The Mountain Fire damaged and destroyed hundreds of buildings in Ventura County last November.
It started on November 6, 2024, and within a couple of hours, the fire had spread into the Camarillo Heights neighborhood, destroying and damaging hundreds of homes, as well as around 12,000 acres of farmland. The cause? An ember from an earlier, much smaller fire in Balcom Canyon in Somis, which had been caused by a tractor fire a week earlier. However, amid strong Santa Ana winds, an ember from the tire reignited and blew outside the perimeter line of the Balcom fire. This process is known as a holdover fire.
"Holdover fires occur when combusting or smoldering material from a previous fire event or ignition isn't fully extinguished," explained Marc Mayes, a senior scientist in ecosystems and remote sensing who studies wildfire hazard mitigation in Ventura County.
There are parallels between the causes of the Mountain Fire and the Palisades fire in January this year, which is also believed to have been caused by an earlier fire reigniting in strong winds.
Holdover fires aren’t uncommon, said Mayes, but experiencing two of that scale within three months is more unusual.
"The physics of it are very common, and they usually don't have much impact. If that fire is within the perimeter of a 20,000 or 30,000-acre event, usually the small holdover sources of reignition just eventually extinguish," explained Mayes.
Essentially, because there is no fuel left as it has already been burned, the fires burn themselves out.
But that's not what happened in the case of the Mountain Fire. The Balcom fire was only 1.8 acres, and with high Santa Ana winds, the Mountain fire investigation determined that those extreme winds dislodged a previously covered pocket of hot debris from the remains of one of the tractor’s tires, gave it the oxygen needed to burn, and carried that fire beyond the containment area where it ignited dry vegetation, starting the Mountain Fire.
Not only were hot spots checked, but rain also fell in the area on November 2.
"There's nothing like incredible natural phenomena to expand our minds from what's possible as far as what can cause holdover fires or any other type of ignition," said Mayes.
"In order to maintain combustion, you need oxygen and you need fuel. And so if something is relatively slow-burning and is still buried in a pocket of aerated space underground, sure, that can continue to smolder. Every single instance of a holdover can just be very hard to find," he said.
The fire department utilized up-to-date drone technology, which Mayes explained can be helpful but does have limits.
"The fire departments in California are some of the best-trained firefighting institutions in the world, and so they are very thorough with their mop-up processes," said Mayes.
"Aerial reconnaissance is experiencing a renaissance in the fire world these days. Technology that used to only largely be available on manned aircraft, like thermal sensors, are being downsized and can now be flown on drones. So that can be an incredible resource for finding hot spots," he said.
"As good a resource as they are, these thermal sensors are measuring heat signatures from the ground surface, but the distance underground that they can sample is really quite limited. And if in the course of mop-up, a source of combusting material gets buried, or in the example of a shrub or some type of vegetation starts burning at its stem and continues combusting underground, up to 6-8 inches or so, thermal imaging won't necessarily be able to detect every hot spot. So thermal imaging is a great resource to enhance finding all that material, but it's by no means perfect."
Of the Mountain Fire, Ventura County Fire Chief Dustin Gardner said, “The Ventura County Fire Department exists to serve our community in support of public safety. We constantly evaluate and update our standards, operations, and equipment to deliver the most effective emergency response we can. Lessons learned from this fire are already shaping how we will prepare for and respond to future wildfires. Our commitment to our community will not waver.”
No lives were lost in the Mountain Fire, but lives were changed forever, and the community is still in the early stages of the long rebuilding process. Hopefully, the lessons learned can assist all of those looking to mitigate wildfire risks.
