RESEARCH LETTER No. 2.5

THE BIG ONE[S]: FIRESTORM!!!

The economically-destructive fires that began on Christmas Eve and spread through the New Year period in the Australian Capital Territory and New South Wales drew media attention for at least three weeks. One television channel had the headline “FIRESTORM” throughout this period. What a “firestorm” is, is the theme of this Research Letter. The following definitions are in chronological order.

1. Firestorm. “Violent convection caused by a large and continuous area of intense fire. Often characterized by destructively violent surface indrafts near and beyond the perimeter, and sometimes by tornadolike whirls” (USDA Forest Service 1956).

2. “A fire storm is produced when many separate fires scattered over a region of fuel interact so that the entire area ignites and burns at the same time. The scale of a fire storm is sufficient to generate very strong induced winds (probably about 100 m.p.h. [160 kmhr-1] at the edge of the Hamburg fire [see below]) and these dominate any external wind. In fact, fire storms probably only develop in conditions of light wind, though this is not certain as few fire storms have been identified positively.” (Morton 1964).

3. It is a high intensity fire, “a stationary fire as compared to the moving fire front, but its burning rate, and hence also its energy production, are at a peak over a sufficient area to create violent indrafts from all directions”. This occurred historically as the result of the multiple-incendiary bomb attack on the timber houses of Hamburg in July 1943 during the second World War. The induced winds were said to have caused cars to tumble end-over-end. “The most significant feature of the firestorm is the merging of many separate fires.” (Brown and Davis 1973).

4.“Fire Storm. Violent convection caused by a large continuous area of intense fire; often characterized by destructively violent surface indrafts, a towering convection column, long-distance spotting, and sometimes by tornado like vortices.” (Chandler et al. 1983).

5. “ A large continuous area of intense burning characterized by violent fire-induced convection resulting in gale-force indraft surface winds near and beyond the fire perimeter, a towering convection column, and the occurrence of large fire whirls.” (Merrill and Alexander 1987).

These definitions suggest that firestorms are unusual phenomena that occur under specific conditions of mass ignition, high fuel loads and light winds to give a large ignited area with a rapid burning rate, extremely strong convection and therefore gale-force in-drafts of wind.

Literature cited

Brown, A.A. and Davis, K.P. (1973). Forest Fire: Control and Use. 2nd Edition. McGraw-Hill, New York.

Chandler, C., Cheney, P., Thomas, P., Trabaud, L. and Williams, D. (1983). Fire In Forestry Volume II. Forest Fire Management and Organization. John Wiley and Sons, New York.

Merrill, D.F. and Alexander, M.E. (eds) (1987). Glossary of Forest Fire Management Terms. Fourth Edition. National Research Council of Canada, Canadian Committee on Forest Fire Management, Ottawa, Ontario. Publication NRCC No. 26516.

Morton, B.R. (1964). Fire and wind. Science Progress 52, 249-258.

USDA Forest Serrvice (1956). Glossary of Terms used in Forest Fire Control. Agriculture Handbook No. 104, Washington, D.C.

A. Malcolm Gill
28 February 2002