Understanding Wildfires

A single spark or the sun's intense heat can ignite a devastating wildfire in moments. The fire quickly spreads, consuming dry plants and everything in its path. What was once a lush forest is transformed into a volatile source of fuel. Within seconds, the wildfire can engulf thousands of acres, endangering both homes and lives nearby.

In the U.S., an average of five million acres are scorched by wildfires annually, inflicting damage worth millions of dollars. Once a fire is sparked, it can race across land at speeds up to 14.29 miles per hour (23 kph), wiping out everything in its wake. As the blaze moves over vegetation, it may seem to take on a life of its own, finding new ways to spread and even starting secondary fires by sending embers miles away. This article delves into the life cycle of wildfires—how they begin, grow, and ultimately die.

On a scorching summer day, when drought conditions are at their peak, a tiny spark—like one from a train wheel hitting the track—can ignite a devastating wildfire. While some fires occur naturally, triggered by the heat of the sun or a lightning strike, most wildfires are caused by human negligence.

The most common causes of wildfires include:

Every material has a temperature at which it will catch fire, known as its flash point. For wood, this flash point is 572 degrees Fahrenheit (300°C). When wood reaches this temperature, it releases hydrocarbon gases, which mix with oxygen in the air, combust, and create fire.

For ignition and combustion to occur, three elements are necessary: fuel to burn, oxygen to support the fire, and a heat source to raise the fuel to its ignition point. These elements form the fire triangle. Firefighters often refer to this concept when battling a blaze. By removing any one of the triangle's elements, they can control and eventually extinguish the fire.

Once combustion occurs and a fire ignites, several factors influence how the fire spreads. These key factors include fuel, weather, and topography. Depending on these elements, a fire might either die down quickly or escalate into a devastating inferno that consumes thousands of acres.

Fuel Loads

Wildfires spread based on the type and amount of fuel around them. Fuel can range from trees, underbrush, and dry grass fields to buildings. The total flammable material surrounding a fire is called the fuel load. Fuel load is typically measured in terms of the weight of available fuel per unit area, often expressed as tons per acre.

A fire with a small fuel load will burn slowly and with low intensity. Conversely, a large fuel load leads to a more intense fire, causing it to spread more rapidly. The higher the heat produced, the faster surrounding materials can catch fire. Additionally, the dryness of the fuel greatly affects fire behavior. When fuel is dry, it burns more quickly, making the fire harder to control.

Here are the fundamental fuel characteristics that determine how it impacts a fire:

Small fuel materials, also known as flashy fuels, such as dry grass, pine needles, fallen leaves, twigs, and dead brush, burn more rapidly than larger logs or stumps (which is why kindling is used to start a fire instead of logs). Chemically, some fuels take longer to catch fire than others. However, in a wildfire, where most of the fuel consists of similar materials, the key factor in ignition time is the fuel’s surface area relative to its volume. A twig, with a surface area nearly equal to its volume, ignites quickly. In contrast, a tree, with far more volume than surface area, requires more time to heat up before igniting.

As the fire advances, it dries out the surrounding material—heat and smoke from the fire cause the moisture in nearby fuels to evaporate. This makes the fuel easier to ignite once the fire reaches it. Fuels spaced farther apart will dry out faster than tightly packed fuels, as more oxygen can reach the dispersed fuel. Tightly packed fuels retain more moisture, which absorbs the fire’s heat and delays ignition.

Weather's Role in Wildfires

Weather significantly influences the ignition, expansion, and extinguishing of wildfires. Drought conditions create highly favorable circumstances for wildfires, and winds accelerate their spread—weather conditions can make the fire travel faster, covering more land, while also complicating efforts to contain it. Three weather factors that can impact wildfires are:

As previously mentioned, temperature plays a key role in wildfire ignition because heat is one of the three essential elements of the fire triangle. The sticks, trees, and underbrush on the ground absorb radiant heat from the sun, which dries out potential fuels. Higher temperatures allow these fuels to ignite and burn more rapidly, accelerating the wildfire's spread. Consequently, wildfires are most intense during the afternoon when temperatures peak.

Wind likely has the greatest impact on a wildfire's behavior and is also the most unpredictable factor. Winds provide the fire with extra oxygen, further dry out potential fuel, and push the fire forward, making it spread more quickly.

Dr. Terry Clark, a senior scientist at the National Center for Atmospheric Research, has developed a computer model that illustrates how winds behave on a small scale. Since 1991, he has been adapting this model to incorporate wildfire dynamics, including fuel and heat exchange between fires and the atmosphere.

"We focus on what’s known as coupled fire-atmosphere dynamics, where the fire and the atmosphere interact with one another," Clark explained. "Our research investigates how fires interact with their environment, and through our modeling, we’ve gained insights into fire spread and behavior."

