What containment and other wildfire related terms mean

When a wildfire comes to your area, it can be challenging to understand how firefighters describe their progress and environmental conditions.

Here's a guide explaining common words and phrases officials typically use to describe wildfires:

Containment

One of the most common terms authorities use to describe a wildfire is containment. 

Usually posted next to acreage, it describes crews' progress in controlling the spread of flames and is typically listed as a percentage. 

For example, if a fire has 30% containment, crews have established barriers, also known as control lines, around 30% of the fire's perimeter. The barriers could either be natural or man-made. 

Natural examples include roads, rivers, and oceans. Firefighters create barriers using equipment like bulldozers, hoses, and shovels to remove vegetation from the bare soil so the fire has nothing to burn.

A control line does not have to be connected. For example, firefighters can contain 10% the northern portion and 10% of a southern portion of a fire, bringing the grand total to 20% 

Full or 100% containment does not necessarily mean the fire is fully extinguished. Rather, it means barriers have completely encircled the fire, effectively stopping the spread and allowing firefighters to put out hot spots still active inside of the perimeter. 

Defensible space

Defensible space is a buffer zone that helps prevent a fire from burning a building or property. Similar to control lines, defensible space can potentially stop the flames from spreading to homes while also creating a safe space for firefighters to control the blaze. 

Firefighters typically recommend a minimum of 30 feet of defensible space surrounding a property. 

Residents can create buffers by removing flammable brush or vegetation surrounding their homes, such as dead or dying plants. Firefighters have created step-by-step guides for building defensible spaces.

Complex fire

When two or more fires burn in the same general area, authorities may pool their resources and assign them to a single incident commander or unified command. Once firefighters make this move, the multiple fires are combined into a single incident name, referred to as a complex fire.

The largest fire in California history, the August Complex Fire, started after 12,000 lightning strikes ignited 37 fires that spanned over 1 million acres across seven counties.

Evacuation warnings vs orders

Residents living near a wildfire may receive an evacuation warning or order. 

A warning indicates that a potential threat or danger is approaching your area and that you will likely need to evacuate. 

At this point, people who will need extra time to evacuate — those with special needs, the elderly, or livestock owners — should start leaving. Others should prepare to evacuate. 

The U.S. Forest Service said residents should prepare and consider the 6-P's: plan, personal items, prescriptions, photographs, pets and papers.

An order means the danger has arrived; leave immediately. Authorities will typically close the area to public access. Residents can leave but cannot return to their homes until authorities lift the order. 

A warning typically comes before an order, but because of the unpredictability of fires, orders could be immediately issued, such as what some residents reported during the Bridge Fire in the San Gabriel Mountains.

Red flag warning

The National Weather Service will issue red flag warnings to indicate climate conditions have increased the risk of fires. The forecast will typically bring warmer temperatures, very low humidities and stronger winds, creating an environment prone to dangerous wildfire growth. 

NWS recommends that anyone starting outfires outside drown them in plenty of water and make sure everything is cold to the touch. Do not leave live charcoal or fires unattended.

Air Quality Index

The Air Quality Index, commonly abbreviated as AQI, measures the amount of pollution in the air. Regional air quality management districts typically issue smoke advisories during major wildfires to warn residents about unhealthy pollution levels. 

There are six levels: good, moderate, unhealthy for sensitive groups, unhealthy, very unhealthy and hazardous. 

When levels reach "unhealthy for sensitive groups" or higher, officials recommend that residents stay indoors and avoid vigorous physical activity outside. If people have to go outside, they should wear a respirator or mask. 

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The U.S. Forest Service created a glossary of fire terms. If you want to learn more about common fire terms, visit their website.