Personal tools
  •  
You are here: Home Wildland Resources Wildland Fire Management and Planning Unit 9: Extreme Fire Behavior

Unit 9: Extreme Fire Behavior

Document Actions
  • Content View
  • Bookmarks
  • CourseFeed

Terminology   ::   Tragedy Fires   ::  Predicting Behavior  ::   Safety Precautions   ::   Exercises

Predicting Severe or Extreme Fire Behavior

It's not enough to know what's causing such tragedies, but we must be able to do something to prevent them in the future. The first step is to recognize when fire conditions can and may change for the worse and threaten lives and property. We have already identified the fire processes that contribute most to fatalities--crowning, spotting, fire whirls, and blowups. This section will present guidelines to predicting whether any of these might occur on your fire.

Crown Fires

more

Factors affecting crown fire development are:

  1. Dead and live fuel moisture contents.
  2. Volatility of foliage.
  3. Crown closure.
  4. Intensity of surface fire.
  5. Vertical continuity or ladder fuels.
  6. Winds at upper crown level.
  7. Steepness of slope.

Firewhirl

more

Firewhirls develop as a result of:

  1. Surface heating : Clear, sunny skies. Favorable exposure to sun. Exposed soil or burned over area.
  2. Light but gusty winds : Lee side or ridge. Intersecting ridge, slope, or canyon winds.
  3. Fire whirls develop in light but gusty winds or winds converging, shifting, or eddying. Whirls usually get their start from two winds coming from different directions and occur more frequently when winds are light to moderate. They can result from any disturbance in the air flow.

  4. Atmospheric instability : Cumulus cloud development. Smoke rising to great heights. Rough, bumpy air encountered by aircraft.
  5. Intense heat from fire : Heavy, dry fuel concentrations. Very low relative humidity. Flame lengths over 11 feet. Rolling convection columns with strong indrafts.

Convection Columns

While discussing spotting potential and fire whirl development, we brought in their relationships to the fire convection column. Now we'll cover that aspect in more detail. We've described the convection column as a thermally produced ascending column of gases, smoke, and debris produced by a fire. How high will a column rise? Well, on the Sundance Fire, convection columns rose to over 30,000 feet.

The height a fire convection column will rise into the atmosphere is dependent on:

  1. The degree of instability in the atmosphere.
  2. The heat energy output of the fire.
  3. The speed of winds aloft.

Blowup Fires

When we talk about convection columns of great heights, we usually associate them with blowup fire situations. As fire combustion processes increase, so do the rates of heat energy output and the magnitude of the smoke convection columns. Our concern, then, is, "When are blowup conditions likely to develop?"

Factors that contribute most to blowup fires are:

  1. Unstable atmosphere.
  2. Very dry, heavy fuels.
  3. Strong winds with spotting.
  4. Steep terrain.

Humidity and Moisture Related to Ignition Potential

The table below gives fire severity or fire activity levels based on relative humidity and fuel moistures for 1 hour and 10-hour time lag fuels. This table gives six levels. Take some time to study this table and see how it could be used as a guide to predicting fire behavior; then return to the text.

Relative Humidity % 1-Hour Fuel Moisture % 10-Hour Fuel Moisture % Relative ease of chance ignition and spotting, general burning conditions
> 60 > 20 > 15 Very little ignition; some spotting may occur with winds above 9 miles per hour.
45 - 60 15 - 19 12 - 15 Low ignition hazard - campfires become dangerous; glowing brands cause ignition when relative humidity is ‹50 percent.
30 - 45 11 - 14 10 - 12 Medium ignitability - matches become dangerous; "easy" burning conditions.
26 - 40 8 - 10 8 - 9 High ignition hazard - matches always dangerous; occasional crowning, spotting caused by gusty winds; "moderate" burning conditions.
15 - 30 5 - 7 5 - 7 Quick ignition, rapid buildup, extensive crowning; any increase in wind causes increased spotting, crowning, loss of control; fire moves up bark of trees igniting aerial fuels; long distance spotting in pine stands; dangerous burning conditions.
< 15 < 5 < 5 All sources of ignition dangerous, aggressive burning, spot fires occur often and spread rapidly, extreme fire behavior probable; critical burning conditions.
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_5.html. This work is licensed under a Creative Commons License Creative Commons License