How Do Wildfires Start and Spread?
Wildfires do sometimes occur naturally, either ignited by the sun’s heat or a lightning strike. However, most wildfires are cause by humans, such as unattended campfires discarded cigarettes, arson and more. So, do you know how wildfires start and spread?
How Do Wildfires Start?
Regardless of the exact cause, there are three components that wildfires require to ignite and eventually spread: fuel, heat and oxygen.
Fire is the effect of a chemical reaction known as combustion. This occurs between oxygen in the air and some sort of fuel that has been heated to its flash point, or the lowest temperature at which it will ignite.
Fuel is any kind of flammable material, which can include trees, grasses, shrubs, wood and even houses. These materials emit a vapor. Heat brings these fuels to their flash point, causing vapor to evaporate and mix with oxygen.
Oxygen is the naturally occurring element needed for igniting and sustaining a fire. When burning fuel is exposed to oxygen from the air, a chemical reaction occurs that releases heat and generates combustion.
For fire to occur, all three of these components must react together. So in order for the fire to be extinguished, at least one of the components much be removed.
How Do Wildfires Spread?
There are ultimately three factors that influence that intensity and movement of a wildfire: fuel, weather and topography.
A fuel’s composition helps determine how quickly a fire will spread and at what temperature. This includes moisture level, chemical makeup and density. For example, vegetation with a low moisture level and is very dry will cause a fire to burn faster and more intensely because the heat does not have to eliminate water. Some plants, trees and shrubs contain oils and resins that cause them to burn more quickly and intensely.
The size and amount of fuel also affects wildfire behavior. Small fuel sources such as grasses typically burn quicker and do not generate as much heat as trees and other large fuel sources. A small amount of fuel will cause a fire to spread slower with lower intensity, while a large amount of fuel will cause a fire to spread faster with more intensity.
The weather, such as wind, temperature and humidity, also plays a major role in the behavior of a wildfire. Wind can supply the fire with additional oxygen, causing a fire to move across a landscape at a faster rate. High temperatures and low humidity also dry out fuel sources and cause them to ignite and burn faster. This is why wildfires typically become more intense and spread faster in the afternoon, when the air is hottest. If the humidity is very high, fire is not going to do much because the heat has to burn off moisture in the air to ignite the fuel.
Topographical features of a landscape can also facilitate wildfire progression. This includes slope, direction of the slope and elevation. If a fire ignites at the bottom of a steep slope, it could spread more quickly uphill because heat rises. As that hot air rises, it preheats fuels that are further uphill, causing them to rapidly ignite once the fire reaches them.
Elevation and direction of the slop can determine the temperature and moisture levels in fuels across a landscape. For example, fuel that is on a south-facing slop and at a lower elevation receives more sunlight, which means it is warmer and dries out fast. This can lead to more intense wildfires.
Source: North Carolina State University
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Image: Pilots and crewmembers of the 2nd General Support Aviation Battalion, 4th Aviation Regiment, 4th Combat Aviation Brigade, 4th Infantry Division, go into the thick smoke to release water onto the burning fires during their bambi bucket mission at Black Forest, Colo., June 12, 2013. (Photo by Sgt. Jonathan C. Thibault, 4th Combat Aviation Brigade Public Affairs Office, 4th Infantry Division; The U.S. Army)