Absolutely stable: an atmospheric stability condition that exists when the environmental lapse rate is less than the moist adiabatic lapse rate.

Absolutely unstable: an atmospheric stability condition that exists when the environmental lapse rate is greater than the dry adiabatic lapse rate.

Absorption: the removal of energy or particles from a beam by the medium through which it propagates.

Acid precipitation: precipitation, as rain, snow or sleet containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.

Acid rain: see acid precipitation.

Acoustic sounder: see sodar.

Adiabat: a line on a thermodynamic chart relating the pressure and temperature of a substance (such as air) that is undergoing a transformation in which no heat is exchanged with its environment.

Adiabatic compression: the increase in air density that occurs thermodynamically when an air parcel is brought to a lower altitude and thus a higher pressure without gain or loss of heat.

Adiabatic lapse rate: the rate of decrease of temperature experienced by a parcel of air when it is lifted in the atmosphere under the restriction that it cannot exchange heat with its environment. When lifted, unsaturated parcels cool at the dry adiabatic lapse rate, while saturated parcels cool at the moist adiabatic lapse rate.

Adiabatic process: a process which occurs with no exchange of heat between a system and its environment.

Advect: to move by the process of advection.

Advection: the horizontal transport of atmospheric properties.

Advection fog: a fog that forms when warm air flows over a cold surface and cools from below until saturation is reached.

Aeroallergen: any of a variety of allergens such as pollens, grasses or dust carried by winds.

Aerosol: a system of colloidal particles dispersed in a gas, such as smoke or fog.

Air mass: a body of air covering a relatively wide area and exhibiting horizontally uniform properties.

Air mass thunderstorm: a thunderstorm produced by local convection within an unstable air mass, as opposed to thunderstorms associated with fronts or instability lines.

Air pollutant: harmful substance or product introduced into the atmosphere.

Air pollution potential: the meteorological potential for air pollution problems, considered without regard to the presence or absence of actual pollution sources.

Air quality model: mathematical or conceptual model used to estimate present or future air quality.

Air toxic: toxic air pollutant.

Albedo: reflectivity; the fraction of radiation striking a surface that is reflected by that surface.

Along-valley wind system: a closed, thermally driven, diurnal mountain wind whose lower branch blows up or down the axis of a valley. The upper branch blows in the opposite direction, thereby closing the circulation.

Alto-: a prefix used in naming mid-level clouds.

Altocumulus: a cloud of a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. These clouds are of medium altitude, about 8000-20,000 ft (2400-6100 m).

Altostratus: a cloud of a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. These clouds are of medium altitude, about 8000 to 20,000 ft (2400-6100 m).

Ambient: of the surrounding area or environment.

Anemometer: an instrument for measuring wind speed.

Aneroid barometer: an instrument for measuring atmospheric pressure in which a needle, attached to the top of an evacuated box, is deflected as changes in atmospheric pressure cause the top of the box to bend in or out.

Anthropogenic: caused or produced by humans.

Anticyclogenesis: the formation or intensification of an anticyclone or high pressure center.

Anticyclone: a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.

Anti-wind: the upper or return branch of an along-valley wind system, confined within a valley and blowing in a direction opposite to the winds in the lower altitudes of the valley.

Area source: an array of pollutant sources, so widely dispersed and uniform in strength that they can be treated in a dispersion model as an aggregate pollutant release from a defined area at a uniform rate. Compare line source and point source.

Aspect (angle): the cardinal direction or bearing angle toward which a slope faces.

Atmospheric boundary layer: see boundary layer.

Azimuth angle: 1. the direction or bearing toward which a sloping surface faces (e.g., a north-facing slope has an azimuth angle of 360°; a northeast-facing slope, an azimuth angle of 45°). 2. the arc of the horizon measured clockwise from north to the point where a vertical circle through a given heavenly body intersects the horizon (e.g., used for solar azimuth angle).

Backfire: 1. a fire started to stop an advancing fire by creating a burned area in its path. 2. a firing pattern or fire ignition pattern for a prescribed fire, in which a fire is ignited across the leeward edge of an area and burns into the wind. Backfires can be ignited in parallel strips to protect an area from fire escapement.

Banner cloud: a cloud plume produced by rising motions in an eddy that forms downwind of steep, isolated mountain peaks; may occur even on otherwise cloud-free days.

Barometer: an instrument for measuring atmospheric pressure.

Barometric pressure: the pressure of the atmosphere as indicated by a barometer.

Barrier jet: a jet wind current that forms when a stably-stratified low-level airflow approaches a mountain barrier and turns to the left to blow parallel to the longitudinal axis of the barrier.

Belt weather kit: belt-mounted case with pockets fitted for anemometer, compass, sling psychrometer, water bottle and forms. Used to provide on-site weather observations for the fire weather forecaster or fire behavior analyst.

Bernoulli effect: see Venturi effect.

Billow cloud: a cloud consisting of broad parallel bands oriented perpendicular to the wind. The cloud forms when wind shear occurs across a sharp change in temperature in a cloudy atmosphere. When wind shear is strong, the clouds can resemble breaking waves.

Blizzard: a severe weather condition characterized by low temperatures and strong winds (32 mph or higher) bearing a great amount of snow (including snow picked up from the ground) that reduces visibility to less than 500 ft.

Blocked (flow): flow approaching a mountain barrier at elevations below the barrier height that is too weak or too stable to be carried over the barrier.

Blowdown: a tree or stand of timber that has been blown down by the wind. Same as windthrow.

Blowup fire: a fire that suddenly increases in intensity or rate of spread. It is usually accompanied by intense convection and adversely affects fire control activities or fire suppression plans.

Bora: a regional downslope wind whose source is so cold that it is experienced as a cold wind, despite compression heating as it descends the lee slope of a mountain range.

Boundary layer: the layer of fluid in the immediate vicinity of a bounding surface; in the atmosphere, the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the surface of the earth.

Box model: a computer model used to calculate air pollution concentrations by assuming that pollutants emitted into a box-shaped volume are immediately and uniformly dispersed throughout the volume. The sides and bottom of the box, for example, could be defined by the sidewalls and floor of a valley and the top could be defined by the mixing depth.

Brocken specter: an optical effect characterized by concentric rings of color (red outermost and violet innermost) surrounding the shadow of an observer's head when the shadow is cast onto a cloud deck below the observer's elevation. Same as glory.

Build: to increase in pressure: a building ridge or high (anticyclone).

Canopy: the cover formed by the leafy upper branches of the trees in a forest; height stratum containing the crowns of the tallest vegetation present (living or dead).

Canyon wind: 1. a foehn wind that is channeled through a canyon as it descends the lee side of a mountain barrier. Example: Wasatch wind. 2. any along-canyon wind.

Cap cloud: a stationary lens-shaped cloud forming over a mountain peak, with cloud base below the mountain top.

Capping inversion: an elevated inversion layer that caps a convective boundary layer, keeping the convective elements from rising higher into the atmosphere.

Ceilometer: a device using a laser or other light source to determine the height of a cloud base. An optical ceilometer uses triangulation to determine the height of a spot of light projected onto the base of the cloud; a laser ceilometer determines the height by measuring the time required for a pulse of light to be scattered back from the cloud base.

Centerfire: a firing pattern or fire ignition pattern for prescribed fires, in which a fire is ignited at a chosen spot and then in concentric circles or spirals around the spot to produce a fire with strong central convection, inward spread and high fire intensity. This firing pattern is modified on slopes to account for the faster speed of fire propagation up the slope.

Chemistry model: a computer model used in air pollution investigations that simulates chemical and photochemical reactions of pollutants during their transport and diffusion.

Chinook: the name given to the foehn in North America, especially used on the eastern side of the Rocky Mountains and Sierras.

Chinook arch: a foehn cloud formation appearing as a bank of altostratus clouds east of the Rocky Mountains, heralding the approach of a chinook. It forms in the rising portion of standing waves on the lee side of the mountains. An observer underneath or east of the cloud sees an arch of clear air between the cloud’s leading edge and the mountains below. The cloud appears to converge with the mountains to the north and south due to a perspective effect.

