Glossary / Science

Weather Glossary

Absolute Humidity: A type of humidity that considers the mass of water vapor present per unit volume of space. Also con
Absolute Instability: When the lapse rate of a column of air is greater than the dry adiabatic lapse rate. The term absolu
Absolute Temperature Scale: A temperature scale with a freezing point of +273°K (Kelvin) and a boiling point of +373°K.
Absolute Zero: Considered to be the point at which theoretically no molecular activity exists or the temperature at
Absorption: The process in which incident radiant energy is retained by a substance. The absorbed radiation is t
Abyssal Plain: The flat, gently sloping or nearly level region of the sea floor.
Adiabatic Process: A thermodynamic change of state in a system in which there is no transfer of heat or mass across the
Advection: The horizontal transfer of any property in the atmosphere by the movement of air (wind). Examples in
Advection Fog: Fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew
Advisory: Statements that are issued by the National Weather Service for probable weather situations of inconv
Afos: Acronym for Automation of Field Operations and Services. It is the computer system that links Nation
Air: This is considered the mixture of gases that make up the earth's atmosphere. The principal gases tha
Air Mass: An extensive body of air throughout which the horizontal temperature and moisture characteristics ar
Air Mass Thunderstorm: A thunderstorm that is produced by convection within an unstable air mass through an instability mec
Air Pollution: The soiling of the atmosphere by contaminants to the point that may cause injury to health, property
Air Quality Standards: The maximum level which will be permitted for a given pollutant. Primary standards are to be suffici
Alaskan Winds: The downslope air flow that blows through the Alaskan valleys. It is usually given local names, such
Albedo: The ratio of the amount of radiation reflected from an object's surface compared to the amount that
Alberta Clipper: A fast moving, snow-producing weather system that originates in the lee of the Canadian Rockies. It
Aleutian Low: A semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Isla
Altimeter: An instrument used to determine the altitude of an object with respect to a fixed level. The type no
Altimeter Setting: The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude
Altitude: In meteorology, the measure of a height of an airborne object in respect to a constant pressure surf
Altocumulus: Composed of flattened, thick, gray, globular masses, this middle cloud genus is primarily made of wa
Altocumulus Castellanus: A middle cloud with vertical development that forms from altocumulus clouds. It is composed primaril
Altostratus: This middle cloud genus is composed of water droplets, and sometimes ice crystals, In the mid-latitu
American Meteorological Society: An organization whose membership promotes the education and professional advancement of the atmosphe
Anabatic Wind: A wind that is created by air flowing uphill. Valley breezes, produced by local daytime heating, are
Anemometer: An instrument that measures the speed or force of the wind.
Aneroid Barometer: An instrument for measuring the atmospheric pressure. It registers the change in the shape of an eva
Anomalous Propagation: This refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmos
Antarctic: Of or relating to the area around the geographic South Pole, from 90° South to the Antarctic Circle
Antarctic Ocean: Although not officially recognized as a separate ocean body, it is commonly applied to those portion
Anticyclone: A relative pressure maximum. An area of pressure that has diverging winds and a rotation opposite to
Anvil: The upper portion of a cumulonimbus cloud that becomes flat and spread-out, sometimes for hundreds o
Aphelion: The point on the earth's orbit that is farthest from the sun. Although the position is part of a 21,
Apogee: The point farthest from the earth on the moon's orbit. This term can be applied to any other body or
Arctic: Of or relating to the area around the geographic North Pole, from 90° North to the Arctic Circle at
Arctic Air Mass: An air mass that develops around the Arctic, it is characterized by being cold from surface to great
Arctic Jet: The jet stream that is situated high in the stratosphere in and around the Arctic or Antarctic Circl
Arctic Sea Smoke: A type of advection fog that forms primarily over water when cold air passes across warmer waters.
Argon (A): A colorless, odorless inert gas that is the third most abundant constituent of dry air, comprising 0
Arid: A term used for an extremely dry climate. The degree to which a climate lacks effective, life-promot
Asos: Acronym for Automated Surface Observing System. This system is a collection of automated weather ins
Astronomical Twilight: The time after nautical twilight has commenced and when the sky is dark enough, away from the sun's
Atmosphere: The gaseous or air portion of the physical environment that encircles a planet. In the case of the e
Atmospheric Pressure: The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several
Aurora: It is created by the radiant energy emission from the sun and its interaction with the earth's upper
Autumn: The season of the year which occurs as the sun approaches the winter solstice, and characterized by
Avhrr: Acronym for Advanced Very High Resolution Radiometer. It is the main sensor on the U.S. polar orbiti
Aviation Weather Center: As one of the National Centers for Environmental Prediction, it is the national center for weather i
Awips: Acronym for Advanced Weather Interactive Processing System. It is the computerized system that proce
Azores High: A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean that migrates east a
Backing: A counterclockwise shift in the wind direction in the Northern Hemisphere at a certain location. In
Backscatter: A radar echo that is reflected, or scattered, at 180 degrees to the direction of the incident wave.
Ball Lightning: A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which mov
Baroclinity: The state of stratification in a fluid in which surfaces of constant pressure intersect surfaces of
Barograph: An instrument that continuously records a barometer's reading of atmospheric pressure.
Barometer: An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the m
Barometric Pressure: The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several
Barotropy: The state of a fluid in which surfaces of constant density or temperature are coincident with surfac
Barrier Winds: Refers to the westerly flow of air along the northern slope of the Brooks Range in northern Alaska t
Bathythermograph: A device used to obtain a record of temperature against depth (pressure) in the ocean. May be referr
Beaufort Wind Scale: A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which
Bellot Winds: Refers to the winds in the Canadian Arctic that blow through the narrow Bellot Strait between Somers
Bermuda High: A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean that migrates east a
Bernoullis Theorem: A statement of the conservation of energy for a steady, nonviscous, incompressible level flow. It is
Biosphere: The transition zone between the earth and the atmosphere within which most terrestrial life forms ar
Black Blizzard: A local term for a violent duststorm on the south-central Great Plains that darkens the sky and cast
Black Ice: Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also re
Blizzard: A severe weather condition characterized by low temperatures, winds 35 mph or greater, and sufficien
Blocking High: The development of a warm ridge or cutoff high aloft at high latitudes which becomes associated with
Blowing Dust: Dust that is raised by the wind to heights of six feet or greater. It is reported as 'BLDU' in an ob
Blowing Sand: Sand that is raised by the wind to heights of six feet or greater. It is reported as 'BLSA' in an ob
Blowing Snow: Snow that is raised by the wind to heights of six feet or greater. It is reported as 'BLSN' in an ob
Blowing Spray: Salt spray that is raised by the wind to heights of six feet or greater. It is reported as 'BLPY' in
Blue Norther: Refers to a swift-moving cold frontal passage in the southern Great Plains, marked by a dark, blue-b
Boiling Point: The temperature at which a liquid changes to a vaporous state. The temperature at which the equilibr
Boulder Wind: A local name referring to an extremely strong downslope wind in the Front Range of the Rocky Mountai
Boundary Layer: The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the ear
Bow Echo: A radar echo signature often associated with severe thunderstorms, especially those that produce win
Boyles Law: States that when the temperature is held constant, the volume of a gas is inversely proportional to
Bright Band: A narrow, intense radar echo due to water-covered ice particles at the melting level where reflectiv
Broken: The amount of sky cover for a cloud layer between 5/8ths and 7/8ths, based on the summation layer am
Bubble High: A small high that may be created by precipitation and vertical instability associated with thunderst
Buys Ballots Law: Describes the relationship of the horizontal wind direction to the pressure distribution. In the Nor
Bwer: Acronym for Bounded Weak Echo Region. Refers to radar echo signatures with low reflectivity in the c
Calm: Atmospheric conditions devoid of wind or any other air motion. In oceanic terms, it is the apparent
Calorie: In meteorology, it is the amount of heat required to raise the temperature of one (1) gram of water
Cap: Composed of a layer of warmer, dryer air aloft which may suppress or delay the development of thunde
Cape: Acronym for Convective Available Potential Energy. The amount of energy available to create convecti
Cape Verde Islands: A group of volcanic islands in the eastern Atlantic Ocean off the coast of West Africa. A Cape Verde
Carbon Dioxide (Co2): A heavy, colorless gas that is the fourth most abundant constituent of dry air, comprising 0.033% of
Catalina Eddy: A weak low pressure circulation that may form off the Southern California coast.
Ceiling: The lowest cloud layer that is reported as broken or overcast. If the sky is totally obscured, then
Ceiling Light: An instrument consisting of a drum and an optical system that projects a narrow vertical beam of lig
Ceilometer: An instrument that is used to measure the angular elevation of a projected light on the base of a cl
Celestial Equator: The projection of the plane of the geographical equator upon the celestial sphere.
