Glossary of Meteorology

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advection

The horizontal transport of some property of the atmosphere or ocean, such as thermal energy, humidity, or salinity. In the context of meteorology, the related term convection generally refers to vertical transport.

actinoform

Also spelled actiniform.

Describing a collection of low-lying, radially structured clouds with distinct shapes (resembling leaves or wheels in satellite imagery), and typically organized in extensive mesoscale fields over marine environments. They are closely related to and sometimes considered a variant of stratocumulus clouds.

actinometer

A scientific instrument used to measure the heating power of radiation, particularly solar radiation.

adiabatic process

Any idealized hypothetical process by which energy is transferred between a thermodynamic system and its surroundings only as work, without a corresponding transfer of heat or mass. In the context of meteorology, the assumption of adiabatic isolation is often used to explain the processes of adiabatic heating and adiabatic cooling, which involve changes in temperature as a parcel of air is compressed or expanded by interaction with its surroundings.

aerobiology

The branch of biology 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 of weather charts.

aeronomy

The branch of meteorology that studies the upper regions of the Earth's or other planetary atmospheres, 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 liquid droplets in air or another gas. Examples of natural aerosols include mist, clouds, fog, and dust.

air current

Any concentrated area of winds that develops due to differences in pressure and/or temperature between adjacent air 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 of air defined by its temperature and content of water vapor.

air parcel

In fluid dynamics, any amount of air that remains identifiable throughout its dynamic history while moving with an associated air flow.

air-mass thunderstorm

Any thunderstorm that is generally weak and usually not severe. 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 produce lightning and heavy rainfall. They derive their energy from solar radiation and commonly develop in temperate zones during summer afternoons.

almanac

An annual publication of calendar events.

anemometer

A scientific instrument used to measure wind speed.

anticyclone

Any large-scale air mass characterized by outward spiraling winds which circulate around a strong center of high atmospheric pressure. Surface-based anticyclones generally bring about cool, dry air and clear skies and are often implicated in weather phenomena such as fog and haze. Contrast [cyclone](https://en.wikipedia.org/wiki/Glossary_of_meteorology#cyclone).

anticyclogenesis

The development or strengthening of an anticyclonic circulation in the atmosphere, which may result in the formation or maintenance of a high-pressure area. Contrast [cyclogenesis](https://en.wikipedia.org/wiki/Glossary_of_meteorology#cyclogenesis).

Atlantic hurricane

A tropical cyclone (locally known as a hurricane) that forms in the Atlantic Ocean and achieves one-minute maximum sustained 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 the Atlantic hurricane season.

atmosphere

The various layers of gases surrounding the Earth and held in place by gravity. The Earth's atmosphere is the origin of the weather phenomena studied in meteorology. Atmospheric composition, temperature, and pressure vary across a series of distinct sublayers including the troposphere and stratosphere.

atmospheric circulation

The global-scale movement of air masses within the Earth's atmosphere. All meteorological phenomena are a consequence of the atmospheric circulation, which manifests as a network of both latitudinal and longitudinal "cells" of convective activity; together with ocean circulation, these cells are the primary means by which thermal energy from the Sun is redistributed across the Earth's surface.

atmospheric pressure

Also called barometric pressure.

The pressure exerted by the Earth's atmosphere. 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 with elevation above sea level. The average atmospheric pressure at sea level on Earth is equal to approximately 1 standard atmosphere (atm), which is defined as exactly 101,325 pascals (760 mmHg).

atmospheric science

Sometimes called aerology.

The collective of scientific disciplines that studies the Earth's atmosphere and its processes, including the effects other systems have on the atmosphere and those the atmosphere has on other systems. Meteorology and climatology are sub-disciplines.

atmospheric sounding

A measurement of the vertical distribution of physical properties through an atmospheric column, usually including pressure, temperature, wind speed and direction, moisture content, ozone concentration, and pollution, among others.

Aviation Area Forecast (FA or ARFOR)

Also simply called an area forecast.

A former message product of the U.S. National Weather Service issued to provide information to pilots and aviation routes about weather 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.

backscatter

The diffuse reflection of waves, particles, or signals back to the same direction from which they originated. Backscattering is the principle underlying all weather radar systems, which can distinguish radar returns backscattered from target aerosols such as raindrops and snowflakes because the strength of the returns depends largely on the size and reflectivity of the targets.