Chinook pause: same as foehn pause.

Cirro-: a prefix used in naming high clouds.

Cirrocumulus: a cirriform cloud characterized by thin, white patches, each of which is composed of very small granules or ripples. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).

Cirrostratus: a cloud of a class composed of ice crystals and appearing as a whitish and usually somewhat fibrous veil, often covering the whole sky and sometimes so thin as to be hardly discernible. Haloes are often seen in cirrostratus. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).

Cirrus: a cloud of a class characterized by thin white filaments or narrow bands and composed of ice crystals. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).

Class I Area: geographic areas designated by the Clean Air Act where only a small amount or increment of air quality deterioration is permissible.

Clean Air Act: a U.S. law designed to protect and enhance the quality of the nation’s air and to protect public health and welfare.

Clear ice: a thin coating of ice on terrestrial objects caused by rain that freezes on impact. The ice is relatively transparent, as opposed to rime ice, because of large drop size, rapid accretion of liquid water or slow dissipation of latent heat of fusion.

Climate: the composite or generally prevailing weather conditions of a region throughout the year, averaged over a series of years.

Climatology: the science that deals with the phenomena of climates or climatic conditions.

Cloud condensation nucleus: a particle, either liquid or solid, upon which water condenses to form cloud droplets.

Cold air avalanche: downslope flow pulsations that occur at more or less regular intervals as cold air builds up on a peak or plateau, reaches a critical mass and then cascades down the slopes.

Cold air damming: a process in which a shallow cold air mass is carried up the slope of a mountain barrier, but with insufficient strength to surmount the barrier. The cold air, trapped upwind of the barrier alters the effective terrain configuration of the barrier to larger-scale approaching flows.

Cold front: a zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.

Cold occlusion: a frontal zone formed when a cold front overtakes a warm front. When the air behind the cold front is colder than the air ahead of the warm front, the cold air slides under the warm front, lifting it aloft. Compare warm occlusion.

Collection efficiency: the fraction of droplets approaching a surface that actually deposit on that surface.

Colorado Low: a low pressure storm system that forms in winter in southeastern Colorado or northeastern New Mexico and tracks northeastward across the central plains of the U.S. over a period of several days, producing blizzards and hazardous winter weather.

Complex terrain: mountainous terrain. In general usage, it may also refer to coastal regions and heterogeneous landscapes.

Compression heating: the temperature increase produced thermodynamically when an air parcel is compressed by bringing it to a lower altitude and therefore a higher pressure.

Conditionally unstable: An atmospheric stability condition that exists when the environmental lapse rate is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate.

Conduction: flow of heat in response to a temperature gradient within an object or between objects that are in physical contact.

Coning: pattern of plume dispersion in a neutral stability atmosphere, in which the plume attains the form of a cone with its vertex at the top of the stack.

Continental Divide: the line of summits in the Rocky Mountains that separate streams flowing toward the Gulf of California and Pacific from those flowing toward the Gulf of Mexico, Hudson Bay and the Arctic Ocean.

Continentality: the degree to which the climate of a region typifies that of the interior of a large landmass.

Contrail: condensation trail from a jet aircraft.

Convection: 1. vertical air circulation in which warm air rises and cool air sinks, resulting in vertical transport and mixing of atmospheric properties. 2. flow of heat by this circulation.

Convection column: a rising column of gases, smoke, fly ash, particulates and other debris produced by a fire.

Convection-dominated fire: see plume-dominated fire.

Convective boundary layer: the boundary layer that forms at the surface and grows upward through the day as the ground is heated by the sun and convective currents transfer heat upwards into the atmosphere. Same as unstable boundary layer.

Convective cloud: cloud formed by convection, with strong vertical development.

Convective overdevelopment: convection that covers the sky with clouds, thereby cutting off the sunshine that produces convection.

Convergence: a net flow of air into a given region. Compare divergence.

Coriolis force: a fictitious force used to account for the apparent deflection of a body in motion with respect to the earth, as seen by an observer on the earth. The deflection (to the right in the Northern Hemisphere) is caused by the rotation of the earth.

Corona: a white or colored circle or set of concentric circles of light of small radius seen around a luminous body, esp. around the sun or moon. The color varies from blue inside to red outside and the phenomenon is attributed to diffraction of light by thin clouds or mist (distinguished from halo).

Cross-valley wind system: a thermally driven wind that blows during daytime across the longitudinal axis of a valley toward the heated sidewall.

Crown: 1. verb, (of a fire) to spread from the understory into the crown of a forest canopy; to spread rapidly across the overstory of a forest. 2. noun, the top or highest part of the forest canopy.

Crown fire: a fire where flames travel from tree to tree at the level of the trees' crowns or tops.

Cumuliform: (of cloud elements) having approximately equal vertical and horizontal extent; resembling cotton balls.

Cumulonimbus: a cloud indicative of thunderstorm conditions characterized by large, dense towers that often reach altitudes of 30,000 ft (9000 m) or more, cumuliform except for their tops, which appear fibrous because of the presence of ice crystals.

Cumulus: a cloud characterized by dense individual elements in the form of puffs, mounds or towers, with flat bases and tops that often resemble cauliflower. They are found at a lower altitude than altocumulus, usually below 8000 ft (2400 m).

Cumulus stage: The first phase in the life cycle of a thunderstorm, characterized by warm, moist air rising in a buoyant plume or in a series of convective updrafts.

Cyclone: a large-scale circulation of winds around a central region of low atmospheric pressure, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.

Cyclogenesis: the formation or intensification of a cyclone or low-pressure storm system.

Damp haze: haze in which the individual particles have grown by the absorption of water, often when relative humidity is below 100%.

Dart leader: a faint, negatively charged lightning channel that travels more or less directly and continuously from cloud to ground.

Day length: duration of the period from sunrise to sunset.

Dead fuel moisture: moisture content of non-living fuels. For dead fuels, moisture content is governed primarily by exposure to precipitation or soil moisture and by the tendency of the dead fuels to approach equilibrium with the relative humidity of the surrounding air.

Deepen: to decrease in atmospheric pressure: a deepening trough or low (cyclone).

Decaying stage: The third and final phase in the life cycle of a thunderstorm, characterized by downdrafts throughout the cloud.

Density: mass per unit volume.

Dew-point temperature: the temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur.

Diagnostic model: a computer model used to calculate air pollution concentrations. A diagnostic model produces a wind field over an area by interpolating in space and time from actual wind observations.

Diffusion: the spreading of atmospheric constituents or properties by turbulent and molecular motions of the air.

Dispersion: transport and diffusion of pollutants.

Diurnal: daily, especially pertaining to actions which are completed in 24 hours and are repeated every 24 hours.

Diurnal mountain circulation: see diurnal mountain winds.

Diurnal mountain winds: a diurnally reversing closed cellular wind current resulting from horizontal temperature contrasts caused by different rates of heating or cooling over adjacent surfaces; includes slope, cross-valley, along-valley, mountain-plain and sea breeze circulations. Also called thermally driven winds.

Divergence: the net flow of air from a given region. Compare convergence.

Dividing streamline height: in the blocked flow region upwind of a mountain barrier, the height above ground of the streamline that separates the blocked flow region near the ground from the air aloft which flows over the barrier.

Domain: in numerical models, the geographical area over which a simulation is performed.

Downburst: a strong downdraft of air from a cumulonimbus cloud, of limited duration and often associated with intense thunderstorms.

Downslope wind: a thermally driven wind directed down a mountain slope and usually occurring at night; part of the slope wind system.

Downslope windstorm: a windstorm on the lee side of a mountain barrier produced by foehn or chinook winds.

Down-valley wind: a thermally driven wind directed down a valley’s axis, usually occurring during nighttime; part of the along-valley wind system.

Downwash: a deflection of air downward behind a flow obstruction.

