National Weather Service United States Department of Commerce

Hazardous Heat Across the Western US into the Northern Plains; Heavy Rain and Flooding Possible in the South

Hazardous heat will persist over much of interior California, the central Great Basin, northern Rockies and Northern Plains into Thursday. Heavy to excessive rainfall will persist across the Southeast US. through mid-week. The Southwest Monsoon will continue to bring a flash flooding threat to the Four Corners Region this week. Read More >

Glossary of Fire Weather Terms


Aerial coverage - The amount or percentage of fuels above the surface as viewed in a horizontal plane.

Aerial fuels - All live and dead vegetation above, but not contiguous with, the surface.

Aspect - The direction a slope faces.

Available fuels - Fuels which will burn during the passage of a flaming front under specific environmental conditions.

Combustion - The process by which fuels burn.

Components of a fuel complex - These include the physical characteristics of the fuels (size, shape, and arrangement) as well as the dynamic characteristics (moisture content).

Critical values - The threshold value of any weather or environmental parameter at which fires may ignite and/or spread rapidly.

Critical fire weather pattern - Patterns that can quickly increase fire danger and trigger rapid fire spread.

Crown fires - Fires which spread through the tops of trees or any other aerial fuels.

Crowning - When fires move rapidly from surface fuels to aerial fuels and then spread as crown fires.

Curing - The natural seasonal drying process of annual and perennial vegetation (fuels).

Dead fuel - Any non-living organic matter that will burn.

Dead fuel moisture - The amount of water in dead fuels.

Dispatcher - A person responsible for assigning (dispatching) personnel and/or equipment to a fire or other project.

Dispersion Description - A simple interpretative conversion from the numerical Dispersion Index

Dispersion Index - A numerical rating of how well smoke will disperse. Dispersion Table

Duff - A mat of partially decomposed organic matter immediately above the mineral soil, consisting of fallen foliage, live or dead herbaceous vegetation, and decaying wood.

Extended forecast - A forecast of weather conditions for a period extending beyond two days from the day of issue.

Fine fuel - Small diameter (0-1/4 in or 0-0.64cm) fuels such as grass, leaves, pine needles, fern, tree moss, and some kinds of slash which, when dry, ignite readily and are consumed rapidly.

Fire behavior - The manner in which fuels ignite, flames develop, and fire spreads.

Fire behavior analyst - The member of an incident team that predicts fire behavior emphasizing the direction and rates of fire spread for fire suppression or control. 

Fire danger - A subjective expression of an objective assessment of environmental (fuels and weather) factors which influence whether fires will start and how they may spread.

Fire environment - The environmental conditions, influences, and modifying forces that determine the behavior of fires.

Fire frequency - The number of fires occurring in a given area over a given period of time.

Fire growth - The increase in fire size (acres or hectares).

Fire ignition - The initiation of a fire.

Fire intensity - A measure of the heat intensity of the flaming zone of a fire front. Objectively expressed as BTU/square meter. Subjectively expressed in terms of fuel consumption, e.g., total, partial, etc.

Fire lines - Fire control lines constructed by various mechanized equipment (e.g., heavy tractors, aerial retardant, etc.) or by crews with hand tools.

Fire manager - The member of the incident team that oversees land management based on available resources, required protection, and the role of fire in that environment.

Fire spread - The movement of fire in all directions.

Fire weather watch - A watch issued when the forecaster feels reasonably confident that red flag conditions will develop in the next 12 to 48 hours.

Firebrands - Pieces of burning debris such as twigs, branches, logs or pine cones that may contribute to fire spread by rolling downhill or being carried aloft in the fire convection column.

Flame length - The distance measured from the tip of the flame to the middle of the flaming zone at the surface.

Foehn - A strong downslope wind characterized by the effects of adiabatic (compressional) warming and drying.

Forecast funnel - The methodology used in preparing a weather forecast by starting with large-scale features and working down in scale to smaller features to determine the weather that will affect a specific region.

Fuel - Any organic matter on the ground or above the surface that will ignite and burn.

Fuel bed - The total volume of fuel in a given area.

Fuel bed depth - The depth of the fuel bed including any duff and surface fuels.

Fuel complex - The composite of the various combustible components of vegetation, live and dead, that occur on a site.

Fuel loading - The amount of organic material in a fuel complex measured in metric tons/hectare.

Fuel moisture content - The amount of water in a fuel, expressed as the percentage of its weight without moisture.

Ground fuels - See surface fuels.

Haines Index - A lower atmospheric stability and dryness index that can be one predictor of large fire growth due to plume-dominated fires.

Headline - A brief statement at the beginning of a forecast that highlights dangerous or changing weather conditions.

