How Homes Ignite

To understand how homes ignite and what can be done to prevent that, first consider what a fire needs and what factors contribute to how a fire will behave. Fire needs heat, fuel, and oxygen to burn. If even one of these is taken away, the fire no longer can burn. Firefighters focus on removing fuels by creating fuel breaks and removing oxygen and heat with water and fire retardant. Homeowners can help by removing flammable vegetation and objects around their home and creating defensible space long before a wildfire threatens.

The Fire Environment

Fuels, weather, topography, and human behavior are the components of the Fire Environment in Wisconsin, all influencing the likelihood of a fire start, speed and direction at which a fire will travel, intensity at which a wildfire burns, and the ability to control and extinguish a wildfire. Weather cannot be changed. Topography generally remains the same. Fuels and human behavior can be altered. The greatest opportunity to reduce the threat of wildfire lies in active management of wildland vegetation and changing human behavior.

Fuels

There are many types of fuels in the wildland-urban interface. Fire moves quickly through light fuels such as grass, fallen pine needles and leaves, and garden mulch. Fire lingers and burns more intensely in heavier fuels such as wood decks and fences, firewood stacks, and lawn furniture. Very heavy fuels such as trees and buildings can burn for long periods of time and spread fire by producing radiant heat and flying embers.

Fuels are arranged horizontally and vertically. Ground fuels are all 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 are all materials lying on or immediately above the ground including pine needles, leaves, grass, downed logs, stumps, tree limbs, and low shrubs. "Surface fires" burn in surface fuels. Aerial fuels are all green and dead materials located in the upper forest canopy including tree branches and crowns, snags, moss, and taller shrubs. "Crown fires" burn in aerial fuels. Fires in conifer stands and pine plantations tend to be very intense and difficult to control.

The continuity of fuels around a home can help determine the survivability of structures. Unmowed grass, unraked leaves, and dead branches can supply 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.

Weather

Weather greatly influences the possibility of wildfires and their behavior. Temperature, relative humidity, and wind speed are the three most 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 easier 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 both 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 of wildfire ignition and spread. The chance for wildfire increases 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

Topography plays a big role in how a fire will behave. Steep slopes cause rapid fire spread. Minimizing fuels downslope 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, is also a factor. North facing slopes tend to be more shaded, moist, and have heavy fuels such as trees. South facing slopes tend to be sunnier and dry, with more light fuels such as grasses.

Human Behavior

Another factor that affects the Fire Environment is human behavior. When people are living in fire-prone environments, the human built environment 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 are examples of increased risk to people living with the threat of wildfire. The risk of wildfire increases when people use fire in a wildland environment. Burning household garbage and yard waste and lighting campfires and warming fires all increase the risk of wildfire.