SAN FRANCISCO (KRON) — California’s peak wildfire season is right around the corner.

Cal Fire said its crews have already fought more than 2,000 wildfires this year. But the most destructive and deadly infernos, “mega-fires,” usually ignite between July and October.

The state’s three biggest wildfires of 2021 — Dixie Fire, Caldor Fire, and Monument Fire — all ignited in Northern California between July and August.

Tens of thousands of wildland firefighters are called to the front lines to battle these blazes. State and federal fire agencies use a long list of terminology to describe what is happening as evacuations and emergencies unfold.

For example, fire agencies often refer to “containment” to let communities know about how much progress has been made in a firefight. But “100% containment” does not mean that a fire is “out.”

The U.S. Forest Service recently tweeted a glossary of fire terms to help Californians be in-the-know before the next big mega-fire.

What fire terms mean

KRON4 compiled the most useful and interesting fire terms from the U.S. Forest Service and Cal OES below:

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

Blow-up: A sudden increase in fire intensity or rate of spread strong enough to prevent direct control or to upset control plans. Blow-ups are often accompanied by violent convection and may have other characteristics of a fire storm.

Dark black smoke indicates a “blow up.” (AP Photo/ File / Nick Ut)

Buffer Zones: An area of reduced vegetation that separates wildlands from vulnerable residential or business developments. This barrier is similar to a greenbelt in that it is usually used for another purpose such as agriculture, recreation areas, parks, or golf courses.

Burn Out: Setting fire inside a control line to widen it or consume fuel between the edge of the fire and the control line.

Burning Period: That part of each 24-hour period when fires spread most rapidly, typically from 10 a.m. to sundown.

Complex: Two or more individual incidents located in the same general area which are assigned to a single incident commander or unified command.

Contain a fire: A fuel break around the fire has been completed. This break may include natural barriers or manually and/or mechanically constructed line.

Control a fire: The complete extinguishment of a fire, including spot fires. Fireline has been strengthened so that flare-ups from within the perimeter of the fire will not break through this line.

Control Line: All built or natural fire barriers and treated fire edge used to control a fire.

Defensible Space: An area either natural or manmade where material capable of causing a fire to spread has been treated, cleared, reduced, or changed to act as a barrier between an advancing wildland fire and the loss to life, property, or resources. In practice, “defensible space” is defined as an area a minimum of 30 feet around a structure that is cleared of flammable brush or vegetation.

Dozer Line: Fire line constructed by the front blade of a dozer.

Evacuation Order: Immediate threat to life. A lawful order to leave now. The area is lawfully closed to public access.

Evacuation Warning: Potential threat to life and/or property. Those who require additional time to evacuate, and those with pets and livestock should leave now.

Entrapment: A situation where personnel are unexpectedly caught in a fire behavior-related, life-threatening position where planned escape routes or safety zones are absent, inadequate, or compromised. An entrapment may or may not include deployment of a fire shelter for its intended purpose. These situations may or may not result in injury. They include “near misses.”

Dixie Fire
A “fire whirl” forms in the Dixie Fire north of Greenville in Plumas County, Calif., on Aug. 3, 2021. The blaze ignited July 14, 2021. (AP Photo / Noah Berger)

Extreme Fire Behavior: “Extreme” implies a level of fire behavior characteristics that ordinarily precludes methods of direct control action. 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 Front: The part of a fire within which continuous flaming combustion is taking place. Unless otherwise specified the fire front is assumed to be the leading edge of the fire perimeter. In ground fires, the fire front may be mainly smoldering combustion.

Fire Perimeter: The entire outer edge or boundary of a fire.

Fire Storm: Violent convection caused by a large continuous area of intense fire. Often characterized by destructively violent surface indrafts, near and beyond the perimeter, and sometimes by tornado-like whirls.

Fire Weather Watch: A term used by fire weather forecasters to notify using agencies, usually 24 to 72 hours ahead of the event, that current and developing meteorological conditions may evolve into dangerous fire weather.

Fire Whirl: Spinning vortex column of ascending hot air and gases rising from a fire and carrying aloft smoke, debris, and flame. Fire whirls range in size from less than one foot to more than 500 feet in diameter. Large fire whirls have the intensity of a small tornado.

Flare-up: Any sudden acceleration of fire spread or intensification of a fire. Unlike a blow-up, a flare-up lasts a relatively short time and does not radically change control plans.

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

Firefighter Tanner McDaniel battles the Dixie Fire from the ground on Aug. 21, 2021. (AP Photo /Ethan Swope)

Hotspot: A particular active part of a fire.

Infrared Detection: The use of heat sensing equipment, known as Infrared Scanners, for detection of heat sources that are not visually detectable by the normal surveillance methods of either ground or air patrols.

Mop-up: To make a fire safe or reduce residual smoke after the fire has been controlled by extinguishing or removing burning material along or near the control line, felling snags, or moving logs so they won’t roll downhill.

Peak Fire Season: That period of the fire season during which fires are expected to ignite most readily, to burn with greater than average intensity, and to create damages at an unacceptable level.

Prescribed Fire: Any fire ignited by management actions under certain, predetermined conditions to meet specific objectives related to hazardous fuels or habitat improvement. A written, approved prescribed fire plan must exist, and NEPA requirements must be met, prior to ignition.

Red Flag Warning: Term used by fire weather forecasters to alert forecast users to an ongoing or imminent critical fire weather pattern.

A long exposure photograph shows embers lighting up hillsides as the Dixie Fire burned near Milford in Lassen County, Calif. on Aug. 17, 2021. (AP Photo /Noah Berger)

Relative Humidity: The ratio of the amount of moisture in the air, to the maximum amount of moisture that air would contain if it were saturated. The ratio of the actual vapor pressure to the saturated vapor pressure.

Run (of a fire)
The rapid advance of the head of a fire with a marked change in fire line intensity and rate of spread from that noted before and after the advance.

Shelter in Place: Go indoors. Shut and lock doors and windows. Prepare to self-sustain until further notice and/or contacted by emergency personnel for additional direction.

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

Uncontrolled Fire: Any fire which threatens to destroy life, property, or natural resources.

Wildland Urban Interface: The line, area or zone where structures and other human development meet or intermingle with undeveloped wildland or vegetative fuels.