Glossary

Absolute Humidity: Amount of moisture in the atmosphere by weight.
Aerial Fuels: Standing and supported live and dead forest combustibles not in direct contact with the ground consisting mainly of foliage, twigs, branches, cones, bark, stems, and vines (See Draped Fuels, Ladder Fuels).
Aerial Ignition: Ignition of fuels by dropping incendiary devices or materials from aircraft.
Age of Rough: Time in years Since the forest floor was last reduced by fire.
Air Stagnation Advisory (ASA): A statement issued by a National Weather Service office when atmospheric conditions are stable enough that the potential exists for pollutants to accumulate in a given area.
Anchor Point: An advantageous location or point, usually a barrier to fire spread, from which to start constructing fireline. It is used to minimize the chance of being flanked by the fire while the line is being constructed.
Anemometer: General name for instruments designed to measure windspeed.
Area Ignition: Igniting, throughout an area to be burned, a number of individual fires either simultaneously or in rapid succession and so spaced that they soon influence and support each other to produce a hot, fast-spreading fire throughout the area.
Aspect: The direction a slope is facing; i.e., its exposure in relation to the sun.
Atmospheric Stability: A measure of the degree to which the atmosphere resists turbulence and vertical motion. In prescribed fire activities the atmosphere is usually described as stable, neutral, or unstable.
Attack: To take action on a fire to slow it down or stop its spread.
Available Fuel: Those fuels which will burn during a passage of a flaming front under specific burning, weather and fuel conditions.; Back to top
Backing Fire: A fire spreading or set to spread into (against) the wind, or downhill. (See Flanking Fire, Heading Fire).
Backfiring: When attack is indirect, intentionally setting fire to fuels inside the control line to contain a fire. Backfiring rapidly widens the control line providing a wide defense perimeter. Backfiring is a tactic which makes possible a strategy of locating control lines at places advantageous to the firefighter.
Barrier: Any obstruction to the spread of fire; typically an area or strip devoid of flammable fuel such as a creek, field or rock outcrop.
BEHAVE: A system of interactive computer programs for modeling fuel and fire behavior comprised of two subsystems: BURN and FUEL.
Belt Weather Kit: Belt mounted canvas case with fitted pockets for anemometer, compass, sling psychrometer, slide rule, water bottle, pencils, and book of weather report forms.
Blackline: Preburning of fuels, either adjacent to a control line before igniting the main prescribed fire, or along a roadway as a deterrent to human-caused fires. Blackline denotes a condition in which there is no unburned fine fuel.
Blackline concept: Fuels that remain between the main fire and a fireline are burned out to insure safety of control forces and security of control lines.
Blowup: Sudden increase in fire intensity or rate of spread sufficient to preclude direct control or to upset existing control plans. Often accompanied by violent convection and may have other characteristics of a fire storm.
Broadcast Burn: Prescribed fire that burns over a designated area, generally in the absence of a merchantable overstory, to consume debris that has not been piled or windrowed.
Brown & Burn: Application of herbicide to desiccate living vegetation prior to burning.
Brownspot Control: A prescribed burn to control a fungal infection (brownspot disease) of longleaf pine in the "grass" (small seedling) stage.
Buildup: Cumulative effects of long-term drying on current fire danger.
Buildup Index (BUI): A relative number expressing the cumulative effect of daily drying factors and precipitation on fuels with a 10-day timelag constant.
Burning Boss: Person responsible for managing a prescribed fire from ignition through mopup.
Burning Index (BI): A relative number related to the contribution fire behavior makes to the amount of effort needed to contain a fire within a given fuel type. A doubling of the BI indicates twice the effort will be needed to contain a fire in that fuel type as was previously required.
Burning Out: When attack is direct, or parallel with the control line tied at points of the fire, intentionally setting fire to fuels inside the control line to strengthen the line. Burning out is almost always done as a part of line construction if personnel are available. The control line is considered incomplete unless there is no fuel between the fire and the line.
Burning Period: That part of each 24-hour period when fires will spread most rapidly. Typically, this is from about mid-morning to about sundown or late afternoon.; Back to top
Canopy: The stratum containing crowns of tallest vegetation (living or dead) usually above 20 feet.
Category Day: A numerical index related to the ability of the atmosphere to disperse smoke. For example, in South Carolina the current scale, based on Ventilation Factor, ranges from I (poor) to 5 (excellent).
Catface: Defect on the surface of a tree resulting from a wound where healing has not re-established the normal cross-section.
