A firestorm in Hamburg (Germany) during World War II
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, forest
fires, and wildfires. The Great Peshtigo Fire is one example of a firestorm.
Firestorms can also be deliberate effects of targeted explosives such as
occurred as a result of the aerial bombings of Dresden and Tokyo during World
Mechanism of firestorms
A firestorm is created as a result of the "chimney 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, or when an atmospheric temperature inversion cap is
pierced by it. As the updraft mushrooms, strong gusty winds develop around
the fire, directed inward. This would seem to prevent the firestorm from spreading
on the wind, but for the fact that tremendous turbulence is also created
by the strong updraft which causes the strong surface inflow winds to change
direction erratically. This wind shear is capable of producing small tornadoes
or dust devils 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.
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 ignites flammable material
at a distance ahead of the fire itself.
Besides the enormous ash cloud produced by a firestorm,
under the right conditions, it can also induce condensation, forming a cloud
called a pyrocumulus or "fire cloud". A large pyrocumulus can produce lightning,
which can set off further fires. Apart from forest fires, pyrocumuluses 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 fire-storm than a simple forest fire. The trees are full of oil to survive
dry conditions. The oil evaporates during the day too, leaving a layer of
flammable gas above the treeline. The oil fuels the fire and the fires are
very difficult to bring under control, with firefighters resorting to saving
buildings and lives when the hot dry days during summer encourage the occurrence
of enormous fires.
Firestorms in wildfires
The firestorms often appear in thalwegs or crests or
on plateaus. The warning signs include:
- Decreased visibility;
- Decreased sound conduction;
- Breathing difficulties (firefighters do not use
SCBA on wildfires);
- Roasting (pyrolysis) of the leaves by the radiated
The plants protect themselves from the heat by two mechanisms:
evapotranspiration, and emission of volatile organic compounds (VOC). In case
of drought, especially when the humidity is less than 30 %, the emission of
VOC is more important as evapotranspiration is drastically reduced.
When a fire comes nearer, the emission of VOC is increased
to fight the rise of temperature; at 170 °C, the rosemary plant emits
55 times more terpene than at 50 °C. A temperature of 170 °C is considered
a critical temperature, at which the emission of VOC can lead to an explosive
mix with the air and thus to a flash over. Additionally, the fire itself
emits pyrolysis gases that are not burnt, and that mix with the VOC; the
explosive mix can be reached faster.
The topography has a complex influence. A closed relief,
such as a small valley or a dry river, concentrates the heat and thus the
emission of VOC, especially for rosemary, rockrose or Aleppo Pine. Contrarily,
the kermes oak emits more VOC on an open relief such as plain or plateau.
Other factors that influence the occurrence of a firestorm
are the natural heat, especially above 35 °C in the shadow, a humidity
less than 30% and no strong wind. These conditions are met in climates such
as the Mediterranean forest.
The firestorms can be classified in several types:
- Thermal bubble: at the bottom of a small valley
rich in combustible materials (plants), the combustible gas forms a bubble
that cannot mix with the air because its temperature is too high; this bubble
moves randomly, pushed by the wind.
- Fire carpet: in a deep and opened small valley,
the whole valley catches fire.
- Confinement by a layer of cold air: a strong and
cold wind prevents the pyrolysis gas from rising, which leads to the explosive
- Pyrolysis of the opposite slope: the fire progresses
down a slope, but the radiated heat pyrolyses the plants on the facing slope,
which catches fire seemingly spontaneously.
- Bottom of a small valley: the gases accumulate in
the bed of a dry river; when the fire comes, it completes the fire triangle
and the bottom of the valley catches fire.
Firestorms in cities
The same underlying combustion physics can also apply
to man-made structures such as cities.
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 Great Fire of Chicago, and the fires resulting from the 1906 San Francisco
Earthquake and the Great Kanto Earthquake. Firestorms were also created by
the firebombing raids of World War II in Hamburg, Dresden, Tokyo, Kassel,
Darmstadt, Pforzheim, Braunschweig, Hildesheim and Stuttgart. (see also:
firebombing of Dresden, Tokyo, Kassel, and Operation Gomorrah).
|City / Event
||Date of the firestorm
|Great Fire of London
||2 September 1666 - 5 September 1666
||Most of the city of London burned to the ground; death toll unknown.
|Great Chicago Fire
|8 October 1871
||Hundreds killed in Chicago from 8 October to 10 October; up to 2,500
killed in Peshtigo, Wisconsin; others killed in similar fires in Holland and
||10 May 1943
||24 July 1943
||31 July 1943
||23 October 1943
||14 July 1944
||15 October 1944
||5 August 1944
||11 September 1944
||12 September 1944
||6 December 1944
||17 December 1944
||13 February 1945
||23 February 1945
||27 February 1945
||9 March 1945
||16 March 1945
||17 March 1945
||23 March 1945
|Oakland Hills Firestorm
||20 October 1991
||25 dead, $1.5 billion in damages
During the course of World War II, the Allies refined
the technique of fire-bombing
: the first wave of bombers
would drop high explosives to expose the timbers within buildings and to rupture
water mains. This was followed immediately by a wave dropping incendiary cluster
bombs (early in the war phosphorus was used, though napalm came into usage
by the end of the war) to start a conflagration. A third wave then followed
after an interval of fifteen minutes or so, dropping fragmentation bombs;
the slight delay allowing time for firefighters and their equipment to be
caught in the open and destroyed, thus preventing efforts to hamper the spreading
fires. The furnace-like conditions created in those firestorms resulting
from the strategic bombing campaigns of World War II were often hot enough
to cremate the corpses they created. Nuclear weapons can also create firestorms
in urban areas. This was responsible for a large portion of the destruction
The author, Kurt Vonnegut, who was a prisoner of war
in Dresden at the time of its fire-bombing, described some of the carnage
of this incident in his novel Slaughterhouse-Five.