Celestial Sphere: The apparent sphere of infinite radius having the earth as its center. All heavenly bodies (planets,
Celsius Temperature Scale: A temperature scale where water at sea level has a freezing point of 0°C (Celsius) and a boiling po
Central Pressure: The atmospheric pressure at the center of a high or low. It is the highest pressure in a high and th
Centrifugal Force: The apparent force in a rotating system that deflects masses radially outward from the axis of rotat
Centripetal Force: The force required to keep an object moving in a curved or circular path. It is directed inwards tow
Charles Law: States that when the pressure is held constant, the volume of a gas varies directly with the tempera
Chemosphere: A vaguely defined region of the upper atmosphere in which photochemical reactions take place. It inc
Cheyenne Fog: An upslope fog formed by the westward flow of air from the Missouri River Valley, producing fog on t
Chinook: A type of foehn wind. Refers to the warm downslope wind in the Rocky Mountains that may occur after
Chocolatta North: A West Indian gale that blows from the northwest.
Chromosphere: A thin layer of relatively transparent gases above the photosphere of the sun. It is observed best d
Circulation: The flow or motion of a fluid in or through a given area or volume. In meteorology, it is used to de
Circulation Cells: Large areas of air movement created by the rotation of the earth and the transfer of heat from the e
Cirriform: Clouds composed of small particles, mostly ice crystals. Because the particles are fairly widely dis
Cirrocumulus: A cirriform cloud with vertical development, appearing as a thin sheet of small white puffs which gi
Cirrostratus: A cirriform cloud that develops from cirrus spreading out into a thin layer, creating a flat sheetli
Cirrus: One of the three basic cloud forms (the others are cumulus and stratus). It is also one of the three
Civil Twilight: The time between the moment of sunset, when the sun's apparent upper edge is just at the horizon, un
Clear: The state of the sky when no clouds or obscurations are observed or detected from the point of obser
Clear Air Turbulence: Name given to turbulence that may occur in perfectly clear air without any visual in warning in the
Clear Ice: A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled in w
Climate: The historical record and description of average daily and in seasonal weather events that help desc
Climate Analysis Center (CAC): The U.S. National Weather Service division that applies new technology and approaches to the analysi
Climate Prediction Center (CPC): A branch of the National Centers for Environmental Prediction,the Center maintains a continuous watc
Climatology: The study of climate. It includes climatic data, the analysis of the causes of the differences in cl
Clinometer: An instrument used to measure angles of inclination. Used in conjunction with a ceiling light, it de
Closed Low: A region of low pressure distinguished by a center of counterclockwise circulation (in the Northern
Cloud: A visible collection of minute particle matter, such as water droplets and/or ice crystals, in the f
Cloud Bank: A well-defined cloud mass that can be observed at a distance. It covers the horizon, but is not dire
Cloudburst: A sudden, heavy rainfall of a showery nature.
Coalescence: The merging of two water drops into a single larger drop.
Cold: A condition marked by low or decidedly subnormal temperature. The lack of heat.
Cold Advection: The horizontal movement of colder air into a location. Contrast with warm advection.
Cold Air Funnel: Funnel clouds, usually short-lived, that develop from relatively small showers or thunderstorms when
Cold Core Thunderstorms: Thunderstorms formed primarily due to steep lapse rates, especially when very cold air aloft overlie
Cold Front: The leading edge of an advancing cold air mass that is under running and displacing the warmer air i
Cold High: A high pressure system that has its coldest temperatures at or near the center of circulation, and h
Cold Low: A low pressure system that has its coldest temperatures at or near the center of circulation, and is
Cold Wave: A rapid fall in temperature within twenty-four hours to temperatures requiring substantially increas
Collada: A strong, steady wind blowing from the north or northwest in the upper part of the Gulf of Californi
Colorado Low: A low pressure disturbance that forms in the lee of the Rocky Mountains, usually in southeastern Col
Comma Cloud: A feature seen on satellite images with a distinctive comma-shape. This is indicative of a synoptic
Condensation: The process by which water vapor undergoes a change in state from a gas to a liquid. It is the oppos
Condensation Funnel: A funnel-shaped cloud consisting of condensed water drops that has possible rotation.
Condensation Nuclei: A particle upon which condensation of water vapor occurs. It may be either in a solid or liquid stat
Conditional Instability: Stable unsaturated air that will result in instability in the event or on the condition that the air
Conduction: The transfer of heat through a substance by molecular action or from one substance by being in conta
Confluence: A rate at which wind flow comes together along an axis oriented normal to the flow in question. The
Constant Pressure Chart: A chart of a constant pressure surface in which atmospheric pressure is uniform everywhere at any gi
Constant Pressure Surface: A surface along which the atmospheric pressure is equal everywhere.
Continent: A large land mass rising abruptly from the deep ocean floor, including marginal regions that are sha
Continental Air Mass: An air mass with continental characteristics. It is a secondary characteristic of an air mass classi
Continental Shelf: The zone around the continents extending from the low-water mark seaward, typically ending in steep
Contrail: Acronym for CONdensation TRAIL. A cloud-like streamer or trail often seen behind aircraft flying in
Convection: Motions in a fluid that transport and mix the properties of the fluid. These properties could be hea
Convective Condensation Level (CCL): The height at which a parcel of air, if heated sufficiently from below, will rise adiabatically unti
Convergence: Wind movement that results in a horizontal net inflow of air into a particular region. Convergent wi
Cooling Degree Day: A cooling degree day is given for each degree that the daily mean temperature departs above the base
Coriolis Effect: A force per unit mass that arises solely from the earth's rotation, acting as a deflecting force. It
Coromell: The prevailing evening land breeze which takes place from November to May in the vicinity of La Paz,
Corona: A pastel halo around the moon or sun created by the diffraction of water droplets. The droplets in t
Corposant: A luminous, sporadic, and often audible, electric discharge. It occurs from objects, especially poin
Crepuscular Rays: Contrasting, alternating bright and dark rays in the sky. Sunlight is scattered by molecules and par
Crystallization: The process of a substance going directly from a vapor form (water vapor) to a solid (ice) at the sa
Cumuliform: Clouds composed of water droplets that exhibit vertical development. The density of the droplets oft
Cumulonimbus: A vertically developed cumulus cloud, often capped by an anvil-shaped cirriform cloud. Also called a
Cumulonimbus Mammatus: A portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud
Cumulus: One of the three basic cloud forms (the others are cirrus and stratus). It is also one of the two lo
Cumulus Congestus: A strongly sprouting cumulus cloud with generally sharp outlines and often with great vertical devel
Cumulus Fractus: Cumulus clouds that appear in irregular fragments, as if they had been shred or torn. Also appears i
Cumulus Humilis: Cumulus clouds with little or no vertical development characterized by a generally flat appearance.
Cumulus Mediocris: Cumulus clouds characterized by moderate vertical development with upper protuberances not very mark
Current: A horizontal movement of water, such as the Gulf Stream off the east coast of North America, or air,
Cut-Off High: A warm high which has become displaced and is on the polarward side of the jet stream. It occurs mos
Cut-Off Low: A closed cold core low completely removed from the primary westerly flow. Cutoff lows may remain det
Cyclogenesis: The process that creates a new low pressure system or cyclone, or intensifies a pre-existing one. It
Cyclone: An area of closed pressure circulation with rotating and converging winds, the center of which is a
Cyclonic Flow: Winds that blow in and around a cyclone, that is counterclockwise in the Northern Hemisphere, clockw
D-Value: The deviation of actual altitude along a constant pressure surface from the standard atmosphere alti
Daily Mean: The average temperature for a day computed by averaging either the hourly readings or, more commonly
Daltons Law: States that the total pressure exerted by a mixture of gases is equal to the sum of the partial pres
Data Buoys: Buoys placed throughout the Gulf of Mexico and along the Atlantic and Pacific coasts of the United S
Dawn: The first appearance of light in the eastern sky before sunrise. It marks the beginning of morning t
Day: Considered a basic unit of time as defined by the earth's motion. It represents the time needed for
Debris Cloud: Considered a rotating cloud of debris or dust that is on the ground or near the ground. The debris c
Deepening: Used in describing the history of a low pressure system or an area of cyclonic circulation, it means
Degree: A measure of temperature difference representing a single division on a temperature scale.
Degree Day: A measure of the departure of the mean daily temperature from a given standard. That is one degree d
Dense Fog Advisory: Advisory issued when fog reduces visibility to 1/8 mile or less, creating possible hazardous conditi
Density: The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to
Density Altitude: The altitude at which a given density is found in the standard atmosphere. Used in aviation, it is c
Depression: In meteorology, it is another name for an area of low pressure, a low, or trough. It also applies to
Derecho: A line of intense, widespread, and fast-moving thunderstorms that moves across a great distance. The
Dew: Condensation in the form of small water drops that forms on grass and other small objects near the g
Dew Point: The temperature to which air must be cooled at a constant pressure to become saturated.
