Fire
Rapid oxidation of combustible materials accompanied by a release of energy in the form of heat and light.

Fire Behavior
Manner in which a fire reacts to the variables of fuel, weather, and topography.

Fire Behavior Forecast
Prediction of probable fire behavior, usually prepared by a fire-behavior officer in support of fire suppression or prescribed-burning operations.

Fire Curtain
Aluminized device on a rod designed to be unrolled to reflect radiant heat from operators or crew members on some apparatus, bulldozers, or tractor-plows.

Fire Edge
Boundary of a fire at a given moment.

Fire Front
Part of a fire within which continuous flaming combustion is taking place; assumed to be the leading edge of the fire perimeter. In surface fires, the fire front may be mainly smoldering combustion.

Fire Hazard Severity Rating System
System of adjectives used to describe fire danger to the public. Adjectives range from low to extreme.

Fire Season
Period(s) of the year during which fires are likely to occur, spread, and damage wildland values sufficient to warrant organized fire suppression.

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
Weather conditions that influence fire ignition, behavior, and suppression.

Fire Weather Forecast
Weather prediction specially prepared for use in wildland fire control.

Fireline
Part of a control line that is scraped or dug to mineral soil; OR a general term for the area where fire fighting activities are taking place, the wildland equivalent of the term "fireground" as used in structural fire fighting.

Fireline Intensity
Rate of heat energy released per unit time per unit length of fire front. Numerically, it is the product of the heat of combustion, quantity of fuel consumed in the fire front, and the rate of spread of a fire in Btu per second per foot (kilojoules per second per meter) of fire front. Synonymous with Intensity.

Firewhirl
Spinning vortex column of ascending hot air and gases rising from a fire and carrying smoke, debris, and flame aloft.

Flame Depth
Depth of the fire front; horizontal distance between leading and trailing edge of fire front.

Flame Height
Average maximum vertical extension of flames at the leading edge of the fire front. Occasional flashes that rise above the general level of flames are not considered. This distance is less than the flame length if flames are tilted due to wind or slope.

Flame Length
Distance between the flame tip and the midpoint of the flame depth at the base of the flame (generally the ground surface); an indicator of fire intensity.

Flammability
Fuel's susceptibility to ignition.

Flare-Up
Any sudden acceleration in rate of spread or intensification of a fire. Relatively short duration and does not radically change existing control plans.

Flashy Fuels
Wildland fuels that are easily ignited and that burn rapidly when dry. Some examples are grass, leaves, pine needles, fern, tree moss, and some kinds of slash.

Friendly Fire
Fully contained and controlled fire started for useful and nondestructive purposes.

Front
In meteorology, the boundary between two air masses of differing densities.

Frontal Winds
Winds generated by the movement of an air mass (front) across the earth's surface.

Fuel
Flammable and combustible substances available for a wildland fire to consume.

Fuel Break
Wide strip or block of land on which the native vegetation has been modified so that fires burning into them can be more readily extinguished. It may or may not have a fireline constructed in it prior to fire occurrence.

Fuel Characteristics
Factors that make up fuels such as compactness, loading, horizontal continuity, vertical arrangement, chemical content, size and shape, and moisture content.

Fuel Continuity
Degree or extent of continuous or uninterrupted distribution of fuel particles in a fuel bed, thus affecting a fire's ability to sustain combustion and spread. This applies to aerial fuels as well as surface fuels.

Fuel Loading
Amount of fuel present expressed quantitatively in terms of weight of fuel per unit area. This may be available fuel (consumable fuel) or total fuel and is usually dry weight.

Fuel Management
Manipulation of fuel prior to an incident to prevent the occurrence or slow the spread of wildland fire. Synonymous with Vegetation Management or Weed Abatement.

Fuel Model
Simulated fuel complex for which all fuel descriptors required for the solution of a mathematical rate-of-spread model have been specified.

Fuel Moisture
Quantity of moisture in fuel expressed as a percentage of the weight when thoroughly dried at 212^oF.

Fuel Volume
Quantity of fuel per unit area; usually expressed in tons per acre (tons per hectare).

General Winds
Large-scale winds caused by high- and low-pressure systems but generally influenced and modified in the lower atmosphere by terrain.