Center Firing: A method of broadcast burning in which fire(s) are set in the center of the area to create a convection column with strong surface indrafts. Usually additional fires are then set progressively nearer the outer control lines as the indraft builds up, to draw the flames and smoke toward the center of the burn.
Chain: Unit of measure in land survey equal to 66 feet; 80 chains equal 1 mile.
Clearcutting: Removal of the entire standing, merchantable timber crop.
Climate: The prevalent or characteristic meteorological conditions of any place or region, and their extremes.
Cold Front: The leading edge of a relatively cold air mass that displaces warmer air. Following a cold front passage, westerly winds of 10 to 20 mph, or more, often continue for 12 to 24 hours.
Combustion: The rapid oxidation of combustible materials that produces heat energy.

Combustion Period: Total time required for a specified fuel component to be completely burned.
Compactness: The spacing between fuel particles. This can be especially important in the surface layer of fuels where the amount of air circulation affects rate of drying, rate of combustion, etc.
Conduction: The transfer of heat within the material itself.
Control Line: An inclusive term for all constructed or natural fire barriers and treated fire edge used to control a fire.
Continuity: The distribution of fuel particles or extent of the fuel bed, thus affecting a fire’s ability to sustain combustion and spread. This applies to aerial fuels as well as surface fuels.
Convection: Transfer of heat by flow of liquids or gases. In meteorology, atmospheric motions that are predominantly vertical because of heating at the surface.
Convection Column: The rising column of gases, smoke and debris produced by a fire. The column has a strong vertical component indicating that buoyant forces override the ambient surface wind (See Smoke plume).
Convergence Zone: The area of increased flame heights and fire intensity produced when two or more flame fronts burn together.
Creeping: Fire burning with a low flame and spreading slowly.
Crown Fire: A fire that advances from top to top of trees or shrubs more or less independently of the surface fire. Sometimes crown fires are classed as either running or dependent, to distinguish the degree of independence from the surface fire.
Crown Scorch: Browning of needles or leaves in the crown of a tree or shrub caused by heat from a fire.
Cumulus: A principal, low cloud-type in the form of individual cauliflower-like cells of sharp non-fibrous outline and less vertical development than cumulonimbus.
Cup Trench: A fireline trench, cut on the downhill side of fire burning on steep slopes, which is deep enough to catch rolling firebrands that could threaten the security of the fireline.
Cured: Debris or herbaceous vegetation that has dried and lost its green color.; Back to top
DAID (Delayed Aerial Ignition Device): See Ping-pong Ball System.
Debris Burning: Any prescribed fire used to dispose of scattered, piled, or windrowed dead woody fuel in the absence of an overstory. Such a burn often accomplishes the objectives of a Site Prep Burn as well.
Dew Point: Temperature to which air must be cooled to reach saturation at a constant atmospheric pressure. The dew point is always lower than the wet-bulb temperature, which in turn is always lower than the dry-bulb temperature. The only exception to this is when the air is saturated (i.e., relative humidity is 100 percent), in which case all three values are equal.
Direct Attack: A method of suppression that treats the fire at the burning edge, by wetting, cooling smothering, or chemically quenching the fire or by mechanically separating the fire from unburned fuel. Action is directly against or near the fire’s edge.
Dispersion: The decrease in concentration of airborne pollutants as they spread throughout an increasing volume of atmosphere.
Dispersion Index: As used in this manual, a numerical index developed by Lee Lavdas (Southern Forest Fire Laboratory). This index is an estimate of the atmosphere7s capacity to disperse smoke from prescribed burns over a 1,000-square-mile area. It is related to the Ventilation Factor, but also considers the rate of pollutant dispersion.
Draped Fuels: Needles, leaves, twigs, etc., that have fallen from above and have lodged on lower branches and brush. Part of aerial fuels.
Drift Smoke: Smoke that has been transported from its point of origin and in which convective motion no longer dominates.
Drip Torch: Hand-held apparatus used to ignite fires by dripping flaming liquid fuel, at an adjustable rate, on the materials to be burned. The fuel is generally a mixture of 65 to 80 percent diesel and 20 to 35 percent gasoline.
Drought Index (Keetch-Byram Drought Index): A numerical rating of the net effect of evapotranspiration and precipitation in producing cumulative moisture depletion in deep duff or upper soil layers.
Dry-bulb Temperature: The temperature of the air.
