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
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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