Wildfire Terms Defined: What It Means When a Blaze Is 30 Percent Contained

Many areas of the United States, especially the West, are in the cross hairs of devastating wildfires again this year. Amid a summer of searing temperatures and dry winds, firefighters have for weeks tried to contain one escalating fire after another.

In news conferences and alerts to residents, firefighters might rattle off figures on how many thousands of acres have burned and speak of how “red flag conditions” are fueling “extreme fire behavior” that is hampering their efforts to increase the percentage of a “complex fire” that is “contained.”

Here is a guide to help you understand some of the terms officials use when discussing wildfires:

When fire officials report that a fire is, say, 30 percent contained, that means that 30 percent of the blaze’s boundary is hemmed in by barriers like rivers, streams, interstate highways or areas that are already scorched, leaving no more vegetation to ignite. Other times, these containment lines are 10- to 12-foot-wide trenches that crews have dug along the fire’s edge — sometimes with bulldozers — to stop the fire from spreading.

When officials say a fire is 100 percent contained, that does not mean it has been extinguished. It means only that firefighters have it fully surrounded by a perimeter; it could still burn for weeks or months. Once a fire is declared “controlled,” then it’s over.

A red flag warning is the highest alert issued by the National Weather Service for conditions that may result in extreme fire behavior within 24 hours. Forecasters announce such a warning when warm temperatures (more than 75 degrees), very low humidity (less than 25 percent), and stronger winds (at least 15 m.p.h.) join forces to produce a heightened risk of fire danger.

If you live in an area under a red flag warning, you should make sure that you:

  • Clear dead weeds and vegetation around your home.

  • Empty your roof and gutters of dry leaves and other debris.

  • Remove flammable household items outside, like brooms and cushions on lawn furniture.

  • Don’t use lawn mowers on dry land.

Generally, extreme fire behavior includes some or all of the following:

  • A high rate of spread

  • Flames growing through the branches and leaves on trees as well as shrubbery, unaided by the blaze on the ground

  • The existence of fire whirls, which are vortexes of hot air and gases rising from a ground fire and carrying debris, flames and smoke into the air. They range from less than one foot to more than 500 feet in diameter. The largest resemble the intensity of a small tornado.

  • The presence of a convection column, which sends gases, smoke, fly ash, particulates and other debris produced by a fire straight into the air, spreading vertically, instead of horizontally

When there are two or more wildfires burning close together in the same area, they are often called a “complex” and attacked by firefighters under a unified command.

In the summer of 2020, a siege of dry lightning strikes sparked about 40 fires in three national forests in northwestern California. They all merged to become the August Complex fire. It burned more than one million acres in total, leading to a new term: “gigafire.”

When you hear of a 100,000-acre fire, that is a description of the total area that has been burned, not what is actively on fire at the time.

But, as Ernesto Alvarado, a professor of wildland fire sciences at the University of Washington, explained, “There’s no way you can map 100,000 acres with people on the ground.”

The authorities instead turn to airplanes, which use infrared cameras, and weather satellites that can snap an image of a fire zone every five minutes or so. Firefighters are able to create real-time maps from these data troves, which can then be supplemented by ground information to map any major fire.