Duff: The layer of decomposing organic materials lying below the litter layer and immediately above the mineral soil. It is comprised of the Fermentation (F) and Humus (H) layers of the forest floor.; Back to top
Eddy: A whirl or circling current of air or water, different and differentiated from the general flow.
Edge: As used in this course, the boundary between two fairly distinct fuel types.
Emission Factor: The amount of pollution (pounds per ton) released to the atmosphere per unit weight of dry fuel consumed during combustion.
Emission Rate: The quantity of pollutant released to the atmosphere per unit length of fire front per unit time.
Environment: Anything surrounding an individual or community of plants or animals, including man, that influences it in any way.
Equilibrium Moisture Content (EMQ): The moisture content that a fuel would eventually attain if exposed for an infinite period to specified constant values of Dry-bulb Temperature and Relative Humidity.
Extreme Fire Behavior: In this case “extreme” implies a level of wildfire behavior characteristics that ordinarily precludes methods of direct control action. One or more of the following is usually involved: High rates of spread; prolific crowning or spotting; presence of fire whirls; a strong convection column. Predictability is difficult because such fires often exercise some degree of influence on their environment, behaving erratically, sometimes dangerously.; Back to top
Fine Fuels (Flash fuels): Fast-drying, dead fuels which have a Timelag constant of 1 hour or less. These fuels ignite readily and are consumed rapidly when dry. Included are grass, leaves, draped pine needles, and small twigs.
Fire Behavior: The manner in which fuel ignites, flame develops, fire spreads and exhibits other characteristics. The combined effects of the fire’s environment on its behavior. A general descriptive term used to designate what a fire is doing.
Firebrand: Any flaming or smoldering material such as leaves, pine cones, or glowing charcoal that could start another fire.
Firebreak: Any natural or constructed discontinuity in a fuelbed used to segregate, stop, or control the spread of fire or to provide a control line from which to suppress a fire.
Fire Danger: Both constant and variable factors that affect the start, spread and difficulty of control of a fire. Usually expressed as an index such as class 1 through class 5 with 5 being the highest or extreme fire danger.
Fire Effects: Physical, biological and ecological impacts of fire on the environment.
Fire Front: The strip within which continuous flaming occurs along the fire perimeter (See Flame Depth).
Fireline Intensity (Byrams Intensity): The rate of heat release per unit time per unit length of fire front. Numerically, it is the product of the heat yield, the quantity of fuel consumed in the Fire Front, and the rate of spread.
Fireline: The part of a control line that is scraped or dug to mineral soil.
Fire Plow: Heavy-duty share or disk plow designed to be pulled by a tractor to construct Firebreaks.
Fire Perimeter: The outer edge or boundary of a fire.
Fire Rake: A long-handled combination rake and cutting tool, the blade of which is usually constructed of a single row of 4 sharpened teeth.
Fire Season: The period or periods of the year during which fires are likely to occur, spread, and do sufficient damage to warrant organized fire control. A period of the year with beginning and ending dates as established by some fire control agencies.
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 Words: Fire woods is a web based glossary by Joe Scott.
Pyrologix LLC
415 N. Higgins Suite 123
Missoula, MT 59802
406.549.2340
joe.scott@pyrologix.com
www.firewords.net

Firewhirl: A spinning, moving column of ascending air rising from a vortex and carrying aloft smoke, debris and flames. These range from a foot or two in diameter to small tornadoes in size and intensity.
Firing Technique: The type(s) of fire resulting from one or more ignition(s), e.g., backing fire, flanking fire, heading fire, (See Grid Ignition, Ignition Pattern).
Flame Depth: The depth of the Fire Front at the fuel surface.
Flame Length: The distance between the flame tip and the midpoint of the Flame Depth at the base of the flame (generally at the ground surface).
Flaming Front: That zone of a moving fire within which the combustion is primarily flaming. Behind this flaming zone combustion is primarily glowing. Light fuels typically have a shallow flaming front, whereas heavy fuels have a deeper front.
Flanking: Attacking a fire by working along the flanks either simultaneously or successively from a less active or anchor point and endeavoring to connect the two lines at the head.
Flanking Fire: A Fire Front spreading, or set to spread at roughly right angles to the prevailing wind.
Flanks of A Fire: The parts of a fire’s perimeter that are roughly parallel to the main direction of spread.
Flare-up: Any sudden acceleration of fire spread or intensification of the fire. Unlike blowup, a flare-up is of relatively short duration and does not radically change existing control plans.
