Glossary of Wildfire Terms

Air Quality Index (AQI): Measures how unhealthy the air is due to smoke or pollution.

Fire Behavior: The manner in which a fire reacts to the influences of fuel, weather, and topography.

Fire Danger Rating (Low / Moderate / High / Very High / Extreme): National system that uses weather, fuels, and fire occurrence data to calculate fire danger for a given area.

Fire Weather Watch: Term used by National Weather Service to alert fire officials and firefighters of potentially dangerous fire weather conditions within the next 18 to 96 hours. They are issued when the there is a high potential for the development of a Red Flag event.

Red Flag Warning: Issued to alert fire officials and firefighters of potentially dangerous fire weather conditions within the next 12 to 24 hours. They are issued when specific extreme weather criteria are met.

Backfire: A fire intentionally set along the inner edge of a fireline to consume the fuel in the path of a wildfire and/or change the direction of the fire.

Containment (%): When a fire is 100% contained, it is completely surrounded by control lines, which could include natural barriers as well as line constructed manually or mechanically. The fire is not extinguished at this point.

Control Line, Fireline: A constructed or natural barrier used to limit the spread of a fire. This could include a linear area where people or machinery have scraped/dug down to mineral soil, a swath of fire retardant, a road, or a natural feature like a river.

Direct Attack: Any treatment of burning fuel, such as by wetting, smothering, or chemically quenching the fire or by physically separating burning from unburned fuel.

Engine: Any ground vehicle providing specified levels of pumping, water and hose capacity.

Extreme Fire Behavior: "Extreme" implies a level of fire behavior characteristics that precludes methods of direct control. One of more of the following is usually involved: high rate of spread, prolific crowning and/or spotting, presence of fire whirls, strong convection column. Predictability is difficult because such fires often exercise some degree of influence on their environment and behave erratically, sometimes dangerously.

Fire Behavior: The manner in which a fire reacts to the influences of fuel, weather and topography.

Flash Fuels, Fine Fuels: Fuels such as grass, leaves, draped pine needles, fern, tree moss and some kinds of slash, that ignite readily and are consumed rapidly when dry.

Heavy Fuels: Fuels of large diameter such as snags, logs, large limb wood, that ignite and are consumed more slowly than flash fuels.

Hotshot Crew: A highly trained fire crew used mainly to build fireline by hand.

Incident Management Team: The incident commander and appropriate general or command staff personnel assigned to manage an incident.

Initial Attack: The actions taken by the first resources to arrive at a wildfire to protect lives and property, and prevent further extension of the fire.

Spot Fire, Spotting: A fire ignited outside the perimeter of the main fire by flying sparks or embers.

Surface Fire: A fire that burns leaf litter, fallen branches and other surface fuels on the forest floor, as opposed to ground fire and crown fire.

Torching: The ignition and flare-up of a tree or small group of trees, usually from bottom to top.

Burn Ban: A declared ban on open air burning within a specified area, usually due to sustained high fire danger.

Defensible Space: An area about 100 feet from a structure or home where combustibles have been removed or altered to reduce wildfire risk and increase the potential for defensive action. The distance may be increased to 150-200 feet if the structure is in steep terrain.

Home Ignition Zone (H.I.Z.): The area where the factors that principally determine home ignition potential during extreme wildfire behavior (high fire intensities and burning embers) are present. The characteristics of a home and its immediate surroundings within 100 feet comprise the H.I.Z. Also called Structure Ignition Zone or Ember Ignition Zone.

Forest Management Plan: Outlines a landowner’s vision for their forest, describes the current forest condition, and outlines a plan of action to achieve their management goals.

Fuel(s): All combustible material within the wildland-urban interface or intermix, including vegetation and structures.

Fuel Management/Fuel Reduction: Manipulation or removal of fuels to reduce the likelihood of ignition and to reduce potential damage in case of a wildfire. Fuel reduction methods include prescribed fire, mechanical treatments (mowing, chipping, mastication), herbicides, biomass removal (thinning or harvesting or trees), and grazing. Fuel management techniques may sometimes be combined for greater effect.

Ladder Fuels: Fuels that provide vertical continuity allowing fire to carry from surface fuels into the crowns of trees or shrubs with relative ease. Ladder fuels may include low-hanging tree branches, small trees, or shrubs underneath larger trees.

Mastication: A mechanical process where vegetation, including small trees and brush, is shredded and left on-site as mulch. This treatment reduces fuel loads, encourages soil moisture retention, and supports the regeneration of desired plant species.

Mitigation: Modifying the environment or human behavior to reduce potential adverse impacts from a natural hazard.

Prevention: Activities directed at reducing the incidence of human-caused fires through public education, rules and regulations, and law enforcement.

Slash: Debris resulting from such natural events as wind, fire, or snow breakage; or such human activities as road construction, logging, pruning, thinning, or brush cutting. It includes logs, chunks, bark, branches, stumps, and broken understory trees or brush. Slash is a common byproduct of vegetation management for wildfire mitigation.

Structure/home hardening: Modifying building materials and design features of a structure for wildfire resistance.

Thinning: A forestry practice aimed at reducing tree density in a stand, which may involve the selective removal of some trees to decrease competition for resources among the remaining trees. Thinning can enhance growth, improve forest health, and reduce fire risk by creating a more open canopy and reducing fuel continuity.

Wildland-Urban Interface (WUI): The area where structures and other human development meets or intermingles with undeveloped wildland fuels and other natural features.