You Asked, We Answer: What Does It Mean When a Fire Is Contained?

Once a fire spews forth the goal is suppression, and the first step is containment. But what does containment mean and why is it so hard to achieve?

“Containment means that there’s some type of barrier between the area that has been burned, which we call ‘the black’ and an area that has not been burned which we refer to as ‘the green,'” says Cal Fire public information officer Jaime Williams.

There are two types of barriers—natural and artificial. A stream or lake can act as a natural barrier. An artificial barrier is often a dirt path dug around the fire. Firefighters will use a bulldozer to create what is called a “dozer line,” or manually carve out a path using picks and shovels, which is called a “hand line.”

“They basically scrape the top layer of the grass off to leave bare mineral soil,” says Williams. “That way the fire stops because there’s nothing to burn.”

Or firefighters will employ a “hose lay,” where they’ll carry a synthetic hose around the fire, periodically spraying the area inside “the black.”

Many of the wildfires burning in Northern California are now partially contained. Of the two largest fires burning in Sonoma and Napa, the Tubbs Fire is 60 percent contained and the Atlas Fire is 56 percent contained.

The “percent contained” indicates how much of the fire’s perimeter is surrounded by a barrier. So, for a fire whose perimeter is 10 miles around, if firefighters create a 5-mile-long dirt area around the fire, the fire is 50 percent contained.

But that doesn’t mean the fire won’t spread beyond the containment line.

“Generally we know that fires spread with the wind,” says Craig Clements, who oversees the Fire Weather Research Laboratory at San Jose State University.

The winds Sunday night when the fires first spread were historic—up to 75 miles an hour in some areas. When the winds pickup, they can cause “spot fires” where an ember flies up and starts another new fire nearby.

“What’s so dangerous about spot fires is that the embers can be transported miles downwind from the main fire front,” says Clements.

And then a fire can go from 50 percent contained to 20 percent contained.

“Once you get spot fires,” Clements says, “it’s like doubling that rate of spread because now you have a fire starting much farther downwind than the initial fire front.”

Even after a fire is contained, there’s still a lot of work to do.

A contained fire can still be flaming inside the perimeter and firefighters must burn out untouched vegetation inside the barrier and cool down hotspots that could flare up. After the hotspots and unburned vegetation are treated and the barrier is expected to hold, a fire is considered “controlled.”

For the North Bay fires, that could take weeks.