Glossary of meteorology

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Thisglossary of meteorology is a list of terms and concepts relevant tometeorology andatmospheric science, their sub-disciplines, and related fields.

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ab-polar current: Anyair current moving away from either the North Pole or the South Pole.
advection: The horizontal transport of some property of theatmosphere orocean, such asthermal energy,humidity, orsalinity. In the context of meteorology, the related termconvection generally refers to vertical transport.
actinoform: Describing a collection of low-lying, radially structuredclouds with distinct shapes (resembling leaves or wheels in satellite imagery), and typically organized in extensivemesoscale fields over marine environments. They are closely related to and sometimes considered a variant ofstratocumulus clouds.
actinometer: A scientific instrument used to measure the heatingpower ofradiation, particularlysolar radiation.
adiabat: A line drawn on athermodynamic diagram along which anair parcel moves as it ascends or descends through the atmosphere,cooling orwarming adiabatically; the path followed by this line depends on whether it is adry adiabat or asaturated adiabat.^[1]
adiabatic cooling: Anadiabatic process of expansional cooling, in which a risingair parcel decreases intemperature as it increases in volume.^[2]
adiabatic heating: Anadiabatic process of compressional warming, in which a sinkingair parcel increases intemperature as it decreases in volume.^[2]
adiabatic lapse rate: The rate at which aparcel of air changestemperature adiabatically as it moves vertically through the atmosphere. The parcel'smoisture content affects this rate: as it rises, a parcel saturated with moisture cools more slowly than a dry parcel because the release of latent heat at the phase change between gas and liquid acts to buffer the temperature decrease caused by the adiabatic expansion.^[1] When not otherwise qualified, the term most often refers to thedry adiabatic lapse rate.
adiabatic process: Any idealized hypothetical process by which energy is transferred between athermodynamic system and its surroundings only aswork, without a corresponding transfer ofheat ormass. Most compressible fluids, including gases in theatmosphere, behave approximately adiabatically, such that meteorologists often use the assumption of adiabatic isolation when describing atmospheric systems. In such systems thetemperature of adry parcel of air changes without any exchange of energy with its surroundings: as the parcel rises, the decrease in the surroundingatmospheric pressure enables the air in the parcel to expand in volume, which decreases its internal energy and therefore its temperature (expansional cooling); as the parcel sinks and is compressed, its temperature increases (compressional warming).^[1]
aerobiology: The branch ofbiology that studies airborne organic particles, such as bacteria, viruses, fungal spores, pollen grains, and very small insects, which are passively transported by the air.
aerography: The production ofweather charts.
aerology: Seeatmospheric science.
aeronomy: The branch of meteorology that studies the upper regions of the Earth's or other planetaryatmospheres, specifically their atmospheric motions, chemical compositions and properties, and interactions with the other parts of the atmosphere and with space.
aerosol: A suspension of fine solid particles or liquiddroplets inair or another gas. Examples of natural aerosols includemist,clouds,fog, anddust.
ageostrophy
air current: Any concentrated area ofwinds that develops because of differences in pressure and/or temperature between adjacentair parcels. They are generally divided into horizontal and vertical currents and exist at a variety of scales and in various layers of the atmosphere.
air mass: A volume ofair defined by itstemperature andmoisture content.
air parcel: Influid dynamics, any amount ofair that remains identifiable throughout its dynamic history while moving with an associated air flow.
air-mass thunderstorm: Anythunderstorm that is generally weak and usually notsevere. Such storms move relatively slowly, are short-lived, and often exist only as single cells (rather than in long continuous lines or complexes), but may still producelightning and heavyrainfall. They derive their energy fromsolar radiation and commonly develop in temperate zones duringsummer afternoons.
Alberta clipper
almanac: An annual publication ofcalendar events.
aloft: Located in the atmosphere at some height (often significantly high) above the Earth's surface. The term is typically used to distinguish anupper-air observation from asurface weather observation, as in "winds aloft".^[2]
altimeter: A scientific instrument used to measure thealtitude of an object (e.g. aweather balloon) with respect to a fixed level such assea level.^[1]
altocumulus castellanus
altocumulus (Ac): A middle-altitudecloud genus characterized by small globular masses, laminae, or rolls, white or gray in color, arranged in patches or extensive sheets at altitudes between 2 and 7 kilometres (6,600 and 23,000 ft), with the individual elements being larger and more distinct than incirrocumulus but smaller than instratocumulus. Like otherstratocumuliform clouds, altocumulus usually signifies convection aloft. It is one of several classic "warning clouds" recorded by the aviation industry as a signal of developingthunderstorms.
altostratus
American Meteorological Society (AMS): A scientific and professional organization in the United States whose mission is to promote and disseminate information about theatmospheric,oceanic, andhydrologic sciences, and advance technologies, applications, and services related to them.
anabatic wind: Awind that blows upslope from the low elevations of a valley to the higher elevations of surrounding hills or mountains as the result of daytime surface heating in the valley, usually at speeds of 12 knots (22 km/h; 14 mph) or less but occasionally at much higher speeds.^[2] Contrastkatabatic wind.
anemometer: A scientific instrument used to measurewind speed.
annular tropical cyclone
anticyclone: Any large-scaleair mass characterized by outward spiralingwinds whichcirculate around a strong center ofhigh atmospheric pressure. Surface-based anticyclones generally bring about cool, dry air and clear skies and are often implicated in weather phenomena such asfog andhaze. Contrastcyclone.
anticyclonic rotation
anticyclonic storm: Anystorm system involving ananticyclone, in whichwinds circulate around a region ofhigh pressure in the direction opposite to that expected around a region oflow pressure. Anticyclonic storms rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
anticyclonic tornado
anticyclogenesis: The development or strengthening of ananticyclonic circulation in the atmosphere, which may result in the formation or maintenance of ahigh-pressure area. Contrastcyclogenesis.
antitriptic wind: Awind generated by the localtopography of a particular place; examples includeanabatic winds andkatabatic winds. Most such winds arediurnal in character.^[1]
apparent temperature: Seeheat index.
arcus cloud
Arctic cyclone
Atlantic hurricane: Atropical cyclone (locally known as ahurricane) that forms in theAtlantic Ocean and achieves one-minute maximumsustained winds exceeding 74 mph (119 km/h; 64 kn). Most of these storms occur between June 1 and November 30 each year, a time period referred to as theAtlantic hurricane season.
atmometer: Seeevaporimeter.
atmosphere: The various layers of gases surrounding the Earth and held in place by gravity. The Earth's atmosphere is the origin of theweather phenomena studied inmeteorology. Atmospheric composition,temperature, andpressure vary across a series of distinct sublayers including thetroposphere andstratosphere.
atmospheric boundary layer (ABL): Seeplanetary boundary layer.
atmospheric circulation: The global-scale movement ofair masses within the Earth'satmosphere. All meteorological phenomena are consequences of the atmospheric circulation, which manifests as a network of both latitudinal and longitudinal "cells" of convective activity; together withocean circulation, these cells are the primary means by which thermal energy from the Sun is redistributed across the Earth's surface.
atmospheric convection
atmospheric density (ρ): Thedensity (mass per unit volume) of the Earth'satmosphere. Atmospheric density generally decreases proportionally with elevation above sea level, and also tends to vary with changes inatmospheric pressure,temperature, andhumidity. According to theInternational Standard Atmosphere, at a pressure of 1atm and a temperature of 15°C, air has a density of approximately 1.225kilograms per cubic metre (kg/m³), about1⁄1000 the density of liquid water.
atmospheric lake: A long-lived pool of water vapor.
atmospheric model
atmospheric pressure (p): Thepressure exerted by the Earth'satmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of the air above the measurement point, and therefore decreases proportionally as altitude increases. The average atmospheric pressure at sea level on Earth is equal to approximately 1standard atmosphere (atm), which is defined as exactly 101,325 pascals (760 mmHg).
atmospheric river
atmospheric science: The collective of scientific disciplines that studies the Earth'satmosphere and its processes, including the effects other systems have on the atmosphere and those the atmosphere has on other systems.Meteorology andclimatology are sub-disciplines.
atmospheric sounding: A measurement of the vertical distribution of physical properties through anatmospheric column, usually includingpressure,temperature,wind speed anddirection,moisture content,ozone concentration, and pollution, among others.
atmospheric temperature: A measure oftemperature at one or more locations within the Earth'satmosphere. Temperatures recorded in the atmosphere can vary widely with altitude,humidity, andsolar irradiance, among other factors.
atmospheric thermodynamics
atmospheric tide: A global-scale periodic oscillation of the Earth'satmosphere caused by gravitational and thermal influences from the Sun and the Moon, analogous tooceanic tides.^[3]
atmospheric window: Any of the ranges of small bandwidths in theelectromagnetic spectrum at which the Earth'satmosphere is nearly transparent, i.e. where absorption by atmospheric gases is nearly zero and transmittance approaches unity both for incoming and outgoing radiation. Examples include theoptical window from ~0.3 to 0.9μm, theinfrared window from ~8 to 13 μm, and themicrowave window at wavelengths longer than ~1mm.^[3] The existence of these windows is vital for the Earth–atmosphere system to be maintained near thermal equilibrium.^[1]
autumn
avalanche
Aviation Area Forecast (FA or ARFOR): A former message product of the U.S.National Weather Service issued to provide information to pilots and aviation routes aboutweather conditions across a large regional area within the United States. FAs were issued three times daily, valid for 18 hours, and covered an area the size of several states. They were replaced by Graphic Area Forecasts (GFAs) in 2017.

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backing: A change ofwind direction in a counterclockwise fashion (e.g. northerly to northwesterly to westerly). Contrastveering.^[2]
backscatter: Thediffuse reflection of waves, particles, or signals back to the same direction from which they originated. Backscattering is the principle underlying allweather radar systems, which can distinguish radar returns backscattered from targetaerosols such as raindrops and snowflakes because the strength of the returns depends largely on the size and reflectivity of the targets.
ball lightning
banana belt: Any segment of a larger geographic region that typically experiences warmer temperatures than the region as a whole, especially during the local winter season, which may prove favorable for agriculture.
barbs
barograph: A scientific instrument used to measure and continuously record changes inatmospheric pressure over time.
baroclinity: A measure of the misalignment between apressure gradient and adensity gradient in a stratified fluid such as theatmosphere. In the context of meteorology, a baroclinic atmosphere is one in whichatmospheric density depends on bothtemperature andpressure, in contrast to abarotropic atmosphere, in which density depends only on pressure. Areas of high atmospheric baroclinity are generally found in the temperate and polar latitudes and are characterized by the frequent formation ofcyclones.
barotropity: The close alignment between apressure gradient and adensity gradient in a stratified fluid such as theatmosphere. In the context of meteorology, a barotropic atmosphere is one in whichatmospheric density depends only onpressure and is more or less independent oftemperature, in contrast to abaroclinic atmosphere. Unlike liquids, gaseous fluids such as the air in the atmosphere are generally not barotropic, but the assumption of barotropity can nonetheless be useful in modeling fluid behavior. Tropical latitudes are more nearly barotropic than the mid-latitudes because air temperature is more nearly horizontally uniform in the tropics.
barometer: A scientific instrument used to measureatmospheric pressure. The two most common types aremercury barometers andaneroid barometers.
barometric pressure: Seeatmospheric pressure.
barrier jet: A low-level core of highwind speeds that sometimes occurs at altitudes of 1,000–1,500 metres (3,300–4,900 ft) in the vicinity of a mountain range, as a consequence of the deceleration of anairflow as it crosses a major topographic barrier and releases latent heat which changes the local thermodynamics of the flow.^[1]
Beaufort scale
Bernoulli's principle: A principle offluid dynamics which states that an increase in the speed of a moving fluid occurs simultaneously with a decrease in thepressure exerted by the fluid or in the fluid'spotential energy.
Bishop's ring
black ice: A thin, nearly transparent coating ofglaze ice on a solid surface, especially a road or walkway, which because of its transparency is often practically invisible and therefore presents a significant hazard to drivers and pedestrians.
blizzard: A severesnowstorm characterized by strongsustained winds of at least 35 mph (56 km/h) andblowing snow, typically lasting three hours or more. They can have an immense size, covering hundreds or thousands of square miles, and occur most often in temperate, polar, or mountainous regions during thewinter.
block: A nearly stationary pattern in theatmospheric pressure field overlying a large geographic area, which effectively "blocks" or diverts the movements ofcyclones and other convective systems. These blocks can remain in place for days or weeks, causing the areas affected by them to experience the same kind of weather for extended periods of time.
blowing dust: Alithometeor phenomenon that occurs when particles of dust are lifted from the Earth's surface bywind and blown about in clouds or sheets. It is classified as an obstruction to vision inMETAR aviation weather observations and is commonly reported if the amount of suspended dust reduces horizontalvisibility to 10 kilometres (6 mi) or less.^[2] Extreme cases may be calleddust storms.^[3]
blowing sand: Alithometeor phenomenon that occurs when grains ofsand are lifted from the Earth's surface bywind and blown about in clouds or sheets. It is classified as an obstruction to vision inMETAR aviation weather observations and is commonly reported if the amount of suspended sand reduces horizontalvisibility to 10 kilometres (6 mi) or less.^[2] Extreme cases may be calledsandstorms.^[3]
blowing snow: Snow blown about bywind, either from falling snow or snow lifted from the surface, to a height of at least 2 metres (6.6 ft), reducingvisibility. It is a defining characteristic ofblizzards.
bounded weak echo region (BWER)
bow echo: A characteristic radar return from amesoscale convective system that is shaped like anarcher's bow and usually associated withsquall lines or lines of convectivethunderstorms. The distinct bow shape is a result of the focusing of a strong flow at the rear of the system. Especially strong bow echoes may develop intoderechos.
breeze: 1. Any generally lightwind.^[1]; 2. Any local-scale air movement that isconvectively forced, e.g. aland breeze orsea breeze.^[1]; 3. On theBeaufort scale, awind speed of force numbers 2 to 6, ranging from 4–27 knots (7–50 km/h; 5–31 mph), and categorized as follows:light breeze, 4–6 knots;gentle breeze, 7–10 knots;moderate breeze, 11–16 knots;fresh breeze, 17–21 knots; andstrong breeze, 22–27 knots.^[2]
brightband
Bulk Richardson Number (BRN): A dimensionless ratio related to the consumption ofturbulence divided by the shear production of turbulence (the generation of kinetic energy caused bywind shear). It is an approximation of the Gradient Richardson Number.
bushfire: Seewildfire.
Buys Ballot's law