Diablo Winds: Dry winds in the Diablo mountain range in central California that can exceed 60 miles per hour. Simi
Diffluence: A rate at which wind flow spreads apart along an axis oriented normal to the flow in question. The o
Diffraction: The result of light waves interfering with other after passing through a narrow aperture, causing th
Directional Shear: The shear created by a rapid change in wind direction with height.
Discontinuity: Comparatively large contrast in meteorological elements over a relatively small distance or period o
Disturbance: This has several applications. It can apply to a low or cyclone that is small in size and influence.
Diurnal: Pertaining to actions or events that occur during a twenty-four hour cycle or recurs every twenty-fo
Divergence: Wind movement that results in a horizontal net outflow of air from a particular region. Divergence a
Dog Days: The name given to the very hot summer weather that may persists for four to six weeks between mid-Ju
Doldrums: Located between 30 degrees North and 30 degrees South latitudes in the vicinity of the equator, this
Doppler Radar: Weather radar that measures direction and speed of a moving object, such as drops of precipitation,
Downburst: A severe localized downdraft from a thunderstorm or shower. This outward burst of cool or colder air
Downdraft: A sudden descent of cool or cold air to the ground, usually with precipitation, and associated with
Downpour: A heavy rain.
Downslope Effect: The warming of an air flow as it descends a hill or mountain slope.
Drainage Wind: A katabatic wind, it is caused by the cooling of air along the slopes of a mountain.
Drifting Snow: Snow particles blown from the ground by the wind to a height of less than six feet.
Drifts: Normally used when referring to snow or sand particles are deposited behind obstacles or irregularit
Drizzle: Slowly falling precipitation in the form of tiny water droplets with diameters less than 0.02 inches
Dropsonde: A radiosonde dropped with a parachute from an aircraft rather than lifted by a balloon to measure th
Drought: Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cau
Dry Adiabat: The line on a Skew T-Log P chart that depicts the lifting of dry air, or air that is unsaturated. As
Dry Bulb Thermometer: A thermometer used to measure the ambient temperature. The temperature recorded is considered identi
Dry Line: The boundary between the dry desert air mass of the Southwest U.S. and the moist air mass from the G
Dry Slot: An area of dry, and usually cloud-free, air that wraps into the southern and eastern sections of a s
Dusk: The period of waning light from the time of sunset to dark.
Dust: Small particles of earth or other matter suspended in the air. It is reported as 'DU' in an observat
Dust Bowl: The term given to the area of the Great Plains including Texas, Oklahoma, Kansas, Colorado, and New
Dust Devil: A small, rapidly rotating column of wind, made visible by the dust, dirt or debris it picks up. It u
Duststorm: A severe weather condition characterized by strong winds and dust-filled air over a large area. Visi
Dynamics: A branch of mechanics that deals with forces and their relations to patterns of motion. In metorolog
Earthlight (Earthshine): The faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the m
Earthquake: A sudden, transient motion or trembling of the earth's crust, resulting from the waves in the earth
Easterlies: 0
Easterly Wave: An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east t
Echo: The energy return of a radar signal after it has hit the target.
Eclipse: The obscuring of one celestial body by another.
Ecliptic: The sun's apparent path across the sky that tracks a circle through the celestial sphere.
Ecology: The study of the relationships between living organisms and their environment.
Eddy: A small disturbance of wind in a large wind flow, which can produce turbulent conditions. They can a
Electromagnetic Radiation: Also called radiation, it is waves of energy propagated though space or through a material media.
Electromagnetic Spectrum: The band of electromagnetic radiation with components that are separated into their relative wave le
Elevation: The measure of height with respect to a point on the earth's surface above mean sea level. Sometimes
Environment: The sum total of all the external conditions that effect an organism, community, material, or energy
Equator: The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the No
Equatorial Trough: The quasi-continuous area of low pressure between the subtropical high pressure areas in both the No
Equinox: The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly eq
Erosion: The movement of soil or rock from one area to another by the action of the sea, running water, movin
Evaporation: The physical process by which a liquid, such as water is transformed into a gaseous state, such as w
Evapotranspiration: The total amount of water that is transferred from the earth's surface to the atmosphere. It is made
Exosphere: This region is considered the very outer limits of the earth's atmosphere. Its lower boundary is oft
Extratropical Cyclone: Any cyclone that is no longer tropical in origin. Generally considered to be a migratory frontal cyc
Eye: The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds
Eye Wall: An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains c
Fahrenheit Temperature Scale: A temperature scale where water at sea level has a freezing point of +32°F and a boiling point of +
Fair: This is a subjective description. Considered as pleasant weather conditions with regard to the time
Fathom: The common unit of depth in the ocean for countries using the English system of measurement. It is s
Feeder Bands: In tropical parlance, the lines or bands of thunderstorms that spiral into and around the center of
Fetch: An area of the water surface over which waves are generated by a wind having a constant direction an
Few: The amount of sky cover for a cloud layer between 1/8th and 2/8ths, based on the summation layer amo
Filling: Used in describing the history of a low pressure system or an area of cyclonic circulation, it means
Firewhirl: A tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volca
First Gust: Another name for the initial wind surge observed at the surface as the result of downdrafts forming
Flanking Line: A line of attached cumulus or towering cumulus clouds of descending height, appearing as stair steps
Flash Flood: A flood that rises and falls quite rapidly with little or no advance warning, usually as the result
Flood: High water flow or an overflow of rivers or streams from their natural or artificial banks, inundati
Flood Plain: Level land that may be submerged by flood waters.
Flood Stage: The level of a river or stream where overflow onto surrounding areas can occur.
Foehn: A warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind desc
Fog: A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of t
Fog Bank: A fairly well-defined mass of fog observed in the distance. Most commonly seen at sea, over a lake,
Fogbow: A whitish semicircular arc seen opposite the sun in fog. The outer margin has a reddish tinge, its i
Forecast: A statement of expected future occurrences. Weather forecasting includes the use of objective models
Fractus: The elements of cumulus and stratus clouds that appear in irregular fragments, as if they had been s
Freezing Drizzle: Drizzle, falling as a liquid, but freezing on impact with the colder ground or other exposed surface
Freezing Fog: Used to describe the phenomena when fog is present and the air temperature is below 0°C. It is repo
Freezing Point: The process of changing a liquid to a solid. The temperature at which a liquid solidifies under any
Freezing Precipitation: Precipitation that is liquid, but freezes upon impact with a solid surface, such as the ground or ot
Freezing Rain: Rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or
Fresh Water: Water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack o
Friction: In meteorology, it is the turbulent resistance of the earth on the atmosphere. Considered as the res
Friction Layer: The thin layer of atmosphere adjacent to the earth's surface. Surface friction is effective in slowi
Front: The transition zone or interface between two air masses of different densities, which usually means
Frontal Passage: It is the passage of a front over a specific point on the surface. It is reflected by the change in
Frontogenesis: The birth or creation of a front. This occurs when two adjacent air masses exhibiting different dens
Frontolysis: The destruction or dying of a front where the transition zone is losing its contrasting properties.
Frost: The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature
Frozen Precipitation: Precipitation that reaches the ground in a frozen state. Examples include snow, snow pellets, snow g
Fujita-Pearson Scale: A scale that classifies the severity of wind damage intensity based on the degree of destruction as
Funnel Cloud: A violent, rotating column of air visibly extending from the base of a towering cumulus or cumulonim
Fusion: The change of state from a solid to a liquid at the same temperature. The heat of fusion is the numb
Gale: On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). For ma
Gale Warning: A warning for marine interests for impending winds from 28 to 47 knots (32 to 54 miles per hour).
Geophysics: The study of the physics or nature of the Earth and its environment. It deals with the composition a
Geosphere: Considered the solid portions of the earth, including the hydrosphere and the lithosphere, as oppose
Geostationary Satellite: An orbiting weather satellite that maintains the same position over the equator during the earth's r
Geostrophic Wind: A steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pres
Glacier Winds: Air flow that descends from glaciers, occasionally at a high rate of speed. Caused by the temperatur
Glaze: A smooth clear icy coating of supercooled water droplets that spread out and freeze onto objects on
Gradient Wind: A steady horizontal air motion along curved parallel isobars or contours in an unchanging pressure o
Graupel: A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water drople
Gravitation: The mutual attraction between two masses of matter. The rotation of the earth and the atmosphere mod
Gravity: The force of attraction of the earth on an object. The direction is downward relative to the earth,
Green Flash: A brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent d
Greenhouse Effect: The overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor
Greenwich Mean Time (GMT): The name of the twenty-four hour time scale which is used throughout the scientific and military com
Ground Clutter: A pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the ra
Ground Fog: Fog created when radiational cooling at the earth's surface lowers the temperature of the air near t
Growing Season: Considered the period of the year during which the temperature of cultivated vegetation remains suff
Gulf Stream: The warm, well-defined, swift, relatively narrow ocean current which exists off the east coast of th
Gully Washer: A heavy rain shower that occurs suddenly, possibly creating a flash flood.
