Fire Weather Glossary

Adiabatic Process: A thermodynamic change of state of a system in which there is no transfer of heat or mass; i.e., compression results in warming, expansion in cooling.

Advection: The transfer of atmospheric properties by horizontal movements of air. The term is used most commonly in reference to transfer of heat, but may also refer to moisture,

Aerial Fuels: All live and dead vegetation located in the forest canopy or above the surface fuels, including tree branches and crowns, snags moss, and high brush.

AFFIRMS: Administrative and Forest Fire Information Retrieval and Management System. It is a user-oriented, interactive computer program that permits entry of fire weather observations and forecasts, and which performs the computation of fire danger indices, both observed and predicted. Additional information and services are available, including data storage.

Airmass: An extensive body of air having to same properties of temperatures and moisture in a horizontal plane.

Altimeter: An aneroid barometer calibrated to indicate altitude instead of pressure.

Anchor Point: An advantageous location or point, usually a barrier to fire spread, from which to start constructing a fireline. It is used to minimize the chance of being flanked by the fire while the line is being constructed.

Anemometer: An instrument for measuring the speed of wind.

Anticyclone: An area of high pressure.

Area Ignition: See simultaneous ignition.

Aspect: The direction a slope is facing; i.e., its exposure in relation to the sun.

Atmosphere: The envelope of air surrounding the earth.

Atmospheric Pressure: The force exerted by the weight of the atmosphere.

Available Fuels: Those fuels which will burn during a passage of a flaming front under specific burning and fuel conditions.

Backfiring: When attack is indirect, intentionally setting fire to fuels inside the control line to slow, knock down, or contain a rapidly spreading fire. Backfiring provides a wide defense perimeter and may be further employed to change the force of the convection column. Backfiring makes possible a strategy of locating control lines at places where the fire can be fought on the firefighter's terms. Except for rare circumstances meeting specified criteria, backfiring is executed on a command decision made through line channels of authority.

Backing Fire: A fire, or that part of a fire, spreading or set to spread against the wind. (See head fire).

Barometer: An instrument for measuring the pressure of the atmosphere. The two principle types are mercurial and the aneroid.

Barrier: Any obstruction to the spread of fire; typically an area or strip devoid of flammable fuel.

Blackline Concept: The principle of fire containment that uses a fire line with all adjacent fuels inside the line burned out for a safe distance.

Blowup: Sudden increase in fire intensity or rate of spread sufficient to prevent direct control or to upset existing control plans. Often accompanied by violent convection, it may also have other characteristics of a fire storm.

Breakover: A fire edge that crosses a control line or natural barrier intended to confine the fire. Also called slopover.

Burning Out: Used when attack is direct, or parallel, and the control line touches points of the fire. Burning out is intentionally setting fire to fuels inside the control line to strengthen the line. Burning out is almost always done by the crew boss as a part of line construction. The control line is considered incomplete unless there is no available 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.

Campaign Fire: A fire normally of a size and/or complexity that requires a large organization and which may take several days or weeks to suppress. Also called a Project Fire.

Canopy: The stratum containing the crowns of the tallest vegetation present, (living or dead) usually above 20 feet.

Celsius: (Centigrade) A temperature scale with zero degrees as the melting point of ice and 100 degrees as the boiling point of water.

Center firing: A technique of broadcast burning in which fires are set in the center of an area to create indraft. Additional fires are then set progressively nearer the outer control lines as indraft builds up to draw them towards the center.

Chemical Properties: Substances in the fuels which can either retard or increase the rate of combustion, such as high mineral content, oils, resins, wax or pitch.

Chinook, or Chinook Wind: A foehm wind blowing down the eastern slopes of the Cascades, Rocky Mountains, and over the adjacent plains in the United States and Canada. In winter, this warm, dry wind causes snow to disappear with remarkable rapidity, and hence it has nicknamed the "snoweater." In hot dry weather, chinook winds can quickly extend fire weather conditions to the "extreme."