Ignition - The initiation of combustion.

Ignition point - The fuel temperature at which spontaneous combustion occurs.

Incident meteorologist - A meteorologist trained and certified to provide on-site weather forecasting support to an incident management team at a fire or other incident.

Key station - A specific weather observing site in a zone.

Key station forecast - A forecast that takes into account an observing station location and the microscale effects on weather parameters in presenting exact values of weather parameters rather than ranges.

Lightning activity level (LAL) - An objective rating system used in the NFDRS that indicates the amount of cloud-to-ground lightning observed or forecast in a given area.

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

Live fuel - Any live organic matter available to burn.

Live fuel moisture - The amount of water in live fuels.

Logging slash - Residue (slash) from logging, such as limbs.

Marine layer - A shallow layer of air with relatively high humidity and cooler temperatures that moves from the ocean over land. It may be associated with diurnal land/sea breeze regimes or other features that increase on-shore pressure gradients.

Mesoscale - A scale that ranges in size from a few kilometers to about 100 kilometers.

Microscale - A scale that covers phenomena smaller than those in the mesoscale range.

Miller-Holzworth method - A method of determining mixing height and transport winds used to characterize smoke dispersal conditions.

Moisture factor - The ambient moisture (relative humidity) that effects fuels and the fire environment.

Moisture of extinction - The fuel moisture content at which combustion will not be sustained independently.

Model output statistics (MOS) - A generation of point specific output from a numerical model.

National fire danger rating system (NFDRS) - A national system used by all land management agencies to assess fuels, weather, and fire potential on a daily basis during fire season.

National fire danger rating system model - A group of 20 models that represents various types of fuels.

Oven dry weight - The resultant weight of a fuel sample after all moisture has been driven off by oven drying.

Peat - Partly decayed, moisture-absorbing plant matter used as a plant covering or fuel.

Plume-dominated fires - Fires with well-developed convection columns whose spread is normally characterized by pulsing, usually in all directions.

Prescribed fire/burn - A natural or human-ignited fire burning under a strict set of predetermined conditions to fulfill specified land management objectives.

Probability of ignition - A rating describing the probability that a firebrand that lands on a fuel will cause a fire.

Red flag criteria - A locally determined set of criteria that expresses environmental and meteorological conditions that would provide for fire starts and rapid, dangerous fire spread.

Red flag warning - A warning issued by fire weather forecasters when red flag criteria are met or expected to be met within 12-24 hours. The warning highlights weather of particular importance to fire behavior and potentially extreme burning conditions or many new fires. It is also used to alert customers to changes in weather that increase the fire danger. Red flag warnings should always be coordinated with the customer.

Short-range forecast - A weather forecast made for a time period generally not greater than 48 hours in advance.

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 small, broken trees or brush.

Slope - The percent of incline or steepness.

Smoke management parameters - The weather parameters used to forecast smoke dispersal.

Spot weather forecast - A forecast tailored for a specific fire. These forecasts are issued on an as-needed basis and are requested by a customer.

Spotting - The process of fire spread by sparks, embers, or firebrand that are carried by the wind beyond an on-going fire.

Stability factor - This is determined by temperature differences between two atmospheric layers.

Suppression - The act and techniques of putting fires out.

Surface fuels - All combustible material on surface, including any duff layers or piled slash.

Synopsis - A statement giving a brief general review or summary.

Thermal belts - An area along the middle of a mountain slope that typically experiences the least diurnal variation in temperature and humidity, thus has the highest daily average temperatures, and the lowest average relative humidity.

Timber litter - Mixed litter, leaves, needles, branches, twigs, or bark fallen from trees.

Timelag - An indication of the rate a dead fuel gains or loses moisture due to changes in its environment.

Topography - A detailed description of surface features including rives, lakes, etc.

Transport winds - Winds in the lower mixed layer, used for smoke dispersal forecasts.

Urban-wildland interface - The region where urban development encroaches on wildlands.

Ventilation index - This is the product of the mixing height and transport wind speed, and is an indicator of dispersion potential.

Wetting rain - A rain event that produces enough rain to sufficiently wet fuels so that ignition or sustained combustion is unlikely.

Wind-driven fires - Fires that spread primarily as a result of winds. These fires typically have elongated elliptical patterns, burn actively due to increased oxygen supply, and have long flame lengths with proportionally short flame heights that cause rapid heating of fuels ahead of the flaming front.

Zone weather forecast - A portion of the general fire weather forecast issued on a regular basis during the normal fire season specifically to fit the requirements of fire management needs. These zones or areas are a combination of administrative and climatological areas, usually nearly the size of an individual forest or district.