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calm: A state of the atmosphere in which there is virtually no horizontal motion of the air. It corresponds to force number 0 on theBeaufort scale, with awind speed less than 1 kn (1.9 km/h). Calm conditions are common in thesubtropical high-pressure belts and in thedoldrums.^[1]
Canadian Meteorological and Oceanographic Society (CMOS): The national society of individuals and organizations dedicated to advancing atmospheric and oceanic sciences and related environmental disciplines inCanada, officially constituted in 1967.
Canadian Meteorological Centre (CMC): Provides forecast guidance to national and regional prediction centres in Canada.
Canterbury arch: SeeNor'west arch.
cap cloud: An approximately stationarycloud on or hovering above an isolated mountain peak. See alsopileus andlenticular cloud.^[2]
capacity: The ability of awind current to transport material, as measured by the maximum amount of detritus (e.g. silt, sand, and/or gravel) carried past a specific point per unit time. Capacity increases withwind speed and decreases as the particle size of the detrital debris increases.^[1]
capping inversion
castellanus: Acloud species that displays at least in its upper partcumuliform protuberances resembling the turrets of a castle, giving acrenellated aspect.
catabatic wind: Seekatabatic wind.
ceiling: A measure of the height above the Earth's surface of the base of the lowest layer ofclouds or obscuring phenomena that covers more than half of the sky (more than fouroktas). An "unlimited" ceiling means either that the sky is mostly free ofcloud cover or that the clouds are sufficiently high so as not to impede aircraft operation byvisual flight rules.
ceiling balloon: A type ofweather balloon used by meteorologists to determine the height of thecloud base above ground level during daylight hours by measuring the time it takes for the balloon, released from the ground and rising at a known rate of ascent, to begin to disappear into the clouds.
ceiling projector: A type ofcloud-height indicator that uses asearchlight to project a beam of light vertically onto acloud base (similar to aceilometer), with the height of the illuminated spot then calculated by the observer using aclinometer oralidade.^[4]^[3]
ceilometer: An instrument that uses a laser transmitter or other light source and a collocated receiver to determine theheight of acloud ceiling orcloud base overhead, or to measure the concentration ofaerosols within the atmosphere.
cell: 1. Anyatmospheric circulation feature that is more or less closed, occurring at any of number of scales, including massive latitudinally oriented circulations such asHadley cells;mesoscale motions that characterizecellular convection and cause the formation ofcellular clouds; andstorm cells formed byupdraft and/ordowndraft loops within athunderstorm.^[1]; 2. Inweather radar, a local maximum in radar reflectivity that undergoes a life cycle of growth and decay, and which often displays an identifiable structure in radar returns. Cells in ordinary convective thunderstorms typically last 20 to 30 minutes, but may form longer-lastingmulticell storms orsupercells.^[3]
cellular cloud: Amesoscale organization ofconvective activity in the form of a quasi-regular pattern ofclouds behaving as individualconvective cells, often stretching horizontally for tens of kilometers. Such patterns may be composed of open or closed cells or both: the open cells consisting of a ring ofcumulus with a clear center, and the closed cells filled withstratocumulus surrounded by a clear rim.^[2]
Center for Analysis and Prediction of Storms (CAPS): Develops techniques for computer-based prediction of high-impact local weather, such as individual spring and winter storms, using Doppler weather radar and other sources. Based in Oklahoma, United States.
central dense overcast (CDO): The large, centralized, contiguous area ofthunderstorms surrounding the rotational center of a strongtropical orsubtropical cyclone. When a cyclone reaches sufficient intensity, a distinguishableeye may develop within the CDO. The strongest winds and heaviest rainfall are usually found beneath the coldest cloud tops in the CDO.
central pressure: Theatmospheric pressure at the center of a recognizablehigh orlow-pressure area at any given instant, i.e. the highest pressure in a high or the lowest pressure in a low.^[2]
ceraunometer: An instrument used for counting the number oflightning discharges within a specific radius.^[2]
chinook wind: A warm, dryföhn wind formed by a rainstorm dropping its precipitation onto the windward side of a mountain, thus drying the air mass before it blows across the leeward side, drops in elevation, and warms byadiabatic heating. Common in the northwestern United States and southwestern Canada, a chinook can cause temperatures to rise from −48 °C (−54.4 °F) to 9 °C (48.2 °F) in 24 hours, an increase of 57 °C (103 °F).
circulation: Common short form ofatmospheric circulation.
cirrocumulus (Cc): Agenus ofcloud with bothstratiform andcumuliform characteristics, signifyingatmospheric convection, and appearing as white, patchy, transient sheets of ripples or tufts organized in undulating rows, usually between 5 and 12 km (16,000 and 39,000 ft) above sea level. Though composed mainly of ice crystals, cirrocumulus is distinguished fromcirrus andcirrostratus by the presence of small amounts of supercooled liquid water droplets.
cirrostratus
cirrus (Ci): Agenus ofcloud characterized by thin, wispy, feather-like strands that appear white or light grey in color and form at very high altitudes, usually between 5 and 13.7 km (16,000 and 45,000 ft) above sea level. Cirrus clouds often develop from theoutflow ofcumulonimbus clouds in advance offronts orthunderstorms, and therefore may indicate the imminent arrival ofprecipitation.
clear ice: A type of solidprecipitation which forms when relatively large drops of water aresupercooled into a dense, transparent coating ofice without air or other impurities. It is similar toglaze andhard rime and, when formed on the ground, is often calledblack ice.
clear-air turbulence
climate: The statistics ofweather in a given region over long periods of time, measured by assessing long-term patterns of variation intemperature,atmospheric pressure,humidity,wind,precipitation, and othermeteorological variables. The climate of a particular location is generated by the interactions of theatmosphere,hydrosphere,cryosphere,lithosphere, andbiosphere and strongly influenced bylatitude,altitude, and localtopography. Climates are oftenclassified according to the averages or typical ranges of different variables, most commonly temperature and precipitation.
climatology: A branch of theatmospheric sciences that studiesclimate, defined asweather conditions averaged over an extended to indefinite period of time. Climatology incorporates aspects ofoceanography,geology,biogeochemistry, and the related field ofmeteorology to understand the long-term dynamics of climate-influencing phenomena and to produce climate models which can be used to estimatepast climates and predict future climates.
cloud: Anaerosol consisting of a visible mass of minute liquid droplets, frozen crystals or other particles suspended in theatmosphere. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of the saturation of anair mass when it is cooled to itsdew point or when it gains sufficient moisture (usually in the form ofwater vapor) from an adjacent source to raise the dew point to the ambient temperature. There are many different types of clouds, which are classified and named according to their shape and altitude.
cloud atlas: A pictorial key to the classification and nomenclature ofclouds.
cloud base: The lowest altitude of the visible portion of acloud.
cloud bow: Seefogbow.
cloud cover: The obscuration of all or part of thesky byclouds as observed from a particular location, or the specific fraction of the sky obscured by clouds as measured inoktas.
cloud drop effective radius
cloud genus: Seecloud type.
cloud iridescence: A type ofphotometeor consisting of colorfuliridescent patterns appearing most commonly near the semi-transparent edges of thinclouds such ascirrus andaltocumulus that are in the general proximity of the Sun or Moon. They are caused by thediffraction of sunlight or moonlight by thin, uniform layers of very small water droplets orice crystals.
cloud species: Any of a set of 14 Latin terms used to describe the shape and internal structure oftropospheric clouds. Cloud species are subdivisions ofcloud genera and are themselves further subdivided intocloud varieties.
cloud tag
cloud type: Any of a set of Latin names used to classify and identifyclouds occurring in thetroposphere, typically by characteristics such as their altitude, shape, and convective activity. A set of 10 or 12 traditional cloud types defined by theWorld Meteorological Organization and further subdivided intocloud species andcloud varieties is widely used in meteorology. Other classification systems have proposed many additional types.
cloud variety
cloudburst: A colloquial term used to describe an excessiveprecipitation event, characterized by brief, sudden, exceptionally heavyrain and/orhail falling from acloud, typically as part of athunderstorm associated with violent upward and downward convective currents.^[1]
col: The point of intersection of atrough and aridge in the pressure pattern of aweather map. It generally takes the shape of asaddle in which the air pressure is slightly higher than that within the low-pressure regions but still lower than that within theanticyclonic zones.
cold front: A type offront located at the leading edge of a coolerair mass as it replaces a warmer air mass. Cold fronts lie within a sharp surfacetrough oflow pressure and the temperature difference between the air masses they separate can exceed 30 °C (86 °F). When enough moisture or instability is present, lines ofrain orthunderstorms may accompany the boundary as it moves. Insurface weather analysis, cold fronts are symbolized by a blue line with triangles pointing in the direction of travel.
cold wave: A period of weather characterized by excessively low temperatures, which may or may not also be accompanied by changes inhumidity. Very cold weather is often only referred to as a cold wave if the temperature, or the rate at which the temperature decreases within a given time period, is abnormal relative to the typical climate for a given location during a given season. Contrastheat wave.
cold-core low
Colorado low: A type oflow-pressure area that forms in southeasternColorado or northeasternNew Mexico, in theUnited States, and then proceeds to move east across theGreat Plains, often producing heavysnow andice when occurring in the winter.
convection: Seeatmospheric convection.
convective available potential energy (CAPE)
convective condensation level
convective inhibition (CIN)
convective instability: The inability of anair mass to resist vertical motion. In a stable atmosphere vertical movement of air is generally difficult, whereas in an unstable atmosphere vertical disturbances can be quite exaggerated, resulting inturbulent airflow andconvective activity that may lead to extensive vertical clouds,storms, andsevere weather.
convective outlooks
convective storm detection
convergence: A pattern of fluid flow that brings about a net inflow of fluid elements into a region, in either the atmosphere or the ocean, accompanied by compensating vertical motion. When convergence occurs in the lower atmosphere, generally below about 550 hectopascals (0.54 atm), the compensatory air motion is upward, with inflow gradually changing tooutflow at higher altitudes; when it occurs in the upper atmosphere, the air motion is downward, withdivergence near the surface.^[1]
convergence zone
corona: An optical phenomenon consisting of apparent concentric, pastel-colored rings around a bright celestial object (such as the Sun or the Moon), which are produced by thediffraction of light by individual water droplets or sometimes smallice crystals in a cloud or on a foggy glass surface. Coronae differ fromhalos in that the latter are formed byrefraction from comparatively large particles.
crepuscular rays
crosswind: Anywind that moves in a direction that is perpendicular to the direction of travel of a reference object, such as an airplane.
Crow instability: Aninviscid line-vortex instability most commonly observed in the skies behind large aircraft such as theBoeing 747. It occurs when thewingtip vortices interact withcontrails from the engines, producing characteristic visual distortions in the shapes of the contrails.
cumuliform: Of or relating to heaped, "puffy"clouds, such ascumulus orcumulonimbus, that form as a result ofatmospheric convection.^[1]
cumulonimbus
cumulus (Cu): Agenus ofcloud characterized by low-level "puffy" or "cotton-like" forms with flat bases (generally opaque white in color but sometimes with grey undersides), which occur individually or multiply in a variety of distinct subforms, usually at altitudes less than 2 km (6,600 ft) above sea level. Cumulus clouds normally produce little or noprecipitation, but can develop into precipitation-bearing clouds such ascumulonimbus when influenced by atmospheric instability, moisture, and temperature gradients.
cumulus congestus
cumulus humilis
cumulus mediocris
cyclone: Any large-scaleair mass characterized by inward spiralingwinds whichcirculate around a strong center oflow atmospheric pressure. Cyclones canform over land or water, can vary in size frommesocyclones such astornadoes tosynoptic-scale phenomena such astropical cyclones andpolar vortices, and may transition between tropical, subtropical, andextratropical phases. Contrastanticyclone.
cyclonic rotation
cyclogenesis: The development or strengthening of acyclonic circulation in the atmosphere. Cyclogenesis may refer to a number of different processes that occur under a variety of conditions and at a variety of scales, all of which result in the formation of some sort ofcyclone; for instance,tornadoes are a type ofmesocyclone whose development may be variously described as cyclogenesis or, more specifically,tornadogenesis. Contrastanticyclogenesis.