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 agricultural production.

barograph

A scientific instrument used to measure and record changes in atmospheric pressure over time.

baroclinity

Also called baroclinicity.

barotropity

Also called barotropicity.

barometer

A scientific instrument used to measure atmospheric pressure.

Bernoulli's principle

A principle of fluid dynamics which states that an increase in the speed of a moving fluid occurs simultaneously with a decrease in the pressure exerted by the fluid or in the fluid's potential energy.

black ice

Also called [clear ice](https://en.wikipedia.org/wiki/Glossary_of_meteorology#clear_ice).

A thin, nearly transparent coating of glaze 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 or pedestrians.

blizzard

A severe snowstorm characterized by strong sustained winds of at least 35 mph (56 km/h) and 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 the winter.

block

Also called a blocking high or blocking anticyclone.

A pattern in the atmospheric pressure field overlying a large geographic area which is nearly stationary, effectively "blocking" or diverting the movements of cyclones 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 snow

Snow blown about by wind, either from falling snow or snow lifted from the surface, to a height of at least 2 metres (6.6 ft), reducing visibility. It is a defining characteristic of blizzards.

bounded weak echo region (BWER)

A radar image showing a distinct bow echo in a line of thunderstorms over Kansas City, Missouri

bow echo

A characteristic radar return from a mesoscale convective system that is shaped like an archer's bow and usually associated with squall lines or lines of convective thunderstorms. 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 into derechos.

Bulk Richardson Number (BRN)

A dimensionless ratio related to the consumption of turbulence divided by the shear production of turbulence (the generation of kinetic energy caused by wind shear). It is an approximation of the Gradient Richardson Number.

bushfire

See [wildfire](https://en.wikipedia.org/wiki/Glossary_of_meteorology#wildfire).

Canadian Meteorological and Oceanographic Society (CMOS)

The national society of individuals and organizations dedicated to advancing atmospheric and oceanic sciences and related environmental disciplines in Canada, officially constituted in 1967.

castellanus

Also called castellatus.

A cloud species that displays at least in its upper part cumuliform protuberances resembling the turrets of a castle, giving a crenellated aspect.

ceiling

A measure of the height above the Earth's surface of the base of the lowest layer of clouds or obscuring phenomena that covers more than half of the sky (more than four oktas). An "unlimited" ceiling means either that the sky is mostly free of cloud cover or that the clouds are sufficiently high so as not to impede aircraft operation by visual flight rules.

ceilometer

An instrument that uses a laser or other light source to determine the height of a cloud ceiling or cloud base or to measure the concentration of aerosols within the atmosphere.

central dense overcast (CDO)

The large, centralized, contiguous area of thunderstorms surrounding the rotational center of a strong tropical or subtropical cyclone. When a cyclone reaches sufficient intensity, a distinguishable eye may develop within the CDO. The strongest winds and heaviest rainfall are usually found beneath the coldest cloud tops in the CDO.

cirrus (Ci)

A genus of cloud 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 the outflow of cumulonimbus clouds in advance of fronts or thunderstorms, and therefore may indicate the imminent arrival of precipitation.

clear ice

A type of solid precipitation which forms when relatively large drops of water are supercooled into a dense, transparent coating of ice without air or other impurities. It is similar to glaze and hard rime and, when formed on the ground, is often called black ice.

climate

The statistics of weather in a given region over long periods of time, measured by assessing long-term patterns of variation in temperature, atmospheric pressure, humidity, wind, precipitation, and other meteorological variables. The climate of a particular location is generated by the interactions of the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere and strongly influenced by latitude, altitude, and local topography. Climates are often classified according to the averages or typical ranges of different variables, most commonly temperature and precipitation.

climatology

Also called climate science.

A branch of the atmospheric sciences that studies climate, defined as weather conditions averaged over an extended to indefinite period of time. Climatology incorporates aspects of oceanography, geology, biogeochemistry, and the related field of meteorology to understand the long-term dynamics of climate-influencing phenomena and to produce climate models which can be used to estimate past climates and predict future climates.

cloud

An aerosol consisting of a visible mass of minute liquid droplets, frozen crystals or other particles suspended in the atmosphere. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of the saturation of an air mass when it is cooled to its dew point or when it gains sufficient moisture (usually in the form of water 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 of clouds.

cloud base

The lowest altitude of the visible portion of a cloud.

cloud cover

The obscuration of all or part of the sky by clouds as observed from a particular location, or the specific fraction of the sky obscured by clouds as measured in oktas.

cloud genus

See [cloud type](https://en.wikipedia.org/wiki/Glossary_of_meteorology#cloud_type).

cloud iridescence

Also called irisation.

cloud species

Any of a set of 14 Latin terms used to describe the shape and internal structure of tropospheric clouds. Cloud species are subdivisions of cloud genera and are themselves further subdivided into cloud varieties.

cloud type

Also called a cloud genus.

