While reading news of the wildfires in Maui, I see that they’re estimating fires are something like 50% or 80% contained. How do they go about calculating that percentage for such a wide area and with the expectation that the fire will continue to grow?
Source: https://www.cnn.com/2023/08/12/us/maui-wildfires-hurricane-dora-saturday/index.html
In: 7
Containment % typically refers to how much of a fire is encircled by a fire guard or a natural fire break (ie. river, rock face, etc.) As a fire burns down and depletes the fuel in an area, that area also becomes a natural fire break, making it very difficult for a fire to double back on itself. Consequently, you’ll almost never see “100% containment” except for fires that have nearly burned out.
It’s talking about the fire’s borders. You go around the outside border edge of the whole fire and say what % of that border is currently stopped from any further spread (by a manmade firebreak or natural feature like a lake) vs. what % is currently open and free to spread and get bigger.
The **containment % is what % of the fire’s** ***perimeter*** **has been contained**. A wildfire with 25% containment means control lines have been completed around 25% of the fire’s perimeter.
A visual example makes it easy. [Here’s a partially contained fire’s borders](https://media.npr.org/assets/img/2017/12/20/2016_10_23-10.05.46.999-cdt-d4ad27ace0faa13829acf042bd37fdadec598cf7-s800-c85.webp). The border that’s in black is contained by “completed fire breaks”. The border that’s in red is “uncontrolled fire edge”. **The containment % is just what fraction of the total border length is currently red (uncontained).**
Aerial firefighter here.
Fires are typically mapped utilizing Infrared (IR) imagery gathered by reconnaissance aircraft. That IR data shows where the perimeter of the fire is and anywhere there is a heat source.
That data is used to create precise calculations of the total acreage and total perimeter length.
Fire is mostly fought by creating a perimeter, or line, that the fire cannot cross. This can be accomplished by a variety of methods from utilizing existing features such as roads, rivers, rocks, etc. Line can also be created by bulldozers, ground crew cutting “hand line”. Burn out, or back burns are also used to consume fuel ahead of the fire to prevent further advance.
Once line is constructed and there is a cold black edge adjacent to the line, it is considered “contained”. There is typically still heat inside the line, but outside of extreme wind events, that heat will not escape. This cold line will be identified on the IR mapping and in conjunction with what the ground based incident command reports, the percentage of containment can be calculated.
What can be misleading or misinterpreted by the public is that even a fire that is listed as being 100% contained, could (say in the case of an extreme wind event) experience a flare up causing it to spot over the containment line and move so quickly that the fire is effectively 0% contained again.
Not really an ELI5 answer, but that’s how it is done.
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