From Good Question
The fires that have wiped out homes in Colorado this fire season have taken the American dreams of hundreds of people and have consumed memories.
Some of the homes have had hardly the mitigation needed to protect them; trees alongside, wood piles under decks, gutters clogged, wood shake roofs. Other people had done nearly everything.
A couple killed in the Lower North Fork fire, Sam and Linda Lucas, had an elaborate fire protection system on their home. The fire got to them before they could get out.
When fires roar over homes they’re often driven by the wind. In the Lower North Fork Fire in Jefferson County in March, the winds were gusting up to 70, driving a fire with heat of an estimated 1300 degrees.
“In front of the flame front you’ve got basically a mass of super heated air and super heated gas.” said Inter-Canyon Fire Chief Dave MacBean. “So hot that it can actually dry things out and cause combustion well in front of the flame front.”
Up to a mile away said MacBean. There’s a steep valley below some of the homes that burned in the North Fork fire. It acted like a chimney.
Big fires push flames and pull in oxygen.
“They can actually create their own weather. They’re that powerful, and with that combustion and the air exchange that’s occurring, they can create these tremendous heat columns and that heat can actually stretch out a great distance.”
Give those blistering heat columns an ignition point and the house will go.
“Exterior surfaces like a deck, underneath a deck. Those gases come up, everything’s superheated, all it needs is a little bit more heat or an ignition point,” said Inter-Canyon Deputy Chief Randy Simpson. “An ember flies up under the deck, lands on the deck. Lands next to the siding. That’s all it takes.”
With incredible heat pushed ahead of a fire there doesn’t even need to be an ignition point.
“It just blew up from the heat,” said Jack Thompson in an interview after he lost his home in the Fourmile Fire in Boulder County in 2010. “So I had done all of the mitigation… everything. Except I didn’t have stucco on my house. It was wood sided.”
He figured the heat was so intense it simply lit the wood from a great distance.
Fires look for anything that will catch. That includes a fence around a home.
“Fire can actually travel that wood fence, right along up to the home and actually light a home on fire,” said MacBean.
The heat columns also drive embers high into the sky where winds aloft carry them well beyond containment lines.
“The Hayman fire for example we had plumes up to 35, 40 thousand feet,” said Simpson. “And those embers could carry for miles.”
And the power of the rising air can lift a lot more than little bits of ash.
“A charcoal briquette size sometimes… can be carried in that air column that’s rising,” said Simpson.
Both MacBean and Simpson wanted to make clear how homes need to be left behind when it comes time to evacuate.
“In this chimney for example,” said Simpson looking out over the steep hillsides in front of a burned home. “It would not be uncommon for that fire to travel at 500 feet per minute.”
Faster than anyone can run uphill.
The radiant heat and flame that caught the home where we stood may have lit it from the inside.
“A lot of these houses in this fire more than likely burned from the inside out,” said Simpson.
The house had a stone exterior on the side from which the fire likely approached.
“It’s the absolute radiant heat, passing through that stone or stucco and will ignite the wood framing on the inside of the house…or the curtains or the carpet.”
There’s no question mitigation around a home helps and firefighters say they often won’t try to fight fire around a home without it. But when the wind drives a fire, they time to get out is short and a home’s time to survive a grace period that will end at the whim of the flames.