Weather Terms

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

Adiabatic Chart: A thermodynamic diagram with temperature as abscissa and pressure as ordinate.

Advection: Transport of an atmospheric property solely by the mass motion of the atmosphere.

Air Mass: A homogenous mass of air, the properties of which can be identified as having been established while the air was situated over a particular region of the earth's surface.

Anabatic Wind: An upslope wind usually applied only when the wind is blowing up hill or mountain as the result of local surface heating, and apart from the effects of the larger scale circulation.

Anemometer: Instrument designed to measure the speed of the wind.

Anticyclone: A closed circulation of the atmosphere, of relative high pressure. (clockwise flow in the northern hemisphere).

Atmospheric Pressure: The pressure exerted by the air as a result of gravity. It is measured by the barometer, and expressed in millibars or inches of mercury.

Backing: A change of wind direction in a counter-clockwise direction (the opposite of veering); i.e. from west to south.

Blocking: Obstruction, on a large scale, of the normal west to east movement of highs and lows.

Calm: Absence of apparent motion of the air.

Climate: The statistical collective of weather conditions over a specified period of time (i.e. usually several decades).

Cloud: The visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earths's surface.

Cold Front: The leading edge of a relatively cold air mass.

Complex: An area of low pressure within which more than one center is found.

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

Conduction: The transfer of energy within and through a conductor by means of molecular activity.

Contour: In meteorology, a line of constant height on a constant pressure surface map.

Convection: In meteorology, atmospheric motions that are predominantly vertical, i.e. usually means upward as opposed to subsidence (downward).

Convergence: Net horizontal inflow of air into a layer. If at the surface, vertical motion results. Associated with low pressure systems.

Coriolis Force: The deflecting influence of the earth's rotation on winds. Deflects winds to the right (with wind at your back) in the northern hemisphere.

Critical Fire Weather Pattern: Patterns that can quickly increase fire danger and trigger rapid fire spread.

Cumulus: A principal cloud type in the form of individual detached elements, sharp non-fibrous outlines, and vertical development.

Cumulonimbus: The ultimate growth of a cumulus cloud into a mushroom shape, with considerable vertical growth, usually fibrous ice crystal tops, and probably accompanied by lightning, thunder, hail and strong winds.

Cut off Low: A cold low which has become displaced to south, out of the basic westerly flow.

Cyclonic: Having a sense of counter-clockwise rotation about the local vertical.

Cyclone: A closed isobaric circulation in the atmosphere, with counter-clockwise rotation in the northern hemisphere.

Deepening: A decrease in the central pressure of a cyclonic, or low pressure system.

Dew Point: The temperature to which a parcel of air must be cooled to reach saturation.

Diffuse front: A front across which the wind shift and temperature change are weakly defined.

Divergence: Downward (subsidence) motion results. Associated with high pressure system.

Diurnal: Daily, especially pertaining to daily cycles of temperature, relative humidity and wind.

Drought: A period of moisture deficiency, extensive in space and time.

Dust Devil: A small but vigorous whirlwind, usually of short duration.

Extended Forecast: A forecast of general weather conditions for days 3 through 5.

Fair: A weather term implying no precipitation and no extreme conditions of clouds, visibility or wind.

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 Weather Watch: Issued when the forecaster feels reasonably confident that red flag conditions will develop in the next 12 to 48 hours.

Fog: Same as cloud except base of cloud is touching earth's surface.

Front: A transition zone between two air masses of different density.

Free Air: That portion of the atmosphere that is not modified by local influence.

Free Air Wind: The wind at the bottom layer of the atmosphere called free air, or just above the portion that is modified by local influences and friction approximately 1,000 feet above the earth's surface.

Friction Layer: The layer of air from the earth's surface to the geostrophic wind level or level of free wind.

Geostrophic Wind: A horizontal wind resultant of the balance of the Coriolis acceleration and the horizontal pressure force.

Gradient Wind: A horizontal wind velocity tangent to the contour line or isobaric surface resulting in a balance of the coriolis, pressure and centrifugal force.

Gravity Wind: A wind directed down a slope caused by greater air density near the slope than at the same height at a distance from the slope. (Also called drainage or downslope wind).

Gust: A sudden brief increase in wind speed.

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.

Heat: Internal energy. A form of energy transferred between systems by virtue of a difference in temperature.

Heat Lightning: The luminosity observed from ordinary lightning too far away for its thunder to be heard.

Heat Low: An area of low pressure due to high temperatures (thermal low).

Heavy: In reference to precipitation, more than a half inch in forecast period.

