A backing fire is a controlled application of fire used by land managers and wildland firefighters in both prescribed burning and active wildfire suppression. This technique involves intentionally setting a line of fire that moves directly against the prevailing wind direction or downhill on a slope. The fundamental principle is to eliminate fuel ahead of a main fire or safely reduce surface fuels in a planned operation.
Defining Characteristics and Movement
The distinguishing feature of a backing fire is its low rate of spread, resulting directly from its movement against the wind. When the fire is set, the wind bends the flames back over the burned area, insulating the unburned fuel ahead of the flame front. This prevents the fire from preheating new fuel, forcing the advancing edge to rely solely on radiant heat for ignition. Consequently, the fire moves very slowly, typically spreading at less than 200 feet per hour, regardless of wind speed.
This process results in a fire of significantly lower intensity compared to a head fire, which moves with the wind and is driven by convection. The flames are shorter and the heat energy is released closer to the ground. Because the intensity is consistently low, a backing fire is considered the most predictable and safest ignition technique.
Primary Role in Fuel Reduction and Control
The slow, low-intensity nature of a backing fire makes it effective for two primary objectives: creating a secure firebreak and achieving thorough fuel consumption. When used for wildfire suppression, a backing fire is ignited along a secured fire line, such as a road or hand-cut line, to burn vegetation in the path of the oncoming wildfire. This process creates a wide area of consumed fuel, known as a “black line,” which the main wildfire cannot cross because it has been starved of its energy source.
In a prescribed burn scenario, the slower movement allows the fire to burn deeper into the forest floor, consuming accumulated layers of leaf litter and duff. This deeper burn enhances the downward heat pulse, which helps control certain pests and achieves a more complete reduction of surface fuels without damaging the tree canopy. Furthermore, the backing fire produces fewer smoke emissions than a higher-intensity fire because more fuel burns in the efficient flaming phase rather than the smoldering phase.
Environmental Requirements for Safe Use
Implementing a backing fire safely requires a precise combination of weather and fuel conditions to ensure the fire remains controllable. A light, steady wind is necessary to consistently push the flames back over the burned area, but the speed must be minimal, often less than 10 kilometers per hour, to prevent the fire from escaping the control line.
The level of moisture in the surface fuels is also a defining factor. The fire will not effectively carry if the surface litter moisture is too high, such as above 15% in hardwood litter. Similarly, high relative humidity, typically above 65% to 70%, can make it difficult to sustain the burn as fuel absorbs moisture from the air.
For these reasons, backing fires are often initiated during the evening or at night when temperatures are naturally lower, winds decrease, and relative humidity begins to rise. These cooler, damper conditions help personnel maintain complete control over the fire’s intensity and direction.