By Geoffrey Mohan
11:36 PM EDT, July 1, 2013
It was small, as wildfires go -- maybe twice the size of Elysian Park surrounding Dodger Stadium. But the Yarnell Hill fire near Prescott, Ariz., claimed 19 lives, a testimony to the brutal physics of wildfire.
“People seem to think fire is sort of an everywhere, all-at-once kind of thing,” said Richard Minnich, a fire ecologist at UC Riverside. “It’s a flame line; that’s all it is. There’s only just a narrow belt of flames and ahead of it is the unburned vegetation and behind it is the burned vegetation. The vegetation is on fire no more than 10 minutes, 15 minutes. And then the fire moves on.”
When that narrow strip of flame moved on near Yarnell, the bodies of 19 firefighters were left behind. And while many will look to abstract forces to blame for fire -- drought, climate change -- for the men who stood in its path, simple physics and biochemistry were enough. Fuel, wind and terrain killed them.
“You’re looking at accumulating energy through photosynthesis building up for day after day for decades and decades and decades -- in other words, a fantastic reservoir of energy that is being unloaded in 15 minutes,” Minnich said.
That fuel is the product of many years of suppressing fire that naturally cycles through forests and chaparral in the western U.S., fire experts agree. Drought can further prepare that fuel for burning, but Minnich, who has chronicled weather and burn patterns, found "the correlation between rainfall and burning rate is random. Nothing less.”
Fire behavior, however extreme it may seem, follows simple laws: Heat spreads through conduction, radiation and convection. Conduction, or heat transfer molecule to molecule, plays almost no role in wildfires. Then there’s the less direct radiant heat, like the warmth we feel from the sun.
“Believe it or not, that’s kind of the minor story,” said Minnich. Energy dissipates exponentially with distance -- think about stepping back just a foot from a campfire on a cold night. “Radiant heat transfer is very inefficient in a flame line,” said Minnich.
It’s convection, or heat transfer by movement of adjacent air, that gives flame lines the ability to accelerate. The horizontal component of convection is essentially a local wind. The vertical component is an updraft that draws in adjacent air. That force is behind many of the extreme behaviors described by witnesses to wildfires over decades -- vortexes or whirlwinds of flame, winds so strong they blow flames horizontally.
Such forces can't be predicted by weather reports, said Minnich, who is also a trained meteorologist.
“You’re looking at a flame line that’s interacting with an atmosphere, and the atmosphere is really volatile," Minnich said. "It’s changeable. None of this changeability at that scale can be predicted by any weather models. And whatever’s going on in the interaction of the atmosphere with the flame line is just as unpredictable.”How the firefighters at Yarnell died may best be answered by the ground where their bodies were recovered Monday. Slope is the final deadly ingredient to wildfires.
If the firefighters were uphill from flames accelerated by convection, it’s unlikely they could have outrun the blaze. That has been the case in countless wildland fire fatalities. A narrow band of flames racing uphill killed 13 firefighters in Montana’s Mann Gulch in 1949, made famous by Norman MacLean’s “Young Men and Fire.” And it killed 14 in 1994, at Colorado’s Storm King Mountain.
The anniversary of the Storm King deaths is Saturday.
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