Flash Fuels: See Fine Fuels.
Flash-Over: A sudden increase in intensity and rate of spread by mass ignition of heated air in front of the fire which had not dispersed because of topography.
Front: A transition zone between two air masses of different densities.
Flying Drip Torch: See Helitorch.
Fuel Loading: The oven dry weight of all existing fuels in a given area. Loading is usually expressed in tons/acre.
Fuel Moisture Content: Is the quantity of moisture in the fuel expressed as a percent of the oven-dried weight.
Fuel Moisture Indicator Sticks: A specially manufactured set of sticks of known dry weight continuously exposed to the weather and periodically weighed to determine changes in moisture content. The changes are an indication of changes in the moisture status and relative flammability of dead fuels that roughly correspond toTen-hour Timelag Fuels.
Fuel Type: An identifiable association of fuel elements of distinctive species, form, size, arrangement, or other characteristics that will cause a predictable rate of fire spread or difficulty of control under specified weather conditions.
Fuel Break: A wide strip or block of land on which the native vegetation has been permanently modified so that fires burning into it can be more readily extinguished. It may or may not have firelines constructed in it prior to fire occurrence.; Back to top
General Winds: Large scale winds caused by high- and low-pressure systems but generally influenced and modified in the lower atmosphere by terrain.
Ground Fire: Fire that consumes the organic material beneath the surface litter of the forest floor, such as peat moss.
Ground Fuels: All combustible material lying beneath the ground surface including deep duff, roots, rotten buried logs, peat and other woody fuels.
Grid Ignitions: Method of igniting fires in which ignition points are set individually at predetermined spacing with predetermined timing throughout.; Back to top
Hazard Reduction: Treatment of living and dead forest fuels to reduce the likelihood of a fire starting, and to lessen its damage potential and resistance to control.
Head of A Fire: The most rapidly spreading portion of a fire’s perimeter, usually to the leeward or upslope.
Heading Fire: A Fire Front spreading or set to spread with the wind or upslope.
Heat Transfer: The exchange of heat by radiation, conduction, or convection.
Heavy Fuels: Fuels of large diameter such as snags, logs, and large limbwood, which ignite and are consumed more slowly than flash fuels. Also called coarse fuels.
Helitorch (Flying Drip Torch): A specialized drip torch hung from, or mounted on a helicopter that dispenses globs of ignited gelled gasoline.
Herbaceous Fuels: Grasses and other plants that contain little woody tissue.
Hotspotting: Checking the spread of fire at points of more rapid spread or special threat. Is usually the initial step in prompt control with emphasis on first priorities.
Humidity: The amount of water vapor content in the air.
Humus: The layer of decomposed organic matter on the forest floor beneath the partially decomposed litter layer (F layer) and directly above the soil.
Hygrothermograph: An instrument that continuously records Dry-bulb Temperature and Relative Humidity.; Back to top
Ignition Pattern: The manner in which a Prescribed Fire is ignited. The distance between ignition lines or points and the sequence of igniting them, as determined by fuel, topography, weather, ignition system, firing technique, and other factors influencing fire behavior and the objectives of the burn (See Firing Technique).
In-stand Wind (Midflame Wind): Windspeed within a stand at about eye level.
Indirect Attack: A method of suppression in which the control line is located along natural firebreaks, favorable breaks in topography, or at considerable distance from the fire and the intervening fuel is backfired or burned out.
Initial Attack: The first action taken on a fire by the first suppression forces to arrive.
Inversion: A layer in the atmosphere where the temperature increases with altitude.; Back to top
Keetch-Byrarn Drought Index: See DroughtIndex.; Back to top
Ladder Fuels: Fuels that provide vertical continuity between the ground and tree crowns, thus creating a pathway for a surface fire to move into the overstory tree crowns.
Land And Sea Breezes: The breezes that, on certain coasts and under certain conditions, blow from the water by day and from the land by night.
Line Ignition: Setting a line of fire as opposed to individual spots.
Litter: The top layer (L layer) of the forest floor directly above the fermentation layer (F layer), composed mainly of recently fallen leaves and pine needles, but also includes dead twigs, bark fragments, etc. (See Duff).
Local Winds: Small-scale convective winds of local origin caused by temperature differences.
Logging Debris: Unwanted tree parts remaining after harvest, including tree crowns, unutilized logs, and uprooted stumps.
Long Range Spotting: Large glowing firebrands are carried high into the convection column and then fall out downwind beyond the main fire starting new fires. Such spotting can easily occur 1/4 mile or more from the firebrands source.