Gust: A sudden significant increase in or rapid fluctuations of wind speed. Peak wind must reach at least
Gust Front: The leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. Sometimes con
Gustnado: A weak, and usually short-lived, tornado that forms along the gust front of a thunderstorm, appearin
Haboob: Sudanese name for duststorm or sandstorm with strong winds that carry small particles of dirt or san
Hail: Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or ir
Halo: The ring of light that seems to encircle the sun or moon when veiled by cirrus clouds. To produce th
Haze: A suspension of fine dust and/or smoke particles in the air. Invisible to the naked eye, the particl
Heat: A form of energy transferred between two systems by virtue of a difference in temperature. The first
Heat Balance: The equilibrium which exists on the average between the radiation received by the earth and atmosphe
Heat Exhaustion: The effect of excessive heat, particularly when combined with high humidity, on a human being. Signs
Heat Index: The combination of air temperature and humidity that gives a description of how the temperature feel
Heat Lightning: Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in dist
Heat Stroke: Introduced to the body by overexposure to high temperatures, particularly when accompanied by high h
Heat Wave: A period of abnormally and uncomfortably hot weather. It could last from several days to several wee
Heating Degree Day: One heating degree day is given for each degree that the daily mean temperature is below 65°F. It i
Helicity: A property of a moving fluid, such as air, representing the potential for helical flow (flow that fo
High Clouds: A term used to signify cirriform clouds that are composed of ice crystals and generally have bases a
High Latitudes: The latitude belt roughly between 60° and 90° North and South.
High Pressure System: An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's
Hoarfrost: Another name for frost. A deposit of hoarfrost occurs when air with a dew point below freezing is br
Hook Echo: A radar reflectivity pattern observed in a thunderstorm, appearing like a fish hook and indicating f
Horizon: One of several lines or planes used as reference for observation and measurement relative to a given
Horse Latitudes: Located between 30° North and 30° South in the vicinity of the equator, this area typically has ca
Hudson Bay Low: An area of low pressure over or near the Hudson Bay area of Canada that often introduces cold air to
Humboldt Current: Also known as the Peru Current, this ocean current flows northward along the western side of South A
Humidity: The amount of water vapor in the air. It is often confused with relative humidity or dew point.
Hurricane: The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in t
Hurricane Warning: A formal advisory issued by forecasters at the National Hurricane Center when they have determined t
Hurricane Watch: A formal advisory issued by forecasters at the National Hurricane Center when they have determined t
Hydrologic Cycle: Often called the water cycle, it is the vertical and horizontal transport of water in all its states
Hydrology: The study of the waters of the earth, especially with relation to the effects of precipitation and e
Hydrometeor: Any any form of atmospheric water vapor, including those blown by the wind off the earth's surface.
Hydrosphere: Considered as the water portion of the earth's surface. Part of the geosphere.
Hygrograph: An instrument that records the hygrometer's measure of water vapor.
Hygrometer: An instrument that measures the water vapor content of the atmosphere.
Hypothermia: This situation occurs when the core temperature of one's body falls below normal. It is the failure
Ice: The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pe
Ice Crystals: Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates.
Ice Fog: Fog that is composed of minute ice particles. It occurs in very low temperatures under clear, calm c
Ice Jam: An accumulation of broken river ice caught in a narrow channel, frequently producing local flooding.
Ice Pellets: Precipitation in the form of transparent or translucent pellets of ice, which are round or irregular
Ice Storm: A severe weather condition characterized by falling freezing precipitation. Such a storm forms a gla
Icelandic Low: A semi-permanent, subpolar area of low pressure in the North Atlantic Ocean. Because of its broad ar
Icicle: Ice that forms in the shape of a narrow cone hanging point down. It usually forms when liquid water
Icing: The forming or depositing of ice on an object.
Inches Of Mercury (Hg): The name comes from the use of mercurial barometers which equate the height of a column of mercury w
Indian Summer: A period of abnormally warm weather in mid to late autumn with clear skies and cool nights. A first
Infrared: The long wave, electromagnetic radiation of radiant heat emitted by all hot objects. On the electrom
Insolation: Solar radiation or heating received at the earth's surface. The name is derived from INcoming SOLar
Instability: The state of equilibrium in which a parcel of air when displaced has a tendency to move further away
Instability: Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature w
Instrument Flight Rules (IFR): Refers to the general weather conditions pilots can expect at the surface and applies to the weather
Instrument Shelter: A boxlike structure designed to protect temperature measuring instruments from exposure to direct su
Intermountain High: An area of high pressure that occurs during the winter between the Rocky Mountains and the Sierra-Ca
International Date Line: The line of longitude located at 180° East or West (with a few local deviations) where the date c
Intertropical Convergence Zone (ITCZ): An area where the Northern and Southern Hemispheric trade winds converge, usually located between 10
Inversion: A departure from the usual increase or decrease of an atmospheric property with altitude. It usually
Ionosphere: A complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilomet
Isallobar: The line of equal change in atmospheric pressure during a certain time period. It marks the change i
Isobar: The line drawn on a weather map connecting points of equal barometric pressure.
Isodrosotherm: The line drawn on a weather map connecting points of equal dew point.
Isohel: A line drawn through geographic points having equal duration of sunshine or another form of solar ra
Isohyet: The line drawn through geographic points recording equal amounts of rainfall during a given time or
Isopleth: A line connecting equal points of value. Also called an isoline.
Isotach: A line connecting equal wind speeds.
Isotherm: The line of equal or constant air temperature. If something is isothermal, it is of equal or constan
Jet Streak: A region of accelerated wind speed along the axis of a jet stream.
Jet Stream: An area of strong winds that are concentrated in a relatively narrow band in the upper troposphere o
K Index: The measure of thunderstorm potential based on the vertical temperature lapse rate, the moisture con
Katabatic Wind: A wind that is created by air flowing downhill. When this air is warm, it may be called a foehn wind
Katafront: A front where the warm air descends the frontal surface, except in the low layers of the atmosphere.
Kelvin Temperature Scale: A temperature scale with the freezing point of +273°K (Kelvin) and the boiling point of +373° K. I
Knot: A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. U
Lake Effect Snow: Snow showers that are created when cold dry air passes over a large warmer lake, such as one of the
Land Breeze: A diurnal coastal breeze that blows offshore, from the land to the sea. It is caused by the temperat
Landfall: The point at which a tropical cyclone's eye first crosses a land mass.
Landspout: A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a
Lapse Rate: The change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies
Latent Heat: The energy released or absorbed during a change of state.
Latitude: The location north or south in reference to the equator, which is designated at zero (0) degrees. Pa
Leeward: The side of an object or obstacle, such as a ship's sail, a mountain, or a hill, furthest away from
Lenticular Cloud: A cloud species which has elements resembling smooth lenses or almonds and more or less isolated. Th
Level Of Free Convection (LFC): The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to r
Lifted Index (LI): A measure of atmospheric instability that is obtained by computing the temperature that the air near
Lifting Condensation Level (LCL): The height at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.
Light Waves: That part of the electromagnetic spectrum that contains visible light. The colors, from longest wave
Lightning: A sudden and visible discharge of electricity produced in response to the build up of electrical pot
Line Echo Wave Pattern (LEWP): A wave-shaped bulge in a line of thunderstorms. It may often be seen as a 'S'-shaped radar echo sign
Lithometeor: Atmospheric phenomena which affect the state of the atmosphere. They constitute dry particles that h
Lithosphere: The solid, outer portion of the earth's crust coupled to the rigid upper mantle. Part of the geosphe
Long Wave Trough: A wave in the prevailing westerly flow aloft which is characterized by a large length and amplitude.
Longitude: The location east or west in reference to the Prime Meridian, which is designated as zero (0) degree
Low Clouds: A term used to signify clouds with bases below 6,000 feet and are of a stratiform or a cumuliform va
Low Latitudes: The latitude belt between 30 and 0 degrees North and South of the equator. Also referred to as the t
Low Level Jet (LLJ): Strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. I
Low Pressure System: An area of a relative pressure minimum that has converging winds and rotates in the same direction a
Lunar Eclipse: An eclipse of the moon occurs when the earth is in a direct line between the sun and the moon. The m
Mackerel Sky: The name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. The r
Macroburst: A large downburst with an outflow diameter of 2.5 miles (4 kilometers) or larger and damaging winds.
Macroscale: The meteorological scale covering an area ranging from the size of a continent to the entire globe.