Drainer: a valley or basin from which air drains continuously during nighttime rather than becoming trapped or pooled.

Drift: sprayed or dusted material that does not deposit on the target area.

Drizzle: precipitation consisting of numerous minute droplets of water less than 0.5 mm (500 micrometers) in diameter.

Dry adiabatic lapse rate: the rate (5.4°F per 1000 ft or 9.8°C per km) at which the temperature of a parcel of dry air decreases as the parcel is lifted, under the assumption that no heat is exchanged with its environment.

Dry bulb thermometer: a thermometer having a dry bulb; used in conjunction with a wet bulb thermometer in a sling psychrometer.

Dry haze: fine, dry dust or salt particles dispersed in the atmosphere. See haze.

Dry line: the roughly north-south boundary between moist air in the Mississippi Valley and dry air on the west side of the Great Plains descending from the Mexican Plateau and Southern Rockies. Thunderstorms often form along this line, which moves eastward during the morning and westward in the evening.

Duff: organic matter in various stages of decomposition on the floor of a forest.

Eddy: swirling currents of air at variance with the main current.

Effective topography: the topography affecting an approaching flow, which may include not only the actual terrain but also cold air masses trapped within or adjacent to the actual topography.

Elevated temperature inversion: a temperature inversion with its base above the surface of the earth.

El Niño: literally, the Christ child, a name given to an extensive ocean warming in the equatorial eastern Pacific along the coast of Peru and Ecuador that often begins around Christmas (hence, the name). The warming brings nutrient-poor tropical water southward along the west coast of South America in major events that recur at intervals of 3-7 years. El Niño is associated with atmospheric circulations that produce wide ranging effects on global weather and climate.

Emissivity: the ability of a surface to emit radiant energy compared to that of a black body at the same temperature and with the same area.

Entrainment zone: a shallow region at the top of a convective boundary layer where fluid is entrained into the growing boundary layer from the overlying fluid by the collapse of rising convective plumes or bubbles.

Environmental temperature lapse rate: the rate of decrease of air temperature with height, usually measured with a radiosonde.

Environmental temperature sounding: an instantaneous or near-instantaneous sounding of temperature as a function of height. This sounding or vertical profile is usually obtained by a balloon-borne instrument, but can also be measured using remote sensing equipment.

Equinox: the time when the sun crosses the earth’s equator, making night and day of approximately equal length all over the earth and occurring about March 21 (the spring or vernal equinox) and September 22 (autumnal equinox).

Evaporation-mixing fog: a fog that forms when the evaporation of water raises the dew point of the adjacent air.

Evapotranspiration: the process of transferring moisture from the earth's surface to the atmosphere by evaporation of water and transpiration from plants.

Extraterrestrial shortwave radiation: the theoretically-calculated radiation flux from the sun at the top of the atmosphere, before losses involved in traversing the atmosphere.

Extreme fire behavior: level of fire behavior that ordinarily precludes direct control of the fire. One or more of the following is usually involved: high rates of spread, prolific crowning or spotting, presence of fire whirls or an intense convection column.

Fall line: the line of steepest descent of a slope.

Fallstreak: same as virga.

Fanning: a pattern of plume dispersion in a stable atmosphere, in which the plume fans out in the horizontal and meanders about at a fixed height.

Fill: to increase in atmospheric pressure: a filling trough or low (cyclone).

Fine fuels: fast-drying dead fuels, generally less than 1/4 inch in diameter and having a time lag of one hour or less. These fuels (e.g., grass, twigs and needles) ignite readily and are consumed rapidly by fire when dry.

Fire behavior: the manner in which a fire reacts to the influences of fuel, weather and topography. Fire behavior is described in terms of the specific characteristics exhibited by a fire, including its rate of spread, spread direction, flame length and rate of heat release.

Fire behavior model: a computer model that uses a set of mathematical equations to predict certain aspects of fire behavior when provided with data on fuel and environmental conditions unique to a site.

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

Fire danger: the exposure to risk or harm from a fire as determined from factors that affect the start, spread, intensity and difficulty of suppression of wildfires and the damage they cause.

Fire intensity: the rate of heat release per unit time and per unit of fire travel distance at the fire front. Numerically, it is the product of the quantity of fuel consumed at the fire front, the heat yield per unit of fuel consumed and the rate of fire spread.

Fire line: a zone along a fire's edge where there is little or no fuel available to the fire.

Fire severity: the degree to which a site has been altered or disrupted by fire. Severity is dependent primarily on the product of fire intensity and duration.

Firestorm: raging fire of great intensity that spreads rapidly.

Fire weather: weather conditions that influence fire ignition, behavior or suppression.

Fire Weather Watch: a term used to alert land mangers to the potential that weather conditions when coupled with critically dry fuels can lead to dangerous wildfires.

Fire whirl: a vortex similar to a dust devil that forms in the fire area, often on the lee edge of the fire. A fire whirl increases fire intensity and the frequency of spot fires.

Fire wind: a wind blowing radially inward toward a fire, produced by horizontal temperature differences (and thus pressure differences)between the heated air above the fire and the surrounding cooler free atmosphere.

Firing pattern: the specific pattern and timing of ignition of a prescribed fire to affect the direction and rate of fire spread and fire intensity.

Flankfire: 1. a fire started to stop an advancing fire by creating a burned area on its flank. 2. a firing pattern or fire ignition pattern for prescribed fires, in which the lee edge of an area to be burned is first protected by a backfire and then fires are ignited in parallel lines leading to windward from the leeward edge.

Flash: a sudden, brief illumination of a conductive channel associated with lightning, which may contain multiple strokes with their associated stepped leaders, dart leaders and return strokes.

Flash density: the number of lightning flashes per unit area in a specified time interval (e.g., one year).

Flash flood: a sudden and destructive rush of water down a narrow gully or over a sloping surface, caused by heavy rainfall.

Flow: 1. noun, wind; volume of transported fluid. 2. verb, to move along, circulate.

Flow separation: the process by which a separation eddy forms on the windward or leeward sides of bluff objects or steeply rising hillsides.

Flow splitting: the splitting of a stable airflow around a mountain barrier, with branches going around the left and right edges of the barrier, often at accelerated speeds.

Flux: the rate of transfer of fluids, particles or energy per unit area across a given surface.

Foehn: a warm, dry wind on the lee side of a mountain range, the warmth and dryness of the air being due to adiabatic compression as the air descends the mountain slopes.

Foehn pause: a temporary cessation of the foehn at the ground due to the formation or intrusion of a cold air layer which lifts the foehn off the ground.

Foehn wall: the steep leeward boundary of an extensive cloud layer that forms as air is lifted over ridges and mountains during foehn conditions.

Foehn wall cloud: same as foehn wall.

Föhn: alternate German spelling of foehn.

Fog: a cloud with its base at the earth’s surface.

Forced channeling: channeling of upper winds along a valley’s axis when upper winds are diverted by the underlying topography. Compare pressure-driven channeling.

Fractocumulus: a cumulus cloud with a ragged, shredded appearance, as if torn.

Fractostratus: a stratus cloud with a ragged, shredded appearance, as if torn. It differs from a fractocumulus cloud in having a smaller vertical extent and darker color.

Free air wind: synoptic-scale wind. In fire and land management, also called general wind.

Free atmosphere: the part of the atmosphere that lies above the frictional influence of the earth’s surface.

Freezing drizzle: drizzle that falls as a liquid but freezes into glaze or rime upon contact with the cold ground or surface structures.

Freezing level: the altitude at which the air temperature first drops below freezing.

Freezing rain: rain that falls as a liquid but freezes into glaze upon contact with the ground.

Front: an interface or zone of transition between two dissimilar air masses.

Frontal inversion: an elevated temperature inversion that develops above a frontal zone when cold air at the surface is overrun by warmer air aloft.

Frontal zone: see front.

Frostbite: human tissue damage caused by exposure to intense cold.

Fuel loading: the amount of fuel present as expressed by the weight of fuel per unit area.