Cold Front: The leading edge of a relatively cold airmass that displaces warmer air. The heavier cold air may cause some of the warm air to be lifted. If the lifted air contains enough moisture, cloudiness, precipitation and even thunderstorms may result. In case both air masses are dry, there may be no cloud formation. Following a cold front passage, often westerly or northwesterly winds of 10 to 20 mph, or more continue for 12 to 24 hours.

Cold Trailing: A method of controlling a partly dead fire edge by carefully inspecting and feeling with the hand to detect any fire, digging out every live spot, and trenching any live edge. No trench is built where the fire edge is dead out.

Combustion: The rapid oxidation of combustible material 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.

Condensation: The process by which a vapor becomes a liquid.

Conduction: The transfer of heat between molecules in contact with one another.

Conflagration: A raging destructive fire. Often used to denote such a fire with a moving front as distinguished from a fire storm.

Continuity: The proximity of fuels to each other that governs the fire's capability to sustain itself. This applies to aerial fuels as well as surface fuels.

Contour Map: A map have contour lines connecting the points on a land surface that have the same elevation.

Control line: An inclusive term for all constructed or natural fire barriers and treated fire edges used to control a fire.

Control Line Standards: The depth and width in which fuels must be treated or removed to control any portion of a wildfire perimeter.

Convection: Vertical air movements resulting in the transport of atmospheric properties. In meteorology, atmospheric motions that are predominantly vertical; i.e., usually upward.

Convection Column: The thermally-produced, ascending column of gases, smoke, and debris produced by a fire.

Convection Winds: All winds-up, down, or horizontal, that have their principal origins in local temperature differences.

Convergence: Net horizontal inflow of air into an area associated with low pressure systems. If convergence occurs at the surface, vertical motions results. The condition exists when the distribution of winds within a given area results in a net horizontal inflow of air into the area. When convergence at lower levels occurs, the removal of the resulting excess air is accomplished by an upward movement of air; consequently, areas of convergent winds are regions favorable to the occurrence of precipitation.

Counter Fire: Fire set between main fire and backfire to hasten spread of backfire. Also called draft fire. The act of setting counter fires is sometimes called front firing or strip firing.

Creeping: Fire burning with a low flame and spreading slowly.

Critical Burnout Time: total time a fuel can burn and continue to feed energy to the base of a forward traveling convection column.

Crown Closure: The spacing between tree crowns; usually expressed as the percent of area covered by tree crowns in the forest canopy region as viewed from above.

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 area classed as either running or dependent to distinguish the degree of independence from the surface fire.

Cup Trench: A fire line trench cut on the downhill side of fire burning on steep slopes deep enough to catch rolling firebrands that could threaten the security of the fire line. (Also called roll-trench).

Dew Point: The temperature to which air must be cooled, at constant pressure and moisture content, in order for saturation to occur.

Direct Attack: A method os suppression that treats the fire as a whole, or all its burning edge, by wetting, cooling, smothering, or by chemically quenching it or mechanically separating it from unburned fuel.

Diurnal: Daily, especially pertaining to cyclic actions which are completed within 24 hours, and which recur every 24 hours.

Divergence: The condition that exists when the distribution of winds within a given results in a net horizontal flow of air outward from the region. In divergence at lower levels, the resulting deficit is compensated by a downward movement of air from aloft; consequently, areas of divergent winds are regions unfavorable to the occurrence of precipitation.

Draft Fire: fire that is set inside a control line and is affected by indrafts from the from the main fire. The draft fire and main fire will burn into each other at an increased rate of speed.

Dry Airmass: A portion of the atmosphere that has a relatively low dew point temperature and where the formation of clouds, fog or precipitation is least likely to occur.

Dry Bulb: A name given to an ordinary thermometer used to determine the temperature of the air.

Dry Lightning Storm: A lightning storm with negligible precipitation reaching the ground.

Duff: A mat of partially decomposed organic matter immediately above the mineral soil, consisting primarily of fallen foliage, herbaceous vegetation and decaying wood.

Eddy: A whirl or circling current of air or water, different and differentiated from the general flow.

Edge Firing: A technique of broadcast burning in which fires are set along the edges of an area and allowed to spread to the center.

Effective Windspeed: The midflame windspeed adjusted for the effect of upslope on fire spread.