Magnetic Poles: Either of the two points on the earth's surface where the magnetic meridians converge. They are not
Mammatocumulus: An obsolete term for cumulonimbus mammatus, it is a portion of a cumulonimbus cloud that appears as
Mares Tail: The name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches or
Marginal Visual Flight Rules (MVFR): Refers to the general weather conditions pilots can expect at the surface. MVFR means Minimum or Mar
Maritime Air Mass: An air mass influenced by the sea. It is a secondary characteristic of an air mass classification, s
Maximum: The greatest value attained by a function, for example, temperature, pressure, or wind speed. The op
Mean Sea Level: The average height of the sea surface water level. For the United States, it is computed by averagin
Mean Temperature: The average of temperature readings taken over a specified amount of time. Often the average of the
Measured Ceiling: A ceiling classification applied when the ceiling value has been determined by an instrument, such a
Melting Level: The altitude at which ice crystals and snow flakes melt as they descend through the atmosphere.
Melting Point: The temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state
Mercurial Barometer: An instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open
Meridional Flow: Atmospheric circulation in which the north and south, or meridional, component of motion is unusuall
Mesocyclone: A area of rotation of storm size that may often be found on the southwest part of a supercell. Its c
Mesohigh: A small, concentrated area of high pressure that may be created by the cold outflow and rain-cooled
Mesolow: A small scale low pressure center, ranging from the size of an individual thunderstorm to many tens
Mesoscale: The scale of meteorological phenomena that range in size from several kilometers to around 100 kilom
Mesoscale Convective Complex (MCC): A large mesoscale convective system (MCS) which is about the size of the state of Ohio or Iowa and l
Mesoscale Convective System (MCS): A large organized convective weather system comprised of a number of individual thunderstorms. It no
Mesosphere: The layer of the atmosphere located between the stratosphere and the ionosphere, where temperatures
Metar: Acronym for METeorological Aerodrome Report. It is the primary observation code used in the United S
Meteorology: The science and study of the atmosphere and atmospheric phenomena. Various areas of meteorology incl
Microbarograph: A instrument designed to continuously record a barometer's reading of very small changes in atmosphe
Microburst: A severe localized wind blasting down from a thunderstorm. It covers an area less than 2.5 miles (4
Microscale: The smallest scale of meteorological phenomena that range in size from a few centimeters to a few ki
Middle Clouds: A term used to signify clouds with bases between 6,000 and 18,000 feet. At the higher altitudes, the
Middle Latitudes: The latitude belt roughly between 35 and 65 degrees North and South. May be referred to as the tempe
Millibar (Mb): The standard unit of measurement for atmospheric pressure used by the National Weather Service. One
Minimum: The least value attained by a function, for example, temperature, pressure, or wind speed. The oppos
Mist: A collection of microscopic water droplets suspended in the atmosphere. It does not reduce visibilit
Mixed Layer: It is the upper portion of the boundary layer in which air is thoroughly mixed by convection. In oce
Mixed Precipitation: Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and fre
Moist Adiabat: The line on a Skew T-Log P chart that depicts the change in temperature of saturated air as it rises
Moisture: Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in
Monsoon: The seasonal shift of winds created by the great annual temperature variation that occurs over large
Mountain Breeze: A katabatic wind, it is formed at night by the radiational cooling along mountainsides. As the slope
Mountain Wave: A wave in the atmosphere caused by a barrier, such as a mountain. Sometimes it is marked by lenticul
Mud Slide: Fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpo
Muggy: A subjective term for warm and excessively humid weather.
Multicell Storm: A thunderstorm made up of two or more single-cell storms.
Multiple Vortex Tornado: A tornado which has two or more condensation funnels or debris clouds, often rotating around a commo
Nadir: The point on any given observer's celestial sphere diametrically opposite of one's zenith.
National Center For Atmospheric Research (NCAR): A division of the University Corporation for Atmospheric Research, the Center plans, organizes, and
National Centers For Environmental Prediction (NCEP): As part of the National Weather Service, the centers provide timely, accurate, and continually impro
National Climatic Data Center (NCDC): The agency that archives climatic data from the National Oceanic and Atmospheric Administration as w
National Hurricane Center (NHC): A branch of the Tropical Prediction Center, it is the office of the National Weather Service that is
National Meteorological Center (NMC): Now incorporated into the National Centers for Environmental Prediction, it was the division of the
National Oceanic And Atmospheric Administration (NOAA): A branch of the U.S. Department of Commerce, it is the parent organization of the National Weather S
National Severe Storms Forecast Center (NSSFC): As of October 1995, the responsibilities of this Center were divided into two branches, the Storm Pr
National Severe Storms Laboratory (NSSL): A branch of the National Oceanic and Atmospheric Administration, it provides accurate and timely for
National Weather Association (NWA): An organization whose membership promotes excellence in operational meteorology and related activiti
National Weather Service (NWS): A primary branch of the National Oceanic and Atmospheric Administration, it is responsible for all a
Nautical Mile: A unit of length used in marine navigation that is equal to a minute of arc of a great circle on a s
Nautical Twilight: The time after civil twilight, when the brighter stars used for celestial navigation have appeared a
Neap Tide: A tide of decreased range, which occurs about every two weeks when the moon is at one quarter or thr
Negative Vorticity Advection: The advection of lower values of vorticity into an area.
Nephelococcygia: A term applied when people find familiar objects within the shape of a cloud.
Newhall Winds: The local name for winds blowing downward from desert uplands through the Newhall Pass southward int
Newton: The unit of force giving a mass of about one kilogram (2.205 pounds) an acceleration of about one me
Nexrad: Acronym for NEXt Generation Weather RADar. A network of advanced Doppler radars implemented in the U
Night: The period of the day between dusk and dawn.
Nimbostratus: This cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be see
Nitrogen (N2): A colorless, tasteless, odorless gas that is the most abundant constituent of dry air. It comprises
Noctilucent Clouds: Rarely seen clouds of tiny ice particles that form approximately 75 to 90 kilometers above the earth
Nocturnal Thunderstorms: Thunderstorms which develop after sunset. They are often associated with the strengthening of the lo
Noreaster: A cyclonic storm occurring off the east coast of North America. These winter weather events are noto
Normal: The recognized standard value of a meteorological element as it has been averaged in a given locatio
North Pacific High: A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in t
Nowcast: A short-term weather forecast for expected conditions in the next few hours.
Numerical Forecasting: The use of numerical models, such as the fundamental equations of hydrodynamics subjected to observe
Obscuration: Any phenomena in the atmosphere, excluding precipitation, that reduces horizontal visibility. Accord
Observation: In meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure
Occluded Front: Also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. I
Ocean: The intercommunicating body of salt water occupying the depressions of the earth's surface, or one o
Oceanography: The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical b
Omega Block: A warm high aloft which has become displaced and is on the polarward side of the jet stream. It freq
Opaque: A condition where a material, such as a cloud, blocks the passage of radiant energy, especially ligh
Orographic Lifting: Where the flow of air is forced up and over barriers such as highlands or mountains. Moist air being
Outflow: Also referred to as an outflow boundary, it is the outward flow of air from a system, such as a thun
Overcast: The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for th
Overrunning: This occurs when a relatively warm air mass is forced above a cooler air mass of greater density. We
Oxygen (O2): A colorless, tasteless, odorless gas that is the second most abundant constituent of dry air, compri
Ozone (O3): A nearly colorless gas and a form of oxygen (O2). It is composed of an oxygen molecule made up of th
Ozone Layer: An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a fi
Palmer Drought Index: A long-term meteorological drought severity index produced by the NOAA/USDA (Department of Agricultu
Palouser: A strong, dangerous, katabatic wind that descends from the mountains into the Palouse River valley i
Parcel: A volume of air small enough to contain uniform distribution of its meteorological properties and la
Parhelion: The scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 deg
Partial Obscuration: Denotes that 1/8th or more of the sky, but not all of the sky, is hidden by any surface-based phenom
Partly Cloudy: The state of the weather when the clouds are conspicuously present, but do not completely dull the s
Pascal: The unit of pressure produced when one newton acts on about one square meter.
Pascals Law: When an external pressure is applied to any confined fluid at rest, the pressure is increased at eve
Peak Gust: The highest instantaneous wind speed observed or recorded.
Perigee: The point nearest the earth on the moon's orbit. This term can be applied to any other body orbiting
Perihelion: The point of the earth's orbit that is nearest to the sun. Although the position is part of a 21,000
Photometer: Any of a number of atmospheric phenomena which appear as luminous patterns in the sky. They do not d
Photosphere: The intensely bright portion of the sun visible to the unaided eye; the 'surface' of the sun. Reachi
Pilot Balloon: A small balloon whose ascent is used to determine the direction and speed of low level atmospheric w
Pilot Report: A report of in-flight weather by an aircraft pilot or crew member. Often referred to as a PIREP.
