Fire behaviour refers to the way a fire ignites and burns and what makes it spread. Understanding fire behaviour is vital for the effective management of wildfires.
What Makes a Fire Burn?
For a wildfire to ignite and burn the following three elements need to be present:
- Fuel in the form of live or dead trees, vegetation and other organic matter;
- Oxygen in the air around us;
- Heat to start and burn, from lightning or human sources.
The Fire Environment
Fire behaviour is influenced by the complex characteristics of, and interactions between, fuel, weather and topography.
The manner in which wildland fuels ignite and burn is dependent on the state of the fuel bed. Fuels can be described based on a range of characteristics, including:
- Moisture content is a critical factor in determining the flammability of fuels;
- Green and/or moist fuels will burn less easily than dead and/or dry fuels;
- Dead fuels can gain or lose moisture depending on the atmospheric conditions surrounding them, subject to the fuel size, exposure to wind and other modifying factors.
- Fuels can be compared generally, in terms of surface area-to-volume ratios;
- Small diameter fuels (referred to as fine fuels), such as grass, foliage, twigs etc. have larger surface area-to-volume ratios, making them more reactive to atmospheric changes and heat sources;
- Large diameter fuels (referred to as heavy fuels), such as logs, stumps and branches have relatively small surface area-to-volume ratios, making them less reactive to atmospheric changes and heat sources;
- Fine fuels can ignite easily and become consumed rapidly, while heavy fuels can take longer (or require a significantly hotter heat source) to reach ignition temperature and then subsequently burn for a relatively longer period of time;
- The continuous (or discontinuous) distribution of fuel can be assessed at varying scales, from the single-tree, micro-scale, up to the landscape-level, macro-scale;
- Vertical fuel continuity is characteristic of certain tree species. For example, black spruce typically has branches that grow along the full length of the tree, from ground to crown, while self-pruning trees such as ponderosa pine, typically have a distinct crown, separated from the ground by a lack of branches along the lower part of the tree;
- The term ladder fuels is often used to describe lower branches or intermediate vegetation, such a shrubs, that provide connectivity for fire to travel upwards from the ground into the crown layer;
Weather conditions, especially wind, temperature and humidity can have a major impact on the behaviour of a wildfire.
- Influences the direction and rate of fire spread;
- Burning embers can be transported by wind and blown onto new fuels ahead of a fire;
- Wind can be influenced locally by terrain features;
- Typically, in mountainous terrain, daytime winds will be up-valley and upslope, and will reverse at night to down-valley and downslope;
- Thunderstorms and the passage of cold fronts can pose challenges to firefighting operations.
Temperature and moisture
- Fuels heated by the sun will ignite and burn faster than cooler fuels;
- The amount of moisture in the air will affect the moisture levels in fuels;
- Dry air (lower relative humidity) will tend to dry out fine fuels, while moist air (higher relative humidity) will tend to add moisture to fine fuels.
- Relative humidity refers to the ratio, in percent, of the amount of moisture in a volume of air to the total amount which that volume can hold at the given temperature and atmospheric pressure;
- Warm air is able to hold more moisture than cooler air;
- Atmospheric stability either assists or suppresses the vertical mixing of air and can have significant influence on fire behaviour;
- Unstable air assists vertical motion whereby air rises, cools and condenses and cumulus type clouds form;
- Stable air resists vertical motion and air can stagnate and develop stratus clouds or fog;
- Unstable air is generally more conducive to greater fire behaviour, compared to stable air;
- The venting index is related to atmospheric stability.
British Columbia is characterised by a range of different topographic features and characteristics that, in turn, influence fire behaviour.
- The steepness of a slope affects both the rate and direction in which a wildfire spreads;
- Fires generally move faster uphill than downhill and the steeper the slope, the faster the fire will move;
- Upslope winds and the preheating of unburned fuel above influence the rate of upslope fire spread;
- Downslope winds and burning material rolling downhill can contribute to downslope fire spread.
- Aspect is the compass direction that a slope faces;
- The aspect of a slope will influence the amount of sunshine the slope receives;
- In the northern hemisphere, south aspect slopes receive more solar radiation than north aspect slopes, which in turn can influence differences in microclimate and vegetation;
- Fire behaviour can change significantly as a fire spreads onto different slope aspects.
- Terrain features can influence air flow across an area;
- Mountains and valleys channel and establish local wind direction;
- As wind passes over a ridgetop, turbulence and eddies can form on the leeward side;
- As wind is forced through a narrow valley or canyon, wind velocity will increase.