Elevation: The height of the terrain above mean sea level, usually expressed in feet.

Equilibrium Vapor Pressure (EMC): the level at which dead fuels neither gain or lose moisture with time, under specific constant temperature and humidity. The water vapor pressure in the air is equal to the vapor pressure in the fuel. A fuel particle, at EMC, will have to net exchange of moisture with its environment.

Evaporation: The transformation of a liquid to the gaseous state. Heat is lost by the liquid during this process.

Extreme Fire Behavior: 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 crowing and/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 and sometimes dangerously.

Fahrenheit: A temperature scale on which 32 degrees denotes the temperature of melting ice, and 212 degrees the temperature of boiling water, both under standard atmospheric pressure.

Fine Fuels: fuels such as grass, leaves, draped pine needles, fern, tree moss and some kinds of slash which, when dry, ignite readily and are consumed rapidly. Also called flash fuels.

Fire Behavior Forecast: Fire behavior predictions prepared for each shift by a fire behavior officer to meet planning needs of fire overhead organization. The forecast interprets fire calculations made, describes expected fire behavior by areas of the fire, with special emphasis on personnel safety, and identifies hazards due to fire for ground and aircraft activities.

Fire Behavior Prediction Model: A set of mathematical equations that can be used to predict certain aspects of fire behavior when provided with an assessment of fuel and environmental conditions.

Fire Danger: A general term used to express an assessment of fixed and variable factors such as fire risk, fuels, weather and topography, which influence whether fires will start, spread, and do damage; also the degree of control difficulty to be expected.

Fire Danger Rating: The process of evaluating fire danger by using a system of numerical scales.

Fire Intensity: The rate of heat release for an entire fire at a specific point in time.

Fire Management: An extension of the concept of wildfire decision making which takes into account resource values, role of fire in the environment, the level of protection required, opportunities for prescribed use of fire, consideration of fire effects, and the efficiency of the fire control operation.

Fire Occurrence: the number of wildland fires started in a given area over a given period of time.

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

Fire Prevention: Activities directed at reducing fire occurrence; includes public education, law enforcement, personal contact and reduction of fire hazards and risks.

Fire Risk: The probability that a wildfire will start as determined by the presence and activities of causative agents.

Fire Scouting: Reconnaissance of a fire and its surroundings by any means to obtain information necessary for fire suppression planning.

Fire Season: The period or periods of the year during which wildfires 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 fire control agencies.

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

Fire Suppression Tactics: the science and air of deploying and maneuvering forces against wildfires.

Firebrand: Any source of heat, natural or manmade capable of igniting wildland fuels. Flaming or glowing fuel particles that can be carried naturally by wind, convection currents, or by gravity into unburned fuels.

Firebreak: A natural or constructed barrier used to stop or check fires that may occur, or to provide a control line from which to work. Sometimes called a firelane.

Firing Out: Also called firing. The act of setting fire to fuels between the control line and the main fire in either a backfiring or burning out operation.

Fire line: the part of a control line that is scraped or dug to mineral soil. Sometimes called firetrail.

Fire line Intensity: the rate of heat energy released during combustion per unit length of fire front. It is usually expressed in BTUs/second/foot.

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.

Flame Height: the average height of flames as measured on a vertical axis. It may be less than flame length if flames are angled.

Flame Length: the distance measured from the tip of the flame to the middle of the flaming zone at base of the fire. It is measured on a slant when the flames are tilted due to effects of wind and slope.

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.

Flank Fire: A fire set along a control line parallel to the wind and allowed to spread at right angles to the line.

Flanks of a Fire: The parts of a fire's perimeter that are roughly parallel to the main direction of spread.

Flareup: Any sudden acceleration of fire spread or intensification of the fire. Unlike blowup, a flareup is of relatively short duration and does not radically change existing plans.

Flash 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. (Also called fine fuels.)

Flashover: rapid combustion and/or explosion of unburned gases trapped at some distance from the main fire front. Usually occurs only in poorly ventilated topography. More commonly associated with structural fire behavior.

