Wildfires, Explained

What is a Wildfire? / Defining a Wildfire

The definition of a wildfire is a large outdoor fire that includes burning vegetation in wildland or rural areas, but may also include or spread into communities with structure and infrastructure.

There are two related terms that help answer the question “what is a wildfire” – wildland fire and wildland urban interface fires, known as WUI fires for short. Some facts on both types of wildfires - a WUI fire is probably the type of wildfire most people are familiar with and is also the type that threatens and burns homes. A wildland fire, on the other hand, mostly burns in the absence of humans. For instance, on US Forest Service land or Bureau of Land Management land – places where humans will largely not be affected. However, a wildland fire can move into an urban area, thus becoming a WUI fire.

Forest fires are a specific type of wildland fire that involve a forest, but they are not the only type of wildland fires, which can also move through dry grass or brush. A forest fire can also become a crown fire – a specific type of forest fire that is very high intensity and one where an entire tree is on fire.

How does a wildfire burn?

In a typical wildfire in North America, a wildfire burns because the heat from the flames heats up the wood or other type of fuel - in this case fuel could be anything flammable – trees, brush, leaves, grass, etc. That fuel is then chemically converted into gaseous fuels that then feed the flames – sustaining the flames, which then heat up and ignite unburnt fuels, allowing them to spread and continue burning.

However, a wildfire can burn differently, depending on the region is located in around the world due to differentiations in ecosystems and climates. One example is in Indonesia, where there are massive peatlands, and when the peatlands catch fire, they burn in a smoldering combustion mode. Smoldering is defined as a flameless combustion mode, similar to the way a cigarette burns – glowing hot, but still burning. Peatlands smoldering fires create a thick, toxic haze that is harmful to human health, and due to the greenhouse gases the fires release into the atmosphere, this type of wildfire also contributes to climate change.

Are wildfires natural?

Wildfires can be both natural and human made.

For most wildland areas, there is a natural cycle of wildfire occurrence, and it is natural for there to be wildfires every so often. But, in the natural realm these fires would be low intensity wildfires that would remove excess fuel and prevent a large wildfire from happening. These smaller, natural wildfires also help fertilize the ground, which leads to renewal of the ecosystem.

However, human intervention has disrupted this cycle, creating what researchers call “the fire paradox.” The fire paradox occurs when humans try to extinguish and suppress any sort of wildfire, but putting out the smaller fires can cause a buildup of fuel. Because of that buildup of fuel, once a wildfire does occur it can turn into a large fire that is more difficult to suppress.

In some ways, this type of wildfire will be a naturally caused wildfire, but it will be unnaturally severe, and possibly turn into a fire that cannot be suppressed.

Generally speaking, the vast majority of wildfires are caused by humans, and are therefore considered unnatural. For natural causes, there are some fires that are started by lightning, but that is generally uncommon.

Many indigenous groups in North America used fire as part of the way they managed the land. “These groups were able to do this in a sustainable fashion for a long period of time, which could allow these fires to be referred to as natural,” said James Urban, Assistant Professor of Fire Protection Engineering at WPI.

What types of scientists study wildfires?

There are many different types of scientists who study wildfires and their impacts. The main types of scientists are fire ecologists, fire engineers, climate scientists, social scientists, and public health scientists.

Urban’s scientific research falls under the fire engineering umbrella. His research focuses on the physical processes of wildfire behavior – how the fires ignite and burn, with the ultimate goal of developing and enabling modeling capabilities. Those capabilities could be simple equations with pen and paper, or large computational models to predict wildfire behavior or develop strategies to prevent against the harmful effects of wildfires. The fire engineering research is motivated by WUI fires primarily, with the goal of preventing structure loss and damage and loss of life.

There is also a subset of fire protection engineers in social science that focuses on how humans evacuate from wildfires. It is a multi-disciplinary field involving the engineering and designing of roads, communication equipment, and protocols for notifying people. Social scientists specializing in fire protection also study how to communicate effectively and notify people during a wildfire.

The major group that studies wildfires in terms of the number of people is fire ecologists. They study how wildfires interplay with the local ecology, which factors in the effects of the local weather, plant species, ecosystems, including pests or different species, and how they influence the characteristics of a wildfire.

While a fire engineer looks at the physics of a fire, fire ecologists look at the ecosystem and how it plays a role in the fire.

Climate scientists study how fire may change the fire risk or its behavior, in terms of the impacts of climate change on these parameters and processes. In some cases, very intense fires might also influence local weather, and climate scientists will also study that aspect.

How do scientists predict, study, and measure wildfires?

There are many ways, but at WPI researchers use experiments ranging from a very small scale -for example, a single tree branch, to the very large scale – researchers can burn one to three trees at a time in the university’s large-scale laboratory.