Any of a set of Latin names used to classify and identify clouds occurring in the troposphere, typically by characteristics such as their altitude, shape, and convective activity. A set of 10 or 12 traditional cloud types defined by the World Meteorological Organization and further subdivided into cloud species and cloud varieties is widely used in meteorology. Other classification systems have proposed many additional types.

col

Also called a saddle point or neutral point.

The point of intersection of a trough and a ridge in the pressure pattern of a weather map. It generally takes the shape of a saddle in which the air pressure is slightly higher than that within the low-pressure regions but still lower than that within the anticyclonic zones.

cold front

A type of front located at the leading edge of a cooler air mass as it replaces a warmer air mass. Cold fronts lie within a sharp surface trough of low 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 of rain or thunderstorms may accompany the boundary as it moves. In surface weather analysis, cold fronts are symbolized by a blue line with triangles pointing in the direction of travel.

cold wave

Also called a cold spell or cold snap.

A period of weather characterized by excessively low temperatures, which may or may not also be accompanied by changes in humidity. 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. Contrast [heat wave](https://en.wikipedia.org/wiki/Glossary_of_meteorology#heat_wave).

Colorado low

A type of low-pressure area that forms in southeastern Colorado or northeastern New Mexico, in the United States, and then proceeds to move east across the Great Plains, often producing heavy snow and ice when occurring in the winter.

convection

See [atmospheric convection](https://en.wikipedia.org/wiki/Glossary_of_meteorology#atmospheric_convection).

convective instability

The inability of an air 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 in turbulent airflow and convective activity that may lead to extensive vertical clouds, storms, and severe weather.

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 the diffraction of light by individual water droplets or sometimes small ice crystals in a cloud or on a foggy glass surface. Coronae differ from halos in that the latter are formed by refraction from comparatively large particles.

crosswind

Any wind that moves in a direction that is perpendicular to the direction of travel of a reference object, such as an airplane.

Crow instability

Also called the vortex Crow instability.

An inviscid line-vortex instability most commonly observed in the skies behind large aircraft such as the Boeing 747. It occurs when the wingtip vortices interact with contrails from the engines, producing characteristic visual distortions in the shapes of the contrails.

cumulus (Cu)

A genus of cloud 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 no precipitation, but can develop into precipitation-bearing clouds such as cumulonimbus when influenced by atmospheric instability, moisture, and temperature gradients.

cumulus mediocris

Very large air masses (and the clouds within them) spiral counterclockwise around a strong center of low atmospheric pressure in this extratropical cyclone over Iceland

cyclone

Any large-scale air mass characterized by inward spiraling winds which circulate around a strong center of low atmospheric pressure. Cyclones can form over land or water, can vary in size from mesocyclones such as tornadoes to synoptic-scale phenomena such as tropical cyclones and polar vortices, and may transition between tropical, subtropical, and extratropical phases. Contrast [anticyclone](https://en.wikipedia.org/wiki/Glossary_of_meteorology#anticyclone).

cyclogenesis

The development or strengthening of a cyclonic 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 of cyclone; for instance, tornadoes are a type of mesocyclone whose development may be variously described as cyclogenesis or, more specifically, tornadogenesis. Contrast [anticyclogenesis](https://en.wikipedia.org/wiki/Glossary_of_meteorology#anticyclogenesis).

dark adaptor goggles

Also called red adaptation goggles.

