Wind, Slope and Preheat
Fire environment is not static, but varies widely in horizontal and vertical space, and in time. The fire environment components and many of their factors are closely interrelated. Thus, the current state of one factor depends on the state of the other factors. Also, a change in one factor can start a chain of reactions that can affect the other factors.
Three major forces heavily influence variations in speed and intensity of wildland fires: wind, slope and the preheating of fuels. Wind and slope both act on a fire by changing the angle and orientation of the flame with respect to the fuel bed. Preheat is a force that alters the fire’s intensity. Solar heated fuels ignite more readily than cooler fuels. Solar preheating is a force multiplier when applied to the wind and slope force vectors. These potent forces can work in cooperation or against each other.
When these three forces align, the fire is going to be at its maximum intensity. When these forces are not in alignment, the intensity is lower than the maximum. This is the alignment of forces concept. Forces that complement each other (producing a cumulative effect) are said to be “in alignment.” When forces are not aligned, and are below their maximum potential to aid the spread or intensity of the fire, the forces are then “out of alignment.” Each side of a fire, the head, flanks and heel, has a different alignment and intensity. For example, if the head is in alignment with the wind, then the flanks and heel will be out of alignment.
Across the fireground, within a single fuel type on any given day, variations can be observed in the fire’s intensity, rate of spread, and consumption of live and dead fuel.
Consider this situation: A fire is burning at the base of a slope, with the wind behind it at 1:00 p.m. on a southwest aspect. In the figure below are three arrows representing the slope, wind direction and the flammability of the fuel. The arrows are pointing in the same direction, aligned with the head of the fire. In fact, this is the very reason why it is the head of the fire.
All the forces that create fire behavior differences are in alignment at the head of the fire. The head of the fire is burning with the greatest intensity. At the heel of the fire, two or three forces are 180 degrees out of alignment.
At the heel, the fire spreads against the wind backing down the slope at the peak of the curve. The minimum intensity occurs at the heel of the fire.
The flanks of the fire are not in alignment with wind and slope. The flanks burn with an intensity less than the head but greater than the heel of the fire. The heel of the fire is out of alignment with wind, slope and burning through the shaded side of the fuel.
To convey the alignment of forces, use the phrases “in full alignment,” “out of alignment,” or “half in alignment” to warn of potential changes of fire behavior caused by the degree of alignment of the primary forces, wind, slope and fuel flammability peak.
For example: If the wind should shift 90 degrees, the flank of the fire shown in the illustration will become the head with intensities as great as the head. The flank will have two of the three forces (wind and peak flammability) in full alignment.
The head will lose the added force of wind and have two of three forces acting in alignment. The result might be equal intensities on one flank and the head. If the fire spreads to another aspect, the force alignment must be determined and result in a prediction of intensity change.
One should be conscious of the alignment of forces acting on the exposures around the fire perimeter. If the fire moves onto another topographic feature where the alignment of forces acting upon the fire are different, then preface a fire behavior prediction with the change of alignment statement. A simple explanation of WHY one thinks the fire will give more trouble on one flank as opposed to the other would be a welcome change from the norm.
Case Study — The Footprint Test Burn
Following is a video of a test fire lit in preparation of a prescribed burn. The plan was to wait until the solar heating dried the grass on the east aspect to 40 degrees surface temperature above air temperature. The test would be on a heated aspect and be free burning.
This fire was a test burn lighted below a small ridge so the fire would burn in alignment with slope and the 3 MPH easterly wind blowing from left to right in the video, from heated, sunlit fuels towards cool, shaded fuels. Wind, slope and preheat are acting on the fire to either help or hinder the spread. When some or all of these forces are in alignment the fire burns with greater intensity, and when the forces are out of alignment the fire behavior ebbs. Observe how the fire intensity changes as the alignment of forces change.
- Two firefighters lit two spots fires (instead of one as instructed).
- The firefighters walked into the wind on a preplanned exit from the fire.
- As the two fires joined the head fire signature—which is the observed fire behavior within the current alignment of forces—was about ten feet of flame.
- As the two fires became one the fire showed three distinct flame signatures: the fire had a head, flanks and a heel—and very different flame lengths and rate of spreads.
The next question is how is the fire going to burn—or exhibit one of these signatures—as it moves over the small ridge, and how will the alignment of forces that are different there cause each signature?
The prediction of change in fire behavior is thus:
The head fire will go out of alignment with wind, slope and onto a cool, shaded aspect. The fires head will reduce and the signature would replicate the lower flank of the fire.
The tactics should one want to suppress the fire would be simple:
- A three-person crew with hand tools could safely extinguish the fire.
- Using the burned out area for safety and anchor the up hill flank as a first action.
- Next suppress the former head next, then the lower flank and heel.
Alignment of forces is a simple way to view the fireground and to explain how the fire will change in intensity. A segment of a wildland fire will gain intensity and speed where it finds a time or place of more favorable alignment. To communicate this change say; “The fire is going into better alignment with —” (naming the force of change.)
There are three primary causative forces present which influence the variations in intensity and rate of spread of a wildland fire. As the fire burns over the topography, the forces change independently. Each force can aid or retard the spread. The forces can work together or cancel each other’s effects out. The three forces usually associated with fire behavior are Weather, Topography, and Fuel, AKA the wildland fire triangle.
Copyright © 1991, 2016 by Doug Campbell.