How Do Wildfires Start? An Interactive Explainer

How do wildfires start? The short answer

A wildfire starts when three things meet in wild land: fuel (dry grass, leaves, shrubs or trees), oxygen from the air, and enough heat to make the fuel catch. Firefighters call this the fire triangle.⁵⁷ Out in nature, fuel and oxygen are almost always there, so what usually starts a wildfire is a source of heat: a spark, a flame or a lightning strike.⁶

In the United States, nearly 85 percent of wildland fires are started by people: campfires left burning, debris burning that escapes, equipment and cars, discarded cigarettes, and arson. Most of the rest are started by lightning.¹² Whether a small fire fizzles out or becomes a huge blaze depends on the fuel, the weather and the shape of the land.⁴

The fire triangle: heat, fuel and oxygen

Fire is a chemical reaction called combustion. It happens between oxygen in the air and a fuel that has been heated to its ignition point, the lowest temperature at which it catches fire.⁵ Once it's burning, the fire releases its own heat, which warms the next bit of fuel, so it keeps going.⁵

Side of the triangle	In a wildfire	How firefighters remove it
🌾 Fuel	Grass, leaves, needles, shrubs, trees, even houses⁵	Clear a strip of trees and brush ahead of the fire (a firebreak)⁷
🔥 Heat	Lightning, a campfire, sparks, hot exhaust²	Cool the fuel with water, foam, retardant or dirt⁷
💨 Oxygen	Air, with fresh supplies brought by the wind⁴	Smother the flames with water, retardant or dirt⁷

Take away any one side and the fire goes out.⁵ That simple rule is behind almost everything firefighters do.

What causes wildfires? People, and lightning

Every year the U.S. has roughly 70,000 wildfires on average.¹¹ The National Interagency Fire Center says the 10-year average of human-caused fires is 88 percent of all wildfires nationally.² From 2016 to 2025, its tables show about 56,600 human-caused fires a year against about 6,800 started by lightning.³

Human causes

Debris and yard-waste burning that escapes, often when wind blows embers into dry plants. This is one of the leading specific causes.²
Campfires left unattended or not fully put out.¹
Vehicles and equipment: hot exhaust, chains dragging on the road and throwing sparks, bits of hot metal, and machines like chainsaws and mowers.²
Power lines that fall or fail.¹⁵
Discarded cigarettes, fireworks, and arson (fires set on purpose).¹²

Natural causes: lightning

Lightning is the main natural cause. Not every strike starts a fire. Fires are usually started by unusually long-lasting "hot" lightning, which carries a smaller current but for a longer time, long enough to set a tree or dry grass alight.¹ (Want to know where lightning comes from? See what causes lightning.)

The good news hidden in these numbers: fires people start are fires people can prevent.²

How do wildfires spread?

Once a fire starts, three things decide how it behaves: fuel, weather and topography (the shape of the land). Together they're called the fire behavior triangle.⁴

Fuel: dry burns fast

Moisture is the most important thing about fuel. Wet plants burn slowly because the fire must use its heat to drive off the water first. Live trees hold a lot of water, and dead logs very little.⁴ Some plants also contain oils or resins that make them burn faster and hotter.⁴

Weather: heat, low humidity and wind

Hot temperatures and low humidity dry fuel out, so it catches and burns more easily. That's why wildfires often spread fastest in the afternoon, when the air is hottest.⁵ Drought, wind and extreme heat can make a fire bigger, faster and more dangerous.⁶

Wind is one of the most important factors: it brings a fresh supply of oxygen and pushes the flames toward new fuel.⁴ Strong wind can also carry glowing embers ahead of the fire, starting new "spot fires", sometimes up to a quarter of a mile away.⁵

Topography: fire races uphill

If a fire starts at the bottom of a steep slope, it spreads much faster upward, because rising hot air preheats the plants above the flames. Upward drafts also make spot fires more likely.⁴ Going downhill, fire spreads slowly.¹⁵ Rocky slopes with little fuel can act as natural firebreaks.⁴

Ground, surface and crown fires

Surface fires burn grass and fallen leaves and are the easiest to put out. Crown fires burn all the way up to the treetops and are the most intense and dangerous. Ground fires smoulder slowly in peat and deep dead plant matter and can even last underground all winter.⁷

How firefighters stop a wildfire

Firefighters can't switch off the wind, so they work on the fire triangle instead. They cool fuels with water, foam, retardant or dirt; cut off oxygen with water, retardant or dirt; and remove fuel by clearing a strip of trees and brush ahead of the fire.⁷ That cleared strip, dug down to bare soil, is a firebreak. In the sandbox above, try it: a firebreak works well on a calm day, but in strong wind embers can jump right over it.⁵

Weather satellites help too. NOAA's GOES satellites can detect the heat of very small fires, sometimes before anyone spots them on the ground.⁶

Why does Canada have so many wildfires?