A type of specialized eyewear used by meteorologists and astronomers for adapting the eyes to the dark prior to an observation made at night, or for aiding with identification of clouds during bright sunshine or when there is a glare from snow.

daytime

The period of the day between sunrise and sunset, during which any given point on the Earth experiences natural illumination from especially direct sunlight, known as daylight.

debris cloud

See [tornado debris signature](https://en.wikipedia.org/wiki/Glossary_of_meteorology#tornado_debris_signature).

deformation

The rate of change of shape of a fluid body such as an air mass. This quantity is very important in the formation of atmospheric fronts, in the explanation of cloud shapes and in the diffusion of materials and properties through the atmosphere.

depression

See [low-pressure area](https://en.wikipedia.org/wiki/Glossary_of_meteorology#low-pressure_area).

dew

Liquid water droplets that commonly appear on thin, exposed surfaces in the morning or evening due to the condensation of atmospheric moisture on radiatively cooled surfaces. When temperatures are low enough, the water droplets freeze into ice particles known as frost.

dew point

(Td)

diamond dust

A ground-level cloud composed of tiny ice crystals. Because it generally forms in sub-freezing temperatures beneath otherwise clear or nearly clear skies, diamond dust is sometimes referred to as clear-sky precipitation.

disdrometer

A scientific instrument used to measure the size distribution and velocity of falling hydrometeors such as raindrops.

Dobson unit (DU)

A unit of measurement used to describe the quantity of a trace gas (primarily atmospheric ozone concentrations) present in a vertical 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 at standard temperature and pressure.

doldrums

See [Intertropical Convergence Zone](https://en.wikipedia.org/wiki/Glossary_of_meteorology#intertropical_Convergence_Zone).

downburst

A surface-level wind system that emanates from an elevated point source and blows radially in all directions upon making contact with the ground. Downbursts are created when rain-cooled air descends rapidly, and can produce very strong damaging winds. They are often confused with tornadoes, though a tornado causes air to move inward and upward whereas a downburst directs it downward and outward. Microbursts, macrobursts, and heat bursts are all types of downburst.

drizzle

A type of light precipitation consisting of liquid water droplets which are smaller than ordinary raindrops, 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

Also called a drouth.

Any prolonged period of below-average precipitation 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 as dry 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 by hot temperatures; in turn, droughts may increase the likelihood of wildfires.

dry lightning

Lightning associated with a dry thunderstorm.

dry thunderstorm

Also called a heat storm.

A thunderstorm that produces thunder and lightning but in which most or all of its precipitation evaporates before reaching the ground. Dry thunderstorms occur necessarily in dry conditions, and their lightning strikes, sometimes referred to as dry lightning, are a major cause of wildfires.

dust storm

Also called a duster or rendered duststorm.

A meteorological phenomenon characterized by very strong winds that blow dust-filled air over an extensive area. Dust storms arise when a gust 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 from drought, it is also used interchangeably with sandstorm and haboob.

echo

On a radar display, the appearance of the radio signal that is scattered or reflected by 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 experiences turbulence, 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 the pressure-gradient force, the Coriolis force, and turbulent drag. Ekman layers occur in both the atmosphere and the ocean.

emagram

One of four thermodynamic diagrams used to display temperature lapse rate and moisture content profiles in the atmosphere. Emagrams have axes of temperature (T) and pressure (p). Temperature and dew point data from radiosondes are plotted on these diagrams to allow calculations of convective stability or convective available potential energy.

Environmental Science Services Administration (ESSA)

The predecessor agency (1965–1970) to the National Oceanic and Atmospheric Administration (1970–present).

extreme weather

Any weather that is unexpected, unusual, unpredictable, unseasonal, or especially severe (i.e. weather at the extremes of an historical distribution).

eye

A typically circular region at the center of a strong tropical cyclone that is the location of the storm's lowest barometric 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 surrounding eyewall and the rest of the storm.

fall wind

See [katabatic wind](https://en.wikipedia.org/wiki/Glossary_of_meteorology#katabatic_wind).

fetch

Also called fetch length.

The length of water over which a given wind blows. Fetch length and wind speed together determine the size of the waves 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 resulting sea state.

field mill

A scientific instrument used to measure the strength of electric fields in the atmosphere.

fire whirl

Also called a fire devil or fire tornado.

