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How wildfire behaves

A wildland fire moves by heating the fuel ahead of it until that fuel catches. How fast it goes, how tall its flames stand and where it turns are set by three things acting together: fuel, weather and topography. When dry, continuous fuel, strong wind and terrain lined up with the wind coincide, fire behavior turns extreme.

The small fires on this page run on the same model as the map: the Rothermel surface fire spread model with standard fuel models, the equations fire behavior software such as BehavePlus and FARSITE are built on.

Fuel

Fine fuels such as cured grass and pine needles have a lot of surface for their volume. They dry out and heat up quickly, so they carry fire fastest. Branches and logs keep burning long after the front has passed, but the spread model weights each size of fuel by its share of surface area, so fine fuel sets the pace of the flaming front.

Dead fuel trades moisture with the air. Fuel under a quarter inch thick gets about two thirds of the way to equilibrium with the surrounding air in an hour; branches 1 to 3 inches thick take about 100 hours. Every fuel type has a moisture of extinction, a dead fuel moisture above which fire stops spreading: 15 percent for short grass, 35 percent for long-needle pine litter. Green grass holds too much water to burn; as it cures it becomes dead fuel and carries fire.

Six fuels in a 10 mph wind, 30 minutes after lighting.

    Weather

    Wind drives the head of a fire forward, and spread rate climbs faster than the wind speed itself. A fire from a single point in a steady wind grows into an ellipse that gets longer and narrower as the wind rises: in short grass, a 14 mph wind measured 20 feet up, about 5 mph at flame height, makes it about two and a quarter times as long as it is wide. Against the wind the fire backs slowly.

    Short grass, 30 minutes after lighting.

    Relative humidity runs opposite to temperature through the day: lowest in the hottest part of the afternoon, highest near dawn. Fine fuels follow the air within hours, and the burning period, when fires spread fastest, typically runs from 10 a.m. to sundown. At night cold, humid air settles in valley bottoms under an inversion, while a warmer, drier band partway up the slope, the thermal belt, can keep a fire active through the night.

    A tenth of an inch of rain counts as a wetting rain. After one, few new fires take hold and those already burning slow down and throw fewer embers, though logs and other heavy fuel need more than one storm to change much. Snow on the ground stops surface spread until it melts.

    Topography

    Fire runs faster uphill. The slope effect in the spread model grows with the square of the steepness, so steep ground matters far more than gentle ground: in calm air, short grass on a 40 percent slope spreads almost seven times as fast uphill as on the flat. Slope matters most when the wind is light, because wind and slope add together. Fire works downhill slowly; field guidance notes that downhill spread can halve for every 20 percent of slope.

    Tall grass in calm air, 45 minutes after lighting.

    Aspect matters too. South- and southwest-facing slopes take more sun, so their fuels run drier than those on north faces, and terrain channels the wind.

    Embers

    Burning embers lofted by the fire land ahead of it and start spot fires. Rivers, roads and firebreaks stop the flaming front, but embers can cross them. Whether an ember catches depends on the air temperature, the shade and the moisture of the fine fuel where it lands: at 90°F in full sun, about 7 in 10 embers start a fire in fuel at 5 percent moisture, and about 2 in 10 at 15 percent. Torching trees loft embers high; in a 45 mph wind, published tables put the farthest spots from torching pines about a mile downwind.

    Tall grass and a river 60 meters wide, 45 minutes after lighting.

    Crown fire

    When surface flames grow tall enough, fire climbs into the tree crowns. Low branches and small trees, called ladder fuels, lower the height it has to reach: for a canopy that starts 3 meters up, it takes a surface fire of about 875 kilowatts per meter of front, with flames about 1.7 meters long. To run from crown to crown, the fire must also move fast enough to keep the canopy burning, and the denser the canopy, the lower that speed. Crowning trees loft embers higher and farther than any surface fire.

    What holds a fire

    Flame length is the quick guide to what can stop a fire edge, from the national fireline handbook’s hauling chart:

    Under 4 feet
    Crews with hand tools can attack the head or flanks, and hand line should hold.
    4 to 8 feet
    Too hot for hand tools at the head. Dozers, engines and retardant aircraft can be effective.
    8 to 11 feet
    Serious control problems: torching, crowning and spotting. Control at the head will probably fail.
    Over 11 feet
    Crowning, spotting and major runs are common. Control at the head does not work.

    The handbook adds that these bands are not guides to personal safety: fire can be dangerous at any intensity.

    Sources