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dark adaptor goggles: A type of specialized eyewear used by meteorologists and astronomers for adapting the eyesto the dark prior to an observation made at night, or for aiding with identification ofclouds during bright sunshine or when there is a glare from snow.
dawn: The first appearance of sunlight in the easternsky beforesunrise, or the time that marks the beginning of the morningtwilight.^[2]
daytime: The period of theday betweensunrise andsunset, during which any given point on the Earth experiences natural illumination from especially directsunlight, known asdaylight.
dBZ: Abbreviation ofdecibel relative to Z
debris cloud: Seetornado debris signature.
deepening: A decrease in thecentral and surrounding sea-levelpressure within the circulation of a pressure system (usually alow-pressure system) over a short period of time, with the result that mass is exported from the total air column overlying the system faster than it is supplied. Deepening of a low is commonly accompanied by the intensification of itscyclonic circulation and hence itswinds, and the term is frequently used to implycyclogenesis.^[1] Contrastfilling.
deformation: The rate of change of shape of a fluid body such as anair mass. This quantity is very important in the formation ofatmospheric fronts, in the explanation ofcloud shapes, and in the diffusion of materials and properties through the atmosphere.
degree-day: A measure of the difference between the mean daily temperature and a specified reference temperature for a given day. For a specified period, e.g. a month or a year, the number of degree-days is the sum of all degree-days within that period.^[1]
dense fog: An advisory issued by the U.S.National Weather Service to caution the public about the possibility that horizontalvisibility may be reduced by densefog to 0.25 miles (0.40 km) or less.^[2]
depression: Anyarea of low atmospheric pressure at a given level in the atmosphere; i.e. a "low" ortrough. The term is used especially frequently to refer to an early stage in the development of atropical cyclone during which the disturbance is only weakly developed or poorly organized; seetropical depression.
derecho: A type of storm that produces widespread, straight-linedsustained winds that are associated withsevere thunderstorms.
dew: Liquid water droplets that commonly appear on thin, exposed surfaces in the morning or evening due to thecondensation of atmosphericmoisture on radiatively cooled surfaces. When temperatures are low enough, the water droplets freeze into ice particles known asfrost.
dew point (T_d): Thetemperature to which anair parcel must be cooled, at constantpressure andmoisture content, in order for saturation to occur. Continued cooling below the dew point will cause condensation of water droplets if atmospheric conditions are favorable. Dew point is often used as a proxy by which to indicate the moisture content of the air.^[1]
dew point depression (T–T_d): The difference between the actualtemperature and thedew point at a certain altitude in the atmosphere. A small dew point depression indicates more moisture and higherrelative humidity, which in the lowertroposphere can result in lowcloud bases andlifted condensation levels, which are important factors contributing to the development ofsevere thunderstorms.
diabatic process: Any thermodynamic process in which thetemperature of anair parcel changes as a result of the transfer of energy (e.g. heat) between the parcel and its surroundings, as opposed to anadiabatic process, in which the temperature changes without any such exchange. Most thermodynamic processes near the Earth's surface are diabatic, owing to the continual mixing of air andturbulence.^[1]
Diablo wind
diamond dust: A ground-levelcloud composed of tinyice crystals. Because it generally forms in sub-freezing temperatures beneath otherwise clear or nearly clear skies, diamond dust is sometimes referred to asclear-sky precipitation.
diffluence: The elongation of a fluid body, such as anair mass, normal to the flow (streamline divergence). It is a flow pattern ofdeformation.
diffuse sky radiation: The component of incomingsolar radiation that is scattered from the direct solar beam by molecules of air,aerosols,clouds, or particulate matter in theatmosphere and subsequently reaches the Earth's surface in nearly equal amounts from nearly all parts of the sky duringdaylight.^[1]
direct circulation: A closed, vertically distributed thermal circulation in the atmosphere, in which warm, lighter air rises and cold, denser air sinks (or, equivalently, a system in which the rising motion occurs at a higherpotential temperature than the sinking motion). Such acell converts heat energy to potential energy and then to kinetic energy.^[2] Contrastindirect circulation.
discontinuity: A horizontal zone across which temperature, humidity, wind speed, or any other meteorological variable changes abruptly, such as afront.^[1]
disdrometer: A scientific instrument used to measure thesize distribution and velocity of fallinghydrometeors such asraindrops.
diurnal: Occurring or varying in the course of a solar day (i.e. daily; completed within and recurring every 24 hours), or during the localdaytime.^[2]
diurnal variation: The range between the maximum and minimum values of a meteorological quantity (e.g. temperature, pressure, relative humidity) observed during the course of a solar day.^[1]
Dobson unit (DU): A unit of measurement used to describe the quantity of a trace gas (primarily atmosphericozone concentrations) present in avertical column of the atmosphere. It is defined as the thickness (in units equivalent to 10 μm) of the layer of pure gas which would be formed if all of the gas molecules in the column could be collected on the surface atstandard temperature and pressure.
doldrums: SeeIntertropical Convergence Zone.
Doppler on Wheels (DOW)
Doppler weather radar
downburst: A surface-levelwind system that emanates from an elevated point source and blows radially in all directions upon making contact with the ground. Downbursts are created whenrain-cooled air descends rapidly, and can produce very strong damaging winds. They are often confused withtornadoes, although a tornado causes air to move inward and upward whereas a downburst directs it downward and outward.Microbursts,macrobursts, andheat bursts are all types of downburst.
downdraft
drifting snow: Particles ofsnow lifted by thewind to a modest height, generally less than 1.8 metres (6 ft) above the ground. Drifting snow does not significantly reducevisibility at eye level below 10 kilometres (6.2 mi), in contrast toblowing snow.^[2]
drizzle: A type of lightprecipitation consisting of liquid water droplets which are smaller than ordinaryraindrops, generally less than 0.05 millimetres (0.002 in) in diameter and falling at a rate of less than 1 millimetre (0.04 in) per day.
drought: Any prolonged period of below-averageprecipitation in a given region that results in shortages in the local water supply, whether of atmospheric, surface water, or ground water. Droughts can last for months or even years, and may be declared after as few as 15 days; annual or seasonal decreases in precipitation, such asdry seasons in the tropics, are sometimes called droughts, though a true drought is by definition abnormal or irregular. Drought conditions result from the confluence of a wide variety of climatic factors and may be exacerbated byhot temperatures; in turn, droughts may increase the likelihood ofwildfires.
dry lightning: Lightning associated with adry thunderstorm.
dry line
dry microburst
dry punch: Meteorological slang for asynoptic-scale ormesoscale weather process. A dry punch that occurs near the Earth's surface may result in adry line bulge, whereas a dry punch aloft may increase the potential forsevere thunderstorms.
dry season: An annual period of relatively low or infrequentprecipitation, during which weather patterns are typically dominated by lengthy periods ofhigh atmospheric pressure, high temperatures, and lowhumidity. The term is primarily used in thetropics, in contrast to thewet season.
dry thunderstorm: Athunderstorm that producesthunder andlightning but in which most or all of itsprecipitation evaporates before reaching the ground. Dry thunderstorms occur necessarily in dry conditions, and theirlightning strikes, sometimes referred to asdry lightning, are a major cause ofwildfires.
dual polarization weather radar
dusk
dust devil
dust storm: A meteorological phenomenon characterized by very strong winds that blow dust-filled air over an extensive area. Dust storms arise when agust front or other strong wind blows loose dirt, sand, and/or small rocks from a dry surface into the atmosphere, drastically reducing visibility. Though the term is sometimes restricted to storms occurring over normally arable land suffering fromdrought, it is also used interchangeably withsandstorm andhaboob.

E

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echo: On aradar display, the appearance of the radio signal that is scattered or reflected back from a target. The distinct characteristics of a radar echo can be used to identify the distance and velocity of the target with respect to the signal source as well as the target's size, shape, and composition.
eddy: The swirling motion of a fluid and the reverse current created when the flow regime experiencesturbulence, such as when an obstacle blocks part of the path of flow.
Ekman layer: The layer in a fluid in which there is a force balance between thepressure-gradient force, theCoriolis force, and turbulentdrag. Ekman layers occur in both theatmosphere and theocean.
Ekman number
Ekman spiral
Ekman transport
energy-helicity index (EHI)
El Niño: The warm phase of theEl Niño–Southern Oscillation (ENSO), associated with the annual development of a band of warm ocean water in the eastern equatorial Pacific, which bringslow pressure and heavyrainfall to the coasts of Central and South America. The El Niño phase of the cycle may last between two and seven years, with local weather patterns recurring every year. The cool phase of the ENSO is calledLa Niña.
El Niño–Southern Oscillation (ENSO): An irregular long-term periodic variation inwinds andsea surface temperatures over the tropical eastern Pacific Ocean which affects the climate of most of the world but especially the tropics and subtropics in a cycle lasting years or decades. The phenomenon, a consequence of theWalker circulation, is marked by two phases: a warming phase,El Niño, during which sea temperatures are above average over a large part of the eastern Pacific Ocean, driving high pressure and dry weather in Asia and low pressure and heavy precipitation in the Americas; and a cooling phase,La Niña, during which sea temperatures are below average in the eastern Pacific and the reverse weather pattern occurs. Each phase can last for several years, with local seasonal weather patterns recurring predictably, though there are also long intervals of "neutral" or average conditions when neither El Niño nor La Niña is active.
electrometeor: Any visible or audible indicator of atmospheric electricity, including all types oflightning discharges,thunder, andaurorae.^[2]
emagram: One of fourthermodynamic diagrams used to display temperaturelapse rate andmoisture content profiles in the atmosphere. Emagrams have axes oftemperature (T) andpressure (p). Temperature anddew point data fromradiosondes are plotted on these diagrams to allow calculations of convective stability orconvective available potential energy.
Enhanced Fujita scale (EF scale)
ensemble forecasting: Aweather forecasting technique in which anumerical weather model generates a set of multiple (often several dozen) forecasts, each based on a slightly different set of initial atmospheric conditions, intended to provide an indication of the range of possible future states of the atmosphere. If the forecasts are consistent, they are usually considered reliable; if they diverge, meteorologists may feel less confident in making specific predictions for the forecast area.^[2]
entrainment: The process by which the air surrounding a developingcloud is mixed into an ascendingconvection current within the cloud, which has the effect of reducing the current's buoyancy. If very dry air is introduced, evaporation of the cloud droplets may cause the cloud system to dissipate completely.^[1]
Environment and Climate Change Canada
environmental lapse rate (ELR): The actual rate at which atmospherictemperature changes withaltitude, as measured by aradiosonde; this is in contrast to the rate predicted by the theoreticalprocess lapse rate. On average, the temperature of thetroposphere decreases with height at a rate of 6.5 °C (11.7 °F) per kilometre, but this rate is influenced by many factors. In general, the ELR is lower nearer to the ground surface, during the localwinter, and over continental landmasses.^[1]
Environmental Modeling Center (EMC)
Environmental Science Services Administration (ESSA): The predecessor agency (1965–1970) to theNational Oceanic and Atmospheric Administration (1970–present).
equivalent potential temperature ( $\theta _{e}$ )
equivalent temperature ( $T_{e}$ ): Thetemperature obtained when anair parcel expandsadiabatically, at constant pressure, until its water vapor content has beencondensed out and the latentheat of condensation is available to raise the air temperature.^[1]
Eulerian equations
European windstorm
evaporimeter: An instrument used to measure the rate of evaporation of water into the atmosphere. The most basic design consists of an open, ground-levelevaporation pan from which water is allowed to evaporate freely.^[1]
explosive cyclogenesis
extratropical cyclone
extreme weather: Anyweather that is unexpected, unusual, unpredictable, unseasonal, or especiallysevere (i.e. weather at the extremes of an historical distribution).
eye: A typically circular region at the center of a strongtropical cyclone that is the location of the storm's lowestbarometric pressure. The eye is usually characterized by light winds, clear skies, and mostly calm weather, in stark contrast to the severe weather that occurs in the surroundingeyewall and the rest of the storm.
eye of the wind: A nautical term used to describe the direction from which thewind is blowing.^[1]
eyewall

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fall wind: Seekatabatic wind.
Fata Morgana
fetch: The length of water over which a givenwind blows. Fetch length andwind speed together determine the size of thewaves that form on the surface of a body of water; the longer the fetch and the stronger the wind, the more wind energy is imparted to the water surface and the larger the resultingsea state.
field mill: A scientific instrument used to measure the strength ofelectric fields in the atmosphere.
fire whirl: Awhirlwind induced by afire and often at least partially composed of flame or ash. They are usually associated with very largewildfires. Fire whirls are seldom classified as truetornadoes, as theirvorticity usually derives from turbulent surface winds and heat-induced lifting rather than from a tornadicmesocyclone aloft.
firestorm: A very largewildfire or otherconflagration which because of its intensity is able to create and sustain its ownstorm-force winds. Firestorms develop when a convectiveupdraft of hot air rising from the burning area draws in strong windgusts from all directions, which supply the fire with additional oxygen and thereby induce further combustion. They are often associated withflammagenitus clouds andfire whirls.
flammagenitus
flash flood: Anyflood which very rapidly inundates low-lying areas such as washes, rivers, dry lakes, and basins, especially one which recedes again in less than six hours. Flash flooding can be caused by heavyrain associated withsevere weather, large amounts ofmeltwater from melting ice or snow, or the sudden collapse of a naturalice ordebris dam.
flash freezing: The process by which objects such as liquidhydrometeors are cooled below their freezing point very quickly, typically upon being subjected to extremely cold atmospheric temperatures or by making contact with a frozen surface.
flood: An overflow of water which submerges land that is usually dry. Flooding may occur when water bodies such as rivers, lakes, or oceans escape their boundaries by overtopping or puncturinglevees, or it may occur whenprecipitation accumulates on saturated ground more rapidly than it can eitherinfiltrate orrun off.
flumen
fog: A visibleaerosol of minute water droplets or ice crystals that is suspended in the air at or near the Earth's surface. Fog is often considered a type of low-lyingcloud and is heavily influenced by local topography, nearby bodies of water, andwind conditions.
fogbow: An optical phenomenon in which a whitish or faintly coloredprimary rainbow, often with red and blue edges, is visible on a background offog or mist at the observer'santi-solar point. It is caused by the refraction, reflection, and diffraction of light from the Sun or Moon by small water droplets with diameters less than 100 micrometres (0.004 in).^[1]
föhn wind: A type of warm, dry, downslopewind that occurs in thelee of a mountain range.
forward-flank downdraft (FFD)
fractus (Fr): Acloud type orspecies consisting of ragged, irregularly shaped patches or shreds ofcumulus orstratus.^[1]
frazil ice
freezing drizzle: A type ofprecipitation in whichdrizzle consisting ofsupercooled liquid water droplets, often falling through atemperature inversion in the lower atmosphere, freezes upon impact with the ground or other cold surfaces to form a coat ofglaze ice.^[1] Comparefreezing rain.
freezing fog: A condition in which supercooled water droplets comprisingfog freeze either while suspended in the air, filling the air with visibleice crystals similar to very lightsnow, or upon contact with sub-freezing surfaces, forming a coating ofrime and/orglaze ice.^[2]
freezing rain: Liquid droplets ofrain that becomesupercooled while falling through a sub-freezingair mass and then freeze upon impact with any surface they encounter; the resultingglaze ice can accumulate to a thickness of several centimeters. Unlikemixed rain and snow,ice pellets, andhail, freezing rain exists entirely as a liquid until it hits a surface.
freshet: 1. A springtime thaw of snow and ice that produces a significant local inundation of rivers, streams, small watercourses, and floodplains as the snowpack melts within a watershed.; 2. Any temporarily inundated or rapidly flowing watercourse or newly created (and often ephemeral) drainage channel resulting from snowmelt.
front: A boundary separating twomasses of air of differentdensities and usually also of differenttemperatures andhumidities. Weather fronts are the principal cause of meteorological phenomena outside thetropics, often bringing with themclouds,precipitation, and changes inwind speed and direction as they move. Types of fronts includecold fronts,warm fronts, andoccluded fronts.
frontogenesis: The meteorological process by which aweather front is created, usually as a result of the narrowing of one or more horizontal temperature gradients across the boundary between two adjacentair masses. Contrastfrontolysis.
frontolysis: The dissipation or weakening of an atmosphericweather front. Contrastfrontogenesis.
frost: A very thin layer ofice crystals on a solid surface, typically restricted to that which forms whenwater vapor in an atmosphere whose temperature is above freezing comes into contact with a surface whose temperature is below freezing. Frost may exhibit a great variety of forms.
Fujita scale
funnel cloud: Afunnel-shapedcloud associated with a rotating column of air and protruding from thebase of a parent cloud but not reaching the ground or a water surface. Funnel clouds form most frequently in association withsupercell thunderstorms and often develop intotornadoes.