However, sometimes even the university’s large lab is not big enough, and for those situations WPI researchers including Professor Albert Simeoni conducts larger scale experiments. These experiments include prescribed burns, or can involve a large wind tunnel that is brought to external sites.

Modeling is also used to test theories about how things burn or how fires spread. It can also be used to determine possible ways in which fires spread. There are different types of modeling – physical modeling and data assimilation modeling. For data assimilation modeling, researchers look at past fire behavior, to create a more data-driven model.

“Predicting” fires can be tricky because there so many variables at play. Instead, the researchers say, they assess the exposure to wildfires, "if you’re truly trying to predict all the physical aspects of a wildfire, you can zoom in and look at a single pine needle that’s burning, and then the fire line is miles long, and you also have the local weather affecting the wildfire – to account for all of those variables is basically not possible,” said Urban. “And so, you have to simplify it enough to get you something that’s informative. We can’t predict the future, so instead we have to consider the possibilities of what can happen, and use that to inform the public and help policy makers and local leaders make better choices.”

Who fights wildfires?

The people who fight wildfires include urban firefighters and specialized wildland firefighters.

Fighting wildfires requires a different approach. In urban fires, the general approach is to control and eventually suppress the fire, primarily using water or other suppression agents.

In a small wildfire, this same method may work before the fire grows too big, but once the wildfire is larger, the approach of suppressing the fire with just a water attack is no longer possible. Instead, the goal is containment.

To contain a wildfire, firefighters create a perimeter around the fire beyond which it can’t spread. The local geography or human made features can help firefighters. For example, a large highway or a large body of water can protect firefighters and the nearby community. Fire also doesn’t spread very well downhill, so if firefighters remove some fuel on a downhill feature, that may prevent the fire from spreading past that point.

Fire services also use planes or helicopters to drop water or fire retardant to subdue the flames and prevent the fire from spreading and slow it down.

There are also two types of wildland firefighters – hotshot crews and smoke jumpers. Hotshot crews typically build, fortify, and defend fire breaks using chainsaws and hand tools. They will also reignite a fire that burns into the approaching fire. “That is basically using a prescribed fire in real time,” says Urban, “which can be effective, but also very dangerous.” When there isn’t an active fire going on, hotshot crews will be involved in maintaining fire breaks or thinning fuel in various strategic areas.

Smoke jumpers perform similar actions to hotshot crews, but they’re deployed in remote locations in smaller groups, and they deploy with parachutes. Smoke jumpers also receive tools and supplies through parachute drops.

In California, prison inmates also fight wildfires. However, they are paid substantially less and ethical questions have been raised about having inmates do this work.

Right now, there is also a labor shortage of wildland firefighters.

In April 2022, Urban co-advised a WPI Major Qualifying Project that built a robot to perform some of the functions of a hotshot crew, to help address the labor shortage and increase firefighter safety. The project was inspired by what are called fire goats – goats that eat vegetation to reduce the potential fuel for wildfires.

Where do forest fires occur?

In short, forest fires occur everywhere. The places where wildfires occur depend on the weather and local ecosystem. In areas that are prone to wildfires, plants have adapted to their occurrence. In the United States, wildfires happen more frequently in the western part of the country, but they can happen anywhere. Generally, wildfires are more intense and more frequent on the west coast.

When are wildfires most common?

Wildfires are most common when a region is in what is called a fire season. That is when conditions are most conducive to fire. Recently, fire seasons have been growing to be longer in duration.

Local weather is also an important factor in when wildfires are most common. In California – the Diablo winds in the northern part of the state and the Santa Ana winds in the southern part of the state make for conditions that are conducive to wildfire spread, because the winds are very fast, warm, and relatively dry.

Winds in general promote wildfire spread, not only by spreading the flames, but also because strong wind can cause power lines to hit together, which leads to sparks shooting down on to the ground, igniting brush or grass. Wildfires that involve power lines in this way tend to be some of the largest fires.

Why do wildfires happen?

Wildfires happen when an ignition source or heat source ignites fuel. “When the conditions are dry and there are high winds, wildlands and WUI areas become like a tinder box, and all you need is a heat source to ignite them” said Urban.

In addition to power lines, fireworks, overheating catalytic converters touching tall grass, and even golf clubs scratching on rocks can cause wildfires. In short, anything that has heat can lead to a wildfire.

Why are wildfires getting worse?

Wildfires are getting worse because of the fire paradox. In the United States, there is a large effort to suppress any type of wildfire, which leads to the build up of fuels, and thus when there is a fire, it can be large and severe.