Fugitive dust: dust that is not emitted from definable point sources such as industrial smokestacks. Sources include open fields, roadways, storage piles, etc.

Full-physics numerical model: a computer model used to calculate air pollution concentrations. A full-physics numerical model uses a full set of equations describing the thermodynamic and dynamic state of the atmosphere and can be used to simulate atmospheric phenomena.

Fumigation: a pattern of plume dispersion produced when a convective boundary layer grows upward into a plume trapped in a stable layer. The elevated plume is suddenly brought downward to the ground, producing high surface concentrations.

Fumigating: the form that an elevated smoke plume takes when undergoing fumigation.

Gap: a major erosional opening through a mountain range.

Gap winds: strong winds driven through low passes or major breaks in mountain barriers by cross-gap pressure gradients that develop regionally. In North America, the term is used mostly for winds in coastal mountain ranges where major river valleys issue onto seaways.

Gaussian plume model: a computer model used to calculate air pollution concentrations. The model assumes that a pollutant plume is carried downwind from its emission source by an average wind. Plume concentrations are obtained by assuming that the highest concentrations occur on the horizontal and vertical midlines of the plume, with the distribution about these mid-lines characterized by Gaussian- or bell-shaped concentration profiles.

Gaussian plume segment model: a modification of the Gaussian plume model in which wind direction changes during plume transport are handled by breaking the plume into segments.

Gaussian puff model: a computer model used to calculate air pollution concentrations. The model assumes that a continuously emitted plume or instantaneous cloud of pollutants can be simulated by the release of a series of puffs that are carried in a time- and space-varying wind field. The puffs are assumed to have Gaussian or bell-shaped concentration profiles in their vertical and horizontal planes.

General circulation: the continuous circulation of wind and ocean currents that acts to moderate the temperature differences between the poles and the equator.

General wind: land management agency term for wind produced by synoptic-scale pressure systems on which a smaller-scale or local convective wind may be superimposed. Also called free air wind.

Geostrophic wind: 1. a wind blowing parallel to isobars with strength proportional to the pressure gradient (i.e., spacing of the isobars). 2. the wind obtained theoretically by a balance between pressure gradient force and coriolis force,

Glaciation: 1. the transformation of cloud particles from water drops to ice crystals. Thus, a cumulonimbus cloud is said to have a “glaciated” upper portion. 2. land surface cover of ice or snow.

Glacier wind: a shallow downslope wind above the surface of a glacier, caused by the temperature difference between the air in contact with the glacier and the free air at the same altitude. The glacier wind does not reverse diurnally like slope and along-valley wind systems.

Glaze: a smooth, clear coating of ice, but sometimes containing some air pockets.

Glory: same as Brocken specter.

Gradient: a rate of change with respect to distance of a variable quantity, such as temperature or pressure, in the direction of maximum change.

Graupel: same as snow pellets or small hail.

Gravity wave: a wave created by the action of gravity on density variations in a stratified atmosphere. A generic classification that includes orographic waves (lee waves and mountain waves) and many other waves that form in the atmosphere.

Greenness: amount of viable or living material in vegetation. Measurements of greeness are obtained from emissivity data collected from remote sensors on satellites.

Greenhouse effect: atmospheric heating caused when solar radiation is readily transmitted inward through the earth's atmosphere but longwave radiation is less readily transmitted outward, due to absorption by certain gases in the atmosphere.

Ground heat flux: the flux of heat from the ground to the earth’s surface; a component of the surface energy budget.

Ground stroke: electrical current propagating along the ground from the point where a direct stroke of lightning hits the ground.

Hail: showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud.

Haines Index: see Lower Atmosphere Severity Index.

Halo: any of a variety of bright circles or arcs centered on the sun or moon, caused by the refraction or reflection of light by ice crystals suspended in the earth’s atmosphere and exhibiting prismatic coloration ranging from red inside to blue outside.

Haze: an aggregation in the atmosphere of very fine, widely dispersed, solid or liquid particles or both, giving the air an opalescent appearance that subdues colors. See also regional haze, layered haze, dry haze and damp haze.

Headfire: a firing pattern or fire ignition pattern for prescribed fires, in which a fire is ignited along a line perpendicular to the wind direction and runs to leeward. The headfire is often ignited in strips with the lee edges protected by backfires to guard against fire escapement.

Heat exhaustion: a mild form of heat stroke, characterized by faintness, dizziness and heavy sweating.

Heat Index: an index that combines temperature and relative humidity to determine an apparent temperature.

Heat low: a shallow low pressure center that forms in response to strong surface sensible heat flux.

Heat stroke: a condition resulting from excessive exposure to intense heat, characterized by high fever, collapse and sometimes convulsions or coma.

Hertz: an international unit of frequency equal to one cycle per second; named after a German physicist.

High: see anticyclone.

High pressure center: a region of high pressure enclosed by a pressure or height contour.

High pressure system: see anticyclone.

Humidity: a general term referring to the air's water vapor content. See relative humidity.

Hydraulic flow: atmospheric flow that is similar in character to the flow of water over an obstacle.

Hydraulic jump: a steady disturbance in the lee of a mountain, where the airflow passing over the mountain suddenly changes from a shallow, high velocity flow to a deep, low velocity flow.

Hydrometeor: liquid water or ice in the atmosphere in various forms, including rain, ice crystals, hail, fog or clouds.

Hygrometer: any instrument that measures the water vapor content of the atmosphere.

Hygroscopic: absorbing or attracting moisture from the air.

Hygrothermograph: an instrument that records, on one record, the variation with time of both atmospheric humidity and temperature.

Hypothermia: a rapid, progressive mental and physical collapse that accompanies the lowering of body temperature.

Ice crystal: precipitation consisting of small, slowly falling crystals of ice.

Ice crystal process: The process by which ice crystals are introduced into a supercooled cloud of water droplets and grow at the expense of the water droplets to produce precipitation.

Ice fog: a fog that consists of small ice crystals rather than water droplets. Ice fog usually forms at temperatures below -20°F (-29°C).

Ice nucleus: any particle that serves as a nucleus in the formation of ice crystals in the atmosphere.

Ice pellets: precipitation consisting of particles of ice less than 5 mm in diameter, occurring either as frozen raindrops or as small hailstones.

Icing: formation of a coating of ice on a solid object. See clear ice and rime.

Ideal gas law: the thermodynamic law applying to perfect gases.

Ignition pattern: see firing pattern.

Impingement: see plume impingement.

Inclination angle: the tilt angle of a surface relative to the horizon.

Insolation: solar radiation received at the earth’s surface.

Inter-tropical convergence zone: the dividing line between the northeast trade winds of the Northern Hemisphere and the southeast trade winds of the Southern Hemisphere where air converges to produce convection and generally rising air.

Inversion: see temperature inversion.

Iridescence: brilliant colored borders or spots of color in clouds, usually red and green, caused by diffraction of light by small cloud particles. The phenomenon is usually observed in thin cirrus clouds within about 30° of the sun and is characterized by bands of color in the cloud that contour the cloud edges.

Isobar: a line of equal or constant pressure; an isopleth of pressure.

Isotach: a line on a weather map or chart connecting points where winds of equal speeds have been recorded.

Isotherm: a line of equal or constant temperature; an isopleth of temperature.

Isothermal: temperature remaining constant with height or time.

Isotropic: of equal physical properties along all axes.

Jet: a fast-moving wind current surrounded by slower moving air.

Jet wind speed profile: a vertical profile of horizontal wind speeds characterized by a relatively narrow current of strong winds with slower moving air above and below. Large wind shears occur above and below the jet speed maximum.

Jet stream: strong, generally westerly winds concentrated in a relatively narrow and shallow stream in the upper troposphere.

Jet stream cirrus: a loose term for filamentous cirrus that appears to radiate from a point in the sky and exhibits characteristics associated with strong vertical wind shear, such as twisted or curved filaments.

