What does it mean when someone says a wildfire is contained? Spoiler alert: It doesn’t mean the fire’s out.

According to the U.S. Department of Agriculture, wildfire containment is “the status of a wildfire suppression action signifying that a control line has been completed around the fire, and any associated spot fires, which can reasonably be expected to stop the fire’s spread.”

If a wildfire is 50% contained, that means fire crews have managed to secure a control line around half of the fire’s perimeter.

“We only call a section of line contained if there are no more hotspots or heat near the fireline,” U.S. Forest Service officials said. “We’re not going to call an area of line contained if there is any chance it could come back to life and allow the fire to grow past the constructed line.”

Fire managers will not consider a portion of the fireline “contained” until they are confident that the fire will not grow further in that direction, but containment can still fluctuate up and down as a wildfire grows

If a solid fire line is constructed along 10 acres of a 100-acre fire’s perimeter, then the fire is considered 10% contained. But if the fire were to grow to 200 acres, the overall containment would drop to 5%.

“Containment doesn’t happen throughout the day,” a Larimer County spokesman said during aSaturday evening news conference about the Alexander Mountain fire. “Containment happens in large acres. … We may not show containment movement each day, but that doesn’t mean progress isn’t happening.”

He said crews have to build the fireline, mop up the area, secure the line and hold it for several days before fire officials can confidently say the area is fully contained. That’s why containment may seem slow or nonexistent even when the fire isn’t growing much.

For wildland firefighters, mopping up describes the physical process of extinguishing or removing burning material near control lines, felling fire-damaged trees and cooling ash pits to secure the fireline and reduce residual smoke, U.S. Forest Service officials said.

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Firefighters will then make sure the fire is completely out along the containment lines by “cold trailing,” according to forest officials. That is, dragging the back of their hands along the ground near the fire line to make sure no heat remains.

If containment is trapping the fire, then mopping up is stomping out the flames.

“It’s important to note that containment does not mean a fire is out, or that the danger has passed,” according to RedZone wildfire mapping officials. “Although it is an indicator of progress, the containment percentage doesn’t always correlate to safety level around or within the fire.”

A wildfire can continue to burn for days or weeks after being fully contained, according to RedZone officials.

Originally Published: August 6, 2024 at 1:18 p.m.