Glossary of meteorology

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 moving away from either the North Pole or the South Pole.

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

actinoform:

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

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

adiabat: A line drawn on a thermodynamic diagram along which an moves as it ascends or descends through the atmosphere, or  ; the path followed by this line depends on whether it is a  or a.

adiabatic cooling: An of expansional cooling, in which a rising decreases in  as it increases in volume.

adiabatic heating:

An of compressional warming, in which a sinking increases in  as it decreases in volume.

adiabatic lapse rate: The rate at which a changes  as it moves vertically through the atmosphere. The parcel's 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. When not otherwise qualified, the term most often refers to the.

adiabatic process: Any idealized hypothetical process by which energy is transferred between a thermodynamic system and its surroundings only as work, without a corresponding transfer of heat or mass. Most compressible fluids, including gases in the, behave approximately adiabatically, such that meteorologists often use the assumption of adiabatic isolation when describing atmospheric systems. In such systems the of a  changes without any exchange of energy with its surroundings: as the parcel rises, the decrease in the surrounding  enables the air in the parcel to expand in volume, which decreases its internal energy and therefore its temperature ; as the parcel sinks and is compressed, its temperature increases.

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.

aerology: See.

aeronomy: The branch of meteorology that studies the upper regions of the Earth's or other planetary, 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,, , and dust.

ageostrophy:

air current: Any concentrated area of that develops because of differences in pressure and/or temperature between adjacent. 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 and.

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 that is generally weak and usually not. 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 and heavy. They derive their energy from and commonly develop in temperate zones during  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 from a, as in "winds aloft".

altimeter: A scientific instrument used to measure the altitude of an object (e.g. a ) with respect to a fixed level such as sea level.

altocumulus castellanus:

altocumulus (Ac): A middle-altitude 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 km, with the individual elements being larger and more distinct than in but smaller than in. Like other clouds, altocumulus usually signifies convection aloft. It is one of several classic "warning clouds" recorded by the aviation industry as a signal of developing.

altostratus:

American Meteorological Society (AMS): A scientific and professional organization in the United States whose mission is to promote and disseminate information about the, oceanic, and hydrologic sciences, and advance technologies, applications, and services related to them.

anabatic wind: A 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 knot or less but occasionally at much higher speeds. Contrast.

anemometer: A scientific instrument used to measure.

annular tropical cyclone:

anticyclone: Any large-scale characterized by outward spiraling which  around a strong center of. Surface-based anticyclones generally bring about cool, dry air and clear skies and are often implicated in weather phenomena such as and. Contrast.

anticyclonic rotation:

anticyclonic storm: Any system involving an, in which circulate around a region of  in the direction opposite to that expected around a region of. Anticyclonic storms rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

anticyclonic tornado:

anticyclogenesis: The development or strengthening of an in the atmosphere, which may result in the formation or maintenance of a. Contrast.

antitriptic wind: A generated by the local topography of a particular place; examples include and. Most such winds are in character.

apparent temperature: See.

arcus cloud:

Arctic cyclone:

Atlantic hurricane: A (locally known as a ) that forms in the Atlantic Ocean and achieves one-minute maximum exceeding 74 mph. 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.

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

atmospheric boundary layer (ABL): See.

atmospheric circulation: The global-scale movement of within the Earth's. 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, 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. Atmospheric density generally decreases proportionally with elevation above sea level, and also tends to vary with changes in,, and. According to the, 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 $1/undefined$ the density of liquid water.

atmospheric lake: A long-lived pool of water vapor.

atmospheric model:

atmospheric pressure (p):

The pressure exerted by the Earth's. 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 Pa.

atmospheric river:

atmospheric science:

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

atmospheric sounding: A measurement of the vertical distribution of physical properties through an column, usually including,, and , ,  concentration, and pollution, among others.

atmospheric temperature: A measure of at one or more locations within the Earth's. Temperatures recorded in the atmosphere can vary widely with altitude,, and, among other factors.

atmospheric thermodynamics:

atmospheric tide: A global-scale periodic oscillation of the Earth's caused by gravitational and thermal influences from the Sun and the Moon, analogous to oceanic tides.