Canada has 369 million hectares of forest, close to 9 percent of all the world's forests.¹⁶ More than 8,000 wildland fires happen there in an average year.⁸ Unlike the U.S., lightning causes slightly less than half of Canada's fires, but those fires burn nearly 67 percent of the area. Lightning fires often start in remote places, and several can start at once, so crews have to choose where to go first.⁷ People cause slightly more than half, usually near towns, where they're spotted and reached quickly.⁷

The 2023 season was historic: 6,623 fires burned more than 15 million hectares, and over 230,000 people were evacuated.⁸ Warmer temperatures, less snowpack, low soil moisture and drought are signs that climate change is affecting how often, how big and where Canada's fires burn.⁸

Is fire always bad? Fire-adapted forests

No. Many ecosystems depend on fire. Lodgepole pines grow serotinous cones, sealed tight with resin that melts in a fire and releases seeds stored for years. The seedlings love the open, sunny ground a fire leaves behind.⁹ Some chaparral plants need fire before their seeds will sprout, and gentle fires clear debris, add nutrients to the soil and let sunlight reach the forest floor.¹⁰

Fire can be "a friend and a foe", as the U.S. Forest Service puts it.¹² Decades of putting out every fire let shrubs and dead wood pile up, which can feed bigger, hotter fires later.¹⁰

Is climate change making wildfires worse?

Many studies have found that climate change has already made wildfire seasons longer and increased how often fires happen and how much land burns. In the U.S., the peak month for burned area has moved from August (1984–2000) to July.¹¹ Warmer springs, longer dry summers and drier soils and plants all play a part.¹¹ The Forest Service says longer fire seasons, bigger fires and more extreme fire behavior have become the norm.¹² For the bigger picture, see what causes climate change.

Wildfire safety: how you can help

Follow fire bans and check local rules before any outdoor fire.
Build campfires only in a cleared spot or fire ring, with water and a shovel nearby.¹³
Never leave a campfire unattended, not even for a minute.¹³
To put it out: drown it with water, stir the ashes, and repeat until it's cold to the touch.¹³ Grey coals can stay hot for hours and flare up in the wind.¹⁴
Don't burn yard waste on windy or very dry days.²
If a wildfire is near you, listen to local officials and leave when they tell you to.

Common myths about wildfires

"Most wildfires are started by lightning." In the U.S., about 85 to 88 percent are started by people.¹²

"Hot weather makes forests burst into flames." Heat dries fuel out, but something still has to light it. Spontaneous combustion is a rare cause.²

"Fire spreads the same in every direction." Wind and slope push it. Fires race uphill and downwind.⁴

"No flames means the campfire is out." Coals can stay hot for hours. Drown, stir and feel.¹³¹⁴

"All fire is bad for forests." Some trees need fire to spread their seeds.⁹

FAQ

What are the top 5 causes of wildfires?

In the United States the main causes are debris and yard-waste burning that escapes, campfires left unattended, sparks and heat from vehicles and equipment (including power lines), arson, and lightning. Lightning is the main natural cause; the rest are human.

Do wildfires ever start naturally?

Yes. Lightning is the main natural cause, usually long-lasting 'hot' lightning strikes. In the U.S. lightning starts only about 12 to 15 percent of wildfires, but in Canada it starts almost half, and those fires burn about two thirds of the forest area that burns.

What is the number one cause of wildfires?

People. Nearly 85 percent of U.S. wildland fires are caused by humans, and the National Interagency Fire Center puts the recent 10-year average at 88 percent. Burning debris and unattended campfires are among the leading specific causes.

How do wildfires start in Canada, and why does Canada have so many?

Canada has close to 9 percent of the world's forests. People cause slightly more than half of its wildfires, and lightning slightly less than half. Lightning often starts several fires at once in remote forests, where they can grow large, and warmer temperatures, less snowpack and drought are making fires bigger and more frequent.

What three things does a fire need?

Heat, fuel and oxygen: the fire triangle. Take away any one of them and the fire goes out. Firefighters cool fires with water, smother them with dirt, or clear a firebreak so the fire runs out of fuel.

Why do wildfires spread so fast uphill?

Heat rises. Flames at the bottom of a slope preheat and dry the plants above them, so those plants catch fire much more easily. Rising air also lifts embers that start spot fires further up the hill.

Can the heat of the sun start a wildfire?

Heat and sunshine dry plants out so they catch fire more easily, but a spark or flame almost always does the actual starting. Spontaneous combustion and sunlight focused through glass are possible but rare causes.

How do wildfires start?

Start a fire, then stop it

Play with fire (safely, on the screen!)

Spring or summer?

Fuel moisture decides

Drop a spark!

Break the fire triangle

Remove one side

Take one away

Who starts wildfires?

Ignition sources by country

Wind and hills

Slope and wind vs. spread rate

Wind and hills

Be the firefighter

Suppression sandbox

Stop the fire!

Open a pine cone

Serotinous cones

Open the pine cone