What Is a Wildfire?

A wildfire is an uncontrolled fire burning through natural or
semi-natural vegetation such as forest, woodland, shrubland, grassland or peat.
Depending on the landscape and region, wildfires may also be called forest fires,
bushfires, brush fires, grassfires or wildland fires.

Some wildfires remain relatively small and slow-moving. Others grow rapidly,
cross roads and rivers, generate enormous smoke plumes, launch burning embers
kilometers ahead of the main fire and threaten entire communities.

Fire is also a natural ecological process in many landscapes. Some ecosystems
evolved with frequent low-intensity fires, while others burn only rarely.
The ecological consequences therefore depend strongly on the ecosystem, fire
intensity, season, frequency and previous fire history.

What Does a Wildfire Need to Burn?

Fire requires three fundamental ingredients: heat, fuel and oxygen.
Together they form the familiar fire triangle.

Wildfire behavior specialists often use a broader concept called the
fire behavior triangle, which emphasizes three interacting controls:

• Fuel — what is available to burn.
• Weather — wind, temperature, humidity and atmospheric stability.
• Topography — slope, aspect, elevation and terrain.

Changes in any one of these can dramatically alter how fast a wildfire spreads
and how difficult it becomes to control.

What Causes Wildfires?

Wildfires require an ignition source. Ignitions can be natural or caused by human activity.

Lightning

Lightning is one of the most important natural wildfire ignition sources.
A lightning strike can ignite dry vegetation, dead trees or organic material
beneath the surface.

Particularly dangerous situations can develop during
dry thunderstorms, when lightning reaches the ground but
little rainfall reaches the surface.

Human-Caused Ignitions

Human activities can ignite fires through campfires, discarded cigarettes,
machinery, vehicles, electrical infrastructure, fireworks, debris burning,
agricultural burning and deliberate fire-setting.

The ignition itself may be tiny. What determines whether it remains small or
becomes a major wildfire depends largely on fuel conditions, weather and terrain.

Volcanic Activity

Lava flows, hot volcanic ejecta and other volcanic processes can occasionally
ignite vegetation, although this is far less common globally than lightning
or human-caused ignition.

Types of Wildfires

Wildfires can burn through different levels of vegetation and soil.
Three broad categories are commonly used.

Ground Fires

Ground fires burn organic material beneath or immediately at the soil surface.
Peat, buried roots and decomposed vegetation can smolder for long periods,
sometimes surviving beneath the surface after visible flames disappear.

Surface Fires

Surface fires burn grasses, leaf litter, fallen branches, shrubs and low vegetation.
Many wildfires begin or spend much of their lifetime as surface fires.

Crown Fires

Crown fires spread through the upper canopy of trees. They can become extremely
intense and fast-moving, particularly when strong winds and dry fuels allow flames
to move from treetop to treetop.

How Do Wildfires Spread?

Wildfire spread occurs as heat from burning vegetation ignites nearby fuels.
Several forms of heat transfer contribute.

Radiation

Flames emit radiant heat that warms nearby vegetation. If fuels become hot
enough, they can ignite even before direct flame contact.

Convection

Hot gases rise above the fire, creating powerful convective currents.
These currents transport heat, smoke and burning material vertically.

Direct Flame Contact

Flames can directly ignite adjacent vegetation as the fire front advances.

Spotting

Winds and convection can lift burning bark, branches and other embers into the air.
These firebrands may land beyond the main fire perimeter and ignite new
spot fires.

Spotting is one of the primary reasons large wind-driven fires can cross roads,
rivers and firebreaks.

Wildfire Fuels: What Actually Burns?

In wildfire science, fuel refers to combustible vegetation and
organic material capable of carrying fire.

Fuels range from tiny blades of grass and pine needles to shrubs, fallen logs,
tree canopies and deep peat deposits.

Fine Fuels

Grass, leaves, needles and small twigs dry rapidly and can ignite easily.
Because of their high surface-area-to-volume ratio, fine fuels can respond
quickly to changes in temperature and humidity.

Heavy Fuels

Large branches, logs and dead trees generally gain and lose moisture more slowly,
but once burning they can release heat for extended periods.

Live Fuels

Living grasses, shrubs and trees contain varying amounts of moisture.
During prolonged heat or drought, live vegetation can become substantially
more flammable.

Dead Fuels

Dead leaves, grass, bark, branches and fallen timber can become extremely dry,
creating highly combustible material capable of supporting rapid fire spread.

Fuel Continuity

Fires spread more easily when fuels form an uninterrupted pathway.
Gaps in vegetation can slow fire movement, while continuous grasses,
shrublands or forest canopies can allow flames to advance rapidly.