Plan Position Indicator: Also known as a PPI Scope, it is a radar indicator scope displaying range and azimuth of targets in
Plow Wind: The spreading downdraft and strong straight-line winds preceding a thunderstorm. So named in the Ame
Polar Air Mass: An air mass that forms over a high latitude region. Continental polar air (cP) is formed over cold s
Polar Front: A semi-continuous, semi-permanent boundary between polar air masses and tropical air masses. An inte
Polar Jet: Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between t
Polar-Orbiting Satellite: A satellite whose orbit passes over both of the earth's between poles.
Poles or Polar: The poles are the geographic point at 90 degrees latitude North and South on the earth's surface. Th
Pollutant: Particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or
Positive Vorticity Advection: The advection of higher values of vorticity into an area. It is also known as cyclonic vorticity.
Pounds Per Square Inch (PSI): A unit for measuring pressure. One PSI equals the pressure resulting from a force of one pound force
Pre-Frontal Squall Line: A line of thunderstorms that precedes an advancing cold front.
Pre-Frontal Trough: An elongated area of relatively low pressure preceding a cold front that is usually associated with
Precipitation: Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This inc
Pressure: The force per unit area exerted by the weight of the atmosphere above a point on or above the earth'
Pressure Altimeter: An aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is rea
Pressure Altitude: The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated
Pressure Change: The net difference between the barometric pressure at the beginning and ending of a specified interv
Pressure Characteristic: The pattern of the pressure change during the specified period of time, usually the three hour perio
Pressure Gradient: The amount of pressure change that occurs over a fixed distance at a fixed altitude.
Pressure Jump: A sudden increase in the observed atmospheric pressure or station pressure.
Pressure Tendency: The pressure characteristic and amount of pressure change during a specified time period, usually th
Prevailing Visibility: It is considered representative of visibility conditions at the observation station. It is the great
Prevailing Wind: A wind that blows from one direction more frequently than any other during a given period, such as a
Profiler: A type of Doppler radar that typically measures both wind speed and direction from the surface to 55
Prognostic Chart: A chart of forecast predictions that may include pressure, fronts. precipitation, temperature, and o
Psychrometer: An instrument used to measure water vapor content of the atmosphere. It consists of two thermometers
Pulse: A very short duration of time. In regard to a radar, it is a brief burst of a electromagnetic radiat
Quantitative Precipitation Forecast (QPF): A forecast of rainfall, snowfall or liquid equivalent of snowfall.
Quasi-Stationary Front: A front which is nearly stationary or moves very little since the last synoptic position. Also known
Radar: Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and
Radarsonde Observation: An upper air observation used to determine winds and other meteorological data, by tracking the rang
Radial Velocity: A type of velocity that expresses motion toward or away from a given location. In Doppler radar, it
Radiation: The process by which energy is propagated through any medium by virtue of the wave motion of that me
Radiation Fog: Fog that is created when radiational cooling at the earth's surface lowers the temperature of the ai
Radiational Cooling: The cooling of the earth's surface and the adjacent air. Although it occurs primarily at night, it h
Radiosonde: An instrument attached to a weather balloon used to measure pressure, temperature, humidity, and win
Rain: Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the dro
Rain Forest: A forest which grows in a region of heavy annual precipitation. There are two major types, tropical
Rain Gauge: An instrument used to measure the amount of rain that has fallen. Measurement is done in hundredths
Rain Shadow: Also referred to as a precipitation shadow, it is the region on the lee side of a mountain or simila
Rainbow: A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue,
Rainfall: The amount of precipitation of any type, primarily liquid. It is usually the amount that is measured
Range Resolution: The ability of radar to distinguish between targets on the same azimuth but at different ranges.
Rawinsonde: An upper air observation that evaluates the winds, temperature, relative humidity, and pressure alof
Reconnaissance (Recco) Code: An aircraft weather reconnaissance code that has come to refer primarily to in-flight tropical weath
Reflectivity: A measure of the process by which a surface can turn back a portion of incident radiation into the m
Refraction: The bending of light or radar beam as it passes through a zone of contrasting properties, such as at
Relative Humidity: A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturatio
Relative Vorticity: The sum of the rotation of an air parcel about the axis of the pressure system and the rotation of t
Resolution: In relation to radar, it is the ability to read two distinct targets separately. The clearer the res
Retrogression: In meteorology, it is the movement of a weather system in a direction opposite to the direction of t
Ridge: An elongated area of high atmospheric pressure that is associated with an area of maximum anticyclon
Rime: The rapid freezing of supercooled water droplets as they touch an exposed object, forming a white op
Rip Current: It is formed by a strong surface water movement, or current, of a short duration that flows seaward
Rocketsonde: A type of radiosonde that is shot into the atmosphere by a rocket, allowing it to collect data durin
Roll Cloud: A relatively rare, low-level, horizontal, tube-shaped cloud. Although they are associated with a thu
Rossby Waves: The movement of ridges and troughs in the upper wind patterns, primarily the jet stream, circling th
Rotation: The spinning of a body, such as the earth, about its axis.
Rotor Cloud: An altocumulus cloud formation that can be found in the lee of a mountain or similar barrier. The ai
Runway Visual Range (RVR): It is the maximum distance at which the runway, or the specified lights or markers delineating it, c
Saffir-Simpson Damage-Potential Scale: Developed in the early 1970s by Herbert Saffir, a consulting engineer, and Robert Simpson, then Dire
Salinity: A measure of the quantity of dissolved salts in sea water. The total amount of dissolved solids in s
Salt Water: The water of the ocean, distinguished from fresh water by its appreciable salinity.
Sand: Loose particles of hard, broken rock or minerals. In observing, sand is reported when particles of s
Sandstorm: A strong wind carrying sand particles through the air. They are low level occurences, usually only t
Santa Ana Winds: The hot, dry winds, generally from the east, that funnel through the Santa Ana river valley south of
Sargasso Sea: An area of the North Atlantic Ocean between Bermuda and the Azores. It is in the middle of the North
Satellite: Any object that orbits a celestial body, such as a moon. However, the term is often used in referenc
Satellite Images: Images taken by a weather satellite that reveal information, such as the flow of water vapor, the mo
Saturate: To treat or charge something to the point where no more can be absorbed, dissolved, or retained. In
Saturation Point: The point when the water vapor in the atmosphere is at its maximum level for the existing temperatur
Scattered: The amount of sky cover for a cloud layer between 3/8ths and 4/8ths, based on the summation layer am
Scattering: The process by which small particles suspended in the air diffuse a portion of the incident radiatio
Scud: Low fragments of clouds, usually stratus fractus, that are unattached and below a layer of higher cl
Sea Breeze: A diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperatu
Sea Breeze Front: A coastal phenomena, it is restricted to large bodies of water and their immediate coast lines. This
Sea Fog: A type of advection fog which forms in warm moist air cooled to saturation as the air moves across c
Sea Ice: Ice that is formed by the freezing of sea water. It forms first as small crystals, thickens into slu
Sea Level: The height or level of the sea surface at any time. It is used as a reference for elevations above a
Sea Level Pressure: The atmospheric pressure at mean sea level, usually determined from the observed station pressure.
Sea Mile: A unit of length distinguished from a nautical mile. One sea mile is equivalent to 1,000 fathoms (6,
Sea Spray: Sometimes called salt spray, it is the drops of sea water (salt water) blown from the top of a wave.
Sea Surface Temperature (SST): The temperature of the water's surface. It is measured using buoy and ship data, infrared satellite
Season: A division of the year according to some regularly recurring phenomena, usually astronomical or clim
Semi-Permanent Pressure Systems: A relatively stable, stationary pressure-and-wind system where the pressure is predominately high or
Severe Thunderstorm: A thunderstorm with winds measuring 50 knots (58 mph) or greater, 3/4 inch hail or larger, or tornad
Severe Weather: Generally, any destructive weather event, but usually applies to localized storms, such as blizzards
Shear: It is the rate of change over a short duration. In wind shear, it can refer to the frequent change i
Shear Line: A line of maximum horizontal wind shear. A narrow zone across which there is an abrupt change in the
Short Wave: A progressive wave of smaller amplitude, wave length, and duration than a long wave. It moves in the
Showalter Stability Index: A measure of the local static stability of the atmosphere. It is determined by lifting an air parcel
Shower: Precipitation from a convective cloud that is characterized by its sudden beginning and ending, chan
Siberian Express: A fierce, cold flow of air that originates in Siberia, then moves into Alaska and northern Canada be
Siberian High: The semi-permanent high pressure area that forms over Siberia during the winter. The average central
Sidereal Time: The measure of time as defined by the diurnal motion of the vernal equinox. A sidereal day is equiva
Skew T-Log P Diagram: A thermodynamic diagram, using the temperature and the logarithm of pressure as coordinates. It is u
Sky: The vault-like apparent surface against which all aerial objects are seen from the earth.
Sky Cover: The amount of the celestial dome that is hidden by clouds and/or obscurations.
