How Do Wildfires Start and Spread?

By Kelly Butler

Wildfires are uncontrolled fires that can decimate homes and the surrounding environment. Whether you’ve witnessed them on the news or in person, their unpredictable nature makes them a threat to communities everywhere. And as the earth’s climate continues to change, the frequency and severity of wildfires will continue to increase. Keep reading to learn how wildfires start and spread, and what you can do to avoid them.

How Do Wildfires Start?

How do wildfires start naturally? Wildfires need three key elements—also known as the fire triangle—to start and eventually spread:

1. Heat (e.g., half-lit cigarettes)
2. Fuel (e.g., dry or dead vegetation)
3. Oxygen (e.g., strong winds)

Let’s start with the heat source. Wildfires ignite either by nature or by humans. How do wildfires start without humans? Lightning strikes cause most natural wildfires, but spontaneous combustion of dry fuel, such as sawdust and leaves, can occur. However, these natural ignitors only make up a small percentage of total wildfires in the United States. According to a study published in the Proceedings of the National Academy of Sciences, humans were to blame for 84% of U.S. wildfires from 1992-2012.

There are many human causes of wildfires. Take a look below at some of the most common ways people start wildfires:

• Powerlines — Strong winds and car crashes can knock down electrical lines and other related equipment. This usually causes a spark big enough to ignite a flame and spread quickly amid high winds.
• Cigarettes — Wildfires caused by tossed cigarette butts became such a problem in California that the state passed a law prohibiting smoking and vaping at state parks and beaches.
• Campfires — People can improperly extinguish campfires or build one in an illegal spot. In either case, the embers can blow from the pit and onto dead vegetation, igniting a fire.
• Arson — Sadly, some wildfires start on purpose. The Los Angeles Police Department recently detained an arson suspect believed to be responsible for triggering an uncontrollable fire in the Pacific Palisades.
• Equipment use and malfunctions — Equipment such as lawnmower blades and weed whackers can create sparks that land on dry vegetation. For example, the Zaca Fire — one of the largest wildfires in California history — started due to sparks from a metal grinder.

When it comes to fuel, wildfires thrive in dry, arid climates and drought-ridden areas where you’ll find dry, dead grasses, leaves, and trees. That’s why forests, grasslands, and wooded areas are so vulnerable to wildfires. They’re essentially a bed of burnable materials. Living vegetation can also serve as wildfire fuel. Pine trees and other evergreen trees contain flammable oils that can easily burn when exposed to a heat source.

What About Other Types of Fire Events

A wildfire is a general term for an uncontrolled fire in a natural area, but within that, there are brush fires and forest fires, depending on where and how it spreads.

• Brush fires usually burn through dry grass or shrubs, often in open areas. They typically start when someone drops a cigarette or uses equipment that throws a spark.
• Forest fires, on the other hand, burn through wooded areas filled with trees and dense vegetation. What causes forest fires? They’re often fueled by fallen branches, pine needles, dry leaves, and even the flammable oils in certain trees. Just like brush fires, many forest fires are sparked by human actions, but natural causes like lightning strikes can also play a role.

How Do Wildfires Spread?

Once a wildfire gains momentum, it doesn’t just stay in one place. It can spread in different directions, and how it moves depends on a few main ingredients—wind, terrain, and fuel.

• Wind: Wind is one of the biggest drivers of wildfire spread. It can carry flames from one patch of vegetation to another and blow hot embers far ahead of the fire front, sometimes even miles away. These embers can land on rooftops, dry grass, or wooded areas, which can start new spot fires before the main blaze arrives.
• Terrain: Terrain is another key factor. Fires usually spread faster uphill because the rising heat warms up the vegetation above, which makes it easier to catch fire. Steeper land can also boost wind speeds and help guide the fire, especially in mountain areas.
• Fuel: This includes dry grass, fallen leaves, pine needles, dead trees, and even things like fences or sheds. The drier the fuel and the more of it there is, the more energy a fire has to grow and move. Forest floors full of dry debris are like giant fire starters, and in grasslands, fire can spread quickly across open space.

When wind, fuel, and dry conditions all come together, the results can be fast-moving and dangerous.

How Fast Do Wildfires Spread?

Wildfires require heat and fuel to start, but flames need wind to gain traction. How fast do wildfires spread? The stronger the wind, the faster the wildfire expands. And since heat rises, fires move more quickly as they go uphill. Once the fire begins and spreads, it can travel at a rate of up to 14.27 miles per hour, destroying everything in its path. Strong winds can also push embers downhill toward new fuel, creating new fires.

A great example of extreme wildfire speed is the Thomas Fire. In 2017, this California wildfire spread so fast that it traveled at a rate equivalent to a football field every second.

How Quickly Can Fire Damage a House?

Now that you know how fast a fire can spread, you may wonder: How long does it take for a house to burn down? According to ready.gov, a fire can completely engulf a house in flames within five minutes. What’s scarier is that a fire can become life-threatening in just two minutes.

If you find yourself in the middle of a burning house that’s out of control, you must act fast. Here are some safety tips to help you get out of harm’s way:

• Leave valuables behind.
• Don’t stop to call 911 — just get out.
• Avoid smoke by crawling low toward the exit.
• If you can get out, yell or call 911 and stay on the line until you’re rescued.
• Stop, drop, and roll if you catch on fire.