Low-Level Jet: See Wind Profile.; Back to top
Microclimate: A small site or habitat in which weather conditions at any given time are essentially uniform.
Midflame Wind: See In-stand wind.
Mineral Soil: Soil layers below the predominantly organic horizons.
Mixing Height: The height to which relatively vigorous mixing of the atmosphere occurs.
Mopup: Extinguishing or removing burning material, especially near control lines after an area has burned to make it safe, or to reduce residual smoke.
Muck: See Organic Soil.; Back to top
National Fire Danger Rating System (NFDRS): The method currently used by the USDA Forest Service, and many other organizations to integrate the effects of topography, fuels, and weather into numerical indices of fire danger on a day-to-day basis.; Back to top
One-Hour Timelag Fuels: Fine fuels consisting mainly of dead herbaceous plants, roundwood less than about 1/4-inch in diameter, and the uppermost Litter Layer.
Organic Soil: Soil that has developed from large amounts of decayed vegetative matter. Usually wet but when dry, will burn underground. It spreads very slowly but is most difficult to suppress.; Back to top
Particulate (Total Suspended Particulate (TSP): Any liquid or solid particles temporarily suspended in the atmosphere. See PM-10.
Peat: See Organic Soil.
Ping-pong Ball System: A method of igniting fires with the use of a Delayed Aerial Ignition Device (DAID). The device is a polystyrene ball, 1.25 inches in diameter that contains a combustible chemical. The balls are fed into a dispenser, generally mounted in a helicopter, where they are injected with another chemical-and drop through a chute leading out of the helicopter. The chemicals react thermally and ignite in about 30 seconds. The space between ignition points on the ground is primarily a function of helicopter speed, gear ratio of the dispenser, and the number of chutes used (up to 4) (See Grid Ignition).
PM-10: Particulate with an aero dynamic diameter smaller than or equal to 10 micrometers.
Prescribed Burning: The controlled application of fire to wildland fuels in either a natural or modified state, under specified environmental conditions which allow the fire to be confined to a predetermined area and at the same time produce the intensity required to attain planned resource management objectives.
Pressure Gradient: The change in atmospheric pressure per unit of horizontal distance.
Project Fire: Usually refers to a fire requiring manpower and equipment beyond the resources of the protection unit on which it originates.
Psychrometer: The general name for instruments designed to determine the moisture content of air. A psychrometer consists of dry-and wet-bulb thermometers that give the Dry-and Wet-bulb Temperatures, which in turn are used to determine Relative Humidity and Dew Point.; Back to top
Radiation: The process by which energy is propagated through any medium by virtue of the wave motion of that medium, as in propagation of heat waves through the atmosphere.
Rate of Spread: The activity of a fire in extending its horizontal dimensions. Usually it is expressed in chains per hour or acres per hour for a specific period in the fire’s history.
Reburn: (1) Subsequent burning of an area in which fire has previously burned but has left flammable fuel that ignites when burning conditions are more favorable. (2) An area that has reburned.
Red Flag Warning: A term used by fire-weather forecasters to call attention to weather of particular importance to fire control.
Relative Humidity: The ratio, expressed as a percentage of the amount of moisture in the air, to the maximum amount of moisture the air is capable of holding under the same conditions.
Residence Time: The time (seconds) required for the Fire Front to pass a stationary point at the surface of the fuel. Numerically, it is the Flame Depth divided by the rate of spread.
Residual Smoke: Smoke produced by smoldering material behind the actively burning Fire Front.
Ring Fire: A fire started by igniting the perimeter of the intended burn area so that the ensuing Fire Fronts converge toward the center of the block.
Rough: The live understory and dead fuels that build up on the forest floor over time.
Running: Behavior of a fire spreading rapidly with a well-defined head.; Back to top
Scorch Height (Scorch Line): The average height to which foliage has been browned by fire.
Scouting: Reconnaissance of a fire and its surroundings to obtain information necessary for fire suppression planning.
Short Range Spotting: A fire producing sparks or embers that are carried by surface winds to start new fires beyond the zone of direct ignition by the main fire. The range of such spotting is usually less than one quarter mile.
Site Prep Burn: A fire set to expose adequate mineral soil and control competing vegetation until seedings of the desired species become established (See Debris Burning).
Size and Shape: Fuel characteristics which affect the fuel moisture, the amount of heat required for ignition and to sustain combustion, and the burnout time of fuels. The surface area to volume ratio is a representation of size and shape.
