# Glossary of meteorology

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This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

## A

ab-polar current
Any air current moving away from either the North Pole or the South Pole.
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.
A line drawn on a thermodynamic diagram along which an air parcel moves as it ascends or descends through the atmosphere, cooling or warming adiabatically; the path followed by this line depends on whether it is a dry adiabat or a saturated adiabat. [1]
An adiabatic process of expansional cooling, in which a rising air parcel decreases in temperature as it increases in volume. [2]

An adiabatic process of compressional warming, in which a sinking air parcel increases in temperature as it decreases in volume. [2]
The rate at which a parcel of air changes temperature adiabatically as it moves vertically through the atmosphere. The parcel's moisture 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 the dry adiabatic lapse rate.
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. Most compressible fluids, including gases in the atmosphere, behave approximately adiabatically, such that meteorologists often use the assumption of adiabatic isolation when describing atmospheric systems. In such systems the temperature of a dry parcel of air changes without any exchange of energy with its surroundings: as the parcel rises, the decrease in the surrounding atmospheric 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 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.
aerology
See atmospheric science .
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.
ageostrophy
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 moisture content.
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.
Alberta clipper
almanac
An annual publication of calendar events.
aloft
Located in the atmosphere at some height (often significantly high) above the Earth's surface. The term is typically used to distinguish an upper-air observation from a surface weather observation, as in "winds aloft". [2]
altimeter
A scientific instrument used to measure the altitude of an object (e.g. a weather balloon) with respect to a fixed level such as sea level. [1]
altocumulus castellanus
altocumulus (Ac)
A middle-altitude cloud 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 in cirrocumulus but smaller than in stratocumulus. Like other stratocumuliform clouds, altocumulus usually signifies convection aloft. It is one of several classic "warning clouds" recorded by the aviation industry as a signal of developing thunderstorms.
altostratus
American Meteorological Society (AMS)
A scientific and professional organization in the United States whose mission is to promote and disseminate information about the atmospheric, oceanic, and hydrologic sciences, and advance technologies, applications, and services related to them.
anabatic wind
A wind 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] Contrast katabatic wind .
anemometer
A scientific instrument used to measure wind speed.
annular tropical cyclone
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 .
anticyclonic rotation
anticyclonic storm
Any storm system involving an anticyclone, in which winds circulate around a region of high pressure in the direction opposite to that expected around a region of low pressure. Anticyclonic storms rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
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 .
antitriptic wind
A wind generated by the local topography of a particular place; examples include anabatic winds and katabatic winds. Most such winds are diurnal in character. [1]
apparent temperature
See heat index .
arcus cloud
Arctic cyclone
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.
atmometer
See evaporimeter .
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 boundary layer (ABL)
See planetary boundary layer .
atmospheric circulation
The global-scale movement of air masses within the Earth's atmosphere. All meteorological phenomena are consequences 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 convection
atmospheric density (ρ)
The density (mass per unit volume) of the Earth's atmosphere. Atmospheric density generally decreases proportionally with elevation above sea level, and also tends to vary with changes in atmospheric pressure, temperature, and humidity. According to the International Standard Atmosphere, at a pressure of 1 atm and a temperature of 15° C, air has a density of approximately 1.225 kilograms per cubic metre (kg/m3), about 11000 the density of liquid water.
Atmospheric lake
A long-lived pool of water vapor.
atmospheric model
atmospheric pressure (p)

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 as altitude increases. 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 river
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.
atmospheric temperature
A measure of temperature at one or more locations within the Earth's atmosphere. Temperatures recorded in the atmosphere can vary widely with altitude, humidity, and solar irradiance, among other factors.
atmospheric thermodynamics
atmospheric tide
A global-scale periodic oscillation of the Earth's atmosphere caused by gravitational and thermal influences from the Sun and the Moon, analogous to oceanic tides. [3]
atmospheric window
Any of the ranges of small bandwidths in the electromagnetic spectrum at which the Earth's atmosphere 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 the optical window from ~0.3 to 0.9 μm, the infrared window from ~8 to 13 μm, and the microwave window at wavelengths longer than ~1 mm. [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)

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.

## B

backing
A change of wind direction in a counterclockwise fashion (e.g. northerly to northwesterly to westerly). Contrast veering . [2]
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.
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 in atmospheric pressure over time.
baroclinity

Also called baroclinicity.

A measure of the misalignment between a pressure gradient and a density gradient in a stratified fluid such as the atmosphere. In the context of meteorology, a baroclinic atmosphere is one in which atmospheric density depends on both temperature and pressure, in contrast to a barotropic 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 of cyclones.
barotropity

Also called barotropicity.