A whirlwind induced by a fire and often at least partially composed of flame or ash. They are usually associated with very large wildfires. Fire whirls are seldom classified as true tornadoes, as their vorticity usually derives from turbulent surface winds and heat-induced lifting rather than from a tornadic mesocyclone aloft.

firestorm

A very large wildfire or other conflagration which because of its intensity is able to create and sustain its own storm-force winds. Firestorms develop when a convective updraft of hot air rising from the burning area draws in strong wind gusts from all directions, which supply the fire with additional oxygen and thereby induce further combustion. They are often associated with flammagenitus clouds and fire whirls.

flammagenitus

Also called pyrocumulus or a fire cloud.

flash flood

Any flood 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 heavy rain associated with severe weather, large amounts of meltwater from melting ice or snow, or the sudden collapse of a natural ice or debris dam.

flash freezing

The process by which objects such as liquid hydrometeors 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 puncturing levees, or it may occur when precipitation accumulates on saturated ground more rapidly than it can either infiltrate or run off.

flumen

Also called a beaver's tail.

fog

A visible aerosol 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-lying cloud and is heavily influenced by local topography, nearby bodies of water, and wind conditions.

föhn wind

Also spelled foehn wind.

A type of warm, dry, downslope wind that occurs in the lee of a mountain range.

forward-flank downdraft (FFD)

Also called a front-flank downdraft.

fractus (Fr)

Often used interchangeably with [scud](https://en.wikipedia.org/wiki/Glossary_of_meteorology#scud).

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.

freezing rain

Liquid droplets of rain that become supercooled while falling through a sub-freezing air mass and then freeze upon impact with any surface they encounter; the resulting glaze ice can accumulate to a thickness of several centimeters. Unlike mixed rain and snow, ice pellets, and hail, freezing rain exists entirely as a liquid until it hits a surface.

front

A boundary separating two masses of air of different densities and usually also of different temperatures and humidities. Weather fronts are the principal cause of meteorological phenomena outside the tropics, often bringing with them clouds, precipitation, and changes in wind speed and direction as they move. Types of fronts include cold fronts, warm fronts, and occluded fronts.

frontogenesis

The meteorological process by which a weather front is created, usually as a result of the narrowing of one or more horizontal temperature gradients across the boundary between two adjacent air masses. Contrast [frontolysis](https://en.wikipedia.org/wiki/Glossary_of_meteorology#frontolysis).

frontolysis

The dissipation or weakening of an atmospheric weather front. Contrast [frontogenesis](https://en.wikipedia.org/wiki/Glossary_of_meteorology#frontogenesis).

frost

A very thin layer of ice crystals on a solid surface, typically restricted to that which forms when water 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

Also simply called the F scale.

funnel cloud

A funnel-shaped cloud associated with a rotating column of air and protruding from the base of a parent cloud but not reaching the ground or a water surface. Funnel clouds form most frequently in association with supercell thunderstorms and often develop into tornadoes.

gale

A strong surface wind, typically used as a descriptor in nautical contexts. Gales are variously defined based on their speed; in the modern Beaufort scale, sustained winds of 28–33 kn (52–61 km/h; 32–38 mph), Beaufort number 7, are designated as near gales and those of 34–47 kn (63–87 km/h; 39–54 mph), spanning Beaufort numbers 8 and 9, as gales.

geopotential height

A measure of the vertical distance or altitude above mean sea level that accounts for variations in gravitational potential as altitude and latitude change. In meteorology and atmospheric science, geopotential height is often used in place of ordinary altitude when calculating the primitive equations in numerical weather prediction and when creating atmospheric models.

geostrophic wind

The theoretical wind that would result from an exact balance between the Coriolis force and the pressure gradient force (known as geostrophic balance). The true wind almost always differs from the geostrophic wind due to the influence of other forces such as friction from the ground.

GPS meteorology

A type of observational meteorology that interprets the effects of atmospheric properties such as total precipitable water vapor on the propagation of Global Positioning System (GPS) radio signals to derive information about the state of the local atmosphere.

graupel

Also called soft hail and snow pellets.

A type of precipitation that forms when supercooled water droplets are collected and freeze on falling snowflakes, forming balls of rime 2–5 mm (0.079–0.197 in) in diameter. Graupel is distinct from hail, small hail, and ice pellets.

Great Salt Lake effect

A lake-effect snow that occurs in the lee of Utah's Great Salt Lake.