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gale: 1. A strong surfacewind, typically used as a descriptor in nautical contexts and variously defined based onspeed. In the modernBeaufort scale, a gale is anysustained wind of Beaufort number 7 or greater, corresponding tonear gale at 28–33 kn (52–61 km/h; 32–38 mph);gale at 34–40 kn (63–74 km/h);strong gale at 41–47 kn (76–87 km/h); andstorm at 48–55 kn (89–102 km/h).^[1]; 2. Any unusually strong wind.^[2]
gale warning
gap wind: A local, low-levelwind that blows along a valley or through acol between mountains, often at speeds as high as 20–40 knots (37–74 km/h; 23–46 mph).^[1]
general circulation
geopotential height: A measure of the vertical distance oraltitude above mean sea level that accounts for variations ingravitational potential as altitude and latitude change. In meteorology and atmospheric science, geopotential height is often used in place of ordinary altitude when calculating theprimitive equations innumerical weather prediction and when creatingatmospheric models.
geostrophic wind: The theoreticalwind that would result from an exact balance between theCoriolis force and thepressure gradient force (known asgeostrophic balance). The true wind almost always differs from the geostrophic wind due to the influence of other forces such as friction from the ground.
glaze: A coating of smooth, clearice, sometimes of considerable thickness, that forms whensupercooled water, usually precipitated asfreezing rain orfreezing drizzle, freezes upon contact with the ground or other exposed surfaces where the temperature (and that of the lower atmosphere) is at or below 0 °C (32 °F).^[1] Glaze is denser, harder, and more transparent thanrime andhoarfrost.^[2]
GPS meteorology: A type of observational meteorology that interprets the effects of atmospheric properties such astotal precipitable water vapor on the propagation ofGlobal Positioning System (GPS) radio signals to derive information about the state of the local atmosphere.
graupel: A type ofprecipitation that forms whensupercooled water droplets are collected and freeze on fallingsnowflakes, forming balls ofrime 2–5 mm (0.079–0.197 in) in diameter. Graupel is distinct fromhail, small hail, andice pellets.
Great Salt Lake effect: Alake-effect snow that occurs in the lee ofUtah'sGreat Salt Lake.
grease ice
green flash: An optical phenomenon consisting of a momentary glimmer of green light occasionally observed near the upper limb of the Sun's apparent disk just as it disappears from view at sunset or just as it appears at sunrise. It is most likely to be seen where there is a low, clear, distant horizon, such as over the ocean.^[1]
ground blizzard: A weather condition that occurs when loose snow or ice on the ground is lifted and blown into the air by strong winds. This can create low-visibility conditions even in the absence of precipitation.
ground truth: Information, such as local weather conditions, provided by direct observation (i.e. empirical evidence) as opposed to information provided by inference.
gust: A brief, sudden increase in thespeed of thewind, usually lasting less than 20 seconds. Gusts are more transient thansqualls and are followed by a lull or slowing of the wind speed. They are generally only reported byweather stations when the maximum wind speed exceeds the average wind speed by at least 10–15 knots (12–17 mph).
gust front: Seeoutflow boundary.
gustnado: A relatively weaktornado associated with theoutflow at the leading edge of athunderstorm cell, and often occurring along agust front. Adebris cloud ordust whirl may indicate the presence of a gustnado.^[2]

H

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haboob
Hadley cell
hail: A type of solidprecipitation that consists of balls or irregular lumps ofice, usually 5–150 mm (0.20–5.91 in) in diameter, each of which is called ahailstone. Hail formation requires environments with strong, upward motion of air and low altitudes at which water freezes, which makes it possible within mostthunderstorms. It is distinct fromgraupel andsleet orice pellets.
hailstorm: Anystorm, usually a strongthunderstorm, which precipitateshail.
Haines Index: A weather index that measures the potential for dry, unstable air to contribute to the development of large or erraticwildland fires. The index derives from data on the stability andmoisture content of the lower atmosphere and is calculated over three ranges ofatmospheric pressure.
halo
hard rime: A type ofrime consisting of opaque, granular masses of ice deposited primarily on vertical surfaces byfreezing fog. Hard rime is more compact and amorphous thansoft rime and usually develops on windward surfaces exposed to high wind speeds and air temperatures between −2 and −8 °C (28 and 18 °F).
Harmattan
haze: Any suspension in the atmosphere of very small, dry particulate matter, including naturalaerosols (e.g. dust, salt, or smoke) as well as man-made pollutants (e.g.smog), the individual particles of which are invisible to the naked eye but collectively produce a milky, often opalescent sky with reducedvisibility at long distances. Haze usually indicates sub-saturated air, whereasfog or mist indicates full saturation.^[2]
hazardous seas warning
hazardous seas watch
heat dome: The effect created by Earth's atmosphere trapping hot ocean air like a lid or cap.
heat burst: A rare phenomenon involving a sudden, localized increase insurface temperature (sometimes 10 °C (18 °F) or more within just a few minutes) associated with a decayingthunderstorm or othermesoscale convective system and possibly accompanied bygusty winds and a rapid decrease inhumidity.^[3]
heat index (HI): A meteorological index that posits theapparent temperature perceived by the average human being who is exposed to a given combination of airtemperature andrelative humidity in a shaded area. For example, when the air temperature is 32 °C (90 °F) with 70% relative humidity, the heat index is 41 °C (106 °F).
heat lightning
heat wave: A period of weather characterized by excessively high temperatures, which may or may not be accompanied by highhumidity or bydrought. Very hot weather is often only referred to as a heat wave if the temperature is abnormal relative to the typical climate for a given location during a given season. Contrastcold wave.
heavy snow warning: A type of weather warning formerly issued by the U.S.National Weather Service to alert areas in which a high rate ofsnowfall (generally 6 in (15 cm) or more in 12 hours) was occurring or wasforecast. The warning was replaced by theWinter Storm Warning for Heavy Snow beginning with the 2008–09 winter storm season.
helicity
high-pressure area
hodograph: A vectorial visual representation of the movement of a body or a fluid, with the position of any data plotted on it proportional to thevelocity of the moving particle. In the context of meteorology, hodographs are used to plotwinds fromatmospheric soundings: for a given vector,wind direction is indicated by the angle from the center axis andwind speed by the distance from the center.
hook echo
horseshoe vortex
humidity: A measure of the amount ofwater vapor present in aparcel of air. By quantifying the saturation of the air with moisture, humidity indicates the likelihood ofprecipitation,dew, orfog occurring. The amount of water vapor needed to achieve full saturation increases as the airtemperature increases. Three primary measurements of humidity are widely employed in meteorology:absolute,relative, andspecific.
humidex
humilis: Seecumulus humilis.
hurricane: The local name for atropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean and achieves one-minute maximumsustained winds exceeding 74 mph (119 km/h; 64 kn).
hurricane hunters
huaico: Amudslide orflash flood caused by torrentialrainfall occurring high in the Andes mountains of South America, especially during the weather phenomenon known asEl Niño.
hydrometeor: Any particulate of liquid or solid water within theatmosphere, encompassing all types ofprecipitation, formations due to condensation such asclouds andhaze, and particles blown from the Earth's surface by wind such asblowing snow andsea spray.
hydrometeorology: A branch ofmeteorology andhydrology that studies the transfer of water and energy between land surfaces and the loweratmosphere.
hydrosphere: The combined mass of allsolid, liquid, and gaseous forms ofwater found on, beneath, or above the surface of the Earth, including all oceans, lakes, streams, groundwater, atmosphericwater vapor, snow, ice caps, and glaciers.
hydrostatic equilibrium
hygrometer: A scientific instrument used to measurehumidity.
hygroscopy: The phenomenon by which a substance attracts and retains water molecules via eitherabsorption oradsorption from the surrounding environment.
hypsometer: A scientific instrument used to measure height or elevation, either by trigonometry or by the principle thatatmospheric pressure influences theboiling point of liquids.

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ice: Waterfrozen into a solid state. Ice is abundant on Earth's surface and in the atmosphere and plays a major role in Earth'swater cycle andclimate. Its natural occurrence in weather phenomena takes many forms, includingsnowflakes,hail,frost,icicles, andice spikes.
iceberg
ice accretion indicator
ice crystal: 1. A minute spicule ofice that forms from water in the atmosphere at temperatures below the freezing point of 0 °C (32 °F). Ice crystals may take on any of a number of macroscopic, crystalline forms depending on the temperature at their formation, including needles, hexagonal prisms, and stars. Their growth occurs by the diffusion ofwater vapor onto them, and they may collide with other ice crystals to formsnowflakes.^[1]; 2. A type ofprecipitation composed of very small, unbranched crystals of ice which fall slowly and often seem to float in the air.^[2]
ice fog: A type offog consisting of a sufficient concentration of tinyice crystals suspended in the atmosphere to reducevisibility to less than 1 kilometre (0.62 mi). Ice fog forms at very low ambient air temperatures, typically −30 °C (−22 °F) or below, usually in calm conditions at high latitudes but sometimes also as the result of mild maritime air blowing across ice- or snow-covered surfaces.^[2]^[1]
ice pellets
ice spike: A rareice formation that consists of a long, slender projection of ice extending upward from the surface of a frozen body of water, often in the shape of an invertedicicle.
ice storm: A type ofwinter storm characterized byfreezing rain which results in the accumulation of at least 6.4 millimetres (0.25 in) ofice on exposed surfaces.
icicle: A long, slender spike ofice formed when water dripping or falling from an object freezes.
incus
Indian summer
inflow: The influx of heat and moisture into astorm system from the surrounding environment. The inflow ofparcels of warm, moist air drives and sustains most types of storms, includingthunderstorms andtropical cyclones. Contrastoutflow.
instrument flight rules (IFR)
International Standard Atmosphere (ISA): A staticatmospheric model of the variations intemperature,pressure,density, andviscosity over a wide range of altitudes within theEarth's atmosphere, established as an international standard by theInternational Organization for Standardization in order to provide a common reference for atmospheric variables relevant to meteorology and atmospheric science.
Intertropical Convergence Zone (ITCZ)
irisation: Seecloud iridescence.

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jet-effect wind: Seecanyon wind.
jet stream: A narrow, fast-flowing, meanderingair current primarily occurring in the upper part of thetroposphere, at altitudes above 9 km (30,000 ft), and usually flowing from west to east. The Northern and Southern Hemispheres each have a predictable though discontinuouspolar jet andsubtropical jet;low-level jets and other types of jet streams can form under certain conditions.
jet streak: The region of maximumwind speed that runs along the elongated axis of ajet stream. In the local winter, the maximum speed in the polar-front jet stream can reach upwards of 200 knots (370 km/h; 230 mph).^[1]

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K-index: An operational atmosphericstability index indicating the potential forthunderstorms, based on temperaturelapse rate,moisture content of the lower troposphere, and the vertical extent of the moist layer.^[3] K-index values of 36 and above suggest a high likelihood of thunderstorm development.^[2]
$K_{DP}$
kata-front: Awarm front orcold front that is overrun by drier air, or in which the warm air subsides, so that any clouds and precipitation tend to be suppressed, making them generally inactive fronts. Contrastana-front.^[1]
katabatic wind: Alocal wind that carries cold, high-density air from a higher elevation downslope under the force of gravity as a result of the radiative cooling of the upland ground surface at night, usually at speeds on the order of 10 kn (19 km/h) or less but occasionally at much higher speeds. Contrastanabatic wind.
Kelvin temperature scale
Kelvin–Helmholtz instability: A phenomenon of instability that occurs occasionally in an atmospheric layer within whichwind speed increases rapidly withaltitude. Kelvin–Helmholtz waves form in this layer of strong verticalwind shear, and are often marked by a distinct train of clouds that resemble breaking ocean waves.^[1]
khamsin: The local name for a dry, hot, seasonalwind, often carrying large quantities ofdust or sand, that occurs in the deserts of Egypt, Israel, Palestine, and Jordan. Comparehaboob,harmattan,sirocco, andsimoom.
kinematics: A branch ofclassical mechanics that describes the motion of points, bodies, and systems of bodies without considering the forces that caused the motion.
knot (kn): A unit of speed commonly used in maritime and aviation disciplines, equivalent to onenautical mile per hour (1.1508 miles per hour or 0.5145 metres per second). It is often used in meteorology for measuringwind speed.
Köppen climate classification

