What Does it Mean When a Wildfire is ‘Contained?’ A Brief Guide to Firefighting and Fire Prevention Jargon

The Dixie Fire, ripping through Butte and Plumas counties, became the 13th-largest fire in California’s history on July 29. The blaze had destroyed 40 buildings, with nearly 11,000 more in its path, according to a Los Angeles Times report. The massive inferno covered 221,000 acres, or 345 square miles. That’s an area larger than all five boroughs of New York City. 

The very next day, the fire consumed another 20,000 acres, about 31 more square miles, and moved up to 11th place on the list of California’s largest fires ever. As if that wasn’t scary enough, state fire officials said they expected the Dixie Fire to form a second pyrocumulonimbus cloud, also known as a “fire cloud” which can generate its own lightning and weather systems, further fueling the flames. The first pyrocumulonimbus cloud produced by the Dixie Fire can be seen in the photo above.

Yet at the same time, the state firefighting agency CalFire said that the fire was 23 percent “contained.” What?

When is it OK to Call a Fire Contained?

“Contained” is one of those terms seen incessantly in news reports every wildfire season, but “containment” may not mean what you think it means. The same holds true for other common firefighting terms. Understanding the jargon could help people, especially homeowners, understand how to respond to wildfires, and how to protect themselves from them in the first place.

“Containment” has a very specific meaning to firefighters. What it does not mean is that a fire has been extinguished, even partially. Fires keep right on burning in the middle of a “contained” area. 

According to CalFire Public Information Officer Mitch Matlow, speaking to reporter Justin Couchot of the Chico Enterprise Record, an area is described as “contained” when it is surrounded by a line wide enough that no heat or flying embers can escape. The burning fire within that line, however, retains the “potential” to jump the line, according to CalFire’s own guide to fire terminology. 

The “line” can be an area that has been stripped of vegetation to deny the fire any new fuel, or it can be a naturally occuring boundary such as a river, which fires are not expected to cross. A line of fire hoses can also contribute to “containing” a fire.

When firefighters determine that the flames inside the line no longer have any ability to escape the boundary, then the fire is said to be “controlled.” It may also be worth bearing in mind that even when CalFire announces that a wildfire is 100 percent contained, they do not mean that they’ve put out the fire. They’ve only prevented it from spreading any further. They hope. 

Winds can pick up sparks and smoldering embers (more on embers, below), carrying them past containment lines. When they land, if there’s enough “fuel” in the area, those red-hot shards of debris can start “spot fires,” whole new areas of flame. That’s why a fire that is listed as, for example, 50 percent contained one day may be only 45 percent contained the next. 

One of the most important procedures firefighters take to bring a wildfire under control is what they call “mop-up.” But as with “containment” and “control,” the process of mopping up does not mean that a fire has been put out. The term refers to the  removal of any debris that is still on fire along the line of containment, or getting rid of logs or other loose vegetation that could roll down a hill, possibly bringing fire with them. 

‘Litter,’ ‘Duff’ and Other Types of Fire ‘Fuel’

The goal is to create a “fuel break,” in other words, an area around a burning wildfire that starves the blaze of new material to burn. What is fuel? It can be live plants such as trees and grass, or it can be “litter,” which does not refer to old Burger King bags or beer bottles. This kind of litter is the loose twigs, fallen leaves, and other not-yet-decomposed vegetation lying on the surface of a forest floor, according to a description by the United States Department of Agriculture Forest Service. Directly underlying the litter is the “duff,” another layer of flammable, loose vegetation that has already begun to decompose. 

While litter is “surface fuel,” the duff along with, according to the Forest Service, “tree or shrub roots, punchy wood, peat, and sawdust” comprises “ground fuel.” These materials may not erupt into flames, but they can still become glowing-hot, a condition which causes fires to spread.

Firefighters alone cannot be solely responsible for removing fuel. They do their mopping up and containing and controlling once fires are already burning. Individual homeowners also need to do their part — and that is what’s meant by the term “hardening.”

Embers — the Reason Why Most Homes Burn in Wildfires

According to CalFire, there are three main reasons homes catch fire during a wildfire. Flames from fire can reach a homeowner’s property or jump across the yard from the house next door. Or a fire can get so hot that it causes any nearby fuel to ignite, even if flames themselves don’t reach that far.

And finally, the most common reason that homes burn during a wildfire, according to CalFire—flying pieces of glowing or burning wood called “embers.” The state fire agency says that embers can set homes ablaze even if the fire that caused them is a full mile away.

The most important aspect of hardening, therefore, is securing one’s home against flying embers. According to the University of California Cooperative Extension, embers often fly straight into a home through a vent, or open window. Once inside, they can set furniture or other ordinary household items on fire, quickly consuming the whole house—even though the yard and other surrounding areas might go untouched.

CalFire recommends covering vents with metallic mesh; plastics can melt, allowing embers to get inside. Obviously, windows should be closed but that’s not enough. CalFire says that dual-pane windows should be installed, with one of the panes made of tempered glass, to cut down the chance of the window breaking, again creating an opening for flying embers.

Roofs and decks are also among the most highly vulnerable areas of a home when it comes to runaway embers. So CalFire wants homeowners to create an “ember-resistant zone” around their decks, and to re-roof with clay, tile, or metallic materials. 

Read CalFire’s full recommendations on fire hardening.