Fire Weather: When the Atmosphere Favors Wildfires

Fire weather describes atmospheric conditions capable of
increasing wildfire ignition potential, fire spread or fire intensity.

Dangerous fire-weather environments often involve some combination of:

• high temperatures,
• low relative humidity,
• strong winds,
• dry vegetation,
• prolonged drought,
• atmospheric instability,
• dry thunderstorms,
• rapid wind shifts,
• strong frontal passages.

Weather does not create every wildfire ignition, but it often determines whether
an ignition remains manageable or evolves into a rapidly spreading fire.

Drought and Wildfires

Drought can strongly increase wildfire potential by reducing moisture in soils,
grasses, shrubs, dead vegetation and sometimes living plants.

As fuels dry, less energy is required to remove moisture before combustion begins.
Ignitions can therefore spread more easily through vegetation.

Long droughts may also increase dead vegetation, tree mortality and landscape-scale
fuel availability.

Drought alone does not guarantee a major wildfire. Ignition, vegetation,
weather and terrain still matter. But drought can create the background conditions
in which dangerous fire weather becomes much more consequential.

Why Wind Can Make Wildfires Explode

Wind is one of the most important short-term controls on wildfire behavior.
Strong winds can tilt flames toward unburned vegetation, transport heat,
increase oxygen supply and push the fire front forward.

Wind can also carry burning embers far ahead of the main fire, producing
multiple spot fires and rapidly expanding the effective fire perimeter.

Santa Ana Winds

Santa Ana winds are strong, dry offshore winds capable of
producing dangerous fire-weather conditions across Southern California when
vegetation is dry.

Their full meteorology, geography and formation are covered in
Santa Ana Winds Explained.

Diablo Winds

Diablo winds are dry, gusty downslope winds affecting parts
of Northern California. When they occur during dry fuel conditions, they can
contribute to rapid wildfire spread.

Explore the atmospheric processes behind them in
Diablo Winds Explained.

How Terrain Influences Wildfires

Wildfire behavior is strongly affected by landscape shape.

Slope

Fires generally spread faster uphill because flames and rising heat preheat
vegetation higher on the slope.

Canyons

Narrow valleys and canyons can channel winds and create complex,
rapidly changing fire behavior.

Aspect

Slopes facing the Sun more directly may become warmer and drier than shaded slopes,
influencing vegetation and fuel moisture.

Mountain Winds

Downslope windstorms and terrain-driven airflow can dramatically increase
fire spread in mountainous regions.

Extreme Fire Behavior

Most wildfires do not display the most dramatic forms of fire behavior.
Under particularly favorable combinations of dry fuels, weather and terrain,
however, fires can undergo rapid transitions.

Extreme fire behavior may involve:

• very rapid rates of spread,
• long flame lengths,
• crown fires,
• mass spotting,
• intense convection columns,
• fire whirls,
• rapid blowups,
• plume-dominated behavior,
• erratic changes in fire direction or intensity.

At these intensities, wildfire behavior can exceed what ground crews can safely
approach or suppress directly.

Ember Storms and Spot Fires

Burning fragments of vegetation and building material are commonly called
embers or firebrands.

Powerful winds and fire-generated convection can carry these fragments far
beyond the visible flame front.

When they land on receptive fuel, embers can ignite new spot fires.
Multiple ignitions ahead of the primary fire can make containment extremely difficult.

During intense events, large numbers of airborne firebrands may produce what is
sometimes described as an ember storm.

Pyrocumulus and Pyrocumulonimbus: When Fires Build Clouds

Large wildfires can release so much heat that rising air creates powerful
convective columns.

If sufficient moisture and atmospheric instability are present, the plume
may develop into a pyrocumulus cloud.

Under more extreme conditions, convection can deepen into a
pyrocumulonimbus cloud, often abbreviated pyroCb.

These wildfire-generated thunderstorms can produce strong winds, lightning,
dramatic vertical smoke transport and rapid changes in fire behavior.

Fire Whirls and Fire Tornadoes

Intense fires can generate rotating columns of hot air, flame and debris known
as fire whirls.

Many are relatively small and short-lived, but exceptionally intense wildfire
environments can produce much larger rotating vortices.

The term fire tornado is often used for particularly intense,
tornado-like fire vortices, although their formation and classification can
differ from ordinary tornadoes produced by supercell thunderstorms.

These vortices illustrate how strongly an intense wildfire can interact with
the atmosphere immediately surrounding it.

Wildfire Smoke: Fires Can Affect Regions Far Away

Wildfire impacts are not limited to the burned area.
Smoke can travel hundreds or thousands of kilometers from the source.