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$L_{DR}$
La Niña
Lagrangian equations
lake-effect snow: A weather phenomenon produced when a coldair mass moves across long expanses of warmer lake water, which causes the lowest layers of air to pick up warmwater vapor from the lake, rise through the upper layers, freeze and then precipitate on the lake'sleeward shores. In combination withorographic lift, the effect produces narrow but very intense bands ofprecipitation, especiallysnow, which can deposit at very high rates and result in very large amounts of snowfall over a region. The same effect can also occur over bodies of salt water, when it is termedocean-effect orbay-effect snow.
laminar flow: A flow in which the particles of a fluid moves smoothly in parallel layers or sheets, i.e. withoutturbulence.^[2]
land breeze: An offshorelocal wind that blows from land to sea, usually at night, a result of the more rapid cooling of the land surface relative to the sea after sunset. It blows in the opposite direction of asea breeze, its daytime counterpart in adiurnal cycle of coastal winds caused by lateral differences in surface temperature between land and sea.^[2]
landfall: The movement of astorm or other weather phenomenon over land after being over water.
landslide
landspout: A type oftornado emerging from a parent cloud that does not contain a pre-existing mid-levelmesocyclone or other rotation. Landspouts share a development process and resemblance withwaterspouts. They are generally smaller and weaker thansupercell tornadoes and are rarely detected byDoppler weather radar.
lapse rate: The rate at which an atmospheric variable, most commonly temperature or pressure, decreases with increasingaltitude.
latent heat: The amount ofheat absorbed or released per unit mass during a change of phase of a substance at constant temperature and pressure. In meteorology, the term usually refers to the amount absorbed or released in the various transformations between the three physical states of water: ice, liquid water, and water vapor. For instance, thelatent heat of vaporization requires about 2.4 × 10⁶Joules per kilogram at 0 °C.^[1] Contrastsensible heat.
latent heat flux: The movement ofwater vapor (a major transporter oflatent heat) from one location to another, e.g. from the tropics toward the poles, where there is a persistent energy deficit relative to lower latitudes. Poleward latent heat flux reaches its global maximum of 1.5 × 10¹⁵watts at latitudes 38 °N and 40 °S.^[1]
law of storms: A general statement of the manner in which the winds of acyclone rotate about the cyclone's center, and the way in which the entiredisturbance moves across the Earth's surface. The development by meteorologists of a "law" describing the general behavior of storms proved important in historical times to sailors navigating during storms at sea.^[2]
layer cloud: Seestratiform.
lee trough: Atrough oflow atmospheric pressure that forms preferentially to thelee or downwind side of a mountain barrier whenair currents flow in directions perpendicular to the barrier and become vertically "squashed" as they cross it. As the column resumes its original depth on the other side of the barrier, it tends to develop a strong spin about its vertical axis, which manifests as a low-pressure center.^[1]
lee wave
Lemon technique: A method used by meteorologists which focuses onupdrafts and usesweather radar to determine the relative strength ofthunderstorm cells in a verticallysheared environment.
length of record: The time interval during which a particular observation or observations in general have been maintained without interruption at a meteorological station, and which therefore serves as the frame of reference for climatic data at that station.^[2]
lenticular cloud: A type of stationarycloud with a distinctlens orsaucer shape which typically forms in an arrangement perpendicular to thewind direction and at altitudes less than 12 kilometres (39,000 ft) above sea level, most commonly above or near very large natural obstructions in the atmosphere, such as mountains and hills.
level of free convection (LFC): The altitude in the atmosphere at which the temperature of the environment decreases faster than themoist adiabatic lapse rate of a saturatedair parcel at the same level. Air masses with one or many LFCs are potentially unstable and may develop convective clouds such ascumulonimbus.
Lidar: Asurveying method that measures the distance to a target by illuminating the target withpulsed laser light and measuring the reflected pulses with a sensor; differences in laser return times and wavelengths can then be used to create digital three-dimensional representations of the target. The name is now used as an acronym oflight detection and ranging.
lifted condensation level (LCL)
lifted index (LI): The difference in temperature between the ambient environment and anair parcel that is liftedadiabatically at a givenpressure height within thetroposphere, typically 500 hectopascals (0.49 atm). When the value of the lifted index is positive, the atmosphere at the given height is stable; when it is negative, the atmosphere isunstable.
light pillar
lightning: A naturally occurringelectrostatic discharge during which two electrically charged regions of the atmosphere or ground temporarily equalize themselves, instantaneously releasing about a billion joules of energy across a wide range of the electromagnetic spectrum, from very hotplasma to brilliant flashes of light visible in the atmosphere. Lightning is often followed by its audible consequence,thunder, and is one of the distinguishing features ofthunderstorms. Lightning phenomena are generally separated into three classes based on where they occur – either inside a single cloud, between two different clouds, or between a cloud and the ground – but many other observational variants have been recognized.
lightning activity level
lightning detection
lightning strike: Anylightning discharge that occurs between the atmosphere and an object (rather than between different parts of the atmosphere). Most lightning strikes arecloud-to-ground, meaning they terminate on the Earth's surface or on an object attached to it, but lightning can also strike airborne objects or travel fromground-to-cloud. The primary electron-conducting channel in such discharges, visible for a fraction of a second as a very bright, "zigzagging" path of light, is sometimes called alightning bolt.
line echo wave pattern (LEWP)
lithometeor
low-level jet
low-level windshear alert system
low-pressure area (L)
low-topped supercell (LT)
lysimeter: An instrument used to measure the total amount ofevapotranspiration that occurs within a certain area of the Earth's surface, usually by recording the amount of precipitation received by the area and the amount of moisture subsequently lost through the soil.

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mackerel sky: A sky that is partially or fullycovered by highaltocumulus orcirrocumulus clouds with a regular pattern of ripples and patches separated by small areas of blue sky, resembling the scales on a mackerel.^[1]
macroburst: A strongdownburst that affects a path longer than 4 kilometres (2.5 mi) and persists for up to 30 minutes, with surface winds reaching as high as 210 kilometres per hour (130 mph).^[2]
macrometeorology: The study of the largest-scalemeteorological processes, i.e. those occurring over very large regions, oceans, continents, or the entire Earth, such as thegeneral circulation, as opposed tomesometeorology andmicrometeorology.^[1] See alsosynoptic-scale meteorology.
MAFOR: A North American system used in the transmission of marineweather forecasts to compress large amounts of information about meteorological and marine conditions, including visibility, expected future wind speed and direction, the "state of sea", and the period of validity of the forecast, into shorter code for convenience during radio broadcasting. MAFOR is an abbreviation ofMArineFORecast.
manometer: A scientific instrument consisting of a liquid column gauge used to measure differences in the pressures of gases, as with amercury barometer.^[1]
marine climate: A regionalclimate that is strongly influenced by its location in relation to a sea or ocean, characterized by relatively smalldiurnal andseasonal temperature variations and high atmosphericmoisture content, which contributes to highprecipitation andhumidity.^[1] Contrastcontinental climate.
marine cloud brightening
marine stratocumulus
mass flow: The movement of a fluid, such as anair mass, down apressure ortemperature gradient.
meridional circulation: The component of the large-scale atmosphericgeneral circulation that is oriented parallel to a meridian or line of longitude, and thus shows large north–south movement.^[1]
mesocyclone
mesohigh
mesolow
mesonet
mesoscale convective complex (MCC)
mesoscale convective discussion (MCD)
mesoscale convective system (MCS)
mesoscale convective vortex (MCV)
mesoscale meteorology
mesosphere: The third major layer of the Earth'satmosphere, above thestratosphere and below thethermosphere. The lower boundary of the mesosphere varies between 50 and 65 km (31 and 40 mi) above the Earth's surface, depending on latitude and time of year.
mesovortices
METAR
Météo-France: The national meteorological agency ofFrance.
meteorology: A branch of theatmospheric sciences which seeks to understand and explain observableweather events, with a major focus onweather prediction. Meteorology uses variables familiar inchemistry andphysics to describe and quantify meteorological phenomena, includingtemperature,pressure,water vapor,mass flow, and how these properties interact and change over time.
microburst
micronet: A weather observation network even denser than amesonet, such as the Oklahoma City Micronet.
microscale meteorology: Meteorological phenomena that occur on a scale of 40 m to 4 km.^[5]
mini-supercell: A distinct kind ofsupercell that is smaller than a typical supercell.
mini-tornado: A fallacious term often used in news media to refer to damaging winds accompanying athunderstorm, indifferently caused bytornadoes ormicrobursts, on a small area.
misocyclone: A vortex with a width between 40 metres (130 ft) and 4 kilometres (2.5 mi),^[6] which in the strictest sense includeswaterspouts andlandspouts.
misoscale meteorology
mixed cloud: Acloud composed of both liquid water droplets andice crystals (e.g.altostratus,cumulonimbus, andnimbostratus), as opposed to awarm cloud.^[1]
mixing ratio: A measure of atmosphericmoisture content, usually expressed as the dimensionless ratio of themass ofwater vapor in a givenparcel of air to the unit mass of dry air (i.e. grams of water vapor per kilogram of dry air).^[1]
mock sun: Seeparhelion.
Modified Fujita Scale: An update to the originalFujita scale from 1971 proposed byTed Fujita in 1992.
moist adiabat: Seesaturated adiabat.
moist adiabatic lapse rate: Seesaturated adiabatic lapse rate.
moisture convergence: An area where moisture concentrates due to the air flow near the surface.
mountain breeze
mountain-gap wind: Seegap wind.
multicellular thunderstorm: Athunderstorm consisting of more than oneconvection cell, i.e. more than one circulating system ofupdrafts anddowndrafts.
multiple-vortex tornado
moisture: The presence of liquid, especially water, within a body or substance, often in trace amounts. Moisture in the air in the form ofwater vapor underlies the concept ofhumidity.
monsoon: 1. An abrupt seasonalwind reversal accompanied by corresponding changes inprecipitation.; 2. Any seasonal change inatmospheric circulation and precipitation associated with the asymmetric heating of land and sea. In this context, the term is often used to refer specifically to therainy phase of such a pattern, and in some places colloquially (and less correctly) to any locally very heavy but short-term rainfall.
Morning Glory cloud
mudflow
murus: Seewall cloud.

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nacreous cloud: A rare type ofpolar stratospheric cloud that forms at altitudes of 24–30 kilometres (79,000–98,000 ft), usually in high-latitude regions. These clouds are normallylenticular in form but may resemblecirrus, and often exhibit brilliantiridescence similar tomother-of-pearl shortly after sunset or before sunrise.^[1]
National Center for Atmospheric Research (NCAR)
National Centers for Environmental Prediction (NCEP)
National Hurricane Center (NHC)
National Oceanic and Atmospheric Administration (NOAA)
National Severe Storms Forecast Center (NSSFC): A predecessor forecasting center to theStorm Prediction Center that was located inKansas City, Missouri.
National Severe Storms Laboratory (NSSL): ANOAA lab inNorman, Oklahoma tasked with researchingsevere weather.
National Tornado Database: The official NOAA record of all knowntornadoes within the United States from 1950 to present.
National Weather Center (NWC)
National Weather Service (NWS): The national meteorological agency of theUnited States, tasked with providingweather forecasts, warnings ofsevere weather, and other weather-related services to organizations and the public for the purposes of protection, safety, and general information.
neap tide: A small-amplitude oceanictide of minimumtidal range occurring semi-monthly near the times when the Moon is in quadrature, i.e. thefirst and third quarters.^[2]
needle ice
negative tilt: The angular displacement of atrough line such that the axis of the trough is rotated clockwise from a north–southmeridian (as opposed to the counterclockwise rotation of apositively tilted trough); in the Northern Hemisphere, negative tilt corresponds to a northwest-to-southeast orientation. Most troughs begin with a positive tilt and gradually become neutral (north–south) and then negatively tilted as the flow of cold air distorts their shape. Positive tilt thus indicates the building phase of the trough, when clouds and precipitation develop, and negative tilt indicates the dissipation of its energy, when the most severe weather occurs.
nephelometer
nephology: The scientific study ofclouds.
nephoscope: A scientific instrument used to measure the altitude, direction, and velocity of atmosphericclouds relative to a point on the ground directly below them.
NEXRAD
nimbostratus (Ns): Agenus ofcloud occurring at low or middle altitudes, typically between 0.5 and 5.5 kilometres (1,600 and 18,000 ft), and often appearing as a dull, dark gray, ragged, nearly uniform sheet or layer that obscures the Sun and produces more or less continuously falling light to moderateprecipitation but no lightning or thunder. Low, raggedfractus clouds frequently occur below nimbostratus and may or may not merge with it.^[1]^[2]
noctilucent cloud
nonadiabatic process: Seediabatic process.
nor'easter: A macro-scaleextratropical cyclone, especially one which impacts the middle and north Atlantic coasts of North America. The name derives from the direction of the winds that most strongly affect the eastern seaboard between the months of October and March. Such storms are often accompanied by very heavy rain or snow, which can cause severecoastal flooding, andhurricane-force winds.
Nor'west arch: A conspicuous high-altitude arch-shaped cloud formation that appears regularly in otherwise clear blue skies above the east coast ofNew Zealand's South Island, when a strong, hot, northwesterlyföhn wind (known as "The Nor'wester") pushes cooling moist air over theSouthern Alps.
normal: The average value of a meteorological element (e.g. temperature, precipitation, humidity) over a givenperiod of time, most commonly three consecutive 10-year intervals totaling 30 years.^[2]
northern lights: Seeaurora.
Novaya Zemlya effect
nowcasting
numerical weather prediction

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obscuring phenomena: Any atmospheric phenomenon exclusive of clouds that restrictsvertical visibility, including varioushydrometeors such asrain andsnow as well aslithometeors such as dust and sand.^[2]
occluded front: A type offront formed during the process ofcyclogenesis when acold front overtakes awarm front. Occluded fronts usually form around maturelow-pressure areas when a warmair mass is physically separated (or "occluded") from the cyclonic center at the Earth's surface by the intervention of a cooler air mass; the warmer air is lifted into atrough of warm air aloft. Insurface weather analysis, occluded fronts are symbolized by various combinations of the symbols for cold and warm fronts.
ocean current: Any regular, permanent or semi-permanent movement or flow ofocean water, either in a cyclic pattern or as a continuous stream along a defined path. Ocean currents are generally driven bywind or bygeostrophic forces related to seawater density gradients. They are major transporters of the heat introduced by solar radiation, usually moving warm water from thetropics to higher latitudes and returning cold water in the opposite direction, by which they exert an important influence onclimate and weather phenomena across the world.^[1]^[2]
oceanic climate: Seemarine climate.
offshore current: Anyocean current that flows parallel to, or away from, the coastline of a landmass.^[1]
offshore wind: Anywind that blows from land out over a body of water, e.g. aland breeze. Contrastonshore wind.
okta: A unit of measurement used to describe the amount ofcloud cover at a given location in terms of how many eighths of thesky are covered in clouds, ranging from 0 oktas (completely clear) to 8 (completelyovercast) or sometimes 9 oktas (indicating that the sky is obstructed from view).
omega equation
onshore wind: Anywind that blows from a body of water to land, e.g. alake orsea breeze. Contrastoffshore wind.
opacity
orographic cloud: Anycloud whose form and extent is determined by the effects of high-elevation terrain upon the passing flow of air, especially the forceduplift of moist air as it passes over hills or mountains. As the rising air mass encounters reduced atmospheric pressures,adiabatic cooling commonly results in condensation andprecipitation. Orographic clouds are usually very slow-moving or stationary; examples includelenticular clouds andcap clouds.
orographic lift: The forced ascent of anair mass as it passes over a topographic barrier such as a range of hills or mountains. If the air is moist, the uplift may result inadiabatic cooling, leading to saturation, condensation, and the formation oforographic clouds and oftenprecipitation.^[1]
orographic precipitation
overcast: The condition ofcloud clover whereinclouds obscure at least 95% of the sky. The type of cloud cover that qualifies as overcast is distinguished from obscuring surface-level phenomena such asfog.
overrunning: The action of anair mass aloft, often relatively warm, moving over another air mass of greater density at the surface, as occurs in awarm front.^[1]
overshooting top: A distinct, bulging protuberance produced by a vigorousupdraft that rises above the top of theanvil of acumulonimbus cloud. Overshooting tops are generally short-lived, but those that persist may indicate the potential for strongthunderstorms andsevere weather.^[1]
outflow: Air that flows outwards (away from) astorm system. Outflow typically radiates fromthunderstorms in the form of a wedge of rain-cooled air, which is often delineated by a low, thick cloud preceded by agust front, apparent both from the ground and inweather radar imagery. The altitude at which the outflow occurs is strongly correlated with the intensity and persistence of large storm systems such astropical cyclones.
outflow boundary: The boundary between the cooledoutflow air from athunderstorm and the air of the surrounding environment, similar to acold front. New thunderstorms often develop along outflow boundaries.^[1]
outflow jet
ozone depletion
ozone layer: A region of the Earth'satmosphere containing relatively high concentrations of the gaseous chemicalozone (O₃) and which is responsible for absorbing more than 97 percent of the Sun's incoming medium-frequencyultraviolet (UV) radiation. The ozone layer is found mainly in the lower portion of thestratosphere, between approximately 15 and 35 kilometres (9.3 and 21.7 mi) in altitude, although its thickness varies seasonally and geographically.