Foehn: (Pronounced "fern") A dry wind characteristic of mountainous regions with a strong downward component. It is usually, but not always, warm for the season. Locally called by various names such as: Santa Ana, Mono, Chinook, etc.

Fog: A cloud at or near the earth's surface. Fog consists of numerous droplets of water which individually are so small that they cannot readily be distinguished by the naked eye.

Friction Layer: the layer of the atmosphere in which the frictional force of the earth's surface exercises and appreciable influence on winds.

Front: A transitional zone between two airmasses of differing densities.

Frost: Crystals of ice formed and deposited like dew, but at a temperature below freezing.

Fuelbreak: A wide strip or block of land on which the native or preexisting vegetation has been permanently modified so that fires burning into it can be more readily extinguished. It may or may not have fire lines constructed in it prior to fire occurrence.

Fuel Energy Available Fore Convection: fuel energy actually fed into the base of the convection column.

Fuel Loading: The weight of fuels in a given area, usually expressed in tons per acre. Fuel loading may be referenced to fuel size or timelag categories; may include surface fuels or total fuels.

Fuel Model: A simulated fuel complex for which all the fuel descriptors required for the solution of the mathematical fire spread model have been specified.

Fuel Moisture Content: The amount of water in a fuel, expressed as a percentage of the oven dry weight of that fuel.

Fuel Type: An identifiable association of fuel elements of distinctive species, form, size, arrangement, or other characteristics. General fuel types area grass, brush timber, and slash.

General Fire Weather Forecast: A forecast issued daily during the regular fire season to resource management agencies that is intended for planning of daily fire management activities, including daily manning levels, prevention programs, and initial attack on wildfires.

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: A wind that flows parallel to the isobars or contours and has a velocity such that the pressure gradient, Coriolis, and centrifugal forces acting in the air are in balance. It does not occur at the earth's surface due to frictional influence, but occurs at a height of roughly 1,500 feet above mean terrain height.

Ground Fire: Fire that consumes the organic material beneath the surface litter of the forest floor, such as peat fire.

Ground Fuels: All combustible materials lying beneath the ground surface including deep duff, roots, rotten buried logs, peat and other woody fuels.

Gust: A sudden, brief increase in the speed of the wind.

Hazard: A fuel complex defined by kind, arrangement, volume, condition, and location that forms a special threat of ignition or of suppression difficulty.

Head Fire: A fire spreading or set to spread with the wind.

Head of Fire: The most rapidly spreading portion of a fire's perimeter, usually to the leeward or upslope.

Heat Per Unit Area: The heat released by a square foot of fuel while the flaming zone of the fire is in that area (BTUs/sq. Ft.)

Heat Transfer: The transfer or exchange of heat energy by radiation, conduction or by convection.

Heavy Fuels: Fuels of large diameter such as snags, logs, and large limbwood that ignite and are consumed much more slowly than flash fuels. Also called coarse fuels.

High: An area of high atmosphere pressure that has a more or less closed circulation; an anticyclone.

Holdover Fire: A fire that remains dormant for a considerable time. (Also called a hangover fire or Sleeper fire.)

Horizontal Continuity: The extent or horizontal distribution of fuels at various levels or planes.

Hotspotting: Checking the spread of fire at points of more rapid spread, or special threat. It is usually the initial step in prompt control with emphasis on first priorities.

Humidity: The measure of water vapor content in the air.

Hygrometer: An instrument for measuring the humidity in the air.

Hygrothermograph: An instrument that records automatically and continuously both temperature and relative humidity.

Ignition: The initiation of combustion.

Ignition Component: A rating of the probability that a firebrand will cause a detectable fire. It is calculated from air temperature, shading, fuel moisture, fuel temperature, wind, slope, and fuel model. (See probability of ignition..)

Ignition Temperature: The lowest temperature of a substance at which sustained combustion can be initiated. (Also called kindling point.)

Indirect Attack: A method of suppression in which the control line is mostly located along natural firebreaks, favorable breaks in topography, or at considerable distance from the fire, and all intervening fuel is backfired or burned out. The strip to be backfired is wider than in the parallel method and usually allows a choice of the time when burnout or backfiring will be done.