Gradient Wind
Upper-level winds that flow around high- and low-pressure cells. Gradient winds flow clockwise around high-pressure cells and counterclockwise around low-pressure cells.

Gravity Wind
Type of general wind that occurs when stable, high-pressure air is forced across and then down the lee slopes of a mountain range. The descending air is warmed and dried due to adiabatic compression; locally called by various names such as Santa Ana, Mono, Chinook, etc. Synonymous with Foehn Wind.

Green
Area of unburned fuels, not necessarily green in color, adjacent to but not involved in a wildland fire.

Green Belt
Landscaped and perhaps irrigated fuel break that is regularly maintained; sometimes put to an additional use (for example, golf course, park, playground, pasture). May also be dedicated but unmaintained open space within or between developments.

Ground Fire
Wildland fire that burns loose debris of the surface; includes dead branches, fallen leaves, needles, duff, stubble, grass, and low vegetation. Synonymous with Surface Fire.

Ground Fuel
Fuel that contacts the surface of the ground; consists of duff, leaf and needle litter, dead branch material, downed logs, bark, tree cones, and low-stature living plants. These are the materials normally scraped away to construct a fireline. Synonymous with Surface Fuel.

Head of a Fire
Most active part of a wildland fire; OR the forward advancing part.

Heat Release Rate (HRR)
(1) Total amount of heat produced per unit mass of fuel consumed per unit time. (2) Amount of heat released to the atmosphere from the convective-lift fire phase of a fire per unit time.

Heat Transfer
Flow of heat from a hot substance to a cold substance. This flow may be accomplished by convection, conduction, or radiation.

Heavy Fuels
Massive natural cover fuels such as logs, snags, and large limbs. Heavy fuels are not easy to ignite; once ignited, they burn slowly and hot.

Heel
Rear portion of a wildland fire. Synonymous with Rear.

Hotshot Crew
Highly trained fire fighting crew used primarily in handline construction.

Infrared Radiation
Radiation with a wavelength outside the visible spectrum at the red end of the spectrum. Thermal radiation from free-burning fires is an example of infrared radiation.

Intensity
Rate of heat energy released per unit time per unit length of fire front. Numerically, it is the product of the heat of combustion, quantity of fuel consumed in the fire front, and the rate of spread of a fire in Btu per second per foot (kilojoules per second per meter) of fire front. Synonymous with Fireline Intensity.

Inversion
Increase of temperature with height in the atmosphere. Vertical motion in the atmosphere is inhibited allowing for smoke buildup. A "normal" atmosphere has temperature decreasing with height.

Ladder Fuels
Fuels that provide vertical continuity between strata, thereby allowing fire to carry from surface fuels into the crowns of trees or shrubs with relative ease. They help initiate and assure the continuation of crowning.

Lapse Rate
Change of an atmospheric variable (temperature unless specified otherwise) with height.

Light Fuels
Fast-drying fuels, with a comparatively high surface-area-to-volume ratio, that are generally less than 1/4inch (6.35 mm) in diameter and have a time lag of 1 hour or less. These fuels readily ignite and are rapidly consumed by fire when dry.

Live Fuels
Living plants, such as trees, grasses, and shrubs, in which the seasonal moisture content cycle is controlled largely by internal physiological mechanisms rather than by external weather influences.

Local Winds
Winds that are generated over a comparatively small area by local terrain and weather. They differ from those that would be appropriate to the general pressure pattern or that possess some other peculiarity.

Medium Fuels
Material available to burn in a geographic area that is in the midrange of size such as various brush species; generally excludes short grasses and large trees.

Mineral Soil
Soil containing little or no combustible material.

Moist Adiabatic Lapse Rate
Rate of decrease in temperature with increasing height of an air mass.

Origin
Point of original ignition of a fire. Synonymous with Point of Origin.

Point of Origin
Point of original ignition of a fire. Synonymous with Origin.

Precipitation
Any or all forms of water particles, liquid or solid, that fall from the atmosphere.

Pyrolysis
Thermal or chemical decomposition of fuel because of heat; the preignition combustion phase of burning during which heat energy is absorbed by the fuel, which in turn gives off flammable tars, pitches, and gases.

