|
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 phenomenon's determining characteristic is a fire with its own storm-force winds from every point of the compass.〔Alexander Mckee's ''Dresden 1945: The Devil's Tinderbox''〕〔(PROBLEMS OF FIRE IN NUCLEAR WARFARE (1961) ''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.'' )〕 The Black Saturday bushfires and the Great Peshtigo Fire are possible examples of forest fires with some portion of combustion due to a firestorm. Firestorms can also occur 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== 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.〔http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD673703&Location=U2&doc=GetTRDoc.pdf Problems of fire in Nuclear Warfare 1961, pg 8 & 9.〕 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.〔http://unintentional-irony.blogspot.no/2007/08/firestorms.html〕〔http://holbert.faculty.asu.edu/eee460/cjc/Thermal_Radiation_Damage.html〕 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.〔http://dge.stanford.edu/SCOPE/SCOPE_28_1/SCOPE_28-1_1.1_Chapter1_1-23.pdf SCOPE report, page 3〕 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, 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Firestorm」の詳細全文を読む スポンサード リンク
|