Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

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4. Firestorm. Under exceptional circumstances, forest fires may develop into the phenomenon known as
"Firestorm."
Although the term was first used by the Police Commissioner of Hamburg to describe the effects of the
RAF raids of 24-25 July, 1943, the phenomenon has been noted in forest fires in America as far back as the great Peshtigo
Fire of Wisconsin in 187 1. A firestorm occurs when a large area (generally greater than 2 1/2 square kilometers) is burning
at one time under meteorological conditions such that the entrainment of air to the fire is confined to a relatively shallow
layer near the surface, and air enters the fire area with an appreciable tendency to rotate. This combination results in the
development of strong surf,-ce winds which act to fan the fire and increase its burning rate. The increased burning rate
causes more rapid air entrainment with a consequent further increase in wind speed, which increases burning rate still
further which increases etc., etc. until all available fuel has been burned. Wind velocities exceeding 100 miles per hour
are often attained during peak firestorm development. 10/
Experts are not in full agreement on the conditions necessary for firestorm formation.
In general, any factor
contributing to a rapid vertical heat release rate, or to marked surface wind vorticity will be favorable. Such factors
include:
I. Heavy fuel loadings of unusually dry material.
2. Many simultaneous ignitions.
3. Intense solar insolation.
4. An unstable atmospheric lapse rate.
5. Marked vertical and/or horizontal wind shear.
Since the winds are blowing into a firestorm area from all sides, the firestorm has no rate of spread in the conventional
sense-the entire firestorm area burns up as a unit. However, a firestorm almost always occurs on only a portion of
a large forest fire. Consequently, the fire may be spreading actively at one location while a firestorm is keeping the fire
stationary at another. Flame heights usually reach 100-150 feet in firestorms, and flashes of flame as high as 500-600 feet
are not uncommon.
Because of the intense heat and high winds, firestorms are extremely destructive. All dead material, regardless of
size, and all living stems and branches smaller than 1-2 inches in diameter are consumed. Firestorms can be expected to
cause extensive casualties to troops in the field. The mortality rate can be high, even among experienced firefighters.
* when even a part of a forest fire reaches firestorm conditions.
LOJ Anderson, H. E. Sundance Fire: An analysis of fire phenomena. USDA - Forest Service Research Paper INT - 56,
37 pp., ilius. 1968.
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