm needing to be above a critical mass 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.

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". The black rain that began to fall at ~20 minutes after the atomic bombing of Hiroshima, produced in total 5–10 cm of rain in a 1-3 hour period. 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.

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.