What it really means for a wildfire to be ‘contained’

What it really means for a wildfire to be ‘contained’ thumbnail

As you follow wildfires in your state and around the country this summer, you’re likely to see progress against fires reported with a single statistic: “percent contained.”

Watching the numbers change over time can give you an idea of how the emergency response is progressing. It’s not always easy to define what containment means. A fire can be contained for several weeks, but not completely. It can also spread in poor conditions, causing the containment percent to drop.

[Related: A network of 1,000 cameras is watching for Western wildfires—and you can, too]

Popular Science spoke to Sean Triplett, the tools and technology lead at the National Interagency Fire Center, which coordinates wildfire responses across the country, to break down the key statistic.

What does wildfire containment mean exactly?

Containment is a method by which firefighters have surrounded the flames using a combination firebreaks, non-flammable area and land that has been burned. It doesn’t mean that the fire is out or that the danger is gone. It’s just unlikely that it will move beyond the containment line.

Percent measures how much of the perimeter remains free to burn and how much is hemmed in. It is most commonly used for fires that have been burning longer than a few days and that require a higher priority response. Triplett warns that fires can quickly grow and become unpredictable during the initial round of firefighting.

Containment forms part of a larger plan to manage a wildfire. Triplett says that fire managers often work to contain the flames within an “operational box” in order to prevent them from reaching homes, infrastructure, or vital watersheds. For example, a fire can be only 50 percent contained, but not pose an immediate threat to the public.

A wildfire containment map of Hermits Peak and Calf Canyon in New Mexico from June 10, 2022
Although the Hermits Peak/Calf Canyon wildfire in New Mexico wasn’t fully contained on Friday, it is contained on the eastern, more populated edge. InciWeb

What counts as contained?

The containment ring around a fire can take many forms depending on the terrain and weather conditions.

“100-percent containment could imply that [fire crews] were tasked to get a line all the way around the fire,” Triplett says–but it could also mean hemming in a fire against the existing landscape, like lakes, rock faces, and even already-burned areas. He explains that a line is basically a way to get down to mineral soil or any type of environmental factor that would stop further burning.

Containment is about as much as the line’s physical properties as it is about its confidence. Triplett states that the fireline counts towards containment only if “[operations managers] feel confident that the fire will not cross the line.

Firefighters are able to clear brush with chainsaws, remove flammable vegetation using shovels or adzes, and in more difficult terrain, bring in bulldozers or huge logging machines that can make a cut in the forest.

[Related: Don’t blame national forests for America’s massive wildfires]

Water tankers and flame retardants are used mainly to support the construction of the containment, Triplett states. They can’t stop fires, but they can cool them down enough to allow crews to get close to it to cut the firebreak and cut off its fuel supply.

In some cases, crews may set fire to the containment area to reinforce the lines. Triplett emphasizes that this takes a lot of preparation. It’s not about setting a fire and watching it burn. First, a crew must build a reliable fireline. They then light the fireline along the break and inside the interior. Triplett explains that as the fire in the line gains momentum, it pulls the entire controlled burning deeper inside, towards the wildfire. “When the main firehead hits this area, it basically runs low on fuel .”

Sometimes, Triplett notes, the goal is to strengthen the boundary until there’s “no heat detected within 100 feet, 300 feet, or even further from the line.” After a fire has passed, they’ll check the boundary by walking into the ashes and testing it with their hands.

Preparing the containment line for fire is key. Triplett explains that incident response teams rely on long-term weather data, climate models, and historical wind patterns to make these calls.

How is it measured?

Firefighters are continuously sending progress reports back from the ground to the incident’s command. These details are recorded by mappers as the fireline is built, inspected, reinforced, or reconstructed to increase the containment percentage.

The maps also include data from drones or planes equipped with infrared sensors that detect spots fires and smoldering. Triplett states that these sensors can detect very small areas heat at temperatures close to the temperature of ignition for woody material. They can also detect fires that aren’t yet raging.

These reports can also push containment in the opposite direction. “You could have 40 percent one day,” Triplett says, “and then the fire makes a run, and that grows your uncontained area.”

A fire can be only 50 percent contained, but not pose an immediate threat to the public.

But the immediate availability of data is a great help in any situation and has transformed wildfire response over two decades, he said. When Triplett first started mapping fires in 2000, everything was done by hand. He recalls that it was mylar and dry-erase markers.

That changed around 2014, when firefighters started to use smartphones to record the progress of containment as they built lines and worked on other maneuvers. Although the system is not perfect, it does allow firefighters to track their progress and sync up with central maps.

Even a few bars can make a huge difference for both the containment crew as well as the people who are in the fire’s path. “We went from knowing what the fire was doing maybe once every 24 hours, to where we are now,” Triplett says. “I hate to use’real-time’ because it doesn’t exist. But we’re getting closer to having an appreciation of what the fire is doing in a dynamic environment.”

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