Fire behaviour

Fire behaviour map of Canada showing head fire intensity. Legend displays energy output at the fire’s front in kilowatts per metre (kW/m), ranging from nil to greater than 30,000. — View fire behaviour maps on the Canadian Wildland Fire Information System (CWFIS).

Understanding fire behaviour is critical for managing wildland fires. It is about how:

fuels ignite
flames develop
fire spreads and is extinguished

Factors such as fuel, weather and topography control:

flame characteristics
rate of fire spread
direction of spread
intensity of a wildland fire

Canadian researchers conduct experimental fires and study wildland fires under various conditions. Over decades, a system of scientific models to predict fire spread, intensity and growth across Canadian landscapes have been developed. These tools are invaluable for preparing for and responding to wildland fire.

Ignition, growth and extinguishment

After a wildland fire ignites, a front develops where flaming combustion occurs. Flame height indicates fire intensity – the amount of heat released, which is highest at the head and decreases along a fire’s flanks and back. During the day, as temperatures rise and humidity drops fire intensity also increases and usually decreases at night as these conditions reverse.

Fire is self-sufficient as long as:

oxygen is available
dry fuel is available
enough energy is produced to reach its combustion temperature

Removing any one element puts out a fire.

Smouldering combustion occurs behind the flame front and can last minutes, hours or days.

Wildland firefighters suppress fire spread by:

applying water, retardants or dirt to:
- cut off oxygen
- extinguish flames
- cool fuels below their combustion temperature
clearing fuels
creating firebreaks using:
- hand tools
- heavy equipment
- backburns

Uncontrolled wildland fires can burn for days to months until fuel is gone or precipitation (rain or snow) raises fuel moisture.

Smoke and firebrands

Incomplete flaming and smouldering combustion produce smoke that contains:

fine particulates
carbon dioxide
carbon monoxide
water
many other organic compounds

Smoke from low-intensity fires typically impacts local air quality. Smoke from large fires can travel hundreds to thousands of kilometres, affecting distant regions.

Intense fires create buoyant smoke plumes that rise tens to thousands of metres, carrying firebrands—partially burned materials capable of igniting new fires. Firebrands are a primary means of fire spread and the leading cause of structural ignitions by wildland fires.

Powerful updrafts from extreme wildland fires can create unpredictable atmospheric conditions, generating their own weather. Extreme fires can form pyro-cumulonimbus (pyro-Cb) clouds—thunderstorm-like clouds towering 10 to 15 kilometres that often produce dry lightning.

“Made in Canada” fire behaviour

Coniferous stand engulfed in flames from the forest floor, up the trunks and meters above the treetops. — Example of FWI #34: Crown fire.

Fire behaviour varies across Canada because of:

diverse fuels (grasslands, shrublands, boreal and temperate forests, peatlands)
weather (wind, air temperature, precipitation, relative humidity)
topography (slope angle and/or direction, elevation)

According to the Fire Weather Index (FWI), fire spread and intensity increase with hot, dry, windy weather, peaking during the day and dropping overnight.

Grass, shrubs or logging slash

Surface fires spread rapidly through organic vegetation. Spread rates can reach up to 12 km/h. Flame heights can exceed 10 metres in extreme conditions.

Forests

Fires often start in organic surface material such as needle and leaf litter. Spread rates are typically less than 0.3 km/h. Moderate flame heights and intensity are managed with firefighting resources.

Deciduous forests: Surface fires occur in spring after snowmelt. Fires spread more slowly after green-up and rarely support crown fires.
Coniferous forests: Intense surface fires can ignite tree canopy. Spread rates for crown fire are up to 6 km/h, with flames several times tree height. Crown fires have extreme intensities and are often uncontrollable.

Peat and humus

Smouldering ground fires burn slowly in deep organic layers and can be difficult to extinguish. During drought, fires can smoulder underground through winter and reemerge in spring.

Fire behavior prediction

Canadian researchers conduct experimental fires and study wildfires under various conditions. Over decades, a system of scientific models to predict fire spread, intensity and growth across Canadian landscapes have been developed. These tools are invaluable for preparing for and responding to wildland fire, as well as managing prescribed fires.

Examples of fire behavior in jack pine

During the 1974 fire season, the Alberta Forest Service, the Canadian Forest Service and three other federal agencies completed a short-term program of experimental fires in the jack pine forests of northeastern Alberta. The results of the study confirmed the strong relationship that exists between the various aspects of fire behavior and the components of Canada’s Fire Weather Index System.

Examples of fire behaviour in an upland jack pine (Pinus banksiana Lamb.) stand in order of increasing FWI levels.