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paleoclimatology
pampero
pan evaporation
pancake ice: A form ofice that consists of round, flat pieces of ice with elevated rims, with diameters ranging from 30 cm (12 in) to 3 m (9.8 ft), and thicknesses of up to 10 cm (3.9 in).
pannus
parhelion: An optical phenomenon in which a patch of bright light is visible along the main 22°halo around the Sun, commonly occurring as a pair of such patches with one on either side of the solar disk; the halo itself is not always visible. More rarely, parhelia may occur at other points on the parhelic circle. They are caused by the refraction of sunlight by airborneice crystals with diameters less than 30 μm (0.0012 in), e.g. those present incirrus orcirrostratus clouds.^[1]
Particularly Dangerous Situation
pascal (Pa): TheSI derived unit ofpressure, defined as onenewton persquare metre. In meteorology, measurements ofatmospheric pressure are often given in hectopascals (hPa) or kilopascals (kPa).
Pascal's law: A hydrostatic principle which states thatpressure applied to a confined incompressiblefluid (e.g.air) is transmitted equally and undiminished to every portion of the fluid and to the walls of the containing vessel.^[2]
Pearson scale: Atornado rating scale developed byAllen Pearson differentiating path length (P) and path width (P) to accompany NOAAFujita scale (F) ratings.
pedestal cloud: Seewall cloud.
pentad: A period of five consecutive days sometimes used in preference to the seven-day week in the analysis of meteorological data because it divides conveniently into the number of days (365) in a standard year.^[1]
period of record: The length of time during which a specific meteorological element (e.g. temperature, humidity, precipitation, etc.) has been officially observed and recorded at a particular place.^[2]
perlucidus: Acloud variety characterized by a widespread sheet or patch of cloud with distinct gaps between the cloud elements such that the Sun, Moon, clear sky, or overlying clouds are visible from the ground. It is most often applied tostratocumulus andaltocumulus.^[1]
permafrost
photometeor: Any bright object or other optical phenomenon appearing in the Earth's atmosphere when sunlight or moonlight creates areflection,refraction,diffraction, orinterference under particular circumstances. Common examples of photometeors includehalos,coronae,rainbows,crepuscular rays, andsun dogs.
physical meteorology: A branch ofmeteorology concerned with the structure and composition of theatmosphere and the various optical, electrical, acoustical, and thermodynamic phenomena that characterize it, includingaerosols andclouds,precipitation, and electromagnetic radiation.^[1]
Phi_DP ( $\Phi _{DP}$ )
pileus: A smallaccessory cloud, appearing as a smooth, shallow,lenticular "cap", that forms above or attached to the top of acumulus orcumulonimbus cloud. Pileus clouds are formed when moist air above the parent cloud is cooled to itsdew point by a strongupdraft, and are good predictors ofthunderstorms; a pileus atop a cumulus cloud often foreshadows its transformation into a cumulonimbus cloud.^[7]
pilot balloon: Seeceiling balloon.
pilot report (PIREP): An inflight report by an aircraft pilot or crew member of theweather experienced by the aircraft. A complete coded report typically includes information about the location and/or extent of reported weather phenomena; the time of observation; a description of the phenomena; the altitude of the phenomena; and the type or status of the aircraft.^[2]
polar high: An extensivehigh-pressure area across the polar latitudes of either the Northern or Southern Hemisphere which acts as a source of very cold and generally dry air. Theanticyclone over the Arctic, known as theArctic high, is generally seasonal, while that over Antarctica, known as theAntarctic high, is semi-permanent.^[1]
polar low: A relatively small-scale, non-frontal, migratorylow-pressure system that occurs in the polar latitudes of either the Northern or Southern Hemisphere. Such systems aresecondary depressions that form over oceans poleward of thepolar front, most commonly during the localwinter, and can produce blustery, snowy conditions.^[1]
polar front: Either of the two semi-permanent, semi-continuousboundaries separating warm, moist tropical air from cold, dry polar air in the middle latitudes of the Northern and Southern Hemispheres. The northern polar front can often be traced as a continuous line of several thousand kilometers over the North Atlantic and North Pacific Oceans.^[1] It is the most significant front in terms of air mass contrast and susceptibility tocyclonic disturbance.^[2]
polar mesospheric cloud (PMC)
polar stratospheric cloud (PSC)
polar vortex: Either of the two very large, persistent, rotating, upper-levellow-pressure areas suspended in the Earth's atmosphere near thegeographic poles. The polar vortices predictably strengthen during their local winter and weaken during their local summer as the temperature contrast between the poles and theEquator changes. When either vortex is weak, high-pressure zones of lower latitudes may push poleward, driving the vortex,jet stream, and masses of cold, dry polar air into the mid-latitudes, which can cause sudden, dramatic drops in temperature known ascold waves.
potential temperature ( $\theta$ )
potential vorticity
power flash: A sudden bright light caused when anoverhead power line is severed or especially when atransformer explodes.Severe weather is one of the most common causes.
precipitable water: The depth of water, in millimeters or inches, that could be measured if all of the water in a column of the atmosphere wereprecipitated asrain.
precipitation: Any product of thecondensation of atmosphericwater vapor that falls by gravity, the main forms of which includerain,sleet,snow,hail, andgraupel. Precipitation occurs when a portion of the atmosphere becomes locally saturated with water vapor such that the water condenses into liquid or solid droplets and thus "precipitates" out of the atmosphere.
pressure gradient: The horizontal or vertical rate of change ofbarometric pressure in theatmosphere, usually expressed inhectopascals (hPa) per metre; the term is also sometimes used more loosely to denote simply the magnitude of the gradient within a pressure field. The three-dimensional pressure gradient vector is usually resolved into its vertical and horizontal components.^[2]
pressure gradient force (PGF): The force experienced by a unit mass ofair in response to differences inatmospheric pressure in either the horizontal or vertical plane, i.e. apressure gradient, such thatair parcels are accelerated away from regions of high pressure and toward regions of low pressure. A strong pressure gradient force leads to intense atmospheric flows and strongwinds.^[1]^[2]
pressure system: A relative peak or lull in the spatial distribution of sea-levelatmospheric pressure.High- andlow-pressure systems evolve by the interactions oftemperature,moisture, andsolar radiation in the atmosphere, and are directly responsible for most localweather phenomena.
prevailing winds: The predominantwinds encountered at a particular point or region of the Earth's surface, identified by their source anddirection. Though wind speed and direction can vary widely for a given location at a given time, the prevailing winds represent the primary trend in the characteristics of local winds averaged over a long period of time. They are influenced both by global patterns of atmosphericair movements and by local topography.
psychrometer
psychrometrics: The field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures, especially the mixture ofair andwater vapor.
Pulse-Doppler radar
pulse storm: Athunderstorm that produces brief but strongupdrafts, common in humid areas of the continental United States during thesummer. These storms are often associated withsevere weather, particularly sudden and intense windgusts, very largehailstones which grow continuously as they are repeatedly moved up and down within the storm, andflash flooding.^[1]
pyranometer: A type ofactinometer used to measuresolar irradiance on a planar surface and solar flux density in the hemisphere above.
pyrgeometer
pyrheliometer

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Q vector: Inquasi-geostrophic and semi-geostrophic theory, a horizontal vector which appears in theomega equation and tends to point in the direction of rising air. If $Q {\displaystyle Q}$ points toward warm air, thegeostrophic flow isfrontogenetic; if it points toward cold air, the geostrophic flow isfrontolytic.^[3]
quantitative precipitation estimation (QPE): A method of estimating the approximate amount or rate ofprecipitation that has fallen at a location or across a region based on radar measurements or satellite data.
quantitative precipitation forecast (QPF): A prediction of the amount ofprecipitation that will fall at a given location within a given time period, expressed in units of depth (e.g. inches).
quasi-biennial oscillation (QBO): A marked oscillation in thezonal winds in the lower part of the equatorialstratosphere, in which thedirection changes gradually from westerly to easterly and back to westerly with a period that fluctuates between approximately 24 and 30 months.^[3]^[1]
quasi-geostrophic approximation: A form of theprimitive equations of motion in which thegeostrophic wind, an idealized approximation to the actualwind, is used to simplify the system of momentum and thermodynamic equations known as thequasi-geostrophic equations. These equations are derived from an expansion of terms in powers of theRossby number, which is presumed small. The quasi-geostrophic approximation is useful in the analysis ofextratropical synoptic-scale systems, but less accurate in situations in which theageostrophic wind plays an importantadvective role, e.g. nearfronts.^[3]
quasi-geostrophic motion: The flow of a fluid in which an approximategeostrophic balance between theCoriolis force and thepressure gradient force holds, but for which other terms such as theinertial terms involving temporal change or advective acceleration still play a key dynamic role despite their relatively small magnitude.^[3]
quasi-geostrophic theory: A theory of atmospheric dynamics that involves thequasi-geostrophic approximation in the derivation of thequasi-geostrophic equations. This theory is relatively accurate forsynoptic-scale atmospheric motions in which theRossby number is less than unity, but it cannot accurately describe some local atmospheric structures such asfronts or small, stronglow-pressure cells as well as other theories.^[3]
quasi-linear convective system (QLCS): Seesquall line.
quasi-stationary front: Afront that isstationary or nearly so; conventionally, a front that is moving at a speed less than about 5 knots (5.8 mph).^[2]