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 the speed of the wind, usually lasting less than 20 seconds. Gusts are more transient than squalls and are followed by a lull or slowing of the wind speed. They are generally only reported by weather stations when the maximum wind speed exceeds the average wind speed by at least 10–15 knots (12–17 mph).

gust front

See [outflow boundary](https://en.wikipedia.org/wiki/Glossary_of_meteorology#outflow_boundary).

haboob

Numerous hailstones accumulated on the ground following a hailstorm

hail

A type of solid precipitation that consists of balls or irregular lumps of ice usually 5–150 mm (0.20–5.91 in) in diameter, each of which is called a hailstone. Hail formation requires environments with strong, upward motion of air and low altitudes at which water freezes, which makes it possible within most thunderstorms. It is distinct from graupel and sleet or ice pellets.

Haines Index

Also called the Lower Atmosphere Severity Index.

A weather index that measures the potential for dry, unstable air to contribute to the development of large or erratic wildland fires. The index derives from data on the stability and moisture content of the lower atmosphere and is calculated over three ranges of atmospheric pressure.

hard rime

A type of rime consisting of opaque, granular masses of ice deposited primarily on vertical surfaces by freezing fog. Hard rime is more compact and amorphous than soft rime and usually develops on windward surfaces exposed to high wind speeds and air temperatures between −2 and −8 °C (28 and 18 °F).

heat index (HI)

Also called the apparent temperature, felt air temperature, or humiture.

A meteorological index that posits the apparent temperature perceived by the average human being who is exposed to a given combination of air temperature and relative 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 wave

A period of weather characterized by excessively high temperatures, which may or may not be accompanied by high humidity or by drought. 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. Contrast [cold wave](https://en.wikipedia.org/wiki/Glossary_of_meteorology#cold_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 of snowfall (generally 6 in (15 cm) or more in 12 hours) was occurring or was forecast. The warning was replaced by the Winter Storm Warning for Heavy Snow beginning with the 2008–09 winter storm season.

hodograph

Also called a velocity diagram.

A vectorial visual representation of the movement of a body or a fluid, with the position of any data plotted on it proportional to the velocity of the moving particle. In the context of meteorology, hodographs are used to plot winds from atmospheric soundings: for a given vector, wind direction is indicated by the angle from the center axis and wind speed by the distance from the center.

humidity

A measure of the amount of water vapor present in a parcel of air. By quantifying the saturation of the air with moisture, humidity indicates the likelihood of precipitation, dew, or fog occurring. The amount of water vapor needed to achieve full saturation increases as the air temperature increases. Three primary measurements of humidity are widely employed in meteorology: absolute, relative, and specific.

hurricane

The local name for a tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean and achieves one-minute maximum sustained winds exceeding 74 mph (119 km/h; 64 kn).

hydrometeor

Any particulate of liquid or solid water within the atmosphere, encompassing all types of precipitation, formations due to condensation such as clouds and haze, and particles blown from the Earth's surface by wind such as blowing snow and sea spray.

hydrometeorology

A branch of meteorology and hydrology that studies the transfer of water and energy between land surfaces and the lower atmosphere.

hydrosphere

The combined mass of all solid, liquid, and gaseous forms of water found on, beneath, or above the surface of the Earth, including all oceans, lakes, streams, groundwater, atmospheric water vapor, snow, ice caps, and glaciers.

hygrometer

A scientific instrument used to measure humidity.

hypsometer

A scientific instrument used to measure height or elevation, either by trigonometry or by the principle that atmospheric pressure influences the boiling point of liquids.

ice

Water frozen into a solid state. Ice is abundant on Earth's surface and in the atmosphere and plays a major role in Earth's water cycle and climate. Its natural occurrence in weather phenomena takes many forms, including snowflakes, hail, frost, icicles, and ice spikes.

ice spike

A rare ice 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 inverted icicle.

ice storm

A type of winter storm characterized by freezing rain which results in the accumulation of at least 6.4 millimetres (0.25 in) of ice on exposed surfaces.

Icicles hanging from the eaves of a building

icicle

A long, slender spike of ice formed when water dripping or falling from an object freezes.

inflow

The influx of heat and moisture into a storm system from the surrounding environment. The inflow of parcels of warm, moist air drives and sustains most types of storms, including thunderstorms and tropical cyclones. Contrast [outflow](https://en.wikipedia.org/wiki/Glossary_of_meteorology#outflow).

International Standard Atmosphere (ISA)

A static atmospheric model of the variations in temperature, pressure, density, and viscosity over a wide range of altitudes within the Earth's atmosphere, established as an international standard by the International Organization for Standardization in order to provide a common reference for atmospheric variables relevant to meteorology and atmospheric science.