Sleet: Also known as ice pellets, it is winter precipitation in the form of small bits or pellets of ice th
Slush: Snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temper
Small Craft Advisory: An advisory issued for marine interests, especially for operators of small boats or other vessels. C
Smoke: Small particles produced by combustion that are suspended in the air. A transition to haze may occur
Snow: Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal
Snow Advisory: A statement or advisory issued when snow is expected to create hazardous travel conditions. It warns
Snow Banner: A plume of snow blown off a mountain crest, resembling smoke blowing from a volcano.
Snow Blindness: Temporary blindness or impaired vision that results from bright sunlight reflected off the snow surf
Snow Cover: The areal extent of ground covered by the snow. It is usually expressed as a percent of the total ar
Snow Creep: A continuous, extremely slow, downhill movement of a layer of snow.
Snow Crust: The crisp, almost icy, surface on fallen snow, usually formed by the slight melting and refreezing o
Snow Depth: The actual depth of snow on the ground at any instant during a storm, or after any single snowstorm
Snow Devil: A small, rotating wind that picks up loose snow instead of dirt (like a dust devil) or water (like a
Snow Eater: Any warm downslope wind, or foehn, that blows over snowy terrain and melts the snow.
Snow Flurry: Light showers of snow, generally very brief without any measurable accumulation. May be reported as
Snow Garland: Snow appearing as a beautiful long thick rope draped on trees, fences and other objects. Formed by t
Snow Grains: Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of
Snow Level: The elevation in mountainous terrain where the precipitation changes from rain to snow, depending on
Snow Line: The lowest elevation area of a perennial snow field on high terrain, such as a mountain range.
Snow Pellets: Frozen precipitation in the form of white, round or conical opaque grains of ice. Their diameter ran
Snow Roller: The product of moist, cohesive snow that when initiated by wind rolls across the landscape, gatherin
Snow Shower: Frozen precipitation in the form of snow, characterized by its sudden beginning and ending. It is re
Snow Squall: A heavy snow shower accompanied by sudden strong winds, or a squall.
Snowburn: A burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface.
Snowfall: The rate at which snow falls, usually expressed in inches of snow depth over a six hour period.
Snowflakes: An ice crystal or an aggregate of ice crystals which fall from clouds.
Snowpack: The amount of annual accumulation of snow at higher elevations.
Solar Day: The complete rotation of the earth in relation to the sun. Although it varies, an average has determ
Solar Eclipse: An eclipse of the sun occurs when the moon is in a direct line between the sun and the earth, castin
Solstice: The point at which the sun is the furthest on the ecliptic from the celestial equator. The point at
Sounding: A plot of the atmosphere, using data rom upper air or radiosonde observations. Usually confined to a
Southern Oscillation: A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El NiÑo events
Specific Humidity: The ratio of the density of the water vapor to the density of the air, a mix of dry air and water va
Spring: The season of the year which occurs as the sun approaches the summer solstice, and characterized by
Spring Tide: A tide of increased range, which occurs about every two weeks when the moon is new or full.
Squall: A sudden onset of strong winds with speeds increasing to at least 16 knots (18 miles per hour) and s
Squall Line: A narrow band or line of active thunderstorms that is not associated with a cold front. It may form
St. Elmos Fire: A luminous, and often audible, electric discharge that is sporadic in nature. It occurs from objects
Stability: Occurs when a rising air parcel becomes denser than the surrounding air. It will then return to its
Stagnation Area: An area that has a combination of stable stratification, weak horizontal wind speed, and little, if
Standard Atmosphere: A standard atmosphere has been defined by the International Civil Aeronautical Organization (ICAO).
Standard Surface Pressure: The measurement of one atmosphere of pressure under standard conditions. It is equivalent to 1,013.2
Standing Cloud: Any type of isolated cloud, generally formed over peaks or ridges of mountainous areas, that appears
Standing Wave: An atmospheric wave that is stationary with respect to the medium in which it is embedded.
Station Elevation: The vertical distance above mean sea level that is the reference level for all current measurements
Station Pressure: The atmospheric pressure with respect to the station elevation.
Stationary Front: A front which is nearly stationary or moves very little since the last synoptic position. May be kno
Steam Fog: A type of advection fog that is produced by evaporation when cool air passes over a warm wet surface
Storm: An individual low pressure disturbance, complete with winds, clouds, and precipitation. The name is
Storm Prediction Center (SPC): A branch of the National Centers for Environmental Prediction, the Center monitors and forecasts sev
Storm Tracks: The path or tracks generally followed by a cyclonic disturbance.
Storm Winds: On the Beaufort Wind Scale, a wind with speeds from 56 to 63 knots (64 to 72 miles per hour).
Straight-Line Winds: Any surface wind that is not associated with rotation. An example is the first gust from a thunderst
Stratiform: Clouds composed of water droplets that exhibit no or have very little vertical development. The dens
Stratocumulus: A low cloud composed of layers or patches of cloud elements. It can form from cumulus clouds becomin
Stratopause: The boundary zone or transition layer between the stratosphere and the mesosphere. Characterized by
Stratosphere: The layer of the atmosphere located between the troposphere and the mesosphere, characterized by a s
Stratus: One of the three basic cloud forms (the others are cirrus and cumulus. It is also one of the two low
Stratus Fractus: Stratus clouds that appear in irregular fragments, as if they had been shred or torn. Also appears i
Sublimation: The process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing dir
Subpolar: The region bordering the polar region, between 50° and 70° North and South latitude. This is gener
Subrefraction: Less than normal bending of light or a radar beam as it passes through a zone of contrasting propert
Subsidence: A sinking or downward motion of air, often seen in anticyclones. It is most prevalent when there is
Subtropical: The region between the tropical and temperate regions, an area between 35° and 40° North and South
Subtropical Air: An air mass that forms over the subtropical region. The air is typically warm with a high moisture c
Subtropical Jet: Marked by a concentration of isotherms and vertical shear, this jet is the boundary between the subt
Summation Layer Amount: The amount of sky cover for each layer is given in eighths of sky cover attributable to clouds or ob
Summer: Astronomically, this is the period between the summer solstice and the autumnal equinox. It is chara
Sun Dog: Either of two colored luminous spots that appear at roughly 22° on both sides of the sun at the sam
Sun Pillar: Horizontal ice crystals in the form of plates, which occur in clouds and ice fog near the earth's su
Sunrise: The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the U
Sunset: The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In
Supercell: A severe thunderstorm characterized by a rotating, long-lived, intense updraft. Although not very co
Supercooling: The reduction of the temperature of any liquid below the melting point of that substance's solid pha
Superrefraction: Greater than normal bending of light or radar beam as it passes through a zone of contrasting proper
Surface Boundary Layer: The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the ear
Surge: The increase in sea water height from the level that would normally occur were there no storm. Altho
Swell: Ocean waves that have traveled out of their generating area. Swell characteristically exhibits a mor
Synoptic Chart: Any map or chart that depicts meteorological or atmospheric conditions over a large area at any give
Synoptic Scale: The size of migratory high and low pressure systems in the lower troposphere that cover a horizontal
Syzygy: The points in the moon's orbit about the earth at which the moon is new or full.
Teleconnections: Information used by forecasters to determine what the weather might be elsewhere when compared with
Temperate Climate: Climates with distinct winter and summer seasons, typical of regions found between the Tropics of Ca
Temperature: The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary
Terminal Doppler Weather Radar (TDWR): Doppler radar installed at major airports throughout the United States to detect microbursts.
Terrestrial Radiation: Long wave radiation that is emitted by the earth back into the atmosphere. Most of it is absorbed by
Texas Norther: Local name in the south-central Great Plains for strong winter winds blowing north or northwest foll
Thaw: A warm spell of weather when ice and snow melt. To free something from the binding action of ice by
Theodolite: An optical instrument used to track the motion of a pilot balloon, or pibal, by measuring the elevat
Thermal Low: Also known as heat low, it is an area of low pressure due to the high temperatures caused by intensi
Thermocline: A vertical negative temperature gradient in some layer of a body of water which is appreciably great
Thermodynamics: Study of the processes that involve the transformation of heat into mechanical work, of mechanical w
Thermograph: Essentially, a self-recording thermometer. A thermometer that continuously records the temperature o
Thermohaline: In oceanography, it pertains to when both temperature and salinity act together. An example is therm
Thermometer: An instrument used for measuring temperature. The different scales used in meteorology are Celsius,
Thermosphere: A thermal classification, it is the layer of the atmosphere located between the mesosphere and outer
Thickness: The thickness of a layer in the atmosphere is proportional to the mean temperature of that whole lay
Thunder: The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-
Thunder Snow: A wintertime thunderstorm from which falls snow instead of rain. Violent updrafts and at or below fr
Thunderstorm: Produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterize
Tide: The periodic rising and falling of the earth's oceans and atmosphere. It is the result of the tide-p
Tilt: The inclination to the vertical of a significant feature of the pressure pattern or of the field of
Tiros: A series of Television InfraRed Observation Satellites that demonstrated the feasibility and capabil
Tornado: A violently rotating column of air in contact with and extending between a convective cloud and the
Tornado Alley: A geographic corridor in the United States which stretches north from Texas to Nebraska and Iowa. In
Towering Cumulus: Another name for cumulus congestus, it is a rapidly growing cumulus or an individual dome-shaped clo
Trace: Generally, an unmeasurable or insignificant quantity. A precipitation amount of less than 0.005 inch
Trade Winds: Two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards
Trajectory: The curve that a body, such as a celestial object, describes in space. This applies to air parcel mo
Translucent: Not transparent, but clear enough to allow light to pass through.