Wildfire smoke contains a complex mixture of gases and particles produced by
combustion.

The amount and composition of smoke depend on fuel type, moisture,
combustion efficiency and fire intensity.

Why Does Wildfire Smoke Travel So Far?

Large fires generate buoyant plumes that transport smoke high into the atmosphere.
Regional winds can then carry the plume over large distances.

Pyrocumulonimbus storms are capable of injecting smoke unusually high,
allowing wildfire material to travel across continents and oceans.

Wildfire Smoke and Air Quality

Dense smoke can sharply reduce visibility and degrade air quality over large areas,
including cities located far from the wildfire itself.

Wildfires and Ecosystems

Wildfire is destructive in some contexts, but fire is also an important ecological
process in many landscapes.

Fire can remove accumulated vegetation, recycle nutrients, create habitat mosaics,
trigger seed release and maintain open ecosystems that would otherwise become
overgrown.

Fire-Adapted Ecosystems

Some plants possess thick bark, underground structures, heat-resistant seeds or
other adaptations that allow them to survive or regenerate after fire.

When Fire Becomes Ecologically Severe

Problems can arise when fires occur with unusual frequency, intensity or timing,
especially in ecosystems that are poorly adapted to repeated burning.

Repeated severe fires can alter vegetation communities, increase erosion,
affect soils and change habitat structure.

Post-Fire Floods and Debris Flows

Severe wildfire can remove vegetation and alter soil conditions.
When intense rainfall later reaches the burned landscape, runoff can increase
dramatically.

Burn scars can therefore become susceptible to flash flooding, erosion,
mudflows and debris flows long after the flames have disappeared.

Explore the related processes in

Debris Flows & Debris Avalanches Explained
.

The Wildland–Urban Interface: Where Wildfires Meet Communities

The wildland–urban interface, commonly abbreviated WUI,
is the zone where homes, roads and other human development meet or mix with
combustible wildland vegetation.

Wildfires entering these areas can transition from burning vegetation to burning
structures, vehicles, fences and other human-made materials.

Embers are especially important because homes can ignite even when the main flame
front never directly reaches the building.

Fire Danger Ratings, Watches and Red Flag Warnings

Fire-danger systems combine information about weather, fuels and landscape
conditions to estimate how readily fires may ignite, spread or become difficult
to control.

Authorities may issue fire-weather watches or red flag warnings when forecasts
indicate conditions capable of producing dangerous wildfire behavior.

Such conditions may involve combinations of very low humidity, strong winds,
dry fuels and unusually hot or unstable weather.

How Wildfires Are Detected and Monitored

Modern wildfire monitoring combines ground observations with aircraft,
satellites, weather stations, cameras and computer models.

Satellite Detection

Satellites can detect thermal anomalies, smoke plumes and active fire zones
across enormous areas.

Weather Stations

Temperature, wind, humidity and precipitation measurements help fire-weather
specialists evaluate rapidly changing conditions.

Aircraft and Drones

Airborne thermal imaging can identify active fire fronts and hotspots hidden
beneath smoke.

Fire-Behavior Models

Models combine information about fuels, terrain and forecast weather to estimate
possible directions and rates of fire spread.

No model can perfectly predict every sudden change because wildfire behavior can
respond rapidly to localized winds, fuel changes and terrain.

Why Do Wildfire Seasons Occur?

Wildfire activity often follows seasonal climate patterns.

A region may experience a wet growing season followed by months of drying,
eventually producing large quantities of flammable vegetation.

Fire seasons may coincide with:

• seasonal drought,
• persistent summer heat,
• dry thunderstorms,
• autumn wind events,
• seasonal vegetation drying,
• periods of low humidity.

Different regions therefore experience peak wildfire conditions at different
times of year.

Historic and Extreme Wildfires

Some wildfires become historic because of their extraordinary size,
speed, intensity, smoke production, ecological effects or destruction
of communities.

Extreme events have occurred across many fire-prone regions, including:

• western North America,
• Mediterranean Europe,
• Australia,
• Siberia,
• South America,
• southern Africa,
• boreal forests and peatlands.

The most damaging fires typically emerge from combinations of unusually dry fuel,
extreme fire weather, difficult terrain, abundant ignition sources and exposure
of communities or infrastructure.

How a Wildfire Can Escalate

Major wildfire disasters often emerge through a chain of interacting conditions
rather than a single trigger.

Wildfires: In-Depth Child Pillars

Use these dedicated guides to explore the major atmospheric, physical
and human dimensions of wildfire behavior.

FAQ: Wildfires