Insolation: Solar radiation received at the earth's surface.

Instability: A state of the atmosphere in which the vertical distribution of temperature is such that an air particle, if given either an upward or a downward impulse, will tend to move vertically away with increasing speed from its original level.

Inversion: A layer in the atmosphere where the temperature increases with altitude.

Isobar: A line that connects points of equal pressure.

Ladder fuels: Fuels which provide vertical continuity between strata. Fire is able to carry from surface fuels by convection into the crowns with relative ease.

Land and Sea Breezes: The breezes that, on certain coasts and under certain conditions, blow from the land by night and from the water by day.

Lapse Rate: The rate of decrease of temperature with height. Normal lapse rate is 3.5 degrees F. per 1000 ft. The dry lapse rate is 5.5 degrees F. per 1000 ft.

Lightning: A sudden visible flash of energy and light caused by electrical discharges from thunderstorms.

Lightning Activity Level: A numerical rating of 1 to 6, keyed to the start of thunderstorms and the frequency and character of cloud-to ground lightning forecasted or observed on a rating area during the rating period.

Line Firing: setting fire to only the border fuel immediately adjacent to the control line.

Line Source Fire Predictions: Predictions that apply to a fire that has become large; that no longer has the basic elliptical shape; that could have significantly different burning conditions along the line; and the growth of which must be considered from more than one point along the fire line.

Litter: The uppermost layer of loose debris composed of freshly fallen or slightly decomposed organic material such as dead sticks, branches, twigs and leaves or needles.

Local Winds: Small-scale convective winds of local origin by temperature differences.

Long Range Spotting: Large flowing 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: An area of low atmospheric pressure, also a depression or cyclone in which winds tend to move in a counterclockwise direction.

Microclimate: A small site or habitat with essentially uniform climate, fuel modifying characteristics, and burning conditions.

Midflame Winds: The wind speed that affects a surface fire and is used in the mathematical fire behavior model. It is usually less than the standard 20 foot wind speed.

Millibar: A unit of pressure equal to a force of 1,000 dynes per square centimeter.

Mixing Layer: That portion of the atmosphere from the surface up to the mixing height. This is the layer of air, usually a sub-inversion layer, within which pollutant are mixed by turbulence and diffusion.

Mobil Weather Unit Forecast: A special fire weather forecast for a specific fire prepared by a meteorologist on site at or near the fire area.

Moist Airmass: A portion of the atmosphere that has a relatively high dew point temperature and relative humidity, and has a higher probability of producing clouds, fog or precipitation.

Moisture of Extinction: The fuel moisture content at which the fire will not spread of spreads only sporadically and in a non predictable manner.

Orographic: Of, or pertaining to, or caused by mountains.

Patrol: 1. To travel a given route to prevent, detect, and suppress fires. 2. To go back and forth watchfully over a length of control line during or after its construction to prevent breakovers, control spot fires, or extinguish overlooked hotspots.

Packing Ratio: the function of a fuel bed occupied by fuels, or the fuel volume divided by bed volume.

Point Source Fire Predictions: predictions that apply to an initiating fire burning during a time when conditions have been relatively constant, or where it can be assumed that the fire will maintain a basically elliptical shape.

Precipitation: the collective name for moisture in either liquid or solid form large enough to fall from the atmosphere and reach the earth's surface.

Pressure Gradient: The change in atmospheric pressure per unit of horizontal distance.

Prescribed Burning: The burning of forest or range fuels on a specific area under predetermined conditions so that the fire is confined to theat area to fulfill silvicultural, wildlife management, sanitary or hazard reduction requirements, or otherwise achieve forestry or range objectives.

Probability of Ignition: A rating of the probability that a firebrand (glowing or flaming) will cause a fire, providing it lands on receptive fuels. It is calculated from air temperature, fuel shading and fuel moisture. (See Ignition Component.).

Project Fire: usually refers to a fire requiring manpower and equipment beyond the resources of the protection unit on which it originates. (Also called a campaign fire.)