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radar echo: The portion of the pulsed beam ofmicrowave energy emitted by aradar transmitter that is reflected back to the receiver after the signal encounters a specific target or obstruction in the atmosphere, such as individual particles ofprecipitation. The term may also refer to thebackscatter produced by these objects.^[2]^[1]
radar imaging: Any method that usesradar technology to map the location and characteristics of selected environmental phenomena by emitting a pulse ofmicrowave radiation at a target and analyzing the portion that is partiallyreturned bybackscattering. Radar imaging is widely used in the atmospheric sciences to create images indicating large-scale spatial patterns of meteorological data, e.g. the intensity and distribution ofprecipitation, or the height and orientation of wind-driven ocean waves.^[1]
radar meteorology: A branch ofmeteorology concerned with the use of primarily ground-basedradar technologies for the analysis andprediction of atmospheric phenomena across a wide variety of spatial scales.^[1]
radar winds: Atmospheric motion detected by usingradar to track a target attached to aradiosonde, or byDoppler radar.^[1]
radiation fog: Fog formed over land, generally at night in moist, calm air under clear skies. The most common type of fog, it is caused by the radiative cooling of the Earth's surface and the lowest layers of the atmosphere when the temperature of the air near the ground decreases below itsdew point. Radiation fog occurs most often in the autumn and winter, and is often deepest around sunrise but usually disperses after dawn when heated by solar radiation.^[1]
radiosonde: A battery-powered scientific instrument released into theatmosphere, usually by aweather balloon, which measures variousatmospheric variables and transmits them by radiotelemetry to a ground receiver. Radiosondes are essential sources of meteorological data.
radius of maximum wind (RMW): The distance between the center of acyclone and its band of strongestwinds, often used as a metric for determining a cyclone's potential intensity.
rain: A type ofprecipitation that occurs when liquid water in the form ofdroplets condenses from atmosphericwater vapor, becoming heavy enough to fall under gravity. Rain is a major component of thewater cycle and is responsible for depositing most of thefresh water on the Earth.
rainband: Acloud andprecipitation structure associated with an elongated area ofrainfall and generated by differences in temperature. Rainbands may develop assquall lines ahead ofcold fronts;tropical cyclones are usually composed of multiple curved rainbands.
rainbow: An optical phenomenon that takes the form of a circular arc of light separated into concentric colored bands consisting of all of the individual colors of thevisible spectrum, which occurs when sunlight is refracted as it passes through water droplets in the atmosphere and is then reflected from the rear of the droplets. In aprimary bow, usually appearing with an angular distance of 42° centered on theanti-solar point, the color separation produces a spectrum with red on the outer edge of the arc and violet on the inner edge; asecondary bow, with an angular distance of 51°, is also sometimes visible, but the colors are typically much dimmer and appear in the reverse order.^[1]
raindrop size distribution (DSD)
rainy season: An annually recurring period of one or more months during whichprecipitation, particularlyrainfall, is at or near its average annual maximum for a certain region. The term is used especially intropical climates, where the rainy season contrasts with thedry season.^[2]
rain and snow mixed: A class ofprecipitation composed of bothrain andsnow, the latter usually partially melted, that is reported in some weather observation formats. It usually occurs only briefly at any one location as a transition phase from rain to snow or vice versa.
rain gauge: An instrument used to collect and measure the amount of liquidprecipitation that occurs within a certain area over a certain period of time.
rain of animals
rain shadow: A relatively and consistently dry area on theleeward side of a significant geographic uplift such as a mountain range. Rain shadows exist because the uplift acts as a barrier to the passage ofprecipitation-producing weather systems: moist air masses crossing high elevations are forced upward byorographic lift, which causes the moisture to condense and precipitate on thewindward side, leaving the air depleted of moisture by the time it reaches the leeward side.
rain showers: Short, intense periods ofrainfall, especially when occurring in widely scattered locations.
rapid intensification
ravine wind: Alocal wind generated as a result of apressure gradient between two ends of a narrow valley, blowing from higher to lower pressure (usually in the downstream direction), with its velocity increased by the funneling effect of the ravine itself.^[1]
rawinsonde: A type ofballoon-borneradiosonde that is tracked using position change as determined byradar orradiotheodolite in order to specifically measurewind speed anddirection aloft, and sometimes also other meteorological variables.^[1]^[2]
rear flank downdraft (RFD)
regional forecast: Aweather forecast for a specified geographic region, usually a wider area than that covered by alocal forecast.^[2]
relative humidity
remote sensing: The acquisition of information about an object or phenomenon without making physical contact with the object and thus in contrast to on-site observation. In meteorology, satellite- or aircraft-based sensor technologies are widely used to detect and classify objects on the surface or within the atmosphere or oceans based on propagated electromagnetic signals.
reshabar: 1. A strong northwesterlywind that blows across the Caucasus Mountains from the Black Sea in the west to the Caspian Sea in the east.^[1]; 2. Alocal wind, cold in winter and hot in summer, that affects northern Syria, northern Iraq, western Iran, and southeastern Turkey.^[1]
retrogression: Any motion of an atmospheric wave orpressure system that opposes, or occurs in a direction opposite to, the normal or typical flow in which it is embedded, e.g. a situation in whichRossby waves move westward, contrary to the generally westerly winds flowing through the pattern.^[1]
Rho_hv ( $\rho _{hv}$ )
ridge: An elongated region of relativelyhigh atmospheric pressure, almost always associated with an area of maximumanticyclonic curvature of wind flow. Ridges may exist at the surface or aloft or both; they may contain the closed circulation of a distinct high-pressure area, and a high may have one or more distinct ridges. Under certain conditions, ridges may alternate withtroughs in a high-amplitude pattern.
rime: A coating ofice on the surface of an object. Seehard rime andsoft rime.
rocketsonde: A type ofradiosonde that is transported into the upper atmosphere, e.g. thethermosphere, byrocket propulsion before being ejected and descending to the Earth's surface by parachute. Rocketsondes are used to makesoundings at altitudes much higher than can usually be obtained byballoon or aircraft. They can provide instantaneous vertical profiles for a number of meteorological variables (temperature, pressure, ozone concentration, wind speed and direction, etc.) as they descend through the layers of the atmosphere.^[1]
rogue wave
roll cloud: An elongated, low-levelaccessory cloud in the shape of a horizontal tube that appears to rotate slowly about its horizontal axis, and is associated with but completely detached from thebase of acumulonimbus cloud above it. Though rare, roll clouds typically occur behind thegust front along the leading edge of athunderstorm orsquall line; they are also sometimes associated withcold fronts.^[1]
Rossby number
Rossby wave: A very large-scale atmospheric wave appearing on an upper-air isobaric analysis of the middle and uppertroposphere. Rossby waves consist of a series ofridges andtroughs with very long wavelengths (typically a few thousand kilometres) stretching around the Earth, principally in the middle latitudes. They are strongly linked to surface weather patterns.^[1]
rotation: Seecyclonic rotation.

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saddle point: Seecol.
Saffir–Simpson hurricane wind scale (SSHWS): A rating system used to classifyhurricanes (tropical cyclones in the Western Hemisphere) into one of five categories according to the intensity of theirsustained winds, measured as themaximum sustained wind speed averaged over a one-minute interval at an altitude of 10 meters above the surface. Category 1, the lowest rating on the scale, indicates average sustained wind speeds of 33–42 metres per second (64–82 kn; 74–94 mph), where the lower limit is also used to define the distinction between atropical storm and a hurricane; Category 5, the highest rating, indicates wind speeds of 70 metres per second (136 kn; 157 mph) or more.
sandstorm: Seedust storm.
sastrugi: Sharp, irregular grooves or ridges formed on asnow surface by wind erosion, saltation of snow particles, and deposition, usually parallel to the prevailing winds. They are often found in the polar regions and in large, open areas such as frozen lakes in cold temperate regions.
satellite sounding: Anatmospheric sounding obtained from instruments on a meteorological satellite in orbit around the Earth.^[1]
satellite tornado: An independenttornado that revolves around a larger, primary tornado (typically a very large and intense one) and interacts with the samemesocyclone. Satellite tornadoes are distinct from the subvortices of amultiple-vortex tornado, though they may still merge into their companion tornado.
saturated adiabat: Acurved line drawn on athermodynamic diagram that traces the path of amoisture-saturated air parcel as it moves through the atmosphereadiabatically. Saturated parcels tend to behave very differently from dry parcels; the latter are instead described by adry adiabat.^[1]
saturated adiabatic lapse rate (SALR)
saturation vapor pressure: The maximum possiblepartial pressure exerted by a quantity ofwater vapor in the atmosphere at a given temperature. Saturation vapor pressure increases non-linearly with air temperature according to theClausius–Clapeyron relation, such that the vapor pressure in millibars at 32 °C (90 °F) is approximately double the value at 21 °C (70 °F).^[1]
scarf cloud: Seepileus.
scavenging: The process by which particulate matter in the atmosphere is captured and removed byprecipitation.^[1]
scud: Seepannus.
sea breeze: An onshorelocal wind that blows from sea to land, a result of the more rapid warming of the land surface relative to the sea during the day. It blows in the opposite direction of aland breeze, its nighttime counterpart in adiurnal cycle of coastal winds caused by lateral differences in surface temperature between land and sea.^[2]
sea spray: Aerosol particles formed directly by theocean, mostly by ejection into the atmosphere by bursting bubbles at the air-sea interface.
sea state
sea surface temperature (SST): The watertemperature of the surface layer of a sea or ocean, usually measured at a depth between 1 millimetre (0.04 in) and 20 metres (70 ft) beneath the surface.Air masses in the atmosphere are strongly influenced by sea surface temperatures within a short distance of the shore.^[1]
season: Any division of the year marked by changes inweather,ecology, and the duration ofdaylight. Seasons result from theEarth's orbit around the Sun and itsaxial tilt relative to theecliptic plane. In temperate and polar regions, four calendar-based seasons –spring,summer,autumn, andwinter – are generally marked by significant changes in the intensity of sunlight that reaches the Earth's surface; these changes become less dramatic as one approaches theEquator, and so many tropical regions have only two or three seasons, such as awet season and adry season. In certain parts of the world, the term is also used to describe the timing of important ecological events, such ashurricane seasons, flood seasons, andwildfire seasons.
secular trend: The slow change (either an increase or a decrease) in the values of one or more climatic elements (e.g. temperature) that takes place over a long period of time, after fluctuations that occur over comparatively short periods have been eliminated.^[1]
seiche: A stationary orstanding wave (i.e. a wave that oscillates in time without moving through space) that occurs in an enclosed or semi-enclosed body of water, such as a lake or bay, or in the atmosphere, continuing to oscillate for some time after the force initiating its formation has ceased (occasionally as long as several days). Seiches may be caused by a variety of forces, including strongwinds,earthquakes,landslides, and sudden changes inatmospheric pressure.^[1]
sensible heat: Theheat absorbed or transmitted by a substance during a change in temperature that is not accompanied by a change ofphase (i.e.enthalpy) and which can be measured or "sensed", e.g. with athermometer. Contrastlatent heat.^[2]
sensible temperature: Thetemperature of the air or an object as it is felt or experienced by an individual. This may differ from the actual measured temperature for any of a number of reasons, e.g. as a result ofhumidity (as with aheat index) orwind speed (as withwind chill).^[1] Compareapparent temperature.
severe thunderstorm: A type ofsevere weather consisting of an especially strong or intensethunderstorm accompanied by locally damagingdowndraft winds exceeding 50 knots (58 mph), heavyrain, frequentlightning, and/or largehailstones with a diameter of at least 20 millimetres (0.79 in). Severe thunderstorms are often capable of producingtornadoes as well.^[2]
severe weather: Any dangerous meteorological phenomena with the potential to cause damage on the ground surface, serious social disruption, or loss of human life. There are many types of severe weather, including strongwinds, excessiveprecipitation,thunderstorms,tornadoes,tropical cyclones,blizzards, andwildfires. Some severe weather may be more or less typical of a given region during a givenseason; other phenomena may beatypical or unpredictable.
sferics: Seeatmospherics.
shade temperature: The airtemperature as measured by a thermometer housed inside an instrument shelter, which allows air to circulate freely around the thermometer while sheltering it from the potentially confounding effects of directsolar radiation, precipitation, and thermal energy emitted from the ground and surrounding objects. Shade temperature is a standard meteorological method for measuring air temperature.^[1]
sheet lightning: A diffuse illumination of the sky caused by alightning discharge in which thebolt form of the discharge is not visible to an observer because of the presence of an obfuscatingcloud.^[1]
shelf cloud: A low, elongated, wedge-shapedaccessory cloud that occurs along agust front, often masking the boundary betweenupdrafts anddowndrafts. Shelf clouds are associated with and attached to the base of acumulonimbus cloud, unlikeroll clouds, which are not attached.^[2]
short wave: Any relatively small, short-wavelength ripple (i.e. atrough or aridge) superimposed upon alonger wave pattern in the planetary-scale movement ofair currents within the middle and uppertroposphere. Short-wave troughs in particular are frequently associated with major cyclonic developments.^[2]
shower: A brief downpour ofprecipitation (especiallyrain, but alsosnow orhail) that starts and ends abruptly and typically lasts less than 10 minutes. Showers are characterized by rapid changes in intensity and are usually associated withconvective clouds (e.g.cumulonimbus) which do not completely cover the sky, such that brightness is frequently evident during showers.^[1]
SIGMET
significant level: In aradiosonde observation, an altitude or elevation (other than amandatory level) for which temperature, pressure, and humidity are reported because temperature and/ormoisture content data at that level are sufficiently important or unusual to warrant the attention of theforecaster, or because they are required for the accurate portrayal of the observation.^[2]
simoom
single cell thunderstorm: Seeair-mass thunderstorm.
sirocco
skew-T log-P diagram
sky
Skywarn: Thestorm spotting program of the U.S.National Weather Service. Skywarn organizations have also been formed inEurope andCanada.
skipping tornado
sleet
slush: Aslurry mixture of small ice crystals (such assnow) and liquid water. Slush forms when ice or snow melts.
snow: A type of solidprecipitation in the form ofice crystals which precipitate from the atmosphere and subsequently undergo changes on the Earth's surface. Snow occurs when particles in the atmosphere attractsupercooled water droplets, whichnucleate and freeze into hexagonal crystals known assnowflakes; upon reaching the ground it may then accumulate intosnowpack orsnowdrifts and, over time, metamorphose bysintering,sublimation, andfreeze-thaw mechanisms. Unless the local climate is cold enough to maintain persistent snow cover on the ground, snow typically melts seasonally.
snow gauge
snow grains
snow roller: A phenomenon in which largesnowballs form naturally as clumps of snow are blown along the ground by strong winds, growing larger as they accumulate material along the way.
Snowbelt: A region near theGreat Lakes of North America where heavy snowfall in the form oflake-effect snow is particularly common.
snowdrift: A deposit ofsnow sculpted bywind into a mound during asnowstorm.
snowflake
snowspout: Seewinter waterspout.
snowsquall: A sudden, moderately heavy snowfall characterized by strong surface windgusts andblowing snow. It is similar to ablizzard but is more local in scale, and snow accumulations may or may not be significant.
snowstorm: A type ofwinter storm accompanied particularly by heavy precipitation in the form ofsnow. Very large snowstorms with strong winds and meeting certain other criteria are calledblizzards.
SODAR
soft hail: Seegraupel.
solar irradiance
solarimeter: Seepyranometer.
sounding: Seeatmospheric sounding.
sounding balloon: Seeweather balloon.
sounding rocket: Asub-orbital rocket carrying scientific instruments designed to record measurements and perform experiments in the upper atmosphere while in flight, usually reaching altitudes ranging from 48 to 145 kilometres (30 to 90 mi) above the surface of the Earth, i.e. higher thanweather balloons but lower thanweather satellites.^[8]
specific humidity
spindrift: Sea spray blown from cresting waves during agale. This spray "drifts" in the direction of the gale and is distinct enough that it is sometimes used to judgewind speed at sea.
spring
sprite
squall
squall line
St. Elmo's fire: A weather phenomenon in which luminousplasma is created by acorona discharge at the tips of long, sharply pointed objects in a strong atmospheric electrical field, such as that generated by athunderstorm.
standard atmosphere
standing cloud: Seecap cloud.
static atmospheric model
station model
stationary front
steam devil
steering: Any influence upon the direction of movement of an atmospheric disturbance that is exerted by another aspect of the state of the atmosphere.^[9]
Stevenson screen
storm: Any disturbed state of an environment oratmosphere especially affecting the ground surface and strongly implyingsevere weather. Storms are characterized by significant disruptions to normal atmospheric conditions, which can result in strongwind, heavyprecipitation, and/orthunder andlightning (as with athunderstorm), among other phenomena. They are created when a center oflow pressure develops within a system ofhigh pressure surrounding it.
storm cell: Anair mass which contains up and downdrafts in convective loops and which moves and reacts as a single entity. It functions as the smallest unit of astorm-producing weather system.
storm chasing
Storm Data and Unusual Weather Phenomena (SD): ANational Climatic Data Center (NCDC) publication beginning in 1959 which details quality-controlled tornado and other severe weather summaries as the officialNOAA record of such events.
storm shelter: A type of underground bunker designed to protect the occupants from violentsevere weather, particularlytornadoes.
storm spotting: A type ofweather spotting in which observers watch for the approach ofstorms andsevere weather and actively relay their findings to local meteorological authorities.
storm surge
Storm Prediction Center (SPC)
Storm Track
straight-line wind: Any very strong and potentially damagingwind that lacks the rotational damage pattern associated with the winds of atornado and hence is said to blow in a "straight line". Straight-line winds commonly accompany thegust front of athunderstorm or originate with adownburst and maygust as high as 130 mph (210 km/h).
stratocumulus
stratocumuliform
stratosphere: The second major layer of the Earth'satmosphere, above thetroposphere and below themesosphere. The lower boundary of the stratosphere varies between 7 and 20 km (4.3 and 12.4 mi) above the Earth's surface, depending on latitude.
stratospheric oscillation: Seequasi-biennial oscillation.
stratus
subtropical high
summer
Stüve diagram
sun dog: Seeparhelion.
sunshine recorder
sunshower: A meteorological phenomenon in whichrain falls while the sun is shining.
supercell
subtropical cyclone
surface weather analysis
surface weather observation
sustained wind
synoptic scale meteorology