Keetch-Byrum Drought Index: a drought index representing the net effect of evapotranspiration and precipitation in producing cumulative moisture deficiency in deep duff and upper organic soil layers. This index is widely used in the southeastern United States.

Kelvin-Helmholtz waves: vertical waves in the atmosphere associated with wind shear across stable layers. Can appear as breaking waves and as braided patterns in radar images and cloud photos.

Lake breeze: a thermally produced wind blowing during the day from the surface of a large lake to the shore, caused by the difference in the rates of heating of water and land.

Lake breeze front: the leading edge of a lake breeze, whose passage is often accompanied by showers, a wind shift and/or a sudden drop in temperature.

Lake-effect storm: a fall or winter storm that produces heavy but localized snowfall over the lee shoreline of a large open-water lake. Air is moistened and warmed as it flows over the lake, and snowfall is produced by convergence when the flow encounters increased roughness at the shoreline and is lifted up rising ground.

Land breeze: a coastal breeze at night blowing from land to sea, caused by the difference in the rates of cooling of land and water.

Lapse rate: the rate of decrease of air temperature with increase of elevation.

Latent heat: heat absorbed or released during a change of water phase (from gas, liquid or solid phases) at constant temperature and pressure.

Latent heat flux: the flux of heat from the earth’s surface to the atmosphere that is associated with evaporation or condensation of water vapor at the surface; a component of the surface energy budget.

Layered haze: haze produced when air pollution from multiple line, area or point sources is transported long distances to form distinguishable layers of discoloration in a stable atmosphere.

Leaf Area Index: projected foliage area per unit area of ground surface.

Lee: the side or part that is sheltered or turned away from the wind.

Leeward: the side away from the wind. Compare windward.

Lee wave: a wavelike oscillation of a flow that occurs in the lee of a mountain range when rapidly moving air is lifted up the steep front of a mountain range and oscillates downwind of the barrier. Compare mountain wave.

Lenticular cloud: a very smooth, round or oval, lens-shaped cloud that is often seen, singly or stacked in groups, near or in the lee of a mountain ridge.

Lidar: a device that is similar in principle and operation to radar but uses a laser to generate pulses of light (rather than radio waves) that are scattered back from aerosols in the atmosphere. The device is used to determine aerosol content and particle movement. From light detection and ranging.

Lightning: a visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air or between a cloud and the ground.

Line source: an array of pollutant sources along a defined path that can be treated in dispersion models as an aggregate uniform release of pollutants along a line. Example: the sum of emissions from individual cars traveling down a highway can be treated as a line source. Compare area source and point source.

Live fuel moisture: the ratio of the amount of water to the amount of dry plant material in living plants.

Local convective wind: in fire weather terminology, local, diurnal, thermally driven winds arising over a comparatively small area and influenced by local terrain. Examples include sea and land breezes, lake breezes, diurnal mountain wind systems and convective currents.

Lofting: a pattern of plume dispersion in a stable boundary layer topped by a neutral stability layer, in which the upper part of the plume disperses upward while the lower part of the plume undergoes little dispersion.

Longwave radiation: a term used to describe the infrared energy emitted by the earth and atmosphere at wavelengths between about 5 and 25 micrometers. Compare shortwave radiation.

Long-wave ridge: a ridge in the hemispheric Rossby wave pattern. Compare short-wave ridge.

Long-wave trough: a trough in the hemispheric Rossby wave pattern. Compare short-wave trough.

Looping: a pattern of plume dispersion in an unstable atmosphere in which the plume undergoes marked vertical oscillations as it is alternately affected by rising convective plumes and the subsiding motions between the plumes.

Loran: Long Range Navigation, a system of long range navigation whereby latitude and longitude are determined from the time displacement of radio signals from two or more fixed transmitters.

Low: see cyclone.

Lower Atmosphere Stability Index: an atmospheric index used to indicate the potential for rapid wildfire growth. This index, also called the Haines Index, contains a stability term and a dryness term.

Low-level jet: a regular, strong, nighttime, northward flow of maritime tropical air over the sloping Great Plains of the central United States, in which the wind increases to a peak in the lowest kilometer and then decreases above.

Low pressure center: a region of low pressure enclosed by a pressure or height contour.

Low pressure system: see cyclone.

Maloja wind: a wind named after the Maloja Pass between the Engadine and Bergell valleys of Switzerland which blows down the valley of the Upper Engadine by day and up the valley at night. These wind directions are contrary to mountain wind system theory and have been explained as the encroachment of winds from the Bergell into the Upper Engadine.

Marine air intrusion: invasion of an air mass with marine characteristics into a continental area.

Marine inversion: temperature inversion produced when cold marine air underlies warmer air.

Massif: a compact portion of a mountain range, containing one or more summits.

Mature stage: the second phase in the life cycle of a thunderstorm, characterized by the presence of both updrafts and downdrafts within the cloud.

Mercury barometer: an instrument for measuring atmospheric pressure. The instrument contains an evacuated and graduated glass tube in which mercury rises or falls as the pressure of the atmosphere increases or decreases.

Mesopause: the top of the mesosphere, corresponding to the level of minimum temperature in the atmosphere found at 70 to 80 km.

Mesoscale: pertaining to meteorological phenomena, such as wind circulations or cloud patterns, that are about 2 to 200 km in horizontal extent.

Mesosphere: the atmospheric layer between about 20 km and about 70 to 80 km above the surface of the earth, extending from the top of the stratosphere (the stratopause) to the upper temperature minimum that defines the mesopause (the base of the thermosphere).

Meteorology: the science dealing with the atmosphere and its phenomena.

Microburst: an intense, localized downdraft of air that spreads on the ground, causing rapid changes in wind direction and speed; a localized downburst.

Microclimate: the climate of a small area such as a cave, house or wooded area that may be different from that in the general region.

Microscale: pertaining to meteorological phenomena, such as wind circulations or cloud patterns, that are less than 2 km in horizontal extent.

Mid-flame wind: wind measured at the midpoint of the flames, considered to be most representative of the wind that is affecting fire behavior.

Millibar: a unit of atmospheric pressure equal to 1/1000 bar or 1000 dynes per square centimeter.

Mixed layer: an atmospheric layer, usually the layer immediately above the ground, in which pollutants are well mixed by convective or shear-produced turbulence.

Mixing depth: vertical distance between the ground and the altitude to which pollutants are mixed by turbulence caused by convective currents or vertical shear in the horizontal wind.

Mixing ratio: a measure of humidity; the ratio of the mass of water vapor in an atmospheric volume to the mass of dry air in the volume.

Moist adiabatic lapse rate: the rate at which the temperature of a parcel of saturated air decreases as the parcel is lifted in the atmosphere. The moist adiabatic lapse rate is not a constant like the dry adiabatic lapse rate but is dependent on parcel temperature and pressure.

Monsoon: a thermally driven wind arising from differential heating between a land mass and the adjacent ocean that reverses its direction seasonally.

Mountainado: a vertical-axis eddy produced in a downslope windstorm by the vertical stretching of horizontal roll vortices produced near the ground by vertical shear of the horizontal wind. Mountainadoes, when carried by the prevailing wind, can produce strong horizontal wind shears and wind gusts that are much more damaging than the prevailing wind speeds.

Mountain meteorology: meteorology of a mountainous or topographically complex area.

Mountain-plain wind system: a closed, large-scale, diurnal, thermally driven circulation between the mountains and the surrounding plain. The mountain-to-plain flow making up the lower branch of the closed circulation usually occurs during nighttime, while the plain-to-mountain flow occurs during daytime.

Mountain wave: 1.a wavelike oscillation of a flow that occurs above and downwind of a mountain range when rapidly flowing air encounters the mountain range’s steep front. 2. generic term for all gravity waves occurring in the vicinity of or caused by mountains. 3. specific term for waves that form above, rather than downwind of, mountains. Compare lee wave.

Mountain wind system: the system of diurnal winds that forms in a complex terrain area, consisting of mountain-plain, along-valley, cross-valley and slope wind systems.