atmospheric window: Any of the ranges of small bandwidths in the electromagnetic spectrum at which the Earth's 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. The existence of these windows is vital for the Earth–atmosphere system to be maintained near thermal equilibrium.

autumn:

avalanche:

Aviation Area Forecast (FA or ARFOR):

A former message product of the U.S. issued to provide information to pilots and aviation routes about 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 in a counterclockwise fashion (e.g. northerly to northwesterly to westerly). Contrast.

backscatter: The diffuse reflection of waves, particles, or signals back to the same direction from which they originated. Backscattering is the principle underlying all systems, which can distinguish radar returns backscattered from target 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 over time.

baroclinity:

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

barotropity:

The close alignment between a pressure gradient and a density gradient in a stratified fluid such as the. In the context of meteorology, a barotropic atmosphere is one in which depends only on and is more or less independent of, in contrast to a  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. The two most common types are mercury barometers and aneroid barometers.

barometric pressure: See.

barrier jet: A low-level core of high that sometimes occurs at altitudes of 1000 – in the vicinity of a mountain range, as a consequence of the deceleration of an as it crosses a major topographic barrier and releases latent heat which changes the local thermodynamics of the flow.

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:

A thin, nearly transparent coating of 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 characterized by strong of at least 35 mph and, typically lasting three hours or more. They can have an immense size, covering hundreds or thousands of square miles, and occur most often in temperate, polar, or mountainous regions during the.

block:

A nearly stationary pattern in the field overlying a large geographic area, which effectively "blocks" or diverts the movements of 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 phenomenon that occurs when particles of dust are lifted from the Earth's surface by and blown about in clouds or sheets. It is classified as an obstruction to vision in aviation weather observations and is commonly reported if the amount of suspended dust reduces horizontal  to 6 mi or less. Extreme cases may be called.

blowing sand: A phenomenon that occurs when grains of sand are lifted from the Earth's surface by and blown about in clouds or sheets. It is classified as an obstruction to vision in aviation weather observations and is commonly reported if the amount of suspended sand reduces horizontal  to 6 mi or less. Extreme cases may be called.

blowing snow: blown about by, either from falling snow or snow lifted from the surface, to a height of at least 2 m, reducing. It is a defining characteristic of.

bounded weak echo region (BWER):

bow echo: A characteristic radar return from a that is shaped like an archer's bow and usually associated with or lines of convective. 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.

breeze: Any generally light. Any local-scale air movement that is forced, e.g. a or. On the, a of force numbers 2 to 6, ranging from 4 –, 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.

brightband:

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

bushfire: See.

Buys Ballot's law:

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, with a less than 1 kn. Calm conditions are common in the and in the.

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.

Canadian Meteorological Centre (CMC): Provides forecast guidance to national and regional prediction centres in Canada.

Canterbury arch: See.

cap cloud:

An approximately stationary on or hovering above an isolated mountain peak. See also ' and '.

capacity: The ability of a 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 and decreases as the particle size of the detrital debris increases.

capping inversion:

castellanus:

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

catabatic wind: See.

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

ceiling balloon:

A type of used by meteorologists to determine the height of the  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 indicator that uses a searchlight to project a beam of light vertically onto a (similar to a ), with the height of the illuminated spot then calculated by the observer using a clinometer or alidade.

ceilometer: An instrument that uses a laser transmitter or other light source and a collocated receiver to determine the of a or  overhead, or to measure the concentration of  within the atmosphere.

cell: Any feature that is more or less closed, occurring at any of number of scales, including massive latitudinally oriented circulations such as ; motions that characterize  and cause the formation of ; and  formed by  and/or  loops within a. In, 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 or.

cellular cloud: A organization of in the form of a quasi-regular pattern of  behaving as individual, 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 with a clear center, and the closed cells filled with  surrounded by a clear rim.