Psychrometer: An instrument for measuring atmospheric temperature and humidity, consisting of a dry-bulb thermometer (bulb covered with a muslin wick); used in the calculation of dew point and relative humidity.

Psychrometric Table: Table prepared from the psysrometric formula and used to obtain relative humidity and dew point from values of we-bulb and dry-bulb temperatures.

Radiation: The process by which energy is propagated through any medium by virtue of the waves motion of that medium, as in propagation of heat waves through the atmosphere.

Radiational Cooling: The cooling of the earth's surface. At night, the earth suffers a net heat loss due to terrestrial cooling.

Radiosonde: a device carried aloft by a balloon with measuring instruments that automatically convert temperature, pressure, and humidity data into electrical impulses and transmits this information ta ground recorder.

Rainfall: A term sometimes synonymous with rain, but most frequently used in reference to amounts of precipitation (including, snow, hail, etc).

Rain Gage: An instrument for measuring precipitation.

Rate of Spread: the relative activity of a fire in extending its horizontal dimensions. It is expressed as rate of increase of the total perimeter of the fire; or as rate of forward spread of the fire front; or as rate of increase in area, depending on the intended used of the information. Usually its (forward) rate of spread is expressed in chains or acres per hour.

Reburn: 1) Subsequent burning of an areas 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 behavior. The purpose is to call attention of forecast users to special conditions of limited duration that may result in extreme burning conditions. In addition to being used when extreme burning conditions are expected, the term may be employed when rapid weather chance is expected to cause an important increase in danger without actually reaching the extreme stage. (A red flag may be specified for a given period or to continue until specifically removed from the forecast statement.)

Relative Humidity: the ratio of the amount of moisture in the air to the amount which the air could hold at the same temperature and pressure if it were saturated; usually expressed in percent.

Resistance to Control: The relative difficulty of constructing and holding a control line as affected by resistance to line construction and fire behavior. (Also called difficulty of control.)

Ridge: An elongated area of relatively high pressure extending from the center of a high pressure region.

Roll Cloud: A turbulent altocumulus type cloud formation found in the lee of some large mountain barriers. The air in the cloud rotates around an axis parallel to the range. Also sometimes refers to part of the cloud base along the leading edge of a cumulonimbus cloud; it is formed by rolling action in the wind shear region between cool downdrafts within the cloud and warm updrafts outside the cloud. (Sometimes called Rotor Cloud.)

Running: Behavior of a fire that is spreading rapidly, usually with a well defined head.

Safety Island: An area for escape in the event the line is outflanked or in case a spot fire causes fuels outside the control line to render the line unsafe. In firing operations, crews progress so as to maintain a safety island close at hand, allowing the fuels inside the control line to be consumed before going ahead.

Saturated Air: air that contains the maximum amount of water vapor it can hold at a given pressure and temperature (relative humidity of 100 percent).

Scorch Height: The maximum vertical height at which lethal scorching of foliage occurs. Below this height, all needles are brown and dead; above it, live and green.

Scratch Line: An unfinished preliminary control line hastily established or constructed as an emergency measure to check or slow the spread of a fire.

Security Weather Watch: Observers are posted at one or more strategic location in the proximity of a fire to detect and warn fire personnel of lending critical weather changes that might significantly affect the fire.

Short Range Spotting: Firebrands, flaming sparks, or embers are carried by surface winds, starting new fires beyond the zone of direct ignition by the main fire. The range of such spotting is usually less than 1/4 mile.

Simultaneous Ignition: A technique of broadcast burning or backfiring by which the fuel on an area to be burned is ignited at many points simultaneously and the sets are so spaced that each receives timely stimulation by radiation from the adjoining sets. By such techniques, all burn together quickly and a hot, clean burn is possible under unfavorable burning conditions where single sets would not spread..

Size and Shape: Fuel characteristics affecting the fuel moisture timelag, 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.

Slash: Debris left after logging, pruning, thinning, or brush cutting; also debris resulting from thinnings, wind, or fire. It may include logs, chunks, bark, branches, stumps and broken understory trees or brush.

Sleeper Fire: A fire that remains dormant for a considerable time after it starts. (See Holdover.)