National Ambient Air Quality Standards: in the United States, national standards for the ambient concentrations in air of different air pollutants; designed to protect human health and welfare.

National Fire Danger Rating System: a uniform fire danger rating system used in the United States that focuses on the environmental factors that impact the moisture content of fuels. Fire danger is rated daily over large administrative areas, such as national forests.

Net all-wave radiation: the net or resultant value of the upward and downward longwave and shortwave radiative fluxes through a plane at the earth-atmosphere interface; a component of the surface energy budget.

Neutral: an atmospheric stability condition that exists in unsaturated (saturated) air when the environmental temperature lapse rate equals the dry (moist) adiabatic lapse rate.

Nieve penitente: a spike or pillar of compacted snow, firn or glacier ice, caused by differential melting and evaporation. The pillars form most frequently on low-latitude mountains where air temperatures are near freezing, dew points are much below freezing and insolation is strong. Penitents are oriented individually toward the noon-day sun and usually occur in east-west lines.

Nimbostratus: a cloud characterized by a formless layer that is almost uniformly dark gray; a rain cloud of the layer type, of low altitude, usually below 8000 ft (2400 m).

Nonattainment Area: an area out of compliance with ambient air quality standards.

Obstruction: the process by which low-level air masses that form on one side or the other of a mountain barrier are prevented from crossing the barrier.

Occluded front: a composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front. Two types of occlusions can form depending on the relative coldness of the air behind the cold front to the air ahead of the warm or stationary front. A cold occlusion results when the coldest air is behind the cold front and a warm occlusion results when the coldest air is ahead of the warm front.

Orographic wave: a wavelike airflow produced over and in the lee of a mountain barrier. Collective term for lee waves and mountain waves.

Ozone: A form of oxygen, O₃. A powerful oxidizing agent that is considered a pollutant in the lower troposphere but an essential chemical in the stratosphere where it protects the earth from high-energy ultraviolet radiation from the sun.

Palmer Drought Severity Index: an index used to gage the severity of drought conditions by using a water balance equation to track water supply and demand. This index is calculated weekly by the National Weather Service.

Particle trajectory model: a computer sub-model that tracks the trajectories of multiple particles that are released into an atmospheric flow model.

Permafrost: a layer of soil at varying depths below the surface in which the temperature has remained below freezing continuously from a few to several thousands of years.

Perturbation model: a computer model used to calculate air pollution concentrations. A perturbation model produces a wind field from solutions to a simplified set of equations that describe atmospheric motions.

Phenomenological model: a computer model used to calculate air pollution concentrations. A phenomenological model focuses on an individual phenomenon, such as plume impingement or fumigation.

Photochemical smog: air pollution containing ozone and other reactive chemical compounds formed by the reaction of nitrogen oxides and hydrocarbons in the presence of sunlight.

Pibal: abbreviation for pilot balloon.

Pilot balloon: a small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction change with altitude. Abbreviated pibal.

Plume: a continuous flow of air pollutants moving horizontally and/or vertically from an emission source and dispersing at a rate determined by atmospheric conditions.

Plume blight: visibility impairment caused by air pollution plumes aggregated from individual sources.

Plume-dominated fire: a fire whose behavior is governed primarily by the local wind circulation produced in response to the strong convection above the fire rather than by the general wind.

Plume impingement: the collision of an air pollution plume with topography that rises above the plume altitude; often a temporary condition that occurs as the plume sweeps by the face of a hill as the wind shifts.

Polar front: the variable frontal zone of mid latitudes separating air masses of polar and tropical origin.

Polar front jet: a strong, generally westerly jet stream wind concentrated in a relatively narrow and shallow current in the upper troposphere above the polar front ; a feature of the general circulation.

Point source: a pollutant source that can be treated in a dispersion model as though pollutants were emitted from a single point that is fixed in space. Example: the mouth of a smokestack. Compare area source and line source.

Powder snow: dry, loose, unconsolidated snow.

Precipitation: liquid or solid water particles that fall from the atmosphere and reach the ground. See drizzle, hail, rain, snow, snow pellets.

Prescribed fire: a management-ignited or natural wildland fire that burns under specified conditions where the fire is confined to a predetermined area and produces the fire behavior and fire characteristics required to attain planned resource management objectives.

Pressure: the exertion of force upon a surface by a fluid (e.g., the atmosphere) in contact with it.

Pressure-driven channeling: channeling of wind in a valley by synoptic-scale pressure gradients superimposed along the valley’s axis. Compare forced channeling.

Pressure gradient force: the force caused by the change in atmospheric pressure per unit of horizontal distance and acting in the direction in which pressure changes most rapidly.

Prevention of Significant Deterioration: a program, specified in the Clean Air Act, whose goal is to prevent air quality from deteriorating significantly in areas of the country that are presently in compliance with ambient air quality standards.

Primary ambient air quality standards: air quality standards designed to protect human health. Compare secondary ambient air quality standards.

Primary pollutant: substances that are pollutants immediately on entering the atmosphere. Compare secondary pollutant.

Psychrometer: an instrument used to determine atmospheric humidity by comparing the readings of two thermometers, the bulb of one being kept moist and ventilated. See sling psychrometer, wet bulb thermometer and dry bulb thermometer.

Pyro-cumulonimbus: cumulonimbus formed in the convection column of a fire.

Pyro-cumulus: cumulus formed in the convection column of a fire.

Radar: a device used to detect and determine the range to distant objects (e.g., hydrometeors) or atmospheric discontinuities by measuring the time for the echo of a radio wave to return from it. From radio detection and ranging.

Radiation: energy transport through electromagnetic waves. See shortwave radiation and longwave radiation.

Radiation fog: a fog that forms when outgoing longwave radiation cools the near-surface air below its dew point temperature.

Radiosonde: an instrument that is carried aloft by a balloon to send back information on atmospheric temperature, pressure and humidity by means of a small, expendable radio transmitter. See also rawinsonde.

Rain: precipitation that falls to earth in droplets with diameters greater than 0.5 mm.

Rain shadow: an area of reduced precipitation on the lee side of a mountain barrier caused by warming of air and dissipation of cloudiness as air descends the barrier.

Rawinsonde: a radiosonde that is tracked by radar, radio direction finding or navigation systems (such as the satellite Global Positioning System) to measure winds.

Red Flag Warning: a term used by fire weather forecasters to alert land managers to an imminent or ongoing weather event that could cause dangerous fire activity.

Regional haze: haze that is mixed uniformly between the surface and the top of a convective boundary layer.

Regional scale: pertaining to meteorological phenomena that are about 500 to 5000 km in horizontal extent.

Relative humidity: the ratio of the actual water vapor pressure at a given time to the vapor pressure that would occur if the air were saturated at the same ambient temperature.

Residence time: the time, in seconds, required for the flaming front of a fire to pass a stationary point at the surface of the fuel.

Residual layer: the elevated portion of a convective boundary layer that remains after a stable boundary layer develops at the ground (usually in late afternoon or early evening) and cuts off convection.

Resonance: the state of a system in which an abnormally large vibration is produced in response to an external stimulus, occurring when the frequency of the stimulus is the same, or nearly the same, as the natural vibration frequency of the system.

Retrograding: a westward shifting of a Rossby wave pattern that normally propagates to the east.

Return stroke: an electrical discharge that propagates upward along a lightning channel from the ground to the cloud.

Ridge: on a weather chart, a narrow elongated area of relatively high pressure.

Ridging: the building or intensification of a ridge or high pressure center.

Rime: an opaque coating of tiny, white, granular ice particles caused by the rapid freezing of supercooled water droplets on impact with an object. Also called rime ice. See also clear ice.

Ringfire: a firing pattern or fire ignition pattern for prescribed fires, in which the fire is ignited in a ring to produce strong central convection, inward spread and high fire intensity; variant of a centerfire.

Rossby waves: a series of troughs and ridges on quasi-horizontal surfaces in the major belt of upper tropospheric westerlies. The waves are thousands of kilometers long and have significant latitudinal amplitude.

