() Wildfires in the LA area have burned more than 29,000 acres and forced the evacuations of more than 180,000 people. The two largest fires are still at zero percent containment.

What is containment?

Containment refers to progress in controlling the spread of fires with the use of barriers, also known as control lines.

The percentage refers to how much of the fire’s perimeter has barriers to keep it from advancing. Control lines do not have to be connected, so a fire may be partially contained in different areas.

What are barriers?

Barriers can be natural or artificial.

Roads, rivers and oceans can serve as natural or preexisting barriers, while firefighters can create control lines in various ways.

Some ways include using heavy equipment or tools to remove vegetation from the bare soil, so the fire has nothing to feed it.

Firefighters may also use hoses to encircle the fire, periodically spraying the inside.

Can a contained fire grow again?

Just because a fire is considered contained does not mean it will stay that way. Winds, which have been a significant issue in the LA-area fires, can carry embers over control lines and spark flames.

This explains why a fire may be marked as mostly contained one day, and the percentage may drop again the next.

What is 100% containment?

Just because a fire is considered 100% contained does not mean it has stopped burning.

Instead, 100% containment refers to having enough barriers in place to completely encircle the fire and keep it from spreading. This allows firefighters to focus on putting out the flames within the perimeter.