The effects of wildfires are also getting worse because more people are moving into WUI areas. Because of that, more people are experiencing wildfires. In reality, by some measures, there aren’t more fires happening, but instead, there are more people who are being affected by them.

Who is affected by wildfires?

People can be affected by wildfires in a variety of ways – they can have their homes burned, and physical harm can also be inflicted upon them by wildfires.

A large number of people can be displaced by wildfires – through losing their homes, or being forced to move to another area.

Smoke affects a much wider area of people. In fact, in California, schools and offices have wildfire pollution days, similarly to the way other parts of the country have snow days.

Firefighters are also affected by wildfires. “Going outside during a wildfire is akin to smoking cigarettes or breathing in second-hand smoke,” says Urban. “Firefighters generally don’t wear masks or breathing protection while doing the manual labor of creating a firebreak, because it is such difficult work and they are breathing heavily.” Due to these factors, wildland firefighters experience smoke significantly.

Are wildfires increasing?

The number of wildfires is not increasing, but the total amount of area burned during a wildfire is increasing. That’s because when wildfires do occur, there are more high-intensity and high-impact fires. Thus, it is not just the number of the fires, but the number of fires that are having an impact.

That impact is also being multiplied because more people are moving into WUI areas, and the presence of people there means more fires could be ignited through daily human activity.

How do wildfires start and spread?

The vast majority of wildfires are started and spread by humans. Fireworks, power lines, arson, runaway campfires, overheating catalytic converters, chains dragging behind vehicles, and people discarding cigarettes can all cause wildfires. Natural causes include lightning and rekindling. Rekindling occurs when a fire wasn’t fully put out, and high wind causes the flames to flare up again.

What happens during a wildfire?

The physical process of what happens during a wildfire is that heat from a fire causes the solid fuels to release flammable gases, which then feed the flames. The flames also heat and ignite fuels that haven’t started to burn, allowing the fire to spread. Wind can cause the flames to be closer to the unburned fuel – making the process happen more rapidly.

Wildfires can also spread through firebrand spotting. Firebrands are burning pieces of wood – small embers that the wind carries substantial distances. Those distances could be anywhere from a few feet to as far as miles away from the fire. Those firebrands can then start a new, smaller fire called a “spot fire.”

The spot fire can grow in size and threaten different areas, or be absorbed by the fire front – allowing the larger fire that spawned the firebrand to spread more rapidly. Firebrands have different effects in wildland and WUI areas. “In wildland fires, firebrands make the fire burn faster, but in WUI fires, they are the major reason homes burn,” said Urban.

Most older homes weren’t built with firebrands in mind. There are some modern construction methods that are better suited to protecting against firebrands, but there are many vulnerable parts of homes, particularly wooden decks. The type of landscaping can also play a large role in how a fire spreads to a home.

During a wildfire, civilians also need to be evacuated from their homes, while firefighters battle the flames. “Oftentimes, people will want to stay and fight the fire to protect their homes, but that can put the fire service in a tough position,” said Urban. “Firefighters want to remove people because the presence of civilians can make it dangerous for both the people whose homes are being threatened by the fire, as well as for the firefighters.”

To make the evacuation safe and orderly, evacuation orders are staggered, one neighborhood at a time, so that there isn’t a traffic jam preventing people from leaving the area. A large portion of the fire protection field is focused on how people evacuate from wildfires, as well as building fires.

What causes a wildfire? What are the causes and effects of a wildfire?

The causes of a wildfire include several different factors, and are not solely confined to the ignition source. Drought conditions, high winds, and the fire paradox all contribute to causing wildfires to be more likely to happen and to be more severe.

The effects of wildfires can be devastating and disastrous – people’s lives, homes, and local ecosystems can all be lost in a wildfire. The effects on ecosystems can be wide ranging and long lasting - if a large area of wildlands is burned, animals may not return to that area.

More effects of wildfires include the smoke, which can spread across a wide area, affecting people miles away from the center of the fire, and people can also be displaced by a wildfire.

Do wildfires cause cancer?

Wildfires can increase the risk of cancer. There is an air pollutant in wildfire smoke called PM2.5 – “PM” stands for “particulate matter” and 2.5 is the size of the particles. PM2.5 are small particles of soot or unburnt fuel that are brought into the air. These particles can cause increased cancer risk in humans.

The effects of wildfires can be devastating and disastrous – people’s lives, homes, and local ecosystems can all be lost in a wildfire. The effects on ecosystems can be wide ranging and long lasting - if a large area of wildlands is burned, animals may not return to that area.

In areas affected by wildfire smoke, Urban recommends that people should use air cleaners, wear N95 masks, and reduce their time outside.