Rotor cloud: a turbulent altocumulus or cumulus cloud formation found in the lee of some mountain barriers when winds cross the barrier at high speed. The air in the cloud rotates around an axis parallel to the mountain range.

Sampling frequency: the rate at which sensor data are read or sampled.

Santa Ana wind: (in southern California) a strong, hot, dust-bearing foehn wind that descends to the Pacific Coast around Los Angeles from inland desert regions.

Sastrugi: ridges of snow formed by wind on a snowfield.

Saturation vapor pressure: the vapor pressure of a system at a given temperature, wherein the vapor of a substance is in equilibrium with a plane surface of that substance’s pure liquid or solid phase.

Scattering: the process in which a beam of light is diffused or deflected by collisions with particles suspended in the atmosphere.

Sea breeze: a thermally driven wind blowing during the day from over a cool ocean surface onto the adjoining warm land, caused by the difference in the rates of heating of the ocean and land surfaces.

Sea breeze convergence zone: the zone at the leading edge of a sea breeze where winds converge. The incoming air rises in this zone, often producing convective clouds.

Sea breeze front: the leading edge of a sea breeze, whose passage is often accompanied by showers, a wind shift or a sudden drop in temperature.

Secondary ambient air quality standards: air quality standards designed to protect human welfare, including the effects on vegetation and fauna, visibility and structures. Compare primary ambient air quality standards.

Secondary pollutant: pollutants generated by chemical reactions occurring within the atmosphere. Compare primary pollutant.

Sensible heat flux: the flux of heat from the earth’s surface to the atmosphere that is not associated with phase changes of water; a component of the surface energy budget.

Separation eddy: an eddy that forms near the ground on the windward or leeward side of a bluff object or steeply rising hillside; streamlines above this eddy go over the object.

Severe thunderstorm: a thunderstorm that produces heavy precipitation, frequent lightning, strong, gusty surface winds or hail. A severe thunderstorm can cause flash floods and wind and hail damage and may spawn tornadoes.

Shortwave radiation: a term used to describe the radiant energy emitted by the sun in the visible and near-ultraviolet wavelengths (between about 0.1 and 2 micrometers). Compare longwave radiation.

Short-wave ridge: a relatively small-scale ridge that is superimposed on and propagates through the longer wavelength Rossby waves.

Short-wave trough: a relatively small-scale trough that is superimposed on and propagates through the longer wavelength Rossby waves.

Sleet: in the U.S., a term used to describe tiny ice pellets that are formed when rain or partially melted snowflakes refreeze before reaching the ground. Colloquial usage of the term coincides with British usage, which defines sleet as a mixture of rain and snow.

Sling psychrometer: an instrument used to measure humidity, in which wet and dry bulb thermometers are mounted on a frame connected to a handle at one end by means of a bearing or a length of chain. The psychrometer is whirled by hand to provide the necessary ventilation to evaporate water from the wet bulb.

Slope wind system: a closed, thermally driven diurnal mountain wind circulation whose lower branch blows up or down the sloping sidewalls of a valley or mountain. The upper branch blows in the opposite direction, thereby closing the circulation.

Small hail: same as snow pellets or graupel.

Smoke management: the use of meteorology, fuel moisture, fuel loading, fire suppression and burn techniques to keep smoke impacts from prescribed fires within acceptable limits.

Snow: precipitation in the form of ice crystals, mainly of intricately branched, hexagonal form and often agglomerated into snowflakes, formed directly from the freezing of the water vapor in the air.

Snow cornice: a mass of snow or ice projecting over a mountain ridge.

Snowflake: an agglomeration of snow crystals falling as a unit.

Snow grain: precipitation consisting of white, opaque ice particles usually less than 1 mm in diameter.

Snow pellets: precipitation, usually of brief duration, consisting of crisp, white, opaque ice particles, round or conical in shape and about 2 to 5 mm in diameter. Same as graupel or small hail.

Snow pillow: a windrow of snow deposited in the immediate lee of a snow fence or ridge.

Sodar: an instrument similar in principle and operation to radar that uses sound to determine the range to distant objects (e.g., hydrometeors) or atmospheric discontinuities in temperature or humidity structure that scatter or reflect sound energy. From sound detection and ranging.

Solstice: either of the two times per year when the sun is at its greatest angular distance from the celestial equator: about June 21 (Northern Hemisphere summer solstice), when the sun reaches its northernmost point on the celestial sphere, or about December 22 (Northern Hemisphere winter solstice) when it reaches its southernmost point.

Sounding: a set of data measuring the vertical structure of one or more atmospheric parameters (e.g., temperature, humidity, pressure, wind) at a given time.

Specific gravity: the ratio of the density of any substance to the density of water.

Specific humidity: the ratio of the mass of water vapor to the mass of moist air in an atmospheric volume.

Spotfire: a firing pattern or fire ignition pattern for prescribed fires, in which a fire is ignited by firing equally spaced spots in rapid succession or using simultaneous ignition. The spacing is chosen so that the fires draw together rather than making individual runs.

Spot fire: a fire ignited outside the perimeter of the main fire by a firebrand.

Spotting: outbreak of secondary fires as firebrands or other burning materials are carried ahead of the main fire line by winds.

Spray block: a geographical area to be sprayed.

Stability: the degree of resistance of a layer of air to vertical motion.

Stable: see absolutely stable.

Stable boundary layer: the stably-stratified layer that forms at the surface and grows upward, usually at night or in winter, as heat is extracted from the atmosphere’s base in response to longwave radiative heat loss from the ground. A stable boundary layer can also form when warm air is advected over a cold surface or over melting ice.

Stable core: post-sunrise, elevated remnant of the temperature inversion that builds up overnight within a valley.

Standard Atmosphere: a hypothetical vertical distribution of temperature, pressure and density which, by international consent, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.

State Implementation Plan: a formal air quality management plan, produced by an individual state, specifying how state air resources will be managed to achieve federal and state standards.

Stationary front: a front between warm and cold air masses that is moving very slowly or not at all.

Station model: a specified pattern for plotting on a weather map the meteorological symbols that represent the state of the weather at a particular observing station.

Steam fog: an evaporation-mixing fog that develops when a cold air mass flows over a warm body of water.

Stepped leader: a faint, negatively charged lightning channel that emerges from the base of a thunderstorm and propagates toward the ground in a series of steps of about 1 microsecond duration and 50-100 meters in length, initiating a lightning stroke.

Stratiform: (of a cloud) having predominantly horizontal development.

Stratocumulus: a cloud characterized by large dark, rounded masses, usually in groups, lines or waves, the individual elements being larger than those in altocumulus and the whole being at a lower altitude, usually below 8000 ft (2400 m).

Stratopause: the boundary between the stratosphere and mesosphere.

Stratosphere: the atmospheric layer above the troposphere and below the mesosphere. It extends from the tropopause, usually 10-25 km high, to a height of approximately 20 to 25 km, where temperature begins to decrease.

Stratus: a cloud characterized by a gray, horizontal layer with a uniform base, found at a lower altitude than altostratus, usually below 8000 ft (2400 m).

Streamline: the path of an air parcel that flows steadily over or around an obstacle.

Subsidence: a descending motion of air in the atmosphere occurring over a rather broad area.

Subsidence inversion: a temperature inversion that develops aloft as a result of air gradually sinking over a wide area and being warmed by adiabatic compression, usually associated with subtropical high pressure areas.

Subtropical jet: a strong, generally westerly wind concentrated in a relatively narrow and shallow stream in the subtropical upper troposphere; a feature of the general circulation.

Supercooled: a liquid cooled below its freezing point without solidification or crystallization.

Surface-based temperature inversion: a temperature inversion with its base at the surface of the earth.

Surface energy budget: the energy or heat budget at the earth’s surface, considered in terms of the fluxes through a plane at the earth-atmosphere interface. The energy budget includes radiative, sensible, latent and ground heat fluxes.