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

Slope Percent: The ratio between the amount of vertical rise of a slope and horizontal distance as expressed in a percent. One hundred feet of rise to one hundred feet of horizontal distance equals 100 percent.

Slopover: (See Breadkover.)

Smoke Management Forecast: A forecast issued daily during specific periods advising fire managers of the atmospheric conditions for the next 36 hours with special emphasis on those elements that will affect the dispersal of pollutants from fire.

Smoke Transport: The movement and dispersion of smoke from a fire as affected by atmospheric conditions of wind, stability, and instability.

Smoldering: Behavior of a fire burning without flame and barely spreading.

Snag: A standing dead tree or part of a dead tree from which at least the leaves and smaller branches have fallen. (Often called stub, if less than about 20 feet tall.)
 

Specific Heat: The heat capacity of a system per unit mass; i.e, the ratio of the heat absorbed (or released) by unit mass of the system to the corresponding temperature rise (or fall).

Spot Fire: Fire set outside the perimeter of the main fire by flying (or rolling) sparks or embers.

Spotting: Behavior of a fire producing sparks or embers that are carried by convection columns and/or the wind and which start new fires beyond the zone of direst ignition by the main fire.

Spot Weather Forecast: A special forecast issued to fit the time, topography, and weather of each specific fire. These forecasts are issued upon request of the user agency and are more detailed, timely, and specific than zone forecasts. Usually special on site weather observations are required of the forecasting office.

Squall: 1) A strong wind characterized by a sudden onset, a usual duration of several minutes, and a rather sudden decrease. 2) A severe local storm considered as a whole; that is, wind, cloud mass and precipitation, thunder and lightning.

Squall Line: Any nonfrontal line or narrow band of active thunderstorms extending across the horizon. It is of importance to fire behavior hue to accompanying strong gusty winds and due to possibility of such a line passing between regular weather observation stations without being reported.

Stability: A state of the atmosphere in which the vertical distribution of temperature is such that an air particle resist vertical displacement from its level. (Stable Air.)

State of Weather: A brief description of current weather that expresses the amount of cloud cover, kind of precipitation, and/or restrictions to visibility being observed at a weather observation site.

Storm Center: The center of an individual disturbance with a complex of pressure, wind, clouds and precipitation. Usually refers to a widespread low pressure system that has intensified sufficiently to produce destructive or unpleasant weather. This definition does not include such local feature as individual thunderstorms or rain squalls; however, such individual features may be part of a larger storm system.

Stratosphere: The layer of the atmosphere between the troposphere and the mesosphere where the air is usually stable (in the mean, the temperature within the layer increases with elevation.

Strip Firing: setting fire to more than one strip of fuel and providing for the strips to burn together. Frequently done in backfiring against a wind where inner strips are fired first to create drafts that pull flames and sparks away from the control line. (Also used in prescribed burning.)

Sublimation: Process by which a gas is changed to a solid or a solid to a gas without going through the liquid state.

Subsidence: An extensive sinking motion of air in the atmosphere, most frequently occurring in polar highs. The subsiding air is warmed by compression and becomes more stable. Of particular importance due to the heating and drying of the air. It is often the cause of very rapid drying of fuels in the smaller size classes.

Suppress a Fire: to extinguish a fire or confine the area it burns within fixed boundaries.

Surface Area-to-Volume Ratio: The ratio of the surface area of a fuel to it's volume, using the same linear unit for measuring volume; the higher the ration, the finer the particle.

Surface Fire: A fire that burns surface litter, debris, and small vegetation.

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.

Surface Wind: The wind measured 20 feet above the average top of the vegetation. It is often a combination of local and general winds.

Temperature: A measure of the degree of hotness or coldness of a substance.

Temperature Lapse Rate: The amount of temperature change with altitude change, expressed as degrees Fahrenheit per 1,000 feet of rise or fall.

Thermal Belt: An area of a mountainous slope that typically experiences the least variation in diurnal temperature, has the highest average temperature, and thus, the lowest average relative humidity.

Thermometer: An instrument for measuring temperature; in meteorology, generally the temperature of the air.