PM2.5 is also a cause for concern for anyone who uses a wood burning stove – recent studies have shown there is a similar concentration of PM2.5 during wildfire season outdoors, as there is during the winter inside homes that use wood burning stoves. Therefore, Urban says, “both wildfires and wood burning stoves have been shown to have similar effects on increasing cancer risks.”

Will wildfires ever stop?

Wildfires will not ever stop because ecosystems depend on wildfire. If wildfires do stop, that would mean there would no vegetation left, and therefore no ecosystem either.

“The goal is to get back to a scenario where we have a more natural wildfire regimen – one that includes low intensity fires that reduce the fuel load, so we don’t have large, destructive fires,” said Urban. “The reality people need to accept if they’re moving into the WUI is that those areas are adapted to wildfires.” Therefore, wildfires will happen there on a regular basis.

How can wildfires be stopped?

In an active wildfire scenario, a wildfire can be stopped through containment - building fire breaks to contain the fire, exploiting local geography, and tactical use of fire retardants.

On a broader scale, wildfires won’t stop completely until there is no vegetation, i.e., if the ecosystem changes to the point where it no longer produces fuel. “So realistically, we need to move to low intensity fires, or other ways of reducing the fuel load to prevent high intensity wildfires,” said Urban.

How can wildfires be prevented?

Wildfires can be prevented by reducing the amount of fuel – vegetation, brush, and debris – that a fire can ignite and burn and as well as through carrying out prescribed burns.

In terms of how wildfires affect communities, there are other strategies that can help prevent wildfires as well. For instance, the “Fire Wise” program helps neighborhoods be more careful about the type of ornamental vegetation they use, and also create what researchers refer to as defensible space – an area around homes that is less conducive to wildfire spread to structures.

Wildfires can also be prevented by updating building codes to require homes to be constructed out of materials that are more resilient to wildfires and firebrands.

New communities and neighborhoods can also be developed with wildfire prevention in mind. In much of California, neighborhoods consist of low-level homes that are tightly packed together, which is problematic from a WUI fire safety point of view. To help prevent wildfires, Urban says homes should be built to be more spread out.

How does climate change affect wildfires?

Climate change does play a role in affecting wildfires, but because there are other human-caused factors that contribute to wildfires, it is sometimes difficult for researchers to differentiate between them.

“Because climate change is such a large, impactful problem, it is hard on a day-to-day basis to quantify the role climate change is playing in wildfires,” says Urban. “Whereas some of the human behavior effects on wildfires can have the same, or arguably even a larger, impact than climate change.”

However, as climate change is leading to more extreme weather events - causing longer periods of drought and more severe storms and heavier rainfalls – those extremes can also lead to more severe wildfires. While rain is often thought to help with suppressing wildfires, when rainstorms are more intense, they can spur the growth of more small vegetation, creating more fuel for fires.

How do wildfires affect the environment?

Wildfires, if they are severe, can affect the environment by damaging the local ecosystem or throwing the ecosystem out of balance. A severe wildfire can cause ripple effects on the ecosystem - if vegetation is destroyed, animals that relied on that plant for food, now don’t have that food source, and the effects grow from there.

Can wildfires cause global warming?

Wildfires can contribute to global warming because trees and plants contain carbon, which then can be released as carbon dioxide (CO2) and carbon monoxide (CO) in a fire.

Can wildfires cause landslides?

Wildfires can cause landslides when plants or trees are killed in a fire. Trees prevent erosion and absorb moisture, so without them could be heavy runoff if there is a strong storm after a wildfire.

Can reforestation happen after a wildfire?

Reforestation can happen after a wildfire – some of it would happen naturally, and human intervention would also likely play a role, specifically through people’s replanting efforts.

Some plants and trees are adapted to wildfires, and their seeds activate when a fire hits, leading to the plant spreading and growing. Eucalyptus trees are one example of this type of plant – surrounding these trees is a flammable bark that falls to the ground during a wildfire. However, eucalyptus trees also have seeds that are resistant to wildfires, thus letting them germinate and grow once the bark falls to the ground.

Wildfires can also contribute to making the soil more fertile, by putting plant minerals back into the ground.

About the Experts

James Urban is Assistant Professor of Fire Protection Engineering. His research is focused on the physical processes controlling how flammable material ignites and understanding how it burns, particularly firebrands – the small pieces of wood are embers that are carried by the wind during a wildfire and can lead to new fires, or larger fires.

Albert Simeoni is Professor and Department Head of Fire Protection Engineering. He is an internationally recognized expert in fire, wildland fires, and fire science. In addition to his academic research, Simeoni’s experience also includes more than a decade as a firefighter in France, where he led firefighting efforts and rescue operations.

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