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 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 –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