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 surrounding the rotational center of a strong or. When a cyclone reaches sufficient intensity, a distinguishable 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 at the center of a recognizable or  at any given instant, i.e. the highest pressure in a high or the lowest pressure in a low.

ceraunometer: An instrument used for counting the number of discharges within a specific radius.

chinook wind: A warm, dry wind formed by a rainstorm dropping its precipitation onto the windward side of a mountain, thus drying the air mass before it blows across the leeward side, drops in elevation, and warms by. Common in the northwestern United States and southwestern Canada, a chinook can cause temperatures to rise from -48 C to 9 C in 24 hours, an increase of 57 C-change.

circulation: Common short form of.

cirrocumulus (Cc): A of with both  and  characteristics, signifying, and appearing as white, patchy, transient sheets of ripples or tufts organized in undulating rows, usually between 5 and 12 km above sea level. Though composed mainly of ice crystals, cirrocumulus is distinguished from and  by the presence of small amounts of supercooled liquid water droplets.

cirrostratus:

cirrus (Ci): A of 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 above sea level. Cirrus clouds often develop from the of  clouds in advance of  or, and therefore may indicate the imminent arrival of.

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

clear-air turbulence:

climate: The statistics of in a given region over long periods of time, measured by assessing long-term patterns of variation in,, , , , and other variables. The climate of a particular location is generated by the interactions of the, , 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:

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

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

cloud base: The lowest altitude of the visible portion of a.

cloud bow: See.

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

cloud drop effective radius:

cloud genus: See.

cloud iridescence:

A type of consisting of colorful iridescent patterns appearing most commonly near the semi-transparent edges of thin such as  and  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.

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

cloud tag:

cloud type:

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

cloud variety:

cloudburst: A colloquial term used to describe an excessive event, characterized by brief, sudden, exceptionally heavy and/or  falling from a, typically as part of a  associated with violent upward and downward convective currents.

col:

The point of intersection of a and a in the pressure pattern of a. 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.

cold front: A type of located at the leading edge of a cooler as it replaces a warmer air mass. Cold fronts lie within a sharp surface of  and the temperature difference between the air masses they separate can exceed 30 C. When enough moisture or instability is present, lines of  or  may accompany the boundary as it moves. In, cold fronts are symbolized by a blue line with triangles pointing in the direction of travel.

cold wave:

A period of weather characterized by excessively low temperatures, which may or may not also be accompanied by changes in. 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.

cold-core low:

Colorado low: A type of 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 and  when occurring in the winter.

convection: See.

convective available potential energy (CAPE):

convective condensation level:

convective inhibition (CIN):

convective instability: The inability of an 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 airflow and  that may lead to extensive vertical clouds,, and.

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 hPa, the compensatory air motion is upward, with inflow gradually changing to at higher altitudes; when it occurs in the upper atmosphere, the air motion is downward, with near the surface.

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 in a cloud or on a foggy glass surface. Coronae differ from in that the latter are formed by refraction from comparatively large particles.

crepuscular rays:

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

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", such as or, that form as a result of.

cumulonimbus:

cumulus (Cu): A of 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 above sea level. Cumulus clouds normally produce little or no, but can develop into precipitation-bearing clouds such as when influenced by atmospheric instability, moisture, and temperature gradients.

cumulus congestus:

cumulus humilis:

cumulus mediocris:

cyclone: Any large-scale characterized by inward spiraling which  around a strong center of. Cyclones can over land or water, can vary in size from  such as  to  phenomena such as  and, and may transition between tropical, subtropical, and  phases. Contrast.

cyclonic rotation:

cyclogenesis: The development or strengthening of a 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 ; for instance, are a type of  whose development may be variously described as cyclogenesis or, more specifically,. Contrast.

D
dark adaptor goggles:

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

dawn:

The first appearance of sunlight in the eastern before sunrise, or the time that marks the beginning of the morning.

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: Abbreviation of decibel relative to Z

debris cloud: See.

deepening: A decrease in the and surrounding sea-level within the circulation of a pressure system (usually a ) 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 and hence its, and the term is frequently used to imply. Contrast.

deformation: The rate of change of shape of a fluid body such as an. This quantity is very important in the formation of, in the explanation of 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.

dense fog: An advisory issued by the U.S. to caution the public about the possibility that horizontal may be reduced by dense  to 0.25 mi or less.

depression: Any at a given level in the atmosphere; i.e. a "low" or. The term is used especially frequently to refer to an early stage in the development of a during which the disturbance is only weakly developed or poorly organized; see.

derecho: A type of storm that produces widespread, straight-lined that are associated with.