Clark's findings reveal that wind doesn’t just influence fire behavior—fires themselves can generate wind patterns. When a fire creates its own weather systems, these patterns can, in turn, affect how the fire spreads. Intense wildfires can produce winds, known as fire whirls, which resemble tornadoes and are formed by the vortices created by the fire's heat. When these horizontal vortices shift to a vertical orientation, fire whirls are created. These powerful whirlwinds can hurl burning logs and debris over great distances.

"Another way to tilt vorticity is without forming fire whirls, creating what are called hairpin vortices or forward bursts," Clark explained. "These are commonly seen in crown fires [fires in the treetops], which create flames racing up hillsides. Forward bursts can reach widths of 20 meters (66 feet) and travel as far as 100 meters (328 feet) at speeds of up to 100 mph (161 kph). These bursts scorch the earth and lead to the fire spreading further."

The stronger the wind, the faster the fire will spread. The fire can generate winds up to 10 times faster than the surrounding ambient wind. These winds can even carry embers through the air, sparking new fires in a phenomenon known as spotting. Wind can also shift the fire's direction, and gusts can push the flames into the treetops, igniting a crown fire.

While wind can propel a fire forward, moisture has the opposite effect. Moisture, whether in the form of humidity or precipitation, helps to slow the fire down and reduce its intensity. Fuels with high moisture content are more difficult to ignite because the moisture absorbs the fire's heat. Wildfires are more likely to occur when the humidity is low, meaning there's less water vapor in the air. The higher the humidity, the less likely it is for the fuel to dry out and catch fire.

Precipitation plays a key role in wildfire prevention by increasing moisture levels in the air and in potential fuels. When the atmosphere becomes saturated with moisture, it often falls as rain. This rain increases the moisture in fuels, making them less flammable and decreasing the likelihood of a wildfire igniting.

Fire on the Mountain

The third major factor influencing wildfire behavior is topography, or the physical layout of the land. While topography remains relatively unchanged, unlike fuel and weather conditions, it can either facilitate or hinder the spread of a wildfire. The most critical topographical feature in wildfire behavior is slope.

Unlike humans, fires tend to move uphill much more quickly than downhill. The steeper the incline, the faster the fire spreads. Fires generally travel in the direction of the prevailing wind, which often blows uphill. Additionally, as the smoke and heat rise, they preheat the fuel further up the slope, aiding the fire's spread. On the other hand, once the fire reaches the top of a hill, it faces difficulty when moving downhill because it cannot preheat the downhill fuel as effectively as it can uphill.

Dr. Clark acknowledges that there are cases where fires travel slower uphill, though this is not typical. Winds can oppose a fire's efforts to climb a slope.

"It depends on which way the wind's blowing," he said. "For instance, I studied a case in Australia where the wind was pushing the fire downhill, away from the slope, until a front moved through, at which point the fire began moving uphill."

Beyond the immediate damage fires cause, they also create long-lasting issues that may not be apparent until months later. When fires strip vegetation from a mountain or hill, the organic material in the soil can be weakened, preventing water from being absorbed. This can lead to severe erosion, triggering dangerous debris flows.

A notable instance occurred after a wildfire in July 1994 that burned approximately 2,000 acres of forest and underbrush on the steep slopes of Storm King Mountain near Glenwood Springs, Colorado. Two months later, heavy rainfall triggered debris flows that dumped mud, rocks, and other debris across a 3-mile stretch of Interstate 70. According to the United States Geological Survey, the debris flows engulfed 30 cars, two of which were swept into the Colorado River.

While wildfires are often seen as destructive, many actually play a vital role in the ecosystem. Some fires clear the underbrush of forests, which can help prevent much larger, more dangerous fires that might occur if this brush were left to accumulate for too long. Wildfires also benefit plant life by curbing the spread of disease, returning nutrients from the burned plants to the soil, and encouraging fresh growth.

Battling the Blaze

Imagine being trapped inside an oven, wearing heavy gear, with smoke filling your lungs. This is a glimpse into the perilous reality that firefighters face when battling a raging wildfire. Every year, thousands of firefighters risk their lives to confront these relentless fires. The elite ground-based firefighters are divided into two main categories:

Along with constructing firebreaks and applying water and fire retardant, ground crews may also deploy backfires. Backfires are intentionally set fires that move toward the active wildfire. The primary purpose of igniting a backfire is to eliminate potential fuel that the advancing wildfire could consume.

While Hotshots, Smokejumpers, and other ground teams battle the fire on the ground, they receive significant support from the air. Air tankers are commonly used to drop large amounts of water and retardant on the flames. The red substance you often see falling from planes and helicopters is a chemical retardant mixed with phosphate fertilizer, which helps to cool and slow the fire’s progress.

Helicopters also play a key role in fighting wildfires by attacking the flames from above. With buckets capable of holding hundreds of gallons of water, these helicopters fly over the fire and release water bombs. In addition, they are crucial for transporting firefighters to and from the fire zones.

Wildfires are immense natural forces that can continue burning as long as they have access to fuel, oxygen, and heat. Firefighters work tirelessly to eliminate at least one side of the fire triangle to halt its spread and reduce the damage it causes.