T

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tail cloud: A ragged band ofcloud and/orfractus extending from awall cloud toward the precipitation core.
temperature: A physical quantity expressing the thermal motion of a substance, such as a mass of air in theatmosphere, and proportional to the average kinetic energy of the random microscopic motions of the substance's constituent particles. Temperature is measured with athermometer calibrated in one or more temperature scales: theKelvin scale is the standard used in scientific contexts, but theCelsius andFahrenheit scales are more commonly used in everyday contexts and forweather forecasting.
temperature gradient: A physical quantity that describes in which direction and at what rate thetemperature changes within or across a particular system or location. It is typically expressed in units of degrees (on a particular temperature scale) per unit length; theSI unit iskelvin per meter (K/m).
temperature inversion
tephigram
terminal aerodrome forecast (TAF): A format for reporting current and forecast weather conditions, particularly as such information relates to aviation. Standard TAFs are issued by major civil airfields at least four times a day (every six hours) and generally apply to a 24- or 30-hour period and an area within approximately 8 kilometres (5.0 mi) from the center of an airport runway complex. TAFs complement and use similar encoding toMETAR reports, but also take into account local geographic influences on weather.
Terminal Doppler Weather Radar (TDWR)
thermal: A column of rising air in the lower altitudes of the Earth's atmosphere. It is a form of atmosphericupdraft created by the uneven heating of the Earth's surface bysolar radiation, and an example ofatmospheric convection.
thermal wind
thermo-hygrograph
thermodynamic diagrams
thermometer: An instrument used to measuretemperature or atemperature gradient.
thermosphere
thunder: The sound produced as a result of the suddenthermal expansion of air within and surrounding the channel of alightning discharge. This expansion creates an audible supersonic shock wave that, depending on the listener's distance from the source, can range from a sharp, loud crack (sometimes called athunderclap orpeal of thunder) to a deep, sustained rumble. Thunder is a defining feature ofthunderstorms.
thundershower: A relatively weakthunderstorm.
thundersnow
thunderstorm: Astorm characterized by the presence oflightning and its acoustic effect on the Earth's atmosphere, known asthunder. Thunderstorms result from the rapid upward movement of warm, moist air, often along afront. They can develop in any geographic location but are most common in themid-latitudes. They are usually accompanied by strongwinds and heavyrain; especially strong orsevere thunderstorms can produce some of the most dangerous weather phenomena, including largehail,downbursts, andtornadoes.
thunderstorm asthma
tilted updraft
tornado: A rapidly rotating column of air that is in contact with both a parentcloud and the surface of the Earth. Tornadoes come in many shapes and sizes, and they are often visible in the form of acondensed funnel originating from the base of acumulonimbus cloud, usually during athunderstorm, with a cloud of rotating dust and debris beneath it. The most extreme tornadoes can achieve wind speeds of more than 480 km/h (300 mph), span more than 3.2 km (2.0 mi) in diameter, and stay on the ground for more than 100 km (dozens of miles) before dissipating.
Tornado Alley
tornado climatology
tornado debris signature (TDS): An area of high reflectivity detected byweather radar that is caused by large amounts ofdebris being lofted into the air, which is often indicative of atornado.
tornado emergency
tornado family
tornadogenesis
tornado outbreak: The occurrence of multipletornadoes (typically at least six to ten) spawned by the samesynoptic scale weather system, usually within the same day and in the same region.
tornado outbreak sequence: A period of continuous or nearly continuoustornado activity consisting of a series oftornado outbreaks spanning multiple days, with very few or no days lacking outbreaks.
tornado preparedness
tornado vortex signature (TVS): A rotation algorithm detected byweather radar that indicates the likely presence of a strongmesocyclone such as atornado. Such signatures can be used to track the location and development of a tornadic rotation within a larger storm.
tornado warning
tornado watch
Tornado and Storm Research Organisation (TORRO)
TORRO scale
Totable Tornado Observatory (TOTO)
trace: An amount ofprecipitation that is too small to reliably or accurately measure.
training
tropical cyclone: A very large,rapidly rotating storm system characterized by alow-pressure center surrounded by a closed low-levelatmospheric circulation, strongwinds, and continuous spiral bands ofthunderstorms that produce heavyrain. Tropical cyclonesdevelop almost exclusively over and derive their strength from warmtropical seas. The strongest systems can last for more than a week, span more than 1,600 km (1,000 mi) in diameter, and cause significant damage to coastal regions with powerful winds,storm surges, and concentrated precipitation that leads toflooding. Depending on its location and strength, a tropical cyclone may be referred to by different names and categorized within a variety of classes.
tropical cyclone scales
tropical cyclogenesis: The process by which atropical cyclone develops and strengthens within the atmosphere. The mechanisms governing cyclone formation in the tropics are distinct from those that govern the development ofsubtropical andextratropical cyclones.
tropical depression
tropical disturbance
tropical storm
tropical wave
tropics: The region of the Earth surrounding theEquator, generally delimited in latitude between theTropic of Cancer (23°26' N) in the Northern Hemisphere and theTropic of Capricorn (23°26' S) in the Southern Hemisphere.
tropopause: The boundary in the Earth's atmosphere between thetroposphere and thestratosphere, on average situated approximately 17 km (11 mi) above equatorial regions and 9 km (5.6 mi) above the polar regions.
troposphere: The lowest layer of the Earth'satmosphere, within which nearly allweather phenomena occur. The troposphere contains approximately 75% of the atmosphere's total mass and 99% of itswater vapor andaerosols. The average height of the troposphere above the Earth's surface varies between 6 and 18 km (3.7 and 11.2 mi) depending on latitude.
trough: An elongated region of relativelylow atmospheric pressure, often associated with afront. Troughs may exist at the surface or aloft or both; the lifting of moist air byconvergent winds usually causes clouds and precipitation to follow immediately behind a trough. Under certain conditions, troughs may alternate withridges in a high-amplitude pattern.
trowal
tsunami
turbulence: Fluid motion characterized by chaotic changes in pressure and flow velocity, caused by excessive kinetic energy in parts of the fluid flow.
twilight: 1. The indirect illumination of the lower atmosphere caused by the scattering of sunlight when the Sun itself is not directly visible because it is below the horizon.; 2. The time period during which such illumination occurs, either between astronomicaldawn andsunrise or betweensunset and astronomicaldusk.
TWISTEX: An acronym forTactical Weather-Instrumented Sampling in/near Tornadoes EXperiment.
typhoon: The local name for atropical cyclone that occurs in the northwestern Pacific Ocean, between 180° and 100°E in the Northern Hemisphere.

U

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unstable air mass: Anyair mass with highconvective instability, characterized by dramatic verticalair currents.
updraft
upper-air chart
upper-air sounding
upper-level low
upper-level outflow
upslope fog
urban heat island (UHI): An urban or metropolitan area within which air temperatures are significantly warmer than in surrounding rural or uninhabited areas as a result of human activities, especially the artificial modification of land surfaces and the generation ofwaste heat by energy usage. Urban heat islands can greatly influence precipitation, air quality, and the likelihood of certain weather phenomena in the vicinity of large cities, though not all cities have a distinct urban heat island.
US Standard Atmosphere
University Corporation for Atmospheric Research (UCAR)

V

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valley breeze
valley exit jet
vertical draft: Seeupdraft.
vertically integrated liquid (VIL): An estimate of the total mass ofprecipitation contained in acloud, obtained by measuring the intensity ofradar echoes returned from the atmosphere.
vertical wind shear
virga
virtual temperature ( $T_{v}$ ): The temperature of a moistair parcel at which a theoretical dry air parcel would have a totalpressure anddensity equal to those of the moist parcel.
visibility
visual flight rules (VFR): A set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going, as opposed toinstrument flight rules, under which operation of the aircraft primarily occurs through referencing the onboard instruments rather than through visual reference to the ground and environs.
Von Kármán constant
Von Kármán vortex street
Von Kármán wind turbulence model
vortex: A region within a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices are a major component ofturbulence and may be observed in many types of meteorological phenomena, including the winds surrounding atropical cyclone,tornado, ordust devil.
vorticity

W

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wall cloud: A large, localized, persistent, and often abrupt lowering ofcloud that develops beneath the surrounding base of acumulonimbus cloud and from whichtornadoes sometimes form.
warm front: A type offront located at the leading edge of a warmerair mass as it overtakes a cooler air mass that is moving more slowly in the same direction. Warm fronts lie within broadertroughs oflow pressure thancold fronts, which sometimes follow them, and the temperature difference between the air masses they separate is often greater.Stratiform clouds,fog, and steadyrain with occasionalthunderstorms often precede the boundary as it moves. Insurface weather analysis, warm fronts are symbolized by a red line with semicircles pointing in the direction of travel.
water vapor: Water in itsgaseous state. Water vapor is ubiquitous in the atmosphere, being continuously generated byevaporation and removed bycondensation, and plays a major role in numerous meteorological processes.
waterspout
weak echo region (WER)
weather: The state of theatmosphere at a given time and location. Weather is driven by a diverse set of naturally occurring phenomena, especiallyair pressure,temperature, andmoisture differences between one place and another, most of which occur in thetroposphere.
weather balloon: Ahigh-altitude balloon used to carry scientific instruments into the atmosphere, which then measure, record, and transmit information about meteorological variables such asatmospheric pressure,temperature,humidity, andwind speed by means of aradiosonde or other measurement device, often one which is expendable. Weather balloons are only feasible in the lower atmosphere and typically do not exceed 40 kilometres (25 mi) in altitude; higher parts of the atmosphere are generally studied withsounding rockets orsatellites.
weather bomb: Seeexplosive cyclogenesis.
weather forecasting: The application of science and technology to predict the conditions of theatmosphere at a given time and location. Weather forecasts are made by collecting quantitative data about the current state of the atmosphere at a given place and then usingmeteorology to project how the atmosphere will change. Forecasting is important to a wide variety of human activities, including business, agriculture, transportation, recreation and general health and safety, because it can be used to protect life and property.
weather front: Seefront.
weather map: A map which displays various meteorological features across a particular area for a particular point or range of time. Weather maps often use symbols such asstation models to conveniently present complicated meteorological data. They are used for both research andweather forecasting purposes.
weather modification
Weather Prediction Center (WPC)
Weather Surveillance Radar (WSR): 1. In the United States,WSR-1,WSR-57,WSR-74, andWSR-88D.; 2. In Canada, theCanadian weather radar network (WKR andCWMN).
weather reconnaissance
weather satellite
weather spotting: The act of observingweather, often on the ground, for the purpose of reporting to a larger group or organization, such as the U.S.National Weather Service.
weather station: Any facility, either on land or at sea, with instruments and equipment for measuring atmospheric conditions in order to provide information forweather forecasts and to study theweather and/orclimate.
weather vane: An instrument (often an architectural ornament) used to indicate thedirection of thewind.
Weatherwise: A photographically adorned general interest weather magazine that frequently publishes articles ontornadoes and othersevere weather.
wet-bulb temperature
wet-bulb globe temperature
wet season
whirlwind: Any vertically oriented rotatingvortex of air that develops as a result ofturbulent air currents created by heating and flow gradients. Examples include major whirlwinds such astornadoes,waterspouts, andlandspouts and minor whirlwinds such asgustnadoes anddust devils.
wildfire
willy-willy: Seedust devil.
wind: The bulk movement of air within the Earth'satmosphere. Wind occurs on a wide range of scales, from very strongthunderstorm flows lasting tens of minutes to milder localbreezes lasting a few hours to globalatmospheric circulations caused by the differential heating of the Equator and the poles and theEarth's rotation. Winds are often referred to by their strength anddirection; the many types of wind are classified according to their spatial scale, theirspeed, the types of forces that cause them, the regions in which they occur, and their effects.
wind chill: A meteorological index that estimates the effect ofwind speed on theapparent temperature perceived by humans, particularly the decrease in human body temperature attributable to the movement of cold air. There is no universally agreed-upon formula for measuring or calculating wind chill, though it is commonly reported as atemperature. It is usually defined only for air temperatures at or below 10 °C (50 °F) and wind speeds above 4.8 km/h (3.0 mph).
wind direction: Thedirection from which awind originates; e.g. a northerly wind blowsfrom the northto the south. Wind direction is usually reported usingcardinal directions or inazimuth degrees measured clockwise fromdue north. Instruments such aswindsocks,weather vanes, andanemometers are commonly used to indicate wind direction.
wind gradient
wind gust: A brief increase in thespeed of thewind, usually lasting less than 20 seconds. Gusts are more transient thansqualls. They are usually only reported by weather stations when the maximum or peak wind speed exceeds the average wind speed by 10–15 knots (12–17 mph).
wind profiler
wind shear: Any difference inwind speed and/ordirection over a relatively short distance in theatmosphere. Atmospheric wind shear is normally described as eithervertical orhorizontal.
wind speed: The measuredspeed of the air comprising awind. Changes in wind speed are often caused byair parcels being exposed topressure andtemperature gradients in the atmosphere. Wind speed is measured with ananemometer, but may also be less precisely classified using theBeaufort scale.
windstorm: Anystorm that produces or is characterized by very strongwinds.
windsock
winter
winter storm: 1. Anystorm which occurs during the localwinter.; 2. Any meteorological event in which varieties ofprecipitation which can only occur at low temperatures are formed, such assnow,sleet, orfreezing rain. Such events are not necessarily restricted to the winter season but may occur in lateautumn or earlyspring, or very rarely in thesummer, as well.
winter waterspout
World Meteorological Organization (WMO)