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An unplanned fire burning in vegetation — forest, grassland, shrubland or peat. The label is about the fuel and the lack of intent, not the size. A quarter-acre grass fire nobody meant to start is a wildfire; a 10,000-acre prescribed burn lit deliberately by an agency is not.

Fire itself needs three things at once: fuel, oxygen, and enough heat to sustain combustion. Remove any one and it stops. Almost everything firefighters do comes down to removing one of those three.

Ground fires burn below the surface in duff, roots and peat. They move slowly, produce little flame, and can smoulder through winter under snow before re-emerging.

Surface fires burn litter, grass and low shrubs. Most fires are surface fires, and most are controllable.

Crown fires burn through the canopy. Once a fire reaches the crowns it moves with the wind rather than through the fuel bed, spreads several times faster, and is effectively uncontrollable by direct attack. What lets a surface fire climb is ladder fuel — the shrubs and low branches bridging the gap.

It is the share of a fire's perimeter enclosed by a control line that crews expect to hold — not the share extinguished. A fire at 60% containment is still burning freely along the other 40%.

A control line can be a hand-dug trench, a bulldozed strip, a road, a river, or an area burned out deliberately ahead of the fire. Containment can also go backwards: a wind shift that breaches a line drops the number.

The first arriving incident commander names the fire, almost always after a nearby geographic feature — a road, creek, ridge or drainage. It is a practical choice, made so radio traffic is unambiguous within minutes.

When several fires burn close enough to be managed as one, they become a complex and take a new combined name. That is why acreage figures sometimes appear to jump overnight without a corresponding spread.

The water content of vegetation, expressed as a percentage of its dry weight. It governs whether fuel ignites at all and how fast it burns.

Dead fuels are classified by how quickly they respond to changing humidity: 1-hour fuels are fine grass and needles that dry within an hour, up through 1,000-hour fuels — logs over three inches thick — which take weeks. A single hot dry afternoon changes the fine fuels; a drought changes the heavy ones, and that is what turns a normal season into a severe one.

The pattern of fire an ecosystem historically experienced — how often it burned, how intensely, in which season, and at what scale.

Some systems burned lightly every few years, which kept fuel loads low. Others burned rarely but catastrophically, and their species are adapted to exactly that. A regime is not a prediction; it is a description of what a landscape is built for. Areas where fire has been excluded for decades are frequently far outside their historical regime, which is the mechanism behind a great many modern large fires.

Because in fire-prone systems, fire is a normal event rather than a disaster. Several strategies recur.

Serotiny — cones or fruit sealed with resin that only open in the heat of a fire, releasing seed onto freshly cleared ground. Thick bark — insulating the cambium so mature trees survive repeated surface fires. Resprouting — regrowing from protected root crowns or lignotubers after the top is destroyed. Fire-cued germination — seeds triggered by heat or by chemicals in smoke.

The zone where development meets undeveloped wildland vegetation. It is where nearly all structure loss occurs, and it has been the fastest-growing land-use category in the United States for decades.

The interface is not only a rural phenomenon. Intermix — houses scattered within continuous vegetation — and interface — dense development pressed against a wildland edge — behave differently, and both exist inside city limits in many western states.

A thunderstorm generated by the fire itself. A large enough fire drives a column of hot air and moisture high into the atmosphere; if it reaches the condensation level it forms a cloud that behaves like a storm cell.

The consequences are serious. These columns produce erratic and violent surface winds, can generate lightning that starts new fires well ahead of the front, and occasionally collapse — sending a downburst outward in every direction at once. It is one of the conditions that makes fire behaviour genuinely unpredictable.

Flames lean toward the slope above them, preheating and drying the fuel ahead. That fuel then ignites faster, which drives the fire further uphill, which preheats more fuel. As a rough rule, a fire's rate of spread roughly doubles for every 10 degrees of slope.

Aspect compounds it. South and west-facing slopes in the northern hemisphere receive more direct sun, so they carry drier fuel and lighter vegetation than a north-facing slope at the same elevation.

A National Weather Service product issued when fire-weather conditions are occurring or imminent — typically some combination of low relative humidity, strong wind and dry fuels.

It does not mean a fire exists. It means that if something ignites, it is likely to spread rapidly and resist control. Agencies use it to pre-position crews and to impose burn restrictions.

For much of the twentieth century US policy aimed to extinguish every fire quickly — formalised at one point as a target of control by 10am the morning after a fire was reported. It worked, in the narrow sense: small fires stopped being common.

The consequence was that fuel which would have burned off regularly in low-intensity fires instead accumulated for decades. Forests grew denser, ladder fuels filled in, and the fires that eventually occurred were far more intense than the systems were adapted to. Modern policy treats fire as a process to be managed rather than eliminated, but the accumulated fuel debt remains across much of the West.