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

dew point:

The to which an must be cooled, at constant  and, 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.

dew point depression: The difference between the actual and the at a certain altitude in the atmosphere. A small dew point depression indicates more moisture and higher, which in the lower can result in low  and, which are important factors contributing to the development of.

diabatic process:

Any thermodynamic process in which the of an changes as a result of the transfer of energy (e.g. heat) between the parcel and its surroundings, as opposed to an, 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.

Diablo wind:

diamond dust: A ground-level composed of tiny 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, normal to the flow (streamline divergence). It is a flow pattern of.

diffuse sky radiation:

The component of incoming that is scattered from the direct solar beam by molecules of air,,, or particulate matter in the and subsequently reaches the Earth's surface in nearly equal amounts from nearly all parts of the sky during.

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 than the sinking motion). Such a converts heat energy to potential energy and then to kinetic energy. Contrast.

discontinuity: A horizontal zone across which temperature, humidity, wind speed, or any other meteorological variable changes abruptly, such as a.

disdrometer: A scientific instrument used to measure the and velocity of falling such as.

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.

diurnal variation:

The range between the maximum and minimum values of a meteorological quantity (e.g. temperature, pressure, relative humidity) observed during the course of a solar day.

Dobson unit (DU): A unit of measurement used to describe the quantity of a trace gas (primarily atmospheric concentrations) present in a 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.

Doppler on Wheels (DOW):

Doppler weather radar:

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

downdraft:

drifting snow: Particles of lifted by the to a modest height, generally less than 1.8 m above the ground. Drifting snow does not significantly reduce at eye level below 10 km, in contrast to.

drizzle: A type of light consisting of liquid water droplets which are smaller than ordinary, generally less than 0.05 mm in diameter and falling at a rate of less than 1 mm per day.

drought:

Any prolonged period of below-average 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 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 ; in turn, droughts may increase the likelihood of.

dry lightning: associated with a.

dry line:

dry microburst:

dry punch: Meteorological slang for a or weather process. A dry punch that occurs near the Earth's surface may result in a bulge, whereas a dry punch aloft may increase the potential for.

dry season: An annual period of relatively low or infrequent, during which weather patterns are typically dominated by lengthy periods of, high temperatures, and low. The term is primarily used in the, in contrast to the.

dry thunderstorm:

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

dual polarization weather radar:

dusk:

dust devil:

dust storm:

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

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, 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 and the ocean.

Ekman number:

Ekman spiral:

Ekman transport:

energy-helicity index (EHI):

El Niño: The warm phase of the (ENSO), associated with the annual development of a band of warm ocean water in the eastern equatorial Pacific, which brings and heavy  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.

El Niño–Southern Oscillation (ENSO): An irregular long-term periodic variation in and 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, is marked by two phases: a warming phase,, 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, , 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 discharges,, and.

emagram: One of four thermodynamic diagrams used to display temperature and profiles in the atmosphere. Emagrams have axes of (T) and  (p). Temperature and data from  are plotted on these diagrams to allow calculations of convective stability or.

Enhanced Fujita scale (EF scale):

ensemble forecasting: A technique in which a 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.

entrainment: The process by which the air surrounding a developing is mixed into an ascending 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.

Environment and Climate Change Canada:

environmental lapse rate (ELR): The actual rate at which atmospheric changes with, as measured by a ; this is in contrast to the rate predicted by the theoretical. On average, the temperature of the decreases with height at a rate of 6.5 C-change per kilometre, but this rate is influenced by many factors. In general, the ELR is lower nearer to the ground surface, during the local, and over continental landmasses.

Environmental Modeling Center (EMC):

Environmental Science Services Administration (ESSA): The predecessor agency (1965–1970) to the (1970–present).

equivalent potential temperature ($\theta_e$):

equivalent temperature ($T_e$): The obtained when an expands, 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.

