Fire is burning hay in the dry season

How Wildfires Start and Spread

Wildfires are uncontrolled fires that can decimate homes and the surrounding environment. Whether you witness them on the news or in person, they’re a frightening sight to behold. And as the earth’s climate continues to change, wildfires are only getting worse. How do these raging infernos start? And how do they get massive enough to destroy acres upon acres of land?

How Do Wildfires Start?

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 naturally? 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. 

How Do Wildfires Spread?

Wildfires require heat and fuel to start, but they need wind to spread the flames and cause millions of acres worth of destruction. 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. 

Are California’s Wildfires Getting More Destructive?

In 2020, California experienced some of the worst wildfires in the state’s history. According to the National Interagency Fire Center (NFIC), California wildfires destroyed over 4 million acres of land, setting a record for the state. Even though the NFIC reports that the number of U.S. wildfires has decreased in recent years, the size and intensity of wildfires are only increasing, leading to more decimated acreage.

Climate change is a huge contributor to the more intense blazes. 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!

Is There Anything That Can Be Done?

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. 

Filing a Claim

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.

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:


FIRESafe Council

Firewise USA