What does it mean to say a wildfire is X% contained

July 31, 2022 4:49 PM

Cal Fire and other agencies will typically report that a wildfire is "X% contained", e.g. at the time of writing the McKinney Fire is 0% contained and the Oak Fire is 64% contained. What does that actually mean? What are the numerator and the denominator? Is it the ratio of two areas? Two rates of change? Something else? Or is it a subjective scale like the Mercalli scale for earthquakes? And in particular, what do 0% and 100% (which I assume are the limits of the scale?) mean? Is "100% contained" the same thing as "completely extinguished" or just "no change"?
posted by caek to Science & Nature (7 answers total)

Best answer:

But containment doesn't necessarily mean a fire isn't still raging. Rather than describing how much of the fire has been put out or beaten back, containment actually refers to the perimeter that firefighters create around the fire to keep it from spreading. Take California's Thomas Fire, for example, which as of this writing is at 60 percent containment. That means that 60 percent of the fire is surrounded by containment lines — any physical barrier that stops the fire from passing a certain point. Those lines can be trenches, natural barriers like rivers, or even already-burned patches of land. Most often, the containment line is a shallow, 10- to 12-feet wide trench firefighters dig into the dirt.

posted by zamboni at 4:54 PM on July 31, 2022 [12 favorites]

Best answer: It is the percentage of the perimeter that the fire agency has created or has control over so that the fire cannot spread in that direction. The perimeter may not be the actual boundary of the actively burning part of the fire, but rather the perimeter that has been set up to control the fire. So they are literally saying they have control of 80% of the perimeter of the fire, meaning the fire is only able to spread from the other 20%.

Forest fires are generally fought by establishing a perimeter that you remove the fuel from (generally with heavy equipment or by burning). The perimeter can also sometimes include existing features (rivers, highways, areas without fuel like the alpine part of mountains, areas that burned long enough ago that they are no longer a danger, etc). Then, if needed, you make sure you have resources at the ready to control the fire when it arrives at the perimeter (people, water, pumps, hoses, heavy equipment, aircraft, etc) and you quickly put out any fires that jump the perimeter. Sometimes that doesn't work though, for instance if the winds change in a way that wasn't anticipated.

Here's an article with some terminology details.
posted by ssg at 5:01 PM on July 31, 2022 [2 favorites]

Yes, it's to do with the firefighting effort, not the state of the fire (which may be burning fiercely while completely contained, or even 'safe'). Here's an Australian version of the terminology, which has differences but is similar to the North American practice.
posted by Fiasco da Gama at 5:07 PM on July 31, 2022 [2 favorites]

Think of 'X% contained' as 'X% of our goal for circumscription is complete'. If you trust their goal, this gives an indication of the likelihood that the fire in question can spread beyond areas currently tagged as lost.
posted by SaltySalticid at 5:20 PM on July 31, 2022

The percentage can go down, too, if the uncontained part of the fire grows a lot. One of the big fires in Northern California a few years ago got to about 30% contained, but then that number kept shrinking because the fire kept getting so much bigger. The firefighters had maintained the containment on one edge, but the other edges kept getting bigger.
posted by lapis at 5:49 PM on July 31, 2022 [1 favorite]

And for smaller fires (1-5 acres) around here lately, I'm seeing that "100% contained" notifications are normally accompanied by "Crews anticipated to be in the area for another 3-4 hours for mop-up and in case of hot spots."
posted by lapis at 5:51 PM on July 31, 2022

As noted, containment is not absolute. Lines that used to be considered essentially impregnable, like 8 lane freeways, have been jumped numerous times by the kinds of firestorms that are becoming all the more regular now.
posted by rockindata at 4:28 AM on August 1, 2022