Eulerian equations:

European windstorm:

evaporimeter:

An instrument used to measure the rate of evaporation of water into the atmosphere. The most basic design consists of an open, ground-level evaporation pan from which water is allowed to evaporate freely.

explosive cyclogenesis:

extratropical cyclone:

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

eye: A typically circular region at the center of a strong that is the location of the storm's lowest. 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 and the rest of the storm.

eye of the wind: A nautical term used to describe the direction from which the is blowing.

eyewall:

F
fall wind: See.

Fata Morgana:

fetch:

The length of water over which a given blows. Fetch length and 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.

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

fire whirl:

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

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

flammagenitus:

flash flood: Any 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 associated with, 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 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 accumulates on saturated ground more rapidly than it can either infiltrate or run off.

flumen:

fog: A visible 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 and is heavily influenced by local topography, nearby bodies of water, and  conditions.

fogbow:

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

föhn wind:

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

forward-flank downdraft (FFD):

fractus (Fr):

A or consisting of ragged, irregularly shaped patches or shreds of  or.

frazil ice:

freezing drizzle: A type of in which consisting of supercooled liquid water droplets, often falling through a  in the lower atmosphere, freezes upon impact with the ground or other cold surfaces to form a coat of. Compare.

freezing fog: A condition in which supercooled water droplets comprising freeze either while suspended in the air, filling the air with visible similar to very light, or upon contact with sub-freezing surfaces, forming a coating of  and/or.

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

freshet: 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. Any temporarily inundated or rapidly flowing watercourse or newly created (and often ephemeral) drainage channel resulting from snowmelt.

front: A boundary separating two of different and usually also of different  and. Weather fronts are the principal cause of meteorological phenomena outside the, often bringing with them, , and changes in speed and direction as they move. Types of fronts include, , and.

frontogenesis: The meteorological process by which a is created, usually as a result of the narrowing of one or more horizontal temperature gradients across the boundary between two adjacent. Contrast.

frontolysis: The dissipation or weakening of an atmospheric. Contrast.

frost: A very thin layer of crystals on a solid surface, typically restricted to that which forms when in an atmosphere whose temperature is above freezing comes into contact with a surface whose temperature is below freezing. Frost may exhibit a great variety of forms.

Fujita scale:

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

G
gale: A strong surface, typically used as a descriptor in nautical contexts and variously defined based on. In the modern, a gale is any of Beaufort number 7 or greater, corresponding to near gale at 28 –; gale at 34 –; strong gale at 41 –; and storm at 48 –. Any unusually strong wind.

gale warning:

gap wind: A local, low-level that blows along a valley or through a col between mountains, often at speeds as high as 20 –.

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 in and when creating.

geostrophic wind: The theoretical that would result from an exact balance between the Coriolis force and the 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:

A coating of smooth, clear, sometimes of considerable thickness, that forms when supercooled water, usually precipitated as or, 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. Glaze is denser, harder, and more transparent than and.

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

graupel:

A type of that forms when supercooled water droplets are collected and freeze on falling, forming balls of 2 – in diameter. Graupel is distinct from, small hail, and.

Great Salt Lake effect: A 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.

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 of the, usually lasting less than 20 seconds. Gusts are more transient than and are followed by a lull or slowing of the wind speed. They are generally only reported by when the maximum wind speed exceeds the average wind speed by at least 10 –.

gust front: See.

gustnado:

A relatively weak associated with the at the leading edge of a, and often occurring along a. A or  may indicate the presence of a gustnado.

H
haboob:

Hadley cell:

hail: A type of solid that consists of balls or irregular lumps of, usually 5 – 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. It is distinct from and  or.

hailstorm: Any, usually a strong, which precipitates.

Haines Index:

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

halo:

hard rime: A type of consisting of opaque, granular masses of ice deposited primarily on vertical surfaces by. Hard rime is more compact and amorphous than and usually develops on windward surfaces exposed to high wind speeds and air temperatures between -2 and -8 C.

