As Engel and I stood along the side of Highway 299 surveying the burnt-out basin below, white ash fell like a light snow though the smoky air. The leaves and grass were mostly gone; what moves in wind tends to burn in fire. Even in the breeze, the charred landscape was still.
Scanning the blackened crescent, Engel was thinking about three things: fuels, topography, and weather. Those are the three basic ingredients of fire behavior, and the three basic factors that a wildfire investigator considers in reconstructing how a fire ignited and spread.
Usually, investigators begin by interviewing firefighters and civilians on the scene to define a general area of origin—a large perimeter, sometimes encompassing as much as two acres of land, that they’re pretty sure contains the ignition point. From there they work inward. When Engel and I arrived at the scene of the Carr Fire, the forest service investigators had already finished their on-site work. A pair of traffic cones and some yellow caution tape marked the origin area, and I could make out a dark scrape mark curving off the road. A metal guardrail beside the cones had twisted as its wooden legs burned out from under it.
In the early afternoon of July 23, a passing trailer with a flat tire had sagged so low it ground against the pavement and sparked. At least one of those sparks leapt into the roadside star thistle, which unseasonable heat and low humidity had primed for ignition. 21-mile-per-hour gusts offered extra encouragement.
Engel traced the fire’s spread away from the origin. A few yards to the left of the cones, he pointed out a stone wedged in the dirt. To its left, some unburned grass had survived; to its right, scorched earth. As the fire advanced on the stone, Engel explained, it diverged, like a stream flowing around a boulder. A week before our visit, other indicators, like ash and fallen grass stems, would also have pointed the way to the fire’s origin. These had since scattered with the wind.
Unlike humans, flames run faster uphill (heat rises), which is why investigators care about topography. But made to choose between favorable slopes and combustible fuels, fire will always pick the latter. Penned in by the unforgiving asphalt of Highway 299 on one side, the flames began to creep downhill.
Wildfires do not tarry. Usually, a flame front passes through a tree in under a minute, consuming what burns easily and sparing what does not. If the fire is growing as it advances, the backside of the tree will show greater damage than the front: In Engel’s lingo, the fire “comes in low and exits high.” When it moves downhill, the burn is more even because fires tend not to grow as they descend. But the Carr Fire did. On the slope down from Highway 299, the sides of the pine trees facing uphill had kept most of their needles, while the sides facing downhill were bare. From this, Engel inferred that as the flames edged down the slope, they reached higher and higher into the foliage.