High Pressure: An anticyclone. An area of atmospheric pressure with closed isobars and relative high pressure at its center. Air flows clockwise around a high.

High Clouds: Cirrus type clouds composed of ice crystals, usually above 25,000 feet.

Homogenous: In reference to an airmass having similar horizontal properties or elements.

Humidity: A measure of the water vapor content of the air.

Hygrothermograph: An instrument that records temperature and relative humidity as a function of time.

Instability: A property of the steady state of a system such that certain disturbances introduced into the steady state will increase in magnitude.

Instability Line: A band of convective activity in the atmosphere, i.e. squall line, a line of active thunderstorms.

Inversion: An increase in temperature with height, i.e. a departure from the usual decrease of temperature with increase of altitude.

Isobar: A line passing through points having equal atmospheric pressure.

Isopleth: A line passing through points having equal or constant values of a given gravity, with respect to either time or space.

Isolated: Affecting less than 20% percent of the area.

Jet Stream: Relatively strong winds concentrated within a narrow stream in the atmosphere.

Katabatic wind: A wind blowing down an incline. If warm, it is called a foehn or chinook. If cold, its called a gravity or mountain wind.

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.

Knot: The unit of speed in the nautical system; a nautical mile per hour. It is equal to 1.15 statute miles per hour.

Lapse Rate: The rate of change of temperature with height. The moist lapse rate is 3.5 degrees per 1000 feet and the dry lapse rate is 5.5 degrees per 1000 feet.

Lee Trough: A low pressure trough formed on the lee side of a mountain range across which the wind is blowing at nearly right angles.

Light: Precipitation, ranging from .11 to .20 inches in a forecast period.

Lightning: All the various visible electrical discharges produced by thunderstorms. It can be cloud to cloud, cloud to ground, or cloud to air.

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.

Local Winds: Winds, which over a small area, differ from those appropriate to the general pressure distribution.

Long Range: An extended forecast for a period greater than 5 days.

Long Wave: A wave in the major belt of westerlies which is characterized by large length and significant amplitude.

Low: An area of low atmospheric pressure having closed isobars. Used interchangeably with cyclone.

Low Aloft: An upper level cyclone (low pressure system).

Major Trough: A long wave trough in the large scale pattern of the upper air.

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.

Meteorology: The study of the phenomena of the atmosphere.

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

Middle Clouds: Clouds of the altocumulus or altostratus family, anywhere from 7,000 to 25,000 feet in elevation.

Minor Trough: A pressure trough in the upper air of smaller scale than a long wave trough. It usually moves rapidly, i.e. short wave.

Mixing Height: Maximum depth to which mixing will occur.

Model Output Statistics: A generation of point specific output from a numerical model.

Moderate: Precipitation, ranging from .21 to .50 inches in a forecast period.

Mountain and Valley Winds: Diurnal winds along the axis of a valley, blowing uphill and valley during the day and downhill and down valley during the night.

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.

Neutral Stability: The state of a parcel of air, which if displaced vertically, will experience no buoyant acceleration.

Normal: The average value of a meteorological element over a period of years, usually 30 in the United States.

Occasional: Occurrence of a meteorological element at infrequent intervals and for short duration.

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

Overcast: A cloud layer that covers most, or all of the sky.

Overrunning: Condition existing in which an air mass is in motion aloft above another air mass of greater density at the surface.

Persistence: The tendency for the occurrence of a specific event to be more probable, at a given time, if that same event has occurred immediately preceding the time period.

Polar Front: The semi-permanent, semi-continuous front separating air masses of polar and tropical origin.

Precipitation: Any or all the forms of water particles, liquid or solid, that fall from the atmosphere and reach the ground.

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

Pressure Center: The center of a high (anticyclone) or low (cyclone) pressure system.

Prevailing Wind Direction: The wind direction most frequently observed during a given period.

Probability: The chance that a prescribed event will occur.

Probability Forecast: A forecast of the probability of occurrence of one or more of a mutually exclusive set of weather contingencies, as distinguished from a series of categorical statements.

Prognostic (Prog) Chart: A chart depicting some meteorological parameter at a specified future time.

Pseudo-adiabatic (chart): A thermodynamic chart (process), same as adiabatic, but with saturation adiabats added.

Psychrometer: An instrument used for measuring the water vapor content of the air.

Quasi-Stationary Front: A front which is stationary, or nearly so,

Radiation: The process by which electromagnetic radiation is propagated through free space.

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

Radiosonde (RAOB): Balloon-borne instument for the measurement and transmission of temperature, humidity and pressure. When tracked by radar, also provides wind direction and velocity (Rawin).