Harmattan:

haze: Any suspension in the atmosphere of very small, dry particulate matter, including natural (e.g. dust, salt, or smoke) as well as man-made pollutants (e.g. ), the individual particles of which are invisible to the naked eye but collectively produce a milky, often opalescent sky with reduced at long distances. Haze usually indicates sub-saturated air, whereas or mist indicates full saturation.

hazardous seas warning:

hazardous seas watch:

heat dome: The effect created by Earth's atmosphere trapping hot ocean air like a lid or cap.

heat burst: A rare phenomenon involving a sudden, localized increase in (sometimes 10 C-change or more within just a few minutes) associated with a decaying or other  and possibly accompanied by  winds and a rapid decrease in.

heat index (HI):

A meteorological index that posits the perceived by the average human being who is exposed to a given combination of air and  in a shaded area. For example, when the air temperature is 32 C with 70% relative humidity, the heat index is 41 C.

heat lightning:

heat wave: A period of weather characterized by excessively high temperatures, which may or may not be accompanied by high or by. 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.

heavy snow warning: A type of weather warning formerly issued by the U.S. to alert areas in which a high rate of (generally 6 in or more in 12 hours) was occurring or was. The warning was replaced by the for Heavy Snow beginning with the 2008–09 winter storm season.

helicity:

high-pressure area:

hodograph:

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

hook echo:

horseshoe vortex:

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

humidex:

humilis: See.

hurricane: The local name for a that occurs in the Atlantic Ocean or northeastern Pacific Ocean and achieves one-minute maximum exceeding 74 mph.

hurricane hunters:

huaico:

A or caused by torrential  occurring high in the Andes mountains of South America, especially during the weather phenomenon known as.

hydrometeor: Any particulate of liquid or solid water within the, encompassing all types of, formations due to condensation such as and , and particles blown from the Earth's surface by wind such as and.

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

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

hydrostatic equilibrium:

hygrometer: A scientific instrument used to measure.

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 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. Its natural occurrence in weather phenomena takes many forms, including,, , , and.

iceberg:

ice accretion indicator:

ice crystal: A minute spicule of that forms from water in the atmosphere at temperatures below the freezing point of 0 C. 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 onto them, and they may collide with other ice crystals to form. A type of composed of very small, unbranched crystals of ice which fall slowly and often seem to float in the air.

ice fog: A type of consisting of a sufficient concentration of tiny suspended in the atmosphere to reduce  to less than 1 km. Ice fog forms at very low ambient air temperatures, typically -30 C 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.

ice pellets:

ice spike: A rare 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.

ice storm: A type of characterized by which results in the accumulation of at least 6.4 mm of  on exposed surfaces.

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

incus:

Indian summer:

inflow: The influx of heat and moisture into a system from the surrounding environment. The inflow of of warm, moist air drives and sustains most types of storms, including  and. Contrast.

instrument flight rules (IFR):

International Standard Atmosphere (ISA): A static of the variations in,, , 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):

irisation: See.

J
jet-effect wind: See.

jet stream:

A narrow, fast-flowing, meandering primarily occurring in the upper part of the, at altitudes above 9 km, and usually flowing from west to east. The Northern and Southern Hemispheres each have a predictable though discontinuous polar jet and subtropical jet; and other types of jet streams can form under certain conditions.

jet streak:

The region of maximum that runs along the elongated axis of a. In the local winter, the maximum speed in the polar-front jet stream can reach upwards of 200 kn.

K
K-index:

An operational atmospheric indicating the potential for, based on temperature, of the lower troposphere, and the vertical extent of the moist layer. K-index values of 36 and above suggest a high likelihood of thunderstorm development.

$K_{DP}$:

kata-front: A or 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.

katabatic wind:

A 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 knot or less but occasionally at much higher speeds. Contrast.

Kelvin temperature scale:

Kelvin–Helmholtz instability: A phenomenon of instability that occurs occasionally in an atmospheric layer within which increases rapidly with. Kelvin–Helmholtz waves form in this layer of strong vertical, and are often marked by a distinct train of clouds that resemble breaking ocean waves.

khamsin:

The local name for a dry, hot, seasonal, often carrying large quantities of, that occurs in the deserts of Egypt, Israel, Palestine, and Jordan. Compare ', ', ', and '.