Intertropical Convergence Zone (ITCZ)

Also called the doldrums or the calms.

irisation

See [cloud iridescence](https://en.wikipedia.org/wiki/Glossary_of_meteorology#cloud_iridescence).

jet stream

A narrow, fast-flowing, meandering air current primarily occurring in the upper part of the troposphere, at altitudes above 9 km (30,000 ft), and usually flowing from west to east. The Northern and Southern Hemispheres each have a polar jet and a subtropical jet; low-level jets and other types of jet streams can form under certain conditions.

katabatic wind

Also called a drainage wind or fall wind, or spelled catabatic wind.

A wind that carries high-density air from a higher elevation down a slope under the force of gravity, usually at speeds of the order of 10 kn (19 km/h) or less but occasionally at much higher speeds.

kinematics

A branch of classical mechanics that describes the motion of points, bodies, and systems of bodies without considering the forces that caused the motion.

lake-effect snow

A weather phenomenon produced when a cold air mass moves across long expanses of warmer lake water, which causes the lowest layers of air to pick up warm water vapor from the lake, rise through the upper layers, freeze and then precipitate on the lake's leeward shores. In combination with orographic lift, the effect produces narrow but very intense bands of precipitation, especially snow, 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 termed ocean-effect or bay-effect snow.

landfall

The movement of a storm or other weather phenomenon over land after being over water.

landspout

A type of tornado emerging from a parent cloud that does not contain a pre-existing mid-level mesocyclone or other rotation. Landspouts share a development process and resemblance with waterspouts. They are generally smaller and weaker than supercell tornadoes and are rarely detected by Doppler weather radar.

Lemon technique

A method used by meteorologists which focuses on updrafts and uses weather radar to determine the relative strength of thunderstorm cells in a vertically sheared environment.

lenticular cloud

A type of stationary cloud with a distinct lens or saucer shape which typically forms in an arrangement perpendicular to the wind 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 the moist adiabatic lapse rate of a saturated air parcel at the same level. Air masses with one or many LFCs are potentially unstable and may develop convective clouds such as cumulonimbus.

Lidar

Also rendered as LIDAR, LiDAR, or LADAR.

A surveying method that measures the distance to a target by illuminating the target with pulsed 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 of light detection and ranging.

lifted index (LI)

The difference in temperature between the ambient environment and an air parcel that is lifted adiabatically at a given pressure height within the troposphere, 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 is unstable.

lightning

A naturally occurring electrostatic 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 hot plasma 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 of thunderstorms. 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 detection

An illustration of how a cloud-to-ground lightning strike occurs when a negatively charged "leader" projecting downward from a cloud meets a positively charged leader projecting upward from the ground

lightning strike

Any lightning discharge that occurs between the atmosphere and an object (rather than between different parts of the atmosphere). Most lightning strikes are cloud-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 from ground-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 a lightning bolt.

lysimeter

An instrument used to measure the total amount of evapotranspiration 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.

MAFOR

A North American system used in the transmission of marine weather 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 of MArine FORecast.

mass flow

The movement of a fluid, such as an air mass, down a pressure or temperature gradient.

mesosphere

The third major layer of the Earth's atmosphere, above the stratosphere and below the thermosphere. 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.

Météo-France

The national meteorological agency of France.

meteorology

A branch of the atmospheric sciences which seeks to understand and explain observable weather events, with a major focus on weather prediction. Meteorology uses variables familiar in chemistry and physics to describe and quantify meteorological phenomena, including temperature, pressure, water vapor, mass flow, and how these properties interact and change over time.

micronet

A weather observation network even denser than a mesonet, such as the Oklahoma City Micronet.

mini-supercell

A distinct kind of supercell that is smaller than a typical supercell.

mini-tornado

A fallacious term often used in news media to refer to damaging winds accompanying a thunderstorm, indifferently caused by tornadoes or microburst, on a small area.

misocyclone

Also called a landspout.

Modified Fujita Scale

An update to the original Fujita scale from 1971 proposed by Ted Fujita in 1992.

moisture convergence

An area where moisture concentrates due to the air flow near the surface.

moisture

Also called moisture content or water content.

