The Science of Wildfires

By Reported by Laura Bradford and David Bjerklie Monday, July 29, 2002
    • DAVID MCNEW/GETTY IMAGES

    A firefighter works on a backfire near the Sequoia National Forest

    HOW THEY START
    Wildfires result from a confluence of fuel, dryness and some kind of trigger. Each factor contributes to the severity of the blaze

  • Fuel means flammable solids — grass, pine needles, undergrowth, smaller trees — that, with oxygen, feed the fire

  • Dryness can be caused by short-term weather patterns with low humidity or by a lengthy drought that parches the landscape

  • Triggers can be as natural as a lightning strike, as innocent as a campfire or as sinister as an arsonist

    FUEL
    It's a dangerous recipe: decades of fighting every forest fire have left many areas dangerously full of fuel — sticks, fallen timber, pine needles and brush. This year's drought has sapped the natural moisture from the ground

    HOW THEY SPREAD
    Weather is the primary force that drives or contains wildfires. But once they start burning, they create their own weather

    1) Smoke and heat from fires can rise thousands of feet in the air

  • Column of rising hot air creates a void below

    2) Then cooler air rushes in to fill the void

  • Fresh air rushes in, bringing more oxygen to fuel the flames


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    3) This convection system creates gale-force hot winds that dry out and preheat fuel ahead of the fire and can propel burning embers as much as half a mile (0.8 km)

  • Blowing embers allow the fire to jump natural barriers such as rivers and valleys

    TORNADO WINDS
    In rare cases, erratic winds within a wildfire create powerful minitornadoes that can shoot spirals of flame into the air and twist trees apart at their trunks

    UPHILL BATTLE
    Wildfires charge rapidly up mountainsides because the heat from the fire rises and is directed at the fuel uphill, drying it out before the flames arrive

    HOW TO FIGHT THEM
    A fire dies when it is deprived of fuel, heat or oxygen. The main strategy for fighting wildfires is containment: surround the fire and starve it

  • Helicopters and tanker airplanes can drop water or chemical retardants to slow the spread of flames

  • Fire fighters can set up fire lines, areas cleared of any fuel that would allow the fire to spread

  • Controlled fires are sometimes set to deny fuel to an approaching blaze

    SOIL INSULATION
    Soil is an excellent insulator that can protect tree roots from a fire's heat, permitting regrowth to begin quickly. But a charred landscape is also vulnerable to erosion

    RETARDANT
    Nitrogen-heavy fertilizer mixed with water coats fuel to prevent burning. Iron oxide in the retardant gives it its orange color

    BACKFIRE
    Backfires are another method of depriving a fire of fuel so that it burns itself out

    1) Using axes, chainsaws and bulldozers, fire fighters create an area called a fire line that is free of all flammable materials

    2) Upwind of the fire line, a fire is started — usually in the morning or evening, when breezes are calmer — that burns parallel to the fire line

    3) Prevailing winds carry the backfire toward the fire line. Everything in between is burned, creating a wide, fuel-free barrier that, with luck, the approaching fire cannot cross

    HOW TO DEAL WITH THEM
    For much of the 20th century, Americans treated forest fires as anathema, to be stamped out wherever they occurred. But experts now believe the zero-tolerance approach did more harm than good. Fires are part of the natural rhythm of the forests, clearing out underbrush and giving trees room to grow. You don't really fight wildfires, according to this view; you only postpone them. If we are going to learn how to coexist with fire, here is how we might start:

    DON'T BUILD
    Fire is part of the natural life of a forest, say environmentalists; vacation houses aren't. If you don't put them up, you won't have to save them

    BUILD SMART
    If you have to build, use fire-resistant materials and clear out any underbrush and other combustibles

    LET IT BURN
    Fires clean out the forest; let them do their job. With more people than ever living in fire-prone areas, however, that's a tough sell

    PRESCRIPTION FIRES
    Set controlled burns on purpose to clear away undergrowth. The risk is that they will burn out of control, like the fire two years ago that burned 200 homes in New Mexico

    CHANGE THE LANDSCAPE
    To reduce fire risk, forests can be thinned by hand. Some experts advocate restoring grasslands that have been taken over by woody vegetation

    Sources: GregAplet, the Wilderness Society; Stephen Pyne, Arizona State University; William Romme, Colorado State University; AP; National Interagency Fire Center; U.S. Forest Service