Equilibrium Moisture Content (EMC)- The level at which dead fuels neither gain nor 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 no net exchange of moisture with its environment.
Equilibrium Vapor Pressure-Occurs when there is no net gain or loss of water molecules between the air and a solid or liquid.
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 crowning and/or spotting; presence of firewhirls; 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 of denotes the temperature of melting ice, and 212 of the temperature of boiling water, both under standard atmospheric pressure.
Fine Fuels-Fuels that are less than 1/4 inch in diameter 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 Perimeter-The entire outer edge or boundary of a fire.
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.
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.
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.
Fireline- The part of a control line that is scraped or dug to mineral soil. Sometimes called Firetrail.
Firewhirl-A spinning, moving column of ascending air which carries aloft smoke, debris and flames. These range from a foot or two in diameter to small tornadoes in size and intensity.
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.
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-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 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.
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.
Front-The transition zone between two different air masses.
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 mayor may not have fire lines constructed in it prior to fire occurrence.
Fuel Moisture Content-The amount of water in a fuel, expressed as a percentage of the oven dry weight of that fuel.
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 area are in balance. It does not occur at the earth's surface due to fractional influence, but occurs at a height of roughly 1,500 feet above mean terrain height. Nearly synonymous with "Free Air" winds.
Ground Fire-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.
Head of a Fire-The most rapidly spreading portion of a fire's perimeter, usually to the leeward or upslope.
Heavy Fuels-Fuels of large diameter such as snags, logs, and large limb wood that ignite and are consumed much more slowly than flash fuels. (Also Called Course Fuels.)
High-An area of relatively high atmospheric pressure that has a more or less closed circulation; an anticyclone. Winds around a high move clockwise in the Northern Hemisphere, while spiraling out from the high toward lower pressure.
Horizontal Continuity-The 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.
Ignition-The initiation of combustion.
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 downward impulse, will tend to move vertically away with increasing speed from its original level. (Unstable air.)
Inversion-A layer in the atmosphere where the temperature increases with altitude.
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 Breeze-A light nighttime breeze which originates over the relatively cool land surface and flows out over the warmer coastal waters.
Lightning-A sudden visible flash of energy and light caused by electrical discharges from thunderstorms.
Litter-The uppermost layer of loose debris composed of freshly fallen or slightly decomposed organic materials such as dead sticks, branches, twigs, and leaves or needles.
Local Winds- Small-scale convective winds of local origin caused by temperature differences.
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-An area of relatively low atmospheric pressure in which winds tend to move in a counterclockwise direction, spiraling in toward the low's center.
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.
Psychrometer-An instrument for measuring atmospheric temperature and humidity, consisting of a dry-bulb thermometer and a wet-bulb thermometer (bulb covered with a muslin wick); used in the calculation of dew point and relative humidity.
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 use of the information. Usually its (forward) rate of spread is expressed in chains or acres per hour.
Reburn-l. 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.
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).
Scratch Line-An unfinished preliminary control line hastily established or constructed as an emergency measure to check or slow the spread of a fire.
Sea Breeze-A daytime breeze in which cooler, higher pressure air from over coastal waters moves on shore to replace heated air rising off the warmer land mass. .
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.
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 thinning, wind, or fire. It may include logs, chunks, bark, branches, stumps, and broken understory trees or brush.
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 100 feet of horizontal distance equals 100 percent.
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.)
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 direct ignition by the main fire.
.Stability-A state of the atmosphere in which the vertical distribution of temperature is such that an air particle will 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.
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 its volume, using the same linear unit for measuring volume; the higher the ratio, 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 temperatures, has the highest average temperatures, 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 anyone storm.
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.
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, and their relationships to one another.)
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 an advancing current (or mass) of relatively warm air which is displacing a retreating colder air mass.
Weather- The short-term variations of the atmosphere in terms of temperature, pressure, wind, moisture, cloudiness, precipitation, and visibility.
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 wet 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 otheraction 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.
Wildland Fuels-Any organic material, living or dead, in the ground, on the ground, or in the air, that will ignite and burn.
Wind-The horizontal movement of air relative to the surface of the earth.
Wind-driven Wildfire-A wildland fire that is controlled by a strong consistent wind.
Windspeed Meter-A handheld device which indicates wind speed, usually in miles per hour. (See Anemometer.)
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