How wildfires start
Three elements are needed for a fire to start:
- fuel (wood, brush, lichen)
- oxygen (from the air)
- ignition source (heat from lightning or human activities)
Each of these three elements are needed to keep a wildfire burning.
These elements are often present across our forests – through trees, grass, dried leaves, and other materials. That’s why fires in these areas are often able to grow.
Wildfire fighting seeks to remove at least one of these elements to prevent a wildfire from growing and spreading into places we don’t want it to.
How wildfires spread?
The primary factors that influence the spread of wildfires are:
- Topography (what the landscape in the area is like)
Fuels and how they help wildfire spread
Fuel is any organic matter, living or dead, in the ground, on the ground, or in the air that can ignite and burn.
Light, small or fast-burning fuels
Dry grass, dead leaves and tree needles, brush and small trees. Light fuels ignite quickly and cause rapid spread of fire. They act as kindling for heavier fuels and burn out faster. Some green fuels such as tree needles have high oil content and are fast-burning when they are not in an active growing stage.
Heavy, large or slow-burning fuels
Logs, stumps, branch wood and deep duff (the topsoil or partly decayed leaves and tree needles found under dense stands of brush or trees). Heavy fuels take longer to ignite, spread slower, burn longer and throw off large volumes of heat when dry.
The ease of ignition and rate of burning are influenced by the size of fuels and how dry it is.
Fuel spacing describes the distribution of fuels in a given area.
Fuel continuity is an important factor in the behaviour of fire because it indicates how quickly and where a fire may spread.
A wide range of fuel continuity conditions will be found in most forested areas. However, for simplicity in determining fire behaviour, two broad classifications of fuel spacing will be discussed: horizontal and vertical.
Horizontal spacing is the spacing of the fuel as it lies on the ground. When fuels are close together, the fire will spread faster.
When fuels are patchy, scattered or separated by natural barriers such as rock, outcropping, streams, or areas of bare ground, the fire will be irregular and spread more slowly.
A common method used in fire suppression is to break the continuity of the fuel by separating burning fuels from unburned fuels.
When fuels are closely spaced vertically, fire will spread rapidly upwards as the fuels are pre-heated prior to ignition. When fuels are spaced far apart vertically, the heat is less able to ignite the fuels above.
As the amount of flammable material in a given area increases, the amount of potential heat produced by the fire also increases.
The hottest fires, as well as those most difficult to control, occur in areas containing the greatest quantity of volatile fuel.
In evaluating fuel volume, it should be noted that a lot of small material such as fine deadwood, means that there is a lot of kindling material to light other fuels.
Weather and how it influences wildfire
One of the most important factors affecting the behaviour of a fire is weather. The four most important components of weather are:
- relative humidity
The stronger the wind, the faster the spread of the fire. Wind brings an additional supply of air to the fire. It flattens the flame which pre-heats the fuel ahead and causes spot fires by blowing sparks and embers ahead of the main fire into new fuel sources.
Winds can be affected by a number of different environmental factors such as topography (upslope and downslope winds or winds being funneled through a canyon), environment (wind speeds slow as they pass through a forest or other obstacles), large bodies of water (the temperature difference between the land and water can slow or hasten local windspeeds) but most of all winds are governed by global weather patterns.
Other wind behaviour characteristics which must be considered:
- cumulonimbus clouds — found most often in thunderstorms — may indicate possible change in wind speed and direction
- winds usually flow out from the edges of a thunderstorm and may reach speeds of 115 km per hour
- gusty winds are very hazardous to fire-fighters because they change speed and direction rapidly
Fuels pre-heated by the sun burn more rapidly than cold fuels. The temperature of the ground also affects the movement of air currents,
Moisture in the form of water vapour is always present in the air. The measurement of that moisture is called humidity and is always expressed as a percentage.
Warm air is capable of holding more water than cool air so as the temperature increases the relative humidity drops without any change in the amount of water in the air. As air cools, the relative humidity increases. The amount of moisture in the air determines its ability to draw moisture from fuels. When the relative humidity of the air is lower than the fuel moisture, fuels begin to dry. This is an important factor in firefighting, since wet and most green fuels will not burn freely.
Precipitation is any rain, sleet, or snow that is part of an area’s weather.
Precipitation will quickly dampen the surface of fuels to the point that fires cannot ignite and no wildfires will occur.
When wildfires are being fought, precipitation falling can make the job easier for wildfire fighters.
When there is less precipitation, or none at all, then fire danger will grow.
If fires do ignite, then a lack of precipitation will make wildfires more difficult to fight.
Lightning and weather monitoring
The role of lightning in wildfire
Most wildfires in the Northwest Territories are the result of lightning. Lightning-caused fires are a natural part of the ecosystem and is the primary source of natural disturbance and natural disturbance is required to maintain healthy forests. Without fire, forests would grow old and decadent, losing their capacity to support wildlife and increasing their susceptibility to other less desirable sources of disturbance, such as pest outbreaks.
Lightning can strike almost anywhere (up to 15 kilometres from a storm cell) so fires started by them may not be affected by rain from thunderclouds. Dry lightning can also occur and is a strong ignition source because there is not enough moisture with the storm to interfere with ignition or fire spread.
The GNWT operates and maintains a lightning detection network in the Northwest Territories. This is a different system than the Canadian Lightning Detection network.
The territorial lightning detection network consists of nine lightning locators that provide complete coverage of the NWT. This network can detect more than 90% of all lightning strikes occurring in the NWT, sometimes recording more than 100,000 strikes in a single day!
How data is collected
To collect the data, the lightning locators continuously detect and record all cloud-to-ground lightning strikes as they occur. After triangulation, the data is sent to a lightning processing server in Forest Management Division headquarters.
Once received, the latitude, longitude, time and polarity of the strike is moved into a database and is available for plotting on computer-generated maps. Wildfire personnel can then check lightning strikes for new wildfires and allocate resources more effectively.
For more information
- Learn more: how to stay safe when lightning strikes
Find where lightning strikes are most likely: view the Canadian Lightning Danger Map
Topography and how it influences wildfire
Slope is the steepness of the land and has the greatest influence on fire behaviour.
The steepness of the slope affects both the rate and direction of the fire spread. Fires usually move faster uphill than downhill and the steeper the slope, the faster the fire will move. This is because:
- On the uphill side, the flames are closer to the fuel the fuels become drier and ignite more quickly than if on the level ground
- Wind currents are normally uphill and this tends to push heat from flames into new fuels
- Convected heat rises along the slope causes a draft which further increases the rate of spread
Burning embers and chunks of fuel may also roll downhill into unburned fuels, increasing spread and starting new fires
Aspect is the direction the land faces - north, south, east or west. The aspect of a slope influences a fire's behaviour in several ways:
- southern aspects receive more direct heat from the sun, drying both the soil and the vegetation
- fuels are usually drier and less dense on southern slopes than fuels on northern slopes
- heating by the sun also causes earlier and stronger slope winds
- on south-facing slopes, there will normally be higher temperatures, stronger winds, lower humidity and lower fuel moistures
All of these conditions can lead to quick starts and a rapid rate of fire spread.
Terrain includes things like hills, mountains, lakes, rivers, and other features on the land.
Each of these items can significantly influence where wind flows as it will always try to follow the path of least resistance. The places winds flow will influence where fire goes.
Bodies of water like lakes and rivers can also serve to limit the spread of surface fires by providing a natural break in flammable fuel.