Thunder: The sound emitted by rapidly expanding gases along the channel of a lightning discharge

Thunderstorm: A storm invariably produced only by a cumulonimbus cloud, and always accompanied by lightning and thunder; usually attended by strong wind gusts, heavy rain, and sometimes hail. It is usually of short duration, seldom over 2 to 3 hours for any one storm.

Timelag: An indication of the rate a dead fuel gains or loses moisture due to changes in its environment. The time necessary for a fuel particle to gain or lose approximately 63 percent of the difference between its initial moisture content and its equilibrium moisture content. Fuels are usually grouped into 1-hour, 10-hour, 100-hour, and 1,000-hour. Timelag categories.

Tinder: Low-density, commonly amorphous solids or aggregates or particles; includes duff, peat and rotten wood. Fuel is compact.

Topography: the configuration of the earth's surface, including its relief and the position of its natural and manmade features.

Torching: Fire burning principally as a surface fire that intermittently ignites the crowns of trees or shrubs as it advances.

Trade Winds: Two belts of winds, one on either side of the equatorial doldrums, where the winds blow almost constantly from easterly quadrants.

Transport Wind Speed: A measure of the average rate of the horizontal transport of air within the Mixing Layer. May also be the wind speed at the final height of the plume rise. Generally refers to the rate at which emissions will be transported from on area to another.

Troposphere: The lower region of the atmosphere, from the ground to the tropopause. The average condition is typified by more or less regular decrease of temperature with increasing altitude; the portion of the atmosphere where the majority of vertical currents, appreciable water vapor content, and weather exist.

Trough: An elongated area of low atmospheric pressure, usually extending from the center of a low pressure system.

Turbulence: Irregular motion of the atmosphere usually produced when air flows over a comparatively uneven surface such as the surface of the earth; or when two currents of air flow past or over each other in different directions of at different speeds.

Undercut Line: A fire line below a fire on a slope. (Also called underslung line.)

Vapor Pressure: Partial pressure, or that part of atmospheric pressure exerted by molecules of water vapor displacing air molecules.

Vertical Arrangement: The relative heights of fuels above the ground and their vertical continuity, which influences fire reaching various levels or strata.

Vertical Development of Column: Depending on fire intensity and atmospheric conditions, the smoke or convection column might rise a hundred feet or many thousands of feet. A low intensity fire with a low smoke column might be termed "two dimensional," whereas a high intensity fire with a well developed convection column rising thousands of feet into the atmosphere can be termed a "three dimensional" fire. (See convection column.)

Virga: Wisps or streaks of water or ice particles falling out of a cloud but evaporating before reaching the earth's surface.

Visibility: The greatest distance that prominent objects can be seen and identified by unaided, normal eyes. (Usually expressed in miles, or fractions of a mile.)

Warm Front: The discontinuity at the forward edge of a advancing current (or mass) of relatively warm air which is displacing a retreating colder air mass.

Waterspout: A tornado occurring over water; rarely, a lesser whirlwind over water, comparable in intensity to a dust devil over land.

Weather: The short term variations of the atmosphere in terms of temperature, pressure, wind, moisture, cloudiness. Precipitation and visibility.

Wet Bulb Depression: The difference in degrees between the dry bulb temperature and the wet bulb temperature.

Wet Bulb Temperature: The lowest temperature to which air can be cooled by evaporating water into it at a constant pressure when the heat required for evaporation is supplied by the cooling of the air. It is measured by the we bulb thermometer, which usually employs a wetted wicking on the bulb as a cooling (through evaporation) device.

Wet Line: A fire control line, usually temporary, prepared by treating the fuels with water and/or chemicals which will halt the spread of the fire.

Wildfire: 1) An unplanned wildland fire requiring suppression action, or other action according to agency policy, as contrasted with a prescribed fire burning within prepared lines enclosing a designated area, under prescribed conditions. 2) A free-burning wildfire unaffected by fire suppression measures.

Wind: The horizontal movement of air relative to the surface of the earth.
 

Wind Speed Meter: A handheld device which indicated windspeed, usually in miles per hour. (See anemometer.)