The close alignment between a pressure gradient and a density gradient in a stratified fluid such as the atmosphere. In the context of meteorology, a barotropic atmosphere is one in which atmospheric density depends only on pressure and is more or less independent of temperature, in contrast to a baroclinic 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 measure atmospheric pressure. The two most common types are mercury barometers and aneroid barometers.
barometric pressure
See atmospheric pressure .
barrier jet
A low-level core of high wind 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 an airflow 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 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.
Bishop's ring
black ice

Also called 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 and pedestrians.
blizzard
A severe snowstorm characterized by strong sustained winds of at least 35 mph (56 km/h) and blowing 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 the winter.
block

Also called a blocking high or blocking anticyclone.

A nearly stationary pattern in the atmospheric pressure field overlying a large geographic area, which effectively "blocks" or diverts 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 dust
A lithometeor phenomenon that occurs when particles of dust are lifted from the Earth's surface by wind and blown about in clouds or sheets. It is classified as an obstruction to vision in METAR aviation weather observations and is commonly reported if the amount of suspended dust reduces horizontal visibility to 10 kilometres (6 mi) or less. [2] Extreme cases may be called duststorms. [3]
blowing sand
A lithometeor phenomenon that occurs when grains of sand are lifted from the Earth's surface by wind and blown about in clouds or sheets. It is classified as an obstruction to vision in METAR aviation weather observations and is commonly reported if the amount of suspended sand reduces horizontal visibility to 10 kilometres (6 mi) or less. [2] Extreme cases may be called sandstorms. [3]
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)
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.
breeze
1.  Any generally light wind. [1]
2.  Any local-scale air movement that is convectively forced, e.g. a land breeze or sea breeze. [1]
3.  On the Beaufort scale, a wind 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; and strong breeze, 22–27 knots. [2]
brightband
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 .

## C

calm
A state of the atmosphere in which there is virtually no horizontal motion of the air. It corresponds to force number 0 on the Beaufort scale, with a wind speed less than 1 kn (1.9 km/h). Calm conditions are common in the subtropical high-pressure belts and in the doldrums. [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 in Canada, officially constituted in 1967.
Provides forecast guidance to national and regional prediction centres in Canada.
Canterbury arch
See Nor'west arch .
cap cloud

Also called a standing cloud.

An approximately stationary cloud on or hovering above an isolated mountain peak. See also pileus and lenticular cloud . [2]
capacity
The ability of a wind 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 with wind speed and decreases as the particle size of the detrital debris increases. [1]
capping inversion
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.
catabatic wind
See katabatic wind .
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.
ceiling balloon

Also called a pilot balloon or pibal.

A type of weather balloon used by meteorologists to determine the height of the cloud 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 of cloud-height indicator that uses a searchlight to project a beam of light vertically onto a cloud base (similar to a ceilometer), with the height of the illuminated spot then calculated by the observer using a clinometer or alidade. [4] [3]
ceilometer
An instrument that uses a laser transmitter or other light source and a collocated receiver to determine the height of a cloud ceiling or cloud base overhead, or to measure the concentration of aerosols within the atmosphere.
cell
1.  Any atmospheric circulation feature that is more or less closed, occurring at any of number of scales, including massive latitudinally oriented circulations such as Hadley cells; mesoscale motions that characterize cellular convection and cause the formation of cellular clouds; and storm cells formed by updraft and/or downdraft loops within a thunderstorm. [1]
2.  In weather 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-lasting multicell storms or supercells. [3]
cellular cloud
A mesoscale organization of convective activity in the form of a quasi-regular pattern of clouds behaving as individual convective 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 of cumulus with a clear center, and the closed cells filled with stratocumulus 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 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.
central pressure
The atmospheric pressure at the center of a recognizable high or low-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 of lightning discharges within a specific radius. [2]
chinook wind
A föhn. A warm and dry wind formed by a rainstorm dropping its precipitation onto the windward side of a mountain, thus drying it, before blowing across the leeward side and dropping in elevation, thus warming it by adiabatic heating. A chinook can cause temperatures to rise from −48 °C (−54.4 °F) to 9 °C (48.2 °F) in 24 hours, a 57 °C (103 °F) increase. Common in southern Alberta (Calgary, etc.) in Canada and in Montana and other states in the United States.
circulation
Common short form of atmospheric circulation .
cirrocumulus (Cc)
A genus of cloud with both stratiform and cumuliform characteristics, signifying atmospheric 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 from cirrus and cirrostratus by the presence of small amounts of supercooled liquid water droplets.
cirrostratus
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.
clear-air turbulence
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 bow
See fogbow .
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 .
cloud iridescence

Also called irisation.