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 mph). It is often used in meteorology for measuring.

Köppen climate classification:

L
$L_{DR}$:

La Niña:

Lagrangian equations:

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

land breeze: An offshore 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, its daytime counterpart in a cycle of coastal winds caused by lateral differences in surface temperature between land and sea.

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

landslide:

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

lapse rate: The rate at which an atmospheric variable, most commonly temperature or pressure, decreases with increasing.

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. Contrast.

latent heat flux: The movement of (a major transporter of ) 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.

law of storms: A general statement of the manner in which the winds of a rotate about the cyclone's center, and the way in which the entire 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.

layer cloud: See.

lee trough:

A of that forms preferentially to the  or downwind side of a mountain barrier when  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.

lee wave:

Lemon technique: A method used by meteorologists which focuses on and uses to determine the relative strength of  cells in a vertically  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.

lenticular cloud: A type of stationary with a distinct lens or saucer shape which typically forms in an arrangement perpendicular to the and at altitudes less than 12 km 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 of a saturated at the same level. Air masses with one or many LFCs are potentially unstable and may develop convective clouds such as.

Lidar:

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 that is lifted at a given  height within the, typically 500 hPa. When the value of the lifted index is positive, the atmosphere at the given height is stable; when it is negative, the atmosphere is.

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,, and is one of the distinguishing features of. 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 discharge that occurs between the atmosphere and an object (rather than between different parts of the atmosphere). Most lightning strikes are, 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. 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):

lithometeor:



low-level jet:

low-level windshear alert system:

low-pressure area (L):

low-topped supercell (LT):

lysimeter: An instrument used to measure the total amount 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 by high or  clouds with a regular pattern of ripples and patches separated by small areas of blue sky, resembling the scales on a mackerel.

macroburst: A strong that affects a path longer than 4 km and persists for up to 30 minutes, with surface winds reaching as high as 210 kph.

macrometeorology: The study of the largest-scale processes, i.e. those occurring over very large regions, oceans, continents, or the entire Earth, such as the, as opposed to and. See also.

MAFOR: A North American system used in the transmission of marine 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.

marine climate:

A regional that is strongly influenced by its location in relation to a sea or ocean, characterized by relatively small and  temperature variations and high atmospheric, which contributes to high  and. Contrast.

marine cloud brightening:

marine stratocumulus:

mass flow: The movement of a fluid, such as an, down a or.

meridional circulation: The component of the large-scale atmospheric that is oriented parallel to a meridian or line of longitude, and thus shows large north–south movement.

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, above the and below the. The lower boundary of the mesosphere varies between 50 and 65 km 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 which seeks to understand and explain observable events, with a major focus on. Meteorology uses variables familiar in chemistry and physics to describe and quantify meteorological phenomena, including,, , , and how these properties interact and change over time.

microburst:

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

microscale meteorology: Meteorological phenomena that occur on a scale of 40 m to 4 km.

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

mini-tornado: A fallacious term often used in news media to refer to damaging winds accompanying a, indifferently caused by or, on a small area.

misocyclone: A vortex with a width between 40 m and 4 km, which in the strictest sense includes and.

misoscale meteorology:

mixed cloud: A composed of both liquid water droplets and (e.g.,, and ), as opposed to a.

mixing ratio: A measure of atmospheric, usually expressed as the dimensionless ratio of the mass of water vapor in a given to the unit mass of dry air (i.e. grams of water vapor per kilogram of dry air).

mock sun: See.

Modified Fujita Scale: An update to the original from 1971 proposed by Ted Fujita in 1992.

moist adiabat: See.

moist adiabatic lapse rate: See.

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

mountain breeze:

mountain-gap wind: See.

multicellular thunderstorm: A consisting of more than one, i.e. more than one circulating system of and.

multiple-vortex tornado:

moisture:

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

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

Morning Glory cloud:

