Wildfire Behavior and Mitigation

Learn all about natural wildfires, including their causes and how to protect your home from these weather realated disasters.

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Each year, millions of acres in the United States and Canada are scorched by wildfires. In recent years, the wildfire problem has gotten significantly worse as the fires have become larger, more intense, and more destructive. Fires have occurred in places previously believed safe. Lives have been lost, and the costs in damage and in fighting the fires have skyrocketed.

By definition, a wildfire or wildland fire is an uncontrolled blaze that starts in vegetation and, at least initially, doesn’t involve structures. Wildfires become a great threat when they cross the so-called “wildland-urban interface,” thus endangering inhabited areas. In the summer of 2018, the Carr Fire in Northern California roared into parts of Redding, killing eight people and destroying more than 1,000 homes. In 2016, Fort McMurray in Alberta, Canada, was devastated by a wildfire — the costliest natural disaster in Canadian history. In November 2016, a wildfire roared through Gatlinburg, Tennessee, killing 14 people and damaging or destroying some 2,400 buildings.

There’s no longer a predictable “fire season,” as wildfires can occur somewhere at any time. Major fires have occurred in the Great Plains during late winter, when strong winds have accompanied warm and dry conditions. In March 2017, one fire burned more than 800,000 acres in Oklahoma. For southern Florida, the greatest fire danger is in early spring, and 2017 went down as one of the worst fire seasons ever. In summer, the risk moves westward and northward. Historic fires have occurred recently in the Rockies from Montana to Arizona. California has also seen its worst fires during the past two years. The Mendocino Complex Fire in 2018 was the largest ever in the state’s history, with more than 400,000 acres burned. Wildfires are also common in Canada during the summer, typically burning millions of acres. In 2017, British Columbia saw its worst fire season ever, with nearly 3 million acres lost, and the summer of 2018 again brought devastating wildfires. Alaska has a similar summer fire season, and a record 5 million acres were burned in 2015.

Wildfire Behavior

The starting fuel for a wildfire (bushes, shrubs, grass, fallen leaves, etc.) must be dry to burn. A lack of precipitation even for just a few days may be enough, but typically we think of longer-term dryness or drought conditions preceding major fires. Seasonal dryness is common in some areas — in the summer out West and in the winter in southern Florida. Sporadic drought can occur in other regions. Considering the size of North America and the size of typical weather systems, drought will likely be occurring somewhere at any given time.

Before people were involved, wildfires were primarily caused by lightning strikes. And many fires in remote areas are still caused by lightning. But where people can be found, fires will be started. Often these are debris burns that get out of hand. Accidents, carelessness, and sometimes even arson can be contributing factors. Regardless of ignition sources, when it’s dry, assume fires will get started.

The basic concern with wildfires is how to control them once they get started. The vast majority of wildfires are easily contained by local fire departments. The ones that can’t be controlled early on become menacing. There are two major factors that affect the ability to control a wildfire.

The first is wind. Strong winds will drive a fire forward at a great speed. Besides the actual forward flank of the fire, burning debris and embers can be blown downwind, starting “spot fires” well ahead of the main line, sometimes a mile or more. This causes the fire to seemingly jump forward. When conditions are at their worst with high winds and dry fuels, wildfires are unstoppable.

The second factor is the location of the fire, especially in terms of terrain. How difficult is it to get to the fire and fight it? Flat ground is typically easy. But once you get into hilly or mountainous terrain, problems arise in terms of getting in people and equipment, and there are greater physical challenges in the actual fighting of the fire. In addition, airflow patterns change. Wind direction can be affected and wind speeds increased on a local scale.

Not all wildfires are alike. They vary because of differences in fuels, weather conditions, and topography. Surface fires burn low, mainly in grasslands or in woods below the main canopy in the leaf litter and lower plants. A crown fire spreads rapidly through the tops of trees or shrubs, and often outraces the surface fire below. Along the fire front, flames can tower hundreds of feet into the sky.

There’s also a difference between “wind-driven” and “plume-dominated” fires. Wind-driven fires are typically low, fast-moving fires that proceed in the direction of the prevailing large-scale wind. The wind dominates the fire. “Plume-dominated” fires have towering convective columns over them, sometimes rising tens of thousands of feet into the atmosphere. The fire spreads erratically. Such fires can produce their own weather. Updrafts and downdrafts generate swirling surface winds when they hit the ground. A cloud, called a “pyrocumulus,” can develop over the fire and can even sometimes produce rain.

One common phenomenon is the fire whirl. Air, intensely heated by the fire, rises and starts to spin. You can observe small fire whirls in a campfire. The heat generated by a wildfire magnifies this effect, and large fire whirls can occur. A rapidly spinning column of air filled with burning debris and extending sometimes hundreds of feet into the sky can exit the fire zone and become a real threat. The Carr Fire produced a monster fire whirl that generated winds estimated over 140 mph near Redding — the equivalent of an EF3 tornado. Although these are sometimes called “fire tornadoes” or “firenadoes,” they’re more similar to dust devils, which are generated by strong surface heating. (True tornadoes form underneath cumulonimbus clouds through a totally different process.)

Control and Containment

Putting out a fire this size is physically impossible. Instead, we try to contain conflagrations to keep them from burning more area. Accomplish this, and the fire will either burn itself out (from a lack of fuel) or rain will put it out (although this may take months). Containment is accomplished by building something called a “fire line,” a type of fire break. A fire break is a strip or piece of land where there’s no fuel (no vegetation) for a fire. The wider the fire break, the more effective it will be in stopping a fire’s forward progress. Fire breaks can be natural, such as a river or lake or a span of rock, or they can be man-made, such as a road. A fire line is constructed by physically removing vegetation both by bulldozer and by hand. Firefighters also use backfires, deliberately setting controllable blazes that will use up potential fuel for the wildfire. In addition, water and fire retardants are often dropped from aircraft to temporarily slow the fire and lessen its intensity.

In recent years, there’s been a major shift in how we view wildfires and how best to control them. For many decades, all wildfires were considered destructive. Then, biologists began to realize that fire was a natural part of ecosystem development, and that fires needed to occur on a regular basis. Fires would be followed by regrowth, and the process would repeat. After we grasped that fire was one of nature’s ways of recycling, we decided not to put out every fire. We understood that preventing regular fires encouraged a buildup of fuels, thereby leading to more intense blazes that caused more damage. We began to “fight fire with fire” — controlled burns. On calm days when fires can be easily contained, fires were deliberately set to lower fuel loads.

Home Protection

The location of your home is critical. Are wildfires a threat in your area? Even if they haven’t occurred in the past, is there fuel (vegetation) available if dry conditions develop? Are there buffer zones — natural or man-made fire breaks, such as rivers, roads, etc. — protecting you? Are there roads that would be accessible if a fire threatens, both to aid your escape and to allow firefighters in? Is there a nearby water supply?

Your dwelling structure should be as fireproof as possible. The exterior walls, including door and window frames, should be non-synthetic; stucco, masonry, cement shingles, concrete, stone, etc. are recommended. Walls should be thick and insulated enough to retard heat penetration. The roof especially should be made of noncombustible materials; metal roofs, concrete shingles and tile, slate shingles, clay tiles, etc. are best. Wood shake roofs need to be treated to fireproof them. Remember, it’s the “attack from above,” the airborne embers and debris ahead of the main fire, that pose a great threat to structures.

Homeowners can take further steps to reduce the risk of wildfire damage. “Fire mitigation” is routinely practiced in the western United States and Canada, but procedures developed there can be utilized anywhere. The key is to develop a “defensible space” around your home or other structure. A defensible space means that firefighters could safely occupy this area while battling the blaze. It will also greatly reduce, but not eliminate, the risk of a wildfire engulfing your home.

Remember, vegetation is the fuel for a wildfire. Although your home may be more aesthetically pleasing when surrounded by shrubs and trees, in a wildfire situation, they can set your house on fire. To be on the safe side, remove flammable vegetation near your house. As for trees, there should be no branches hanging above the structure, and low and dead branches should be trimmed. Further away from your home, thin out lower vegetation and remove low or dead tree limbs. The goal is to stop or at least slow any approaching fire along the ground, and to keep it from crowning. Any debris produced by your work should be disposed of.

Forecasts and Warnings

Knowing that weather is a critical factor in wildfires, meteorologists working with fire-fighting agencies have developed specific forecasts for wildfire potential. In the U.S., the National Significant Wildland Fire Potential Outlook is based on long-range forecasts of dry conditions. Natural Resources Canada also produces a similar product. Local authorities may issue “outdoor burning bans,” restricting the use of fire by residents during particularly dry spells. For notices of immediate hazards, look to the National Weather Service’s “Red Flag Warnings” indicating acute wildfire risk. Similarly, Environment Canada may issue an “Extreme Fire Danger” forecast.

Scientists believe the wildfire problem will get worse in the future. Heat waves and droughts are becoming more common. In turn, wildfires are becoming larger and more intense, and are occurring in areas formerly thought safe.

For 30-plus years, Ed Brotak taught thousands of college students about weather, and helped many of them pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife (also a meteorologist) and his two daughters.

  • Updated on Apr 29, 2022
  • Originally Published on Dec 4, 2018
Tagged with: drought, fire, fire safety, wildfires
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