What Causes California Wildfires?

Climate change is a huge contributor to these intense blazes in California. As the planet warms, it creates conditions conducive to wildfires: hotter temperatures, drier vegetation, and deeper droughts. These factors mean larger wildfires, as well as an earlier and longer wildfire season.

Also, more people and homes in California are dealing with wildfire risk than ever before. The wildland-urban interface (WUI) — the area where houses and wildland vegetation intersect — has become the fast-growing form of land use in the continental United States. According to U.S. Census data, California has 11.2 million people living in the WUI, which is the largest number out of all the states. That’s nearly one-third of the California population in danger!

In 2021, California experienced some of the worst wildfires in the state’s history. According to data from the State of California, wildfires destroyed over 2.5 million acres of land from nearly 9,000 incidents.

How Do Brush Fires Start?

As mentioned earlier, brush fires are a type of wildfire that burn through low-lying vegetation, like dry grass, weeds, and shrubs. They’re especially common in dry, open areas where the ground cover hasn’t seen rain in a while.

So, how do brush fires start? Most often, they’re caused by human activity. Something as small as a lit cigarette or a spark from a lawnmower can ignite dry brush and quickly turn into a fast-moving fire. Here are some of the most common causes:

• Discarded cigarettes
• Yard work and equipment
• Campfires and fire pits
• Fireworks and grilling accidents
• Vehicles

Where are Wildfires Most Common?

Wildfires can happen almost anywhere there’s dry vegetation, heat, and wind, but some regions face a much higher risk than others. In the United States, wildfires are most common in western states where hot, dry summers and drought conditions create the perfect environment for fires to ignite and spread.

Wildfire-prone states include:

• California: The state consistently leads the nation in wildfire frequency and damage. According to CAL FIRE, California saw more than 8,110 wildfires in 2024 alone, burning more than 1 million acres.
• Texas: Texas regularly experiences large-scale wildfires due to its vast open land and dry grasslands. In early 2024, the Smokehouse Creek Fire became the largest in state history, burning in excess of 1 million acres.
• Arizona, Colorado, and New Mexico: These states can face wildfires due to high temperatures, low humidity, and strong winds during fire season.
• Oregon and Washington: The Pacific Northwest is another high-risk region for forest fires due to its dense forests and dry summers.

How Hot Can a Forest Fire Get?

According to the U.S. Geological Survey, fires can reach up to 2,220°F in extreme conditions. But not all wildfires burn the same. Temperatures vary depending on factors like the type of fuel, wind speed, humidity, terrain, and whether the fire is burning on the ground, in tree canopies, or even underground.

Here’s a breakdown of common wildfire temperature ranges:

• Surface fires (burning leaves, brush, and grasses) typically reach 800°F to 1,472°F
• Crown fires (leaping through treetops) can exceed 1,800°F to 2,192°F
• Ground fires (smoldering below the forest floor) burn at lower temperatures but can last much longer

How to Stop Wildfires

Advanced technology like weather satellites has made it easier for firefighters to predict the patterns of pre-existing fires by following smoke and small heat patterns. Additionally, weather satellites can track heat signals of small hazardous fires, giving communities the opportunity to stop potential wildfires.

We know firefighters risk their lives trying to extinguish massive infernos. But you might be wondering: What can I do to help? Here are few things you can do to lend a helping hand:

• Be cautious with fire —Whether you’re departing a campsite, finishing a cigarette, or leaving your backyard fire pit, always remember to extinguish your heat source properly. All it takes is one tiny ember to spark a wildfire.
• Give back — Pick a charity and donate money or belongings to those affected by devastating wildfires. You can also volunteer for an organization like American Red Cross and contribute to its relief efforts.
• Be environmentally conscious — Human-caused climate change is making wildfires worse. Consider carpooling, avoiding single-use items, conserving water, and doing other eco-friendly actions to help mitigate the effects of global warming.

Wildfires cause great destruction to land, property, and people’s lives. If you live in California or any other area prone to wildfires, don’t wait until disaster strikes. Explore Mercury’s Castrophere Center. You’ll find resources covering safety and preparedness tips, so you and your family will know exactly what to do before and after a wildfire.

Does Homeowners Insurance Cover Wildfires?

Yes. Homeowners insurance covers damage and destruction caused by fire, which includes wildfires. It provides coverage for your house, personal belongings, and detached structures on your property — e.g., garages, sheds, swimming pools, etc. Homeowners insurance also offers additional living expenses, which helps cover costs like hotel stays and meals if your home becomes uninhabitable due to a wildfire.

Claims can be reported to Mercury’s Claims Hotline 24 hours a day, seven days a week by calling (800) 503-3724. Learn more by reading our fire insurance claims guide (PDF file 0.6mb).

Mercury Insurance offers wildfire mitigation discounts to California homeowners living in wildfire prone regions. Homeowners who take one or more steps to harden their homes against wildfires or live in a community recognized by the National Fire Protection Association® (NFPA) as a Firewise USA® site will be eligible to receive discounts of up to 18%. For more information about protecting your family, pets and property from violent and deadly firestorms, read Mercury’s FAQ Guide and visit these websites:

CalFire

FIRESafe Council

Firewise USA

IBHS Wildfire Prepared Home