In just seconds, a spark or even the sun's heat alone can set off an inferno. Wildfires spread quickly, consuming thick, dried-out vegetation and almost everything else in their path. What was once a forest becomes a virtual powder keg of untapped fuel. In a seemingly instantaneous burst, a wildfire overtakes thousands of acres of surrounding land, threatening the homes and lives of many in the vicinity.
An average of 5 million acres burn every year in the United States, causing millions of dollars in damage. Once a fire begins, it can spread at a rate of up to 14.29 miles per hour (23 kph), consuming everything in its path. As a fire spreads over brush and trees, it may take on a life of its own -- finding ways to keep itself alive, even spawning smaller fires by throwing embers miles away.
After combustion occurs and a fire begins to burn, three factors control how the fire spreads. Depending on these factors, a fire can quickly fizzle or turn into a raging blaze that scorches thousands of acres. These three factors are:
Wildfires spread based on the type and quantity of fuel that surrounds them. Fuel can include everything from trees, underbrush and dry grass to homes. The amount of flammable material that surrounds a fire is referred to as the fuel load. Fuel load is measured by the amount of available fuel per unit area, usually tons per acre. A small fuel load will cause a fire to burn and spread slowly, with a low intensity. If there is a lot of fuel, the fire will burn more intensely, causing it to spread faster. The faster it heats up the material around it, the faster those materials can ignite.
Because vegetation is the primary fuel for wildfires, the Federal Emergency Management Agency (FEMA) recommends a minimum 30-foot safety zone around your home. You should:
- Limit the number and size of plants within this zone.
- Replace highly flammable species with less flammable vegetation.
- Limb trees from their base up to about 15 feet up the tree.
- Remove any climbing vines or espalier attached to your home.
- Cut grass and prune trees and shrubs in this area regularly.
- Remove plant debris such as broken limbs and fallen leaves.
A second zone, extending to 100 feet from the house, is also suggested. In this zone, you should lower the volume of vegetation and replace highly flammable trees and shrubbery with less flammable varieties.
Wildfires and Wind
Landscape foliage isn't the only culprit to be found around your home. You should also consider what your house is made of and any combustible items you store nearby. If you live in an area that has a history of wildfire activity, your home may already be outfitted with fire-retardant materials. For example, a slate or metal roof is much better than regular shingles. Check any outside storage closets or buildings for flammable items like paint, kerosene, gasoline or propane and move them 10 to 15 feet away from your home or any other structures. This includes that gas grill near your deck.
Temperature has a direct effect on the sparking of wildfires, because heat is one of the three pillars of the fire triangle. Sticks, trees and underbrush on the ground receive radiant heat from the sun, which heats and dries potential fuels. Warmer temperatures allow for fuels to ignite and burn faster, adding to the rate at which a wildfire spreads. For this reason, wildfires tend to rage in the afternoon, when temperatures are at their hottest.
Wind probably has the biggest impact on a wildfire's behavior. It is also the most unpredictable factor. Winds supply the fire with additional oxygen, provide even more dry potential fuel and push the fire across the land at a faster rate.
The stronger the wind blows, the faster the fire spreads. The fire generates winds of its own that are as much as 10 times faster than the ambient wind. It can even throw embers into the air and create additional fires, called spotting. Wind can also change the direction of the fire, and gusts can raise the fire into the trees, creating a crown fire. Obviously, you can't do anything to change the weather, but you can be aware of it. If a wildfire is in your area, you will want to watch the weather and note any changes in wind direction or speed or humidity. When the humidity is low, meaning that there is a low amount of water vapor in the air, wildfires are more likely to start. The higher the humidity, the less likely the fuel is to dry and ignite.
Wildfires and Topography
Another big influence on wildfire behavior is the lay of the land, or topography. Although it remains virtually unchanged, unlike fuel and weather, topography can either aid or hinder wildfire progression. The most important factor in topography as it relates to wildfire is slope.
Unlike humans, fires usually travel uphill much faster than downhill. The steeper the slope, the faster the fire travels. Fires travel in the direction of the ambient wind, which usually flows uphill. Additionally, the fire is able to preheat the fuel further up the hill because the smoke and heat are rising in that direction. Once the fire has reached the top of a hill, it must struggle to come back down because it is not able to preheat the downhill fuel. So, if you live on a hill you will want to follow the steps listed previously, making sure that your zone covers the downhill side of your property. Furthermore, according to FEMA, you should extend the safety zone beyond the minimum 30 feet. Remember, the idea is to interrupt the fuel source so the fire cannot spread.
Another thing you should do, whether you're in the vicinity of wildfire activity or not, is have an evacuation plan. In the event of a wildfire, this plan should not only include getting out of your house - make sure you have fire ladders for upper floors - but also an escape route with alternates just incase any roads are blocked-off.