Transmissometer: An electronic instrument system which provides a continuous record of the atmospheric transmission b
Transparent: A condition where a material is clear enough not to block the passage of radiant energy, especially
Transpiration: The process by which water in plants is transferred as water vapor to the atmosphere.
Triple Point: The point at which any three atmospheric boundaries meet. It is most often used to refer to the poin
Tropic Of Cancer: The most northern point on the earth where the sun is directly overhead, located at approximately 23
Tropic Of Capricorn: The most southern point on the earth where the sun is directly overhead, located at approximately 23
Tropical Air Mass: An air mass that forms in the tropics or subtropics over the low latitudes. Maritime tropical air is
Tropical Cyclone: A warm core low pressure system which develops over tropical, and sometimes subtropical, waters, and
Tropical Depression: A tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or
Tropical Disturbance: An area of organized convection, originating in the tropics and occasionally the subtropics, that ma
Tropical Prediction Center (TPC): A division of the National Centers for Environmental Prediction, the Center issues watches, warnings
Tropical Storm: A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots
Tropical Wave: Another name for an easterly wave, it is an area of relatively low pressure moving westward through
Tropics or Tropical: The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and th
Tropopause: The boundary zone or transition layer between the troposphere and the stratosphere. This is characte
Troposphere: The lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17
Trough: An elongated area of low atmospheric pressure that is associated with an area of minimum cyclonic ci
Tsunami: An ocean wave with a long period that is formed by an underwater earthquake or landslide, or volcani
Tule Fog: Ground fog in the central valley of California and the leading cause of weather-related casualties i
Turbulence: The irregular and instantaneous motions of air which is made up of a number of small of eddies that
Twilight: Often called dusk, it is the evening period of waning light from the time of sunset to dark. The tim
Twister: A slang term used in the United States for a tornado.
Typhoon: The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in t
Ultraviolet: Electromagnetic radiation that has a wavelength shorter than visible light and longer than x-rays. A
Undercast: In aviation, it is an opaque cloud layer viewed from an observation point above the layer. From the
United States Weather Bureau: The official name of the National Weather Service prior to 1970.
Universal Time Coordinate: One of several names for the twenty-four hour time which is used throughout the scientific and milit
University Corporation For Atmospheric Research (UCAR): A non-profit university membership consortium which carries out programs to benefit atmospheric, oce
Unstable: Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature w
Updraft: A small scale current of air with vertical motion. If there is enough moisture, then it may condense
Upper Air: The portion of the atmosphere which is above the lower troposphere. It is generally applied to the l
Upper Level: The portion of the atmosphere which is above the lower troposphere. It is generally applied to the l
Upslope Effect: The cooling of an air flow as it ascends a hill or mountain slope. If there is enough moisture and t
Upslope Fog: Fog that forms when warm, moist surface air is forced up a slope by the wind. It is adiabatically co
Upwelling: The process by which water rises from a lower to a higher depth, usually as a result of divergence a
Upwind: The direction from which the wind is blowing. Also the windward side of an object. The opposite of t
Valley Breeze: An anabatic wind, it is formed during the day by the heating of the valley floor. As the ground beco
Vapor Pressure: The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part
Vapor Trail: A cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor t
Variable Ceiling: Occurs when the height of a ceiling layer increases and decreases rapidly, The ascribed height is th
Veering: A clockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Sou
Vernal Equinox: Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects th
Vertical Temperature Profile: A series of temperature measurements taken at various levels in the atmosphere that show the thermal
Vertical Visibility: The distance an observer can see vertically into an undefined ceiling, or the height corresponding t
Vertical Wind Profile: A series of wind direction and wind speed measurements taken at various levels in the atmosphere tha
Virga: Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evapora
Visibility: A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see promine
Visible Light: The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the
Visual Flight Rules (VFR): Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceil
Vortex: Any circular or rotary flow in the atmosphere that possesses vorticity.
Vorticity: The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it
Vorticity Maximum: A center of vorticity, or the maximum of the vorticity field of a fluid.
Walker Circulation: A deep east-west overturning in the atmosphere normally confined to within about 20 degrees latitude
Wall Cloud: An abrupt lowering of a cloud from its parent cloud base, a cumulonimbus or supercell, with no visib
Warm: To have or give out heat to a moderate or adequate degree. A subjective term for temperatures betwee
Warm Advection: The horizontal movement of warmer air into a location.
Warm Front: The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air
Warm High: A high pressure system that has its warmest temperatures at or near the center of circulation. Contr
Warm Low: A low pressure system that has its warmest temperatures at or near the center of circulation. Also r
Warning: A forecast issued when severe weather has developed, is already occurring and reported, or is detect
Wasatch Winds: Strong winds blowing easterly out of the Wasatch Mountains in Utah, sometimes reaching speeds greate
Watch: A forecast issued well in advance of a severe weather event to alert the public of the possibility o
Water: Refers to the chemical compound, H2O, as well as its liquid form. At atmospheric temperatures and pr
Water Cycle: The vertical and horizontal transport of water in all its states between the earth, the atmosphere,
Water Vapor (H2O): Water in gaseous form. It is one of the most import constituents of the atmosphere. Due to its molec
Waterspout: A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a
Wave: In general, any pattern with some roughly identifiable periodicity in time and/or space. It is also
Wave Cyclone: A cyclone which forms and moves along a front. The circulation around the cyclone's center produces
Wave Length: The least distance between particles moving in the same phase of oscillation of a wave. In oceanogra
Weather: The state of the atmosphere at a specific time and with respect to its effect on life and human acti
Weather Surveillance Radar (WSR-88D): The newest generation of Doppler radars, the 1988 Doppler weather radar. The radar units, with help
Weather Vane: Originally used as a wind vane, it is an instrument that indicates the wind direction. The name deve
Weathering: The decay and breakup of rocks on the earth's surface by natural chemical and mechanical processes.
Wedge: Primarily refers to an elongated area of shallow high pressure at the earth's surface. It is general
West Virginia High: An area of stagnant high pressure located over West Virginia during Indian Summer.
Westerlies: Usually applied to the broad patterns of persistent winds with a westerly component. It is the domin
Wet Bulb Depression: Dependent on the temperature and the humidity of the air, it is the difference between the dry bulb
Wet Bulb Thermometer: A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state
Whirlwind: A small-scale, rapidly rotating column of wind, formed thermally and most likely to develop on clear
Whiteout: When visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and su
Wind: Air that flows in relation to the earth's surface, generally horizontally. There are four areas of w
Wind Chill Index: The calculation of temperature that takes into consideration the effects of wind and temperature on
Wind Direction: The direction from which the wind is blowing. For example, an easterly wind is blowing from the east
Wind Shear: The rate of wind speed or direction change with distance. Vertical wind shear is the rate of change
Wind Shift: The term applied to a change in wind direction of 45 degrees or more, which takes place in less than
Wind Speed: The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In obse
Wind Vane: An instrument that indicates the wind direction. The end of the vane which offers the greatest resis
Wind Wave: An ocean or lake wave resulting from the action of wind on the water's surface. After it leaves its
Windward: The direction from which the wind is blowing. Also the upwind side of an object. The opposite of the
Winter: Astronomically, this is the period between the winter solstice and the vernal equinox. It is charact
Winter Storm: Any one of several storm systems that develop during the late fall to early spring and deposit wintr
World Meteorological Organization (WMO): From weather prediction to air pollution research, climate change related activities, ozone layer de
X-Rays: The portion of the electromagnetic spectrum that has a very short wave length. It has a wave length
Year: The interval required for the earth to complete one revolution around the sun. A sidereal year, whic
Yellow Snow: Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.
Zenith: The point which is elevated 90 degrees from all points on a given observer's astronomical horizon. T
Zodiac: The position of the sun during the course of the year as it appears to move though successive conste
Zonal Flow: The flow of air along a latitudinal component of existing flow, normally from west to east.
Zonal Index: The measure of the strength of the westerly winds of the middle latitudes. It is expressed as the ho
Zulu Time: One of several names for the twenty-four hour time which is used throughout the scientific and milit

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