The presence of liquid, especially water, within a body or substance, often in trace amounts. Moisture in the air in the form of water vapor underlies the concept of humidity.

monsoon

1. An abrupt seasonal wind reversal accompanied by corresponding changes in precipitation.
2. Any seasonal change in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. In this context, the term is often used to refer specifically to the rainy phase of such a pattern, and in some places colloquially (and less correctly) to any locally very heavy but short-term rainfall.

mudflow

Also called a mudslide.

murus

See [wall cloud](https://en.wikipedia.org/wiki/Glossary_of_meteorology#wall_cloud).

National Severe Storms Forecast Center (NSSFC)

A predecessor forecasting center to the Storm Prediction Center that was located in Kansas City, Missouri.

National Severe Storms Laboratory (NSSL)

A NOAA lab in Norman, Oklahoma tasked with researching severe weather.

National Tornado Database

The official NOAA record of all known tornadoes within the United States from 1950 to present.

National Weather Service (NWS)

The national meteorological agency of the United States, tasked with providing weather forecasts, warnings of severe weather, and other weather-related services to organizations and the public for the purposes of protection, safety, and general information.

nephoscope

A scientific instrument used to measure the altitude, direction, and velocity of atmospheric clouds.

nor'easter

Also called a northeaster.

A macro-scale extratropical 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 severe coastal flooding, and hurricane-force winds.

Nor'west arch

Also called the Canterbury arch; associated with nor'wester.

A conspicuous high-altitude arch-shaped cloud formation that appears regularly in otherwise clear blue skies above the east coast of New Zealand's South Island, when a strong, hot, northwesterly föhn wind (known as "The Nor'wester") pushes cooling moist air over the Southern Alps.

occluded front

A type of front formed during the process of cyclogenesis when a cold front overtakes a warm front. Occluded fronts usually form around mature low-pressure areas when a warm air 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 a trough of warm air aloft. In surface weather analysis, occluded fronts are symbolized by various combinations of the symbols for cold and warm fronts.

okta

Also spelled octa.

A unit of measurement used to describe the amount of cloud cover at a given location in terms of how many eighths of the sky are covered in clouds, ranging from 0 oktas (completely clear) to 8 (completely overcast) or sometimes 9 oktas (indicating that the sky is obstructed from view).

overcast

The condition of cloud clover wherein clouds obscure at least 95% of the sky. The type of cloud cover that qualifies as overcast is distinguished from obscuring surface-level phenomena such as fog.

outflow boundary

Also called a gust front.

pancake ice

A form of ice 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

Also called scud; often used interchangeably with [fractus](https://en.wikipedia.org/wiki/Glossary_of_meteorology#fractus_cloud).

parhelia

See [sun dog](https://en.wikipedia.org/wiki/Glossary_of_meteorology#sun_dog).

pascal (Pa)

The SI derived unit of pressure, defined as one newton per square metre. In meteorology, measurements of atmospheric pressure are often given in kilopascals (kPa).

Pearson scale

Also called the Fujita-Pearson scale or F-P-P scale.

A tornado rating scale developed by Allen Pearson differentiating path length (P) and path width (P) to accompany NOAA Fujita scale (F) ratings.

pedestal cloud

See [wall cloud](https://en.wikipedia.org/wiki/Glossary_of_meteorology#wall_cloud).

photometeor

Any bright object or other optical phenomenon appearing in the Earth's atmosphere when sunlight or moonlight creates a reflection, refraction, diffraction, or interference under particular circumstances. Common examples of photometeors include halos, coronae, rainbows, crepuscular rays, and sun dogs.

polar vortex

Either of the two very large, persistent, rotating, upper-level low-pressure areas suspended in the Earth's atmosphere near the geographic poles. The polar vortices predictably strengthen during their local winter and weaken during their local summer as the temperature contrast between the poles and the Equator 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 as cold waves.

power flash

A sudden bright light caused when an overhead power line is severed or especially when a transformer explodes. Severe weather is one of the most common causes.

precipitable water

Also called total precipitable water (TPW).

The depth of water, in millimeters or inches, that could be measured if all of the water in a column of the atmosphere were precipitated as rain.

precipitation

Any product of the condensation of atmospheric water vapor that falls by gravity, the main forms of which include rain, sleet, snow, hail, and graupel. 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 system

A relative peak or lull in the spatial distribution of sea-level atmospheric pressure. High- and low-pressure systems evolve by the interactions of temperature, moisture, and solar radiation in the atmosphere, and are directly responsible for most local weather phenomena.

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