The intent of this article is to "tune-up" your understanding of wildland fire behavior. Understanding fire behavior allows for the correct selection of strategy and tactics. Similar to structural firefighting, knowing where the fire is, how it is behaving, and where it is going are critical. Also, anticipating how the fire will react once it gets to your location allows for safer engagement of the fire.
A wildland fire that threatens structures can become the interface fire. Sometimes the "interface" itself is the entire fire area. Common factors are the vegetative fuels (wildland and ornamental), weather, and topography. Once the fire enters the immediate area around structures, yard debris, outbuildings, and the structures themselves contribute to the fire behavior. In a report published by NWCG, 50 percent of engine burnovers occurred on interface fires.
Safety depends on your ability to:
- Determine the potential worse case scenario of current and expected fire behavior
- Determine how much time you have before the fire arrives.
- Determine when and where you will be in danger of entrapment.
Fire researcher Carl Wilson, in his article "Fatal and Near-Fatal Forest Fires: The Common Denominators," published in The International Fire Chief in 1977 discussed four common denominators of fire behavior that caused fatalities and near-misses on wildland fires:
- On relatively small fires or deceptively quiet areas of large fires.
- In relatively light fuels, such as grass, herbs, and light brush.
- When there is an unexpected shift in wind direction or wind speed.
- When fire responds to topographic conditions and runs uphill.
These continue to be applicable today. Particularly noteworthy is the first common denominator as it more closely resembles a human behavior factor. In other words, the fire itself does not need to be of great size or intensity for it to injure or kill. When you assess fire behavior and determine the facts presented, a logical and safe course of action needs to follow. If all the factors tell you to disengage from the current operation, your approach needs to honor that thought. The situation may dictate that you seek a more advantageous point of engagement.
Wildland Fire Behavior Factors
Fuels, weather, and topography create fire behavior. The factors below, taken from the Incident Response Pocket Guide (IRPG), must be visually noted. These are then evaluated so a more accurate determination of the resulting fire behavior can be predicted.
- Continuous fine fuels to heavy loading of down & dead
- Ladder fuels in brush & timber
- Tree spacing of under 20 feet or numerous snags which can serve as firebrand senders and receptors
- High dead-to-live ratio from frost or bug kill
- Preheated fuels
- Drought stressed or old aged fuels
- Normal seasonal dried fuels
- Fuels in direct sunlight
- Low relative humidity
- High temperature
- Moderate to high wind speed, (foehn, general, and localized/terrain),
- Approaching thunderstorms (tall, building cumulus clouds)
- Approaching cold fronts (cirrus clouds, sudden calm, battling or shifting winds)
- Unstable air mass (clouds and smoke grow vertically to great heights, cumulus-type clouds, gusty winds, good visibility, presence of dust devils and firewhirls, inversion lifts)
- Steep and gentle slopes
- Narrow and box canyons
Emergeing Extreme Fire Behavior Indicators
- Smoke column is well-developed and could be leaning or looks blown out
- Trees are torching and throwing embers (spotting)
- Large areas of ground/surface fuels are on fire
- Firewhirls appear
- Smoldering fires are waking up
- Inversion has lifted out
- Terrain winds have increased
Note: Relative humidity, temperature, and wind speed thresholds have been published within the "pocket cards" which are available from the federal land management agencies.
Interface firefighting by nature is normally a defensive action since unburned fuel exists between the fire and your position. If the fuels are already burnt, they must be carefully examined to determine their potential to re-burn. This is a very important consideration, especially if you are utilizing the burned area for a safety zone.