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 the forecasters when red flag criteria are met or expected to be met within 12 to 24 hours. The warning highlights weather of particular importance to fire behavior and potentially extreme burning conditions or many new fires. Red flag warnings should always be coordinated with the customer.

Relative Humidity: (Humidity) - The ratio of the actual amount of water vapor in the air to the possible amount at that temperature.

Retrograde: Usually used to denote the movement of a weather system in a direction opposite to that of the normal flow in which the system is embedded.

Ridge: An elongated area of relatively high atmospheric pressure.

Saturation: The condition in which the air contains all the water vapor possible at that temperature, i.e. 100% relative humidity.

Sea Level Pressure: The atmospheric pressure at mean sea level.

Severe: Used in reference to thunderstorm intensity. Indicates strong winds and large hail.

Short Wave: (Minor Wave) - A progressive wave in the pattern of upper air motion with dimensions of cyclonic scale, as distinguished from a long wave.

Shower: Precipitation from a convective cloud, characterized by the suddenness with which they start, stop, and change intensity.

Smoke Management Parameters: The weather parameters used to forecast smoke dispersal (mixing height and transport winds).

Spot Weather Forecast: A specialized forecast issued by the National Weather Service for a localized area and time, at the request of the user.

Squall: A strong wind characterized by a sudden onset, of longer duration than gusts, and a rather sudden decrease in speed.

Stability Factor: This is determined by temperature differences between two atmospheric layers.

Stable: A property of the steady state of a system such that a disturbance introduced into the steady state will not increase in magnitude.

Storm: In meteorology, usually refers to cyclonic storms with considerable cloud and precipitation areas.

Stratiform: Descriptive of clouds of extensive horizontal development.

Subsidence: A descending motion of air in the atmosphere, of particular importance due to the heating and drying of the air as it contracts.

Surface Chart: An analyzed map showing the distribution of sea level pressure (isobars) and location of fronts and air masses.

Surface Pressure: The atmospheric pressure at a given location on the earth's surface.

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

Temperature: The degree of hotness or coldness as measured on some definite temperature scale by means of various types of thermometers.

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 temperature and the lowest relative humidity.

Thermal Low: (Heat Low) - An area of low atmospheric pressure due to high temperatures and intensive heating at earth's surface, usually stationary and have weak cyclonic circulation.

Thunderhead: Cumulonimbus or ice top cumulus.

Thunderstorm: A local storm produced by cumulonimbus clouds accompanied by lightning and thunder...often containing heavy rain...and sometimes strong winds and hail.

Topography: A detailed description of surface features including rivers, lakes, etc.

Tornado: A violently rotating column of air, pendant from a cumulonimbus cloud observed as a funnel cloud.

Towering Cumulus: The transitory stage of a cumulus into a cumulonimbus cloud.

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

Tropical Cyclone: The general term for a cyclone that originates over the tropical oceans. The remnants of these storms occasionally recurve and move into the mid-latitude westerlies, bringing considerable moisture with them.

Tropopause: The boundary between the troposphere and the stratosphere, usually characterized by an abrupt change to a small lapse rate.

Trough: An elongated area of relatively low atmospheric pressure, the axis of which is called a trough line.

Turbulence: A state of fluid flow in which the instantaneous velocities exhibit irregular and apparently random fluctuations.

Upper Air: Generally applied to levels above 850 mb (5,000 feet).

Upslope Wind: A wind directed up a slope during the hot part of the day.

Valley Wind: A wind which ascends a mountain valley during the day.

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

Veering Wind: A change in wind direction in a clockwise manner, i.e. south to southwest to west, the opposite of backing.

Virga: Water or ice particles falling out of a cloud but evaporating before reaching the ground.

Visibility: The greatest distance it is possible to see permanent objects with the unaided eye.

Warm Front: A front that moves in such a way that warmer air replaces colder air.

Wave: A disturbance propagated by virtue of periodic motions in the atmosphere.

Weather: The state of the atmosphere, usually short term, with respect to its effects upon life, property and human activities.

Wetting Rain: Precipitation of .10 inches or more over most of the area specified.

Westerlies: The dominant west to east motion of the atmosphere across the mid latitudes.

Wet Bulb Temperature: The temperature an air parcel would have if cooled to saturation. It lies between the dry bulb temperature and the dew point temperature.

Whirlwind: A small scale rotating column of air (dust devil).

Wind: Air in horizontal motion relative to the surface of the earth.

Wind Direction: The direction from which the wind is blowing.

Zonal Flow: The flow of air along a latitude circle.

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.