Size-Up: An analysis or evaluation of the overall fire situation for determining control action to take. The present and potential behavior of the fire is an important and necessary step.
Slash: Debris resulting from such natural events as wind, fire, or snow breakage, or such human activities as logging or road construction.
Smoke Concentration: The weight of combustion products (micrograms per cubic meter) found in a given volume of air.
Smoke Management: Application of knowledge of fire behavior and meteorological processes to minimize air quality degradation during Prescribed Burning.
Smoke Plume: The gases, smoke, and debris that rise slowly from a fire while being carried along the ground because the buoyant forces are exceeded by those of the ambient surface wind (See Convection Column).
Smoke-sensitive Area (SSA): An area in which smoke from outside sources is intolerable.
Smoldering Combustion Phase: Combustion associated with residual burning of forest fuels behind the Fire Front. Emissions are at least twice that of the Fire Front, and consist mainly of tars.
Spot Fire: Fire ignited outside the perimeter of the main fire by a Fire Brand.
Spotting: A fire spreading by spot fires occurring ahead of the main fire by flying sparks and embers or firebrands.
Spot Weather Forecast: Special prediction of atmospheric conditions at a specific site, sometimes requested by the Burning Boss before igniting a prescribed fire.
Stagnant Conditions: Conditions under which pollutants build up faster than the atmosphere can disperse them.
Strip-Heading Fire: A series of lines of fire upwind (or downslope) of a firebreak or backing fire that will burn with the wind toward the firebreak or backing fire.
Surface Fuels: All materials lying on or immediately above the ground including needles or leaves, duff, grass, small dead wood, downed logs, stumps, large limbs, low brush, and reproduction.; Back to top
Ten-Hour Timelag Fuels: Dead roundwood 1/4 to 1 inch in diameter and, to a rough approximation, the top 3/4 inch of the litter layer.
Timelag: The drying time, under specified conditions, required for a dead fuel to loose about 63 percent of the difference between its initial moisture content and its Equilibrum Moisture Content. Providing conditions remain unchanged, a fuel will reach 95 percent of its EMC after four timelag periods.
Topography: The configuration of the earth’s surface and the position of its natural and manmade features. The features of a place or region. Examples are: rivers, mountains, hills, creeks, roads, fences, power lines, swamps, etc.
Torching: Fire burning principally as a surface fire that intermittently ignites the crowns of trees or shrubs as it advances.
Tractor-Plow: Any tracked vehicle, with a plow for exposing mineral soil, with transportation and personnel for its operation.
Transport Wind: A measure of the average rate of the horizontal movement of air throughout the mixing layer.; Back to top
Underburning: Prescribed burning under a timber canopy.
Unstable Atmosphere: When air next to the ground is rising due to surface heating. The heat is absorbed by the air close to the ground through conduction and radiation. Vertical movement is thus created in the atmosphere causing unstable conditions. When very unstable, heated air will rise rapidly. Winds will be gusty and erratic. Fires will become intense and spread rapidly.; Back to top
Vertical Arrangement: The relative heights of fuels above the ground and their vertical continuity, which influences fire reaching various levels or strata. Surface fuels vs. aerial fuels, etc.
Ventilation Factor: An indicator of the lower atmosphere's potential to diffuse and disperse smoke. Numerically, it is the product of the Mixing Height and the Transport Windspeed (See Dispersion Index).; Back to top
Warm Front: The discontinuity at the forward edge of an advancing current of relatively warm air which is displacing a retreating colder air mass.
Wet-bulb Temperature: Technically, the temperature registered by the wet bulb thermometer of a Psychrometer. It is the lowest temperature to which air can be cooled by evaporating water into it at a constant atmospheric pressure.
Wetline: A line of water, or water and chemical retardant, sprayed along the ground and which serves as a temporary control line from which to ignite or stop a low-intensity fire.
Wildfire: (1) A fire requiring suppression action, as contrasted with a prescribed fire burning with prepared lines enclosing a designated area, under prescribed conditions. (2) A free burning fire.
Wind Direction: Compass direction from which the wind is blowing.
Wind Profile: A plot of windspeed over height above the earth's surface. A rapid increase with height to a maximum windspeed within 1,000 feet above ground and then a slow decrease above that peak is commonly called a low-level jet and is one of several adverse wind profiles.
Windrow: Woody debris that has been piled into a long continuous row.

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