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Unit 9: Extreme Fire Behavior

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Crown Fires

The graphic below illustrates most factors affecting crown fires. At the surface, we see rather low flames that are not reaching into the foliage. The convective heat above the fire is certainly drying and scorching the foliage, perhaps to the point of killing it. Until the flames actually reach the foliage, torching and crowning probably will not occur.

Factors
Factors affecting crowning

The wind near the surface has considerable effect on scorch height. Under very light wind conditions, the convected heat goes straight up into the canopy, thus producing a higher scorch height effect. When surface winds are somewhat stronger, the flames and convected heat are angled; thus scorch height can be effectively lowered.

Sustained crowning fires (running) usually require:

  1. Strong winds and/or steep slopes.
  2. Closed canopy or crown cover.
  3. Spotting ahead in continuous aerial fuels.
  4. Long-range spotting can occur when large glowing firebrands are carried high into convection columns and then fall out downwind from the main fire.

Firebrand transport
Firebrand transport from spruce (by wind speeds)

Spotting normally results from firebrands being lifted by convected heat and then carried downwind into new fuels. (See figure above) Windspeed has a direct effect on the distance that spotting might occur. In the illustration, we have chosen a burning spruce tree, approximately 50 feet high, as the firebrand source. Under average burning conditions, firebrands produced might be lifted to about 450 feet. Five windspeeds are illustrated with respective distances that spotting could occur given receptive fuels. With 40 miles-per-hour winds in the lower atmosphere, spotting over 1 mile is possible.

In predicting spotting potential, we must be concerned with three aspects--the production of firebrands, the transporting distance, and the receptiveness of new fuels to ignite and sustain fire. As the intensity of a fire increases, so does the production of firebrands. By considering such factors as-fuel moisture, fuel temperatures, and windspeed, we can have a good indication of whether firebrands will ignite new fuels.

Spotting distance affected by column height

Moderate Winds Strong Winds

The height of the convection column is a factor in spotting distance. The figures above illustrate two columns. The one on the left has developed under moderate wind conditions to an altitude of well over 10,000 feet. Fire-brands lifted to that altitude can travel downwind several miles.

The column on the right is rising in moderate winds at lower altitudes to about 5,000 feet, where it encounters high speed winds aloft, perhaps 40 to 60 miles-per-hour. The column is sheared at that point, breaking up the convective cell, and causing firebrands to fall out. In comparing the two situations, the longer range spotting will be with the taller convection column.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 10). Unit 9: Extreme Fire Behavior. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_9__Extreme_Fire_Behavior_4.html. This work is licensed under a Creative Commons License Creative Commons License