Surface weather chart: an analyzed synoptic chart of surface weather observations. A surface chart shows the distribution of sea-level pressure (therefore, the position of highs, lows, ridges and troughs) and the location and nature of fronts and air masses. Often added to this are symbols for occurring weather phenomena. Although the pressure is referred to mean sea level, all other elements on this chart are presented as they occur at the surface point of observation.

Swath width: the width of the spray swath parallel to the path of the aerial or ground application vehicle over which a desired spray deposition is achieved.

Synoptic-scale: pertaining to meteorological phenomena occurring on the spatial scale of the migratory high and low pressure systems of the lower troposphere, with length scales of 1000 to 2500 km and on time scales exceeding 12 hours.

Temperature: a measure of the warmth or coldness of an object or substance with respect to some standard scale or value.

Temperature inversion: a layer of the atmosphere in which air temperature increases with height.

Terpene: any of a class of monocyclic hydrocarbons of the formula C₁₀H₁₆, obtained from plants.

Terrain-forced flow: an airflow that is modified or channeled as it passes over or around mountains or through gaps in a mountain barrier.

Thermal belt: a zone of high nighttime temperature and relatively low humidity that often occurs within a narrow altitude range on valley sidewalls. The thermal belt is especially evident during clear weather with light winds.

Thermally driven circulation: See diurnal mountain winds.

Thermistor: a resistor whose resistance changes with temperature and can therefore be used as a temperature sensor.

Thermosphere: the atmospheric layer extending from the top of the mesosphere to outer space. It is a region of more or less steadily increasing temperature with height, starting at 70 or 80 km above the surface of the earth.

Thunder: the sound caused by rapidly expanding gases in a lightning discharge.

Thunderstorm: a local storm produced by a cumulonimbus cloud and accompanied by lightning and thunder.

Time constant: the time required for a measuring instrument to respond to 63.2% of a stepwise change in a measured quantity.

Time lag: 1. same as time constant. 2. the time needed under specific atmospheric conditions for a fuel particle to lose about 63 percent of the difference between its initial moisture and its equilibrium moisture contents.

Towering cumulus: a tall cumulus cloud, extending through low and middle cloud levels.

Trade winds: any of the nearly constant easterly winds that dominate most of the tropics and subtropics, blowing mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.

Transpiration: the passage of water vapor into the atmosphere through the vascular system of plants.

Transport wind: for smoke management computations, the average wind speed and direction through the depth of the mixed layer.

Trapper: a valley or basin in which cold air becomes trapped or pooled.

Tropopause: the boundary between the troposphere and stratosphere, characterized by an abrupt change in temperature lapse rate (temperatures decrease with height in the troposphere, but increase or remain constant with height in the stratosphere).

Troposphere: the portion of the earth’s atmosphere from the surface to the tropopause; that is, the lowest 10-20 km of the atmosphere. The troposphere is characterized by decreasing temperature with height and is the layer of the atmosphere containing most clouds and other common weather phenomena.

Trough: on a weather chart, a narrow, elongated area of relatively low pressure.

Troughing: the deepening or intensification of a trough or low pressure center.

Turbulence: irregular motion of the atmosphere, as indicated by gusts and lulls in the wind.

Twenty-foot wind: wind measured or estimated over a 2- to 10-minute period at a standard level 20 feet above the vegetative surface or continuous tree canopy and used as input into fire danger models or for fire suppression activities.

Undersun: an optical effect seen by an observer who is above a cloud deck and is looking toward the sun. Sunlight is reflected upward off the horizontally oriented ice crystals in the cloud deck below.

Unstable: see absolutely unstable.

Unstable boundary layer: see convective boundary layer.

Upper-air weather chart: weather maps that are produced for the portion of the atmosphere above the lower troposphere, generally at and above 850 mb. Isolines on these maps usually represent the heights of a constant pressure surface, such as the 500 mb surface.

Upslope fog: a fog that forms when moist, stable air is cooled as it is lifted up a mountain slope.

Upslope wind: a diurnal thermally driven flow directed up a mountain slope and usually occurring during daytime; part of the slope wind system.

Up-valley wind: a diurnal thermally driven flow directed up a valley’s axis, usually occurring during daytime; part of the along-valley wind system.

Upwelling: the process by which warm, less dense surface water is drawn away from a shoreline by offshore currents and replaced by cold, denser water brought up from the subsurface.

Valley exit jet: a strong elevated down-valley air current issuing from a valley above its intersection with the adjacent plain.

Valley volume effect: the effect of the reduction in volume of a valley or basin (as compared to a volume with a horizontal floor, an equal depth and an equal area at the top) on temperature change. The temperature change for an equal heat flux is greater in the valley volume than in the flat-floor volume.

Vapor pressure: the pressure exerted by the water vapor molecules in a given volume of air.

Ventilation index: product of mixing depth and transport wind speed, a measure of the potential of the atmosphere to disperse airborne pollutants from a stationary source.

Venturi effect: the speedup of air through a constriction due to the pressure rise on the upwind side of the constriction and the pressure drop on the downwind side as the air diverges to leave the constriction. Also called the Bernoulli effect.

Virga: streaks of water droplets or ice particles falling out of a cloud and evaporating before reaching the ground.

Visible spectrum: the portion of the electromagnetic spectrum to which the eye is sensitive, i.e., light with wavelengths between 0.4 and 0.7 micrometers. Compare shortwave radiation and longwave radiation.

Visibility: the distance at which an object can be seen and identified with the naked eye.

Visibility Protection Program: the program specified by the Clean Air Act to achieve a national goal of remedying existing impairments to visibility and preventing future visibility impairment throughout the United States.

Volatile fraction: mass fraction of a sprayed liquid that could evaporate in the atmosphere before depositing.

Vortex: a whirling mass of air in the form of a column or spiral. A vortex can rotate around either a horizontal or a vertical axis.

Wake: the region of turbulence immediately behind a solid body caused by the flow of air over or around the body.

Warm front: a transition zone between a mass of warm air and the colder air it is replacing.

Warm occlusion: a frontal zone formed when a cold front overtakes a warm front. When the air behind the cold front is warmer than the air ahead of the warm front, it leaves the ground and rises up and over the denser, colder air. Compare cold occlusion.

Warm rain process: the collision and coalescence of water droplets in a cloud, producing rain.

Warm sector: the region of warm air within a low pressure center located between an advancing cold front and a retreating warm front.

Wasatch wind: a strong easterly wind blowing out of the mouths of the canyons of the Wasatch Mountains onto the plains of Utah. Also called canyon wind.

Weather: the state of the atmosphere with respect to wind, temperature, cloudiness, moisture, pressure, etc.

Westerlies: the prevailing winds that blow from the west in the mid-latitudes of both the Northern and Southern Hemsipheres.

Wet bulb thermometer: a thermometer having a bulb that is kept moistened when humidity determinations are made with a psychrometer. Compare dry bulb thermometer.

Wet bulb depression: the temperature difference between dry and wet bulb thermometers on a psychrometer, used to determine atmospheric humidity.

Wildfire: an unwanted fire that requires measures of control.

Wind: air in natural motion relative to the earth's surface.

Wind chill: see wind chill equivalent temperature.

Wind chill equivalent temperature: the apparent temperature felt on the exposed human body owing to the combination of temperature and wind speed.

Wind-driven fire: a fire whose behavior is governed primarily by a strong consistent wind, rather than by the convective circulation produced by the fire itself.

Wind field: the three-dimensional spatial pattern of winds.

Wind rose: a diagram, for a given locality or area, showing the frequency and strength of the wind from various directions.

Wind shear: the rate of wind velocity change with distance in a given direction (as, vertically). The shear can be speed shear (where speed but not direction changes between the two points), direction shear (where direction but not speed changes between the two points) or a combination of the two.

Winds of Most Concern to Firefighters: winds that dominate the fire environment, rendering fire suppression activities dangerous or ineffective.

Windthrow: see blowdown.

Windward: the side toward the wind. Compare leeward.