Radiation
Transfer of heat through intervening space by infrared thermal waves.

Rate of Spread (ROS)
Relative activity of a fire in extending its horizontal dimensions. Expressed as rate of increase of the total perimeter of a fire, as rate of forward spread of the fire front, or as rate of increase in area, depending on the intended use of the information. Usually expressed in chains or acres (hectares) per hour for a specific period in the fire's history.

Rear
Rear portion of a wildland fire. Synonymous with Heel.

Relative Humidity
Percentage of moisture in the air compared to the maximum amount of moisture that air will hold at that temperature.

Running Fire
Behavior of a fire spreading rapidly with a well-defined head.

Saddle
Depression or pass in a ridgeline; low area on a ridgeline between two higher points.

Set
(1) Individual incendiary fire. (2) Point or points of origin of an incendiary fire. (3) Material left to ignite an incendiary fire at a later time. (4) Individual lightning or railroad fires, especially when several are started within a short time. (5) Burning material at the points deliberately ignited for backfiring, slash burning, prescribed burning, and other purposes.

Slash
Debris left after logging, pruning, thinning, or brush cutting; includes logs, chunks, bark, branches, stumps, and broken understory trees or brush.

Sling Psychrometer
Meteorological instrument used to determine relative humidity.

Slope
Natural or artificial topographic incline; degree of deviation from horizontal.

Slope Winds
Small-scale convective winds that occur due to local heating and cooling of a natural incline of the ground.

Slopover
Fire edge that crosses a control line. Synonymous with Breakover.

Smoldering
Fire burning without flame and barely spreading.

Snag
Standing dead tree or part of a dead tree from which at least the leaves and smaller branches have fallen.

Spot Fire
Fires starting outside the perimeter of a main fire typically caused by flying sparks or embers.

Spotting
Behavior of a fire producing sparks or embers that are carried by the wind to start new fires beyond the main fire.

Stable Atmosphere
Condition of the atmosphere in which the temperature decrease with increasing altitude is less than the dry adiabatic lapse rate. In this condition, the atmosphere tends to suppress large-scale vertical motion.

Subsurface Fire
Fire that consumes the organic material beneath the ground such as a peat fire or roots burning.

Subsurface Fuel
All combustible materials below the surface litter, such as tree or shrub roots, peat, and sawdust, that normally support smoldering combustion without flame.

Surface Fire
Wildland fire that burns loose debris of the surface; includes dead branches, fallen leaves, needles, duff, stubble, grass, and low vegetation. Synonymous with Ground Fire.

Surface Fuel
Fuel that contacts the surface of the ground; consists of duff, leaf and needle litter, dead branch material, downed logs, bark, tree cones, and lowstature living plants. These are the materials normally scraped away to construct a fireline. Synonymous with Ground Fuel.

Target Hazard
Facilities in which there is a great potential likelihood of life or property loss.

Thermal Belt
Elevation on a mountainous slope that typically experiences the least variation in diurnal temperatures and has the highest average temperatures and, thus, the lowest relative humidity. Its presence is most evident during clear weather with light wind.

Topography
Land surface configuration.

Unfriendly Fire
Uncontained and uncontrolled fire of intentional or accidental origin that may cause injury or damage.

Vegetation Management
Manipulation of fuel prior to an incident to prevent the occurrence or slow the spread of wildland fire. Synonymous with Fuel Management or Weed Abatement.

Virga
Precipitation that evaporates before reaching the ground.

Warm Front
Leading edge of a relatively warm air mass that moves in such a way that warm air displaces colder air. Winds associated with warm-frontal activity are usually light, and mixing is limited. The atmosphere is relatively stable when compared to cold-frontal activity.

Weed Abatement
Manipulation of fuel prior to an incident to prevent the occurrence or slow the spread of wildland fire. Synonymous with Vegetation Management or Fuel Management.

Whirlwind
Small rotating windstorm of limited extent containing sand or dust. Synonymous with Dust Devil.

Wildland Fire
Unplanned and unwanted fire requiring suppression action; an uncontrolled fire, usually spreading through vegetative fuels. These fires can threaten structures or other improvements.

Wind
Horizontal movement of air relative to the surface of the earth.