MSU Extension


Understanding how wildfires behave is important in protecting your rural home. You can determine how vulnerable your home is to wildfires by paying attention to the topography and vegetation around your home, while modifying the vegetation around your home can reduce the associated risk of wildfires. It is also important to consider the type of hills, slopes, and forest or field adjacent to your home.

Historical Background of Wildfires in Michigan

Between 1950 and 1996, the U.S. Forest Service and the Michigan Department of Natural Resources (MDNR) were involved in suppressing wildfires that burned 390,000 acres of the Michigan state forest. According to the state, wildfires occur in every county of Michigan, with approximately 46,100 wildfires damaging or destroying around 100 homes and properties each year. Even small fires can cause significant damage to homes. However, these small fires often go unnoticed in the evening news. Michigan experiences an average of 8,000 to 10,000 wildfires each year, but we typically associate states like California, Arizona, and Colorado as being more prone to wildfires. (Figure 1)

field on fire
Figure 1. Most wildfires are smaller fires that do not make the evening news.

Forest Fire Epidemics

It might be challenging for a residential property owner to extinguish a wildfire once it begins. A garbage fire has the potential to quickly transform into a wildfire due to a brief lapse in attention or an unforeseen shift in the weather. Figure 2 illustrates that one-third of all wildfires are ignited by individuals burning debris (such as household and yard waste). Approximately 98 percent of fires are a result of human actions, although wildfires can commence through various means.

Bin in a clearing
Figure 2. One-third of Michigan wildfires are caused by debris burning.

The Department of Natural Resources and local government units use burn permits to communicate with homeowners when it is safe to burn and provide them with safety information in the form of guidance on what can be legally burned to avoid undue air pollution.

Trains and off-road vehicles, tobacco use, bonfires, and other human factors contribute to fires. Only approximately 2 percent of all wildfires are caused by natural occurrences like lightning.

Behavior of Forest Fires

Your home can also serve as fuel. Vegetation, foliage, undergrowth, and turf supply the necessary fuel. The fire generates heat, while an ignition source initiates the combustion process. Once a wildfire is ignited, it generates its own heat. Oxygen is always readily available in the air and does not limit the spread of wildfires. A wildfire follows the same principles as any other fire, requiring fuel, heat, and oxygen to sustain combustion.

Large wildfires can rapidly move across the landscape, like a tidal wave, if the fuel and heat requirements are met. This is why wildland firefighters typically control wildfires by removing all vegetation down to the mineral soil to create a fuelbreak (Figure 3). If there is no fuel in the area, the fire will not go there.

dirt trench with fire in the background
Figure 3. Creating a fuelbreak.

Movement of Forest Fires

Within approximately 60 seconds, it swiftly surpasses any given location — it has a short duration in any particular area. Although a wildfire can endure for several hours, these petite, delicate combustibles also consume rapidly. These delicate combustibles that transport a wildfire across the terrain are the most arid combustibles on a day with high fire risk. Tiny fragments of combustible material — such as parched grass, foliage, and diminutive branches — undergo the most swift alterations in moisture levels and necessitate less heat to sustain combustion when the fuel is dry. The higher the amount of heat required for a fire to persist and propagate, the greater the moisture content in the fuel. Moisture within the fuel necessitates heat for evaporation, and wildfires promptly react to fluctuations in the moisture levels of the combustible material.

How Homes Catch Fire in Wildfires

Wildfires are propelled upwards by convection, whereas the sparks and ashes that ascend from that bonfire represent the warmth emanating from a bonfire or hearth. Radiation and convection serve as the two methods through which wildfires transmit heat from the fire to your residence. Safeguarding your home from wildfires is a crucial factor in countering this rapid spread over the terrain.

Research has shown that the structure of the touch actually cannot ignite a typical wood-sided home from a wildland fire if it is survived 33 feet away from the flames. In fact, case studies and actual fire simulations have demonstrated that it is impossible for the radiant heat from a wildland fire to ignite a home that is more than 100 feet away. The amount of radiant heat transfer from the fire to your home depends on how far the fire is from the structure and how long it stays there.

Red house
Figure 4. Wildfires cannot burn where there is no fuel.

Agitators and Fresh Fire Initiations

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The weakest spot of the residence, a fresh fire can start here and firebrands can touch down. The region within 3 feet of the residence is the most crucial. These flames then have the potential to ignite the residence and set fire to flammable plants near the residence. They can also land in defective eave and roof openings and enter attics through fissures in the foundation, onto decks and porches, and be carried beneath wind-propelled embers. More so than the flames alone, it is firebrands that present the biggest risk to residences.

Figure 5. Firebrands can start new fires.

Mountains and Inclines

The fire intensifies as it burns uphill. The heat follows the slopes of the land similar to how a fireplace acts on the chimney. This drives off moisture from the fuel and heats it, causing it to ignite quicker. This occurs because the flames of the fire move closer to the unburned fuel in front of it as the fire moves uphill. Wildfires move more uphill than on ground level. (Figure 6)

Figure 6. Fires move faster uphill.

In order to safeguard these residences from the extra warmth, it is necessary to establish an extra distance for fuel downwards. Residences situated at the summit of a steep incline are at a higher risk of wildfires as they experience greater heat exposure for an extended duration compared to what they would encounter on level ground.

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