A type of photometeor consisting of colorful iridescent patterns appearing most commonly near the semi-transparent edges of thin clouds such as cirrus and altocumulus that are in the general proximity of the Sun or Moon. They are caused by the diffraction of sunlight or moonlight by thin, uniform layers of very small water droplets or ice crystals.
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 tag
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.
cloud variety
cloudburst
A colloquial term used to describe an excessive precipitation event, characterized by brief, sudden, exceptionally heavy rain and/or hail falling from a cloud, typically as part of a thunderstorm associated with violent upward and downward convective currents. [1]
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 .
cold-core 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 .
convective available potential energy (CAPE)
convective condensation level
convective inhibition (CIN)
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.
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 to outflow at higher altitudes; when it occurs in the upper atmosphere, the air motion is downward, with divergence 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 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.
crepuscular rays
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.
cumuliform
Of or relating to heaped, "puffy" clouds, such as cumulus or cumulonimbus, that form as a result of atmospheric convection. [1]
cumulonimbus
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 congestus
cumulus humilis
cumulus mediocris
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 .
cyclonic rotation
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 .

## D

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.
dawn

Also daybreak.

The first appearance of sunlight in the eastern sky before sunrise, or the time that marks the beginning of the morning twilight. [2]
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.
dBZ
debris cloud
deepening
A decrease in the central and surrounding sea-level pressure within the circulation of a pressure system (usually a low-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 its cyclonic circulation and hence its winds, and the term is frequently used to imply cyclogenesis. [1] Contrast filling .
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.
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 horizontal visibility may be reduced by dense fog to 0.25 miles (0.40 km) or less. [2]
depression
Any area of low atmospheric pressure at a given level in the atmosphere; i.e. a "low" or trough. The term is used especially frequently to refer to an early stage in the development of a tropical cyclone during which the disturbance is only weakly developed or poorly organized; see tropical depression .
derecho
A type of storm that produces widespread, sustained straight-lined winds that are associated with severe thunderstorms.
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)

Also dewpoint or dew-point.

The temperature to which an air parcel must be cooled, at constant pressure and moisture 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–Td)
The difference between the actual temperature and the dew point at a certain altitude in the atmosphere. A small dew point depression indicates more moisture and higher relative humidity, which in the lower troposphere can result in low cloud bases and lifted condensation levels, which are important factors contributing to the development of severe thunderstorms.
diabatic process

Any thermodynamic process in which the temperature of an air parcel changes as a result of the transfer of energy (e.g. heat) between the parcel and its surroundings, as opposed to an adiabatic 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 and turbulence. [1]
Diablo wind
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.
diffluence
The elongation of a fluid body, such as an air mass, normal to the flow (streamline divergence). It is a flow pattern of deformation.

The component of incoming solar radiation that is scattered from the direct solar beam by molecules of air, aerosols, clouds, or particulate matter in the atmosphere and subsequently reaches the Earth's surface in nearly equal amounts from nearly all parts of the sky during daylight. [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 higher potential temperature than the sinking motion). Such a cell converts heat energy to potential energy and then to kinetic energy. [2] Contrast indirect circulation .
discontinuity
A horizontal zone across which temperature, humidity, wind speed, or any other meteorological variable changes abruptly, such as a front. [1]
disdrometer
A scientific instrument used to measure the size distribution and velocity of falling hydrometeors such as raindrops.
diurnal
Occurring or varying in the course of a solar day (i.e. daily; completed within and recurring every 24 hours), or during the local daytime. [2]
diurnal variation

Also diurnal range.

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 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 .
Doppler on Wheels (DOW)
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, although 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.
downdraft
drifting snow
Particles of snow lifted by the wind to a modest height, generally less than 1.8 metres (6 ft) above the ground. Drifting snow does not significantly reduce visibility at eye level below 10 kilometres (6.2 mi), in contrast to blowing snow. [2]
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 line
dry microburst
dry punch
Meteorological slang for a synoptic-scale or mesoscale weather process. A dry punch that occurs near the Earth's surface may result in a dry line bulge, whereas a dry punch aloft may increase the potential for severe thunderstorms.
dry season
An annual period of relatively low or infrequent precipitation, during which weather patterns are typically dominated by lengthy periods of high atmospheric pressure, high temperatures, and low humidity. The term is primarily used in the tropics, in contrast to the wet season.
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.
dusk
dust devil
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.

## E

echo
On a radar 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 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.
Ekman number
Ekman spiral
Ekman transport
energy-helicity index (EHI)
El Niño
The warm phase of the El Niño–Southern Oscillation (ENSO), associated with the annual development of a band of warm ocean water in the eastern equatorial Pacific, which brings low pressure and heavy rainfall 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 called La Niña.
El Niño–Southern Oscillation (ENSO)
An irregular long-term periodic variation in winds and sea 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 the Walker 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 of lightning discharges, thunder, and aurora. [2]
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.
Enhanced Fujita scale (EF scale)
ensemble forecasting
A weather forecasting technique in which a numerical 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 developing cloud is mixed into an ascending convection 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]
environmental lapse rate (ELR)
The actual rate at which atmospheric temperature changes with altitude, as measured by a radiosonde; this is in contrast to the rate predicted by the theoretical process lapse rate. On average, the temperature of the troposphere 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 local winter, and over continental landmasses. [1]
Environmental Modeling Center (EMC)
The predecessor agency (1965–1970) to the National Oceanic and Atmospheric Administration (1970–present).
equivalent potential temperature (${\displaystyle \theta _{e}}$)
equivalent temperature (${\displaystyle T_{e}}$)
The temperature obtained when an air parcel expands adiabatically, at constant pressure, until its water vapor content has been condensed out and the latent heat of condensation is available to raise the air temperature. [1]
Eulerian equations
European windstorm
evaporimeter

Also called an atmometer.

An instrument used to measure the rate of evaporation of water into the atmosphere. The most basic design consists of an open, ground-level evaporation pan from which water is allowed to evaporate freely. [1]
explosive cyclogenesis
extratropical cyclone
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.
eye of the wind
A nautical term used to describe the direction from which the wind is blowing. [1]
eyewall

## F

fall wind
See katabatic wind .
Fata Morgana
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.
fogbow

Also called a white rainbow, mist bow, or cloud bow.

An optical phenomenon in which a whitish or faintly colored primary rainbow, often with red and blue edges, is visible on a background of fog or mist at the observer's anti-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

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 .

A cloud type or species consisting of ragged, irregularly shaped patches or shreds of cumulus or stratus. [1]
frazil ice
freezing drizzle
A type of precipitation in which drizzle consisting of supercooled liquid water droplets, often falling through a temperature inversion in the lower atmosphere, freezes upon impact with the ground or other cold surfaces to form a coat of glaze ice. [1] Compare freezing rain .
freezing fog
A condition in which supercooled water droplets comprising fog freeze either while suspended in the air, filling the air with visible ice crystals similar to very light snow, or upon contact with sub-freezing surfaces, forming a coating of rime and/or glaze ice. [2]
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.
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 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 .
frontolysis
The dissipation or weakening of an atmospheric weather front. Contrast 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.

## G

gale
1.  A strong surface wind, typically used as a descriptor in nautical contexts and variously defined based on speed. In the modern Beaufort scale, a gale is any sustained wind of Beaufort number 7 or greater, corresponding to near 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); and storm at 48–55 kn (89–102 km/h). [1]
2.  Any unusually strong wind. [2]
gale warning
gap wind
A local, low-level wind that blows along a valley or through a col 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 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.
glaze

Also called glazed frost.

A coating of smooth, clear ice, sometimes of considerable thickness, that forms when supercooled water, usually precipitated as freezing rain or freezing 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 than rime and hoarfrost. [2]
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.
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 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 .

Also called a gust front tornado.

A relatively weak tornado associated with the outflow at the leading edge of a thunderstorm cell, and often occurring along a gust front. A debris cloud or dust whirl may indicate the presence of a gustnado. [2]

## H

haboob

Also called a tropical cell.

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.
hailstorm
Any storm, often a strong thunderstorm, which precipitates hail.
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.
halo
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).
Harmattan
haze
Any suspension in the atmosphere of very small, dry particulate matter, including natural aerosols (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 reduced visibility at long distances. Haze usually indicates sub-saturated air, whereas fog or mist indicates full saturation. [2]
hazardous seas warning
hazardous seas watch
Heat dome
When Earth'satmosphere traps hot ocean air like a lid or cap.
heat burst
A rare phenomenon involving a sudden, localized increase in surface temperature (sometimes 10 °C (18 °F) or more within just a few minutes) associated with a decaying thunderstorm or other mesoscale convective system and possibly accompanied by gusty winds and a rapid decrease in humidity. [3]
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 lightning
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 .
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.
helicity
high-pressure area
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.
hook echo
horseshoe vortex
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.
humidex
humilis
See cumulus humilis .
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).
hurricane hunters
huaico

Also huayco.

A mudslide or flash flood caused by torrential rainfall occurring high in the Andes mountains of South America, especially during the weather phenomenon known as El Niño.
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.
hydrostatic equilibrium
hygrometer
A scientific instrument used to measure humidity.
hygroscopy
The phenomenon by which a substance attracts and retains water molecules via either absorption or adsorption from the surrounding environment.
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.

## I

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.
iceberg
ice accretion indicator
ice crystal
1.  A minute spicule of ice 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 of water vapor onto them, and they may collide with other ice crystals to form snowflakes. [1]
2.  A type of precipitation composed of very small, unbranched crystals of ice which fall slowly and often seem to float in the air. [2]
ice fog
A type of fog consisting of a sufficient concentration of tiny ice crystals suspended in the atmosphere to reduce visibility 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 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.
icicle
A long, slender spike of ice formed when water dripping or falling from an object freezes.
incus
Indian summer
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 .
instrument flight rules (IFR)
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 .

## J

jet-effect wind
See canyon wind .
jet stream

Also simply jet.

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 predictable though discontinuous polar jet and subtropical jet; low-level jets and other types of jet streams can form under certain conditions.
jet streak

Also jet stream core or jet maximum.

The region of maximum wind speed that runs along the elongated axis of a jet 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]

## K

K-index

Also called George's index.

An operational atmospheric stability index indicating the potential for thunderstorms, based on temperature lapse 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]
${\displaystyle K_{DP}}$
kata-front
A warm front or cold 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. Contrast ana-front . [1]
katabatic wind

Also catabatic wind, drainage wind, or fall wind.

A local 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. Contrast anabatic wind .
Kelvin temperature scale
Kelvin–Helmholtz instability
A phenomenon of instability that occurs occasionally in an atmospheric layer within which wind speed increases rapidly with altitude. Kelvin–Helmholtz waves form in this layer of strong vertical wind shear, and are often marked by a distinct train of clouds that resemble breaking ocean waves. [1]
khamsin

Also chamsin, hamsin, and khamaseen.

The local name for a dry, hot, seasonal wind, often carrying large quantities of dust or sand, that occurs in the deserts of Egypt, Israel, Palestine, and Jordan. Compare haboob , harmattan , sirocco , and simoom .
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.
knot (kn)
A unit of speed commonly used in maritime and aviation disciplines, equivalent to one nautical mile per hour (1.1508 miles per hour or 0.5145 metres per second). It is often used in meteorology for measuring wind speed.
Köppen climate classification

## L

${\displaystyle L_{DR}}$
La Niña
Lagrangian equations
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.
laminar flow
A flow in which the particles of a fluid moves smoothly in parallel layers or sheets, i.e. without turbulence. [2]
land breeze
An offshore local 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 a sea breeze, its daytime counterpart in a diurnal cycle of coastal winds caused by lateral differences in surface temperature between land and sea. [2]
landfall
The movement of a storm or other weather phenomenon over land after being over water.
landslide
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.
lapse rate
The rate at which an atmospheric variable, most commonly temperature or pressure, decreases with increasing altitude.
latent heat
The amount of heat 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, the latent heat of vaporization requires about 2.4 × 106 Joules per kilogram at 0 °C. [1] Contrast sensible heat .
latent heat flux
The movement of water vapor (a major transporter of latent 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 × 1015 watts at latitudes 38 °N and 40 °S. [1]
law of storms
A general statement of the manner in which the winds of a cyclone rotate about the cyclone's center, and the way in which the entire disturbance 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
See stratiform .
lee trough

Also called a lee depression, orographic depression, or dynamic trough.

A trough of low atmospheric pressure that forms preferentially to the lee or downwind side of a mountain barrier when air 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 on updrafts and uses weather radar to determine the relative strength of thunderstorm cells in a vertically sheared 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 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 condensation level (LCL)
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.
light pillar
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 activity level
lightning detection
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.
line echo wave pattern (LEWP)
low-level jet
low-pressure area (L)
low-topped supercell (LT)
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.

## M

mackerel sky
A sky that is partially or fully covered by high altocumulus or cirrocumulus 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 strong downburst 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-scale meteorological processes, i.e. those occurring over very large regions, oceans, continents, or the entire Earth, such as the general circulation, as opposed to mesometeorology and micrometeorology. [1] See also synoptic-scale meteorology .
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.
manometer
A scientific instrument consisting of a liquid column gauge used to measure differences in the pressures of gases, as with a mercury barometer. [1]
marine climate

Also maritime climate.

A regional climate that is strongly influenced by its location in relation to a sea or ocean, characterized by relatively small diurnal and seasonal temperature variations and high atmospheric moisture content, which contributes to high precipitation and humidity. [1] Contrast continental climate .
marine cloud brightening
marine stratocumulus
mass flow
The movement of a fluid, such as an air mass, down a pressure or temperature gradient.
meridional circulation
The component of the large-scale atmospheric general 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'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.
mesovortices
METAR
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.
microburst
micronet
A weather observation network even denser than a mesonet, 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 of supercell that is smaller than a typical supercell.
A fallacious term often used in news media to refer to damaging winds accompanying a thunderstorm, indifferently caused by tornadoes or microbursts, on a small area.
misocyclone
A vortex with a width between 40 m and 4 km. [6] Includes (sensu stricto) waterspouts and landspouts.
misoscale meteorology
mixed cloud
A cloud composed of both liquid water droplets and ice crystals (e.g. altostratus, cumulonimbus, and nimbostratus), as opposed to a warm cloud. [1]
mixing ratio
A measure of atmospheric moisture content, usually expressed as the dimensionless ratio of the mass of water vapor in a given parcel of air to the unit mass of dry air (i.e. grams of water vapor per kilogram of dry air). [1]
mock sun
See parhelion .
Modified Fujita Scale
An update to the original Fujita scale from 1971 proposed by Ted Fujita in 1992.
See saturated adiabatic lapse rate .
moisture convergence
An area where moisture concentrates due to the air flow near the surface.
mountain breeze
mountain-gap wind
See gap wind .
multicellular thunderstorm
A thunderstorm consisting of more than one convection cell, i.e. more than one circulating system of updrafts and downdrafts.
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.
Morning Glory cloud
mudflow

Also called a mudslide.

murus
See wall cloud .

## N

nacreous cloud

Also mother-of-pearl cloud.

A rare type of polar stratospheric cloud that forms at altitudes of 24–30 kilometres (79,000–98,000 ft), usually in high-latitude regions. These clouds are normally lenticular in form but may resemble cirrus, and often exhibit brilliant iridescence similar to mother-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 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.
The official NOAA record of all known tornadoes within the United States from 1950 to present.
National Weather Center (NWC)
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.
neap tide
A small-amplitude oceanic tide of minimum tidal range occurring semi-monthly near the times when the Moon is in quadrature, i.e. the first and third quarters. [2]
needle ice
negative tilt
The angular displacement of a trough line such that the axis of the trough is rotated clockwise from a north–south meridian (as opposed to the counterclockwise rotation of a positively 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 of clouds.
nephoscope
A scientific instrument used to measure the altitude, direction, and velocity of atmospheric clouds relative to a point on the ground directly below them.
nimbostratus (Ns)
A genus of cloud 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 moderate precipitation but no lightning or thunder. Low, ragged fractus clouds frequently occur below nimbostratus and may or may not merge with it. [1] [2]
noctilucent cloud
See diabatic process .
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.
normal
The average value of a meteorological element (e.g. temperature, precipitation, humidity) over a given period of time, most commonly three consecutive 10-year intervals totaling 30 years. [2]
northern lights
See aurora .
Novaya Zemlya effect
nowcasting
numerical weather prediction

## O

obscuring phenomena
Any atmospheric phenomenon exclusive of clouds that restricts vertical visibility, including various hydrometeors such as rain and snow as well as lithometeors such as dust and sand. [2]
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.
ocean current
Any regular, permanent or semi-permanent movement or flow of ocean water, either in a cyclic pattern or as a continuous stream along a defined path. Ocean currents are generally driven by wind or by geostrophic forces related to seawater density gradients. They are major transporters of the heat introduced by solar radiation, usually moving warm water from the tropics to higher latitudes and returning cold water in the opposite direction, by which they exert an important influence on climate and weather phenomena across the world. [1] [2]
oceanic climate
See marine climate .
offshore current
Any ocean current that flows parallel to, or away from, the coastline of a landmass. [1]
offshore wind
Any wind that blows from land out over a body of water, e.g. a land breeze. Contrast onshore wind .
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).
omega equation
onshore wind
Any wind that blows from a body of water to land, e.g. a lake or sea breeze. Contrast offshore wind .
opacity
orographic cloud
Any cloud whose form and extent is determined by the effects of high-elevation terrain upon the passing flow of air, especially the forced uplift 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 and precipitation. Orographic clouds are usually very slow-moving or stationary; examples include lenticular clouds and cap clouds.
orographic lift

Also orographic uplift.

The forced ascent of an air 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 in adiabatic cooling, leading to saturation, condensation, and the formation of orographic clouds and often precipitation. [1]
orographic precipitation
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.
overrunning
The action of an air mass aloft, often relatively warm, moving over another air mass of greater density at the surface, as occurs in a warm front. [1]
overshooting top
A distinct, bulging protuberance produced by a vigorous updraft that rises above the top of the anvil of a cumulonimbus cloud. Overshooting tops are generally short-lived, but those that persist may indicate the potential for strong thunderstorms and severe weather. [1]
outflow
Air that flows outwards (away from) a storm system. Outflow typically radiates from thunderstorms in the form of a wedge of rain-cooled air, which is often delineated by a low, thick cloud preceded by a gust front, apparent both from the ground and in weather radar imagery. The altitude at which the outflow occurs is strongly correlated with the intensity and persistence of large storm systems such as tropical cyclones.
outflow boundary

Also called a gust front.

The boundary between the cooled outflow air from a thunderstorm and the air of the surrounding environment, similar to a cold front. New thunderstorms often develop along outflow boundaries. [1]
outflow jet
ozone depletion
ozone layer

Also called the ozone shield and ozonosphere.

A region of the Earth's atmosphere containing relatively high concentrations of the gaseous chemical ozone (O3) and which is responsible for absorbing more than 97 percent of the Sun's incoming medium-frequency ultraviolet (UV) radiation. The ozone layer is found mainly in the lower portion of the stratosphere, between approximately 15 and 35 kilometres (9.3 and 21.7 mi) in altitude, although its thickness varies seasonally and geographically.

## P

paleoclimatology
pan evaporation
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 .

pampero
parhelion

Also called a sun dog or mock sun.

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 airborne ice crystals with diameters less than 30 μm (0.0012 in), e.g. those present in cirrus or cirrostratus clouds. [1]
Particularly Dangerous Situation
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 hectopascals (hPa) or kilopascals (kPa).
Pascal's law

Also called Pascal's principle.

A hydrostatic principle which states that pressure applied to a confined incompressible fluid (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

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 .
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
A cloud 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 to stratocumulus and altocumulus. [1]
permafrost
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.
physical meteorology
A branch of meteorology concerned with the structure and composition of the atmosphere and the various optical, electrical, acoustical, and thermodynamic phenomena that characterize it, including aerosols and clouds, precipitation, and electromagnetic radiation. [1]
Phi_DP (${\displaystyle \Phi _{DP}}$)
pileus

Also called a cap cloud or scarf cloud.

A small accessory cloud, appearing as a smooth, shallow, lenticular "cap", that forms above or attached to the top of a cumulus or cumulonimbus cloud. Pileus clouds are formed when moist air above the parent cloud is cooled to its dew point by a strong updraft, and are good predictors of thunderstorms; a pileus atop a cumulus cloud often foreshadows its transformation into a cumulonimbus cloud. [7]
pilot balloon
See ceiling balloon .
pilot report (PIREP)
An inflight report by an aircraft pilot or crew member of the weather 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

Also called a polar anticyclone.

An extensive high-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. The anticyclone over the Arctic, known as the Arctic high, is generally seasonal, while that over Antarctica, known as the Antarctic high, is semi-permanent. [1]
polar low

Also called a polar-air depression.

A relatively small-scale, non-frontal, migratory low-pressure system that occurs in the polar latitudes of either the Northern or Southern Hemisphere. Such systems are secondary depressions that form over oceans poleward of the polar front, most commonly during the local winter, and can produce blustery, snowy conditions. [1]
polar front
Either of the two semi-permanent, semi-continuous boundaries 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 to cyclonic disturbance. [2]
polar mesospheric cloud (PMC)
polar stratospheric cloud (PSC)
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.
potential temperature (${\displaystyle \theta }$)
potential vorticity
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.
The horizontal or vertical rate of change of barometric pressure in the atmosphere, usually expressed in hectopascals (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]
The force experienced by a unit mass of air in response to differences in atmospheric pressure in either the horizontal or vertical plane, i.e. a pressure gradient, such that air 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 strong winds. [1] [2]
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.
prevailing winds
The predominant winds encountered at a particular point or region of the Earth's surface, identified by their source and direction. 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 atmospheric air movements and by local topography.
psychrometer
psychrometrics

Also called psychrometry and hygrometry.

The field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures, especially the mixture of air and water vapor.
pulse storm
A thunderstorm that produces brief but strong updrafts, common in humid areas of the continental United States during the summer. These storms are often associated with severe weather, particularly sudden and intense wind gusts, very large hailstones which grow continuously as they are repeatedly moved up and down within the storm, and flash flooding. [1]
pyranometer

Also called a solarimeter.

A type of actinometer used to measure solar irradiance on a planar surface and solar flux density in the hemisphere above.
pyrgeometer
pyrheliometer

## Q

Q vector
In quasi-geostrophic and semi-geostrophic theory, a horizontal vector which appears in the omega equation and tends to point in the direction of rising air. If ${\displaystyle Q}$ points toward warm air, the geostrophic flow is frontogenetic; if it points toward cold air, the geostrophic flow is frontolytic. [3]
quantitative precipitation estimation (QPE)
A method of estimating the approximate amount or rate of precipitation 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 of precipitation that will fall at a given location within a given time period, expressed in units of depth (e.g. inches).
quasi-biennial oscillation (QBO)

Also called stratospheric oscillation.

A marked oscillation in the zonal winds in the lower part of the equatorial stratosphere, in which the direction 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

Also geostrophic approximation and pseudogeostrophic approximation.

A form of the primitive equations of motion in which the geostrophic wind, an idealized approximation to the actual wind, is used to simplify the system of momentum and thermodynamic equations known as the quasi-geostrophic equations. These equations are derived from an expansion of terms in powers of the Rossby number, which is presumed small. The quasi-geostrophic approximation is useful in the analysis of extratropical synoptic-scale systems, but less accurate in situations in which the ageostrophic wind plays an important advective role, e.g. near fronts. [3]
quasi-geostrophic motion
The flow of a fluid in which an approximate geostrophic balance between the Coriolis force and the pressure gradient force holds, but for which other terms such as the inertial 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 the quasi-geostrophic approximation in the derivation of the quasi-geostrophic equations. This theory is relatively accurate for synoptic-scale atmospheric motions in which the Rossby number is less than unity, but it cannot accurately describe some local atmospheric structures such as fronts or small, strong low-pressure cells as well as other theories. [3]
quasi-linear convective system (QLCS)
See squall line .
quasi-stationary front
A front that is stationary or nearly so; conventionally, a front that is moving at a speed less than about 5 knots (5.8 mph). [2]

## R

The portion of the pulsed beam of microwave energy emitted by a radar transmitter that is reflected back to the receiver after the signal encounters a specific target or obstruction in the atmosphere, such as individual particles of precipitation. The term may also refer to the backscatter produced by these objects. [2] [1]
Any method that uses radar technology to map the location and characteristics of selected environmental phenomena by emitting a pulse of microwave radiation at a target and analyzing the portion that is partially returned by backscattering. 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 of precipitation, or the height and orientation of wind-driven ocean waves. [1]
A branch of meteorology concerned with the use of primarily ground-based radar technologies for the analysis and prediction of atmospheric phenomena across a wide variety of spatial scales. [1]
Atmospheric motion detected by using radar to track a target attached to a radiosonde, or by Doppler radar. [1]
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 its dew 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]

A battery-powered scientific instrument released into the atmosphere, usually by a weather balloon, which measures various atmospheric variables and transmits them by radio telemetry to a ground receiver. Radiosondes are essential sources of meteorological data.
The distance between the center of a cyclone and its band of strongest winds, often used as a metric for determining a cyclone's potential intensity.
rain
A type of precipitation that occurs when liquid water in the form of droplets condenses from atmospheric water vapor, becoming heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth.
rainband
A cloud and precipitation structure associated with an elongated area of rainfall and generated by differences in temperature. Rainbands may develop as squall lines ahead of cold 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 the visible 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 a primary bow, usually appearing with an angular distance of 42° centered on the anti-solar point, the color separation produces a spectrum with red on the outer edge of the arc and violet on the inner edge; a secondary 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

Also wet season and green season.

An annually recurring period of one or more months during which precipitation, particularly rainfall, is at or near its average annual maximum for a certain region. The term is used especially in tropical climates, where the rainy season contrasts with the dry season. [2]
rain and snow mixed
A class of precipitation composed of both rain and snow, 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

Also called an udometer, pluviometer, and ombrometer.

An instrument used to collect and measure the amount of liquid precipitation that occurs within a certain area over a certain period of time.
rain of animals
A relatively and consistently dry area on the leeward side of a significant geographic uplift such as a mountain range. Rain shadows exist because the uplift acts as a barrier to the passage of precipitation-producing weather systems: moist air masses crossing high elevations are forced upward by orographic lift, which causes the moisture to condense and precipitate on the windward side, leaving the air depleted of moisture by the time it reaches the leeward side.
rain showers

Often simply called showers.

Short, intense periods of rainfall, especially when occurring in widely scattered locations.
rapid intensification
ravine wind
A local wind generated as a result of a pressure 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 of balloon-borne radiosonde that is tracked using position change as determined by radar or radiotheodolite in order to specifically measure wind speed and direction aloft, and sometimes also other meteorological variables. [1] [2]
rear flank downdraft (RFD)
regional forecast
A weather forecast for a specified geographic region, usually a wider area than that covered by a local 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 northwesterly wind that blows across the Caucasus Mountains from the Black Sea in the west to the Caspian Sea in the east. [1]
2.  A local wind, cold in winter and hot in summer, that affects northern Syria, northern Iraq, western Iran, and southeastern Turkey.