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How homes ignite
Avoiding a recipe for disaster
Fire needs heat, fuel and oxygen to burn. If even one of these factors is removed from the equation, the fire is snuffed out. Fuels, weather, topography and human behavior influence the likelihood of a fire starting, as well as its speed, direction, intensity, and the ability to control and extinguish the wildfire. Weather cannot be changed. Topography generally remains the same. But fuels and human behavior can be altered. Thus, the greatest opportunity to reduce a wildfire threat to your home lies in actively managing wildland vegetation and changing your habits.
Fire quickly moves through light fuels like grass, fallen leaves, pine needles and mulch. Fire lingers and burns more intensely in heavier fuels like wood decks and fences, firewood stacks and lawn furniture. Very heavy fuels like trees and buildings can burn for long periods and spread fire by producing radiant heat and flying embers.
Fuels are arranged horizontally and vertically. Ground fuels consist of combustible materials lying beneath the surface including deep duff, roots, rotten buried logs and other organic matter. Fires in ground fuels are usually called "peat fires."
Surface fuels consist of materials lying on or immediately above the ground including pine needles, leaves, grass, downed logs, stumps, tree limbs and low shrubs.
Aerial fuels include green and dead materials in the upper forest canopy: tree branches, crowns, snags, moss and taller shrubs. "Crown fires" burn these aerial fuels. Fires in conifer stands and pine plantations tend to be very intense and difficult to control.
How these fuels are connected around a home can determine the chances of a structure surviving. Unmowed grass, unraked leaves and dead branches are a continuous fuel supply right up a home's siding. Breaking the chain of continuous fuels up to and around a home can serve as a fuel break, slowing a fire and bringing it to the ground where firefighters have a better chance to stop it.
Temperature, relative humidity and wind speed are three significant weather factors affecting wildfire behavior. Higher temperatures preheat fuels by driving off moisture, which allows fuels to burn faster. Lower relative humidity and a lack of precipitation lowers fuels moisture; dry fuels burn more easily than fuels with higher moisture content.
Wind is the most important weather factor since it dries fuel and increases the supply of oxygen. Wind has the greatest influence on the rate and direction of fire spread. In Wisconsin, wind direction almost always changes in a clockwise rotation and winds tend to be the strongest in mid-afternoon.
Wisconsin's wildfire weather is most severe during spring, between the time after the last snowmelt and before the vegetation "greens up." Spring rains and new green growth lessen the likelihood that wildfires will start and spread. The chances increase again during late summer and fall when the vegetation begins to dry out. The combination of hot weather, high wind speed and dry vegetation creates prime conditions for wildfires.
Topography plays a big role in how a fire will behave. Steep slopes spread fire rapidly. Minimizing fuels downhill from a home can make a difference when a wildfire threatens. Fire travels faster uphill and afternoon winds travel upslope as hot air rises, pushing fire even faster. Homes built on a hilltop need larger areas of defensible space, particularly on the downhill side. Aspect, or the direction a slope faces, also is a factor. North facing slopes tend to be more shaded and moister with heavier fuels such as trees. South facing slopes tend to be sunnier and drier, with more light fuels such as grasses.
When people live in fireprone environments, their behavior becomes an important factor in predicting the loss of life and property. Narrow or sandy roads and driveways, limited access, lack of firewise landscaping, inadequate water supplies and poorly planned subdivisions increase risk to people living with the threat of wildfire. Wildfire risk also increases when people burn trash or light warming campfires.
Jolene Ackerman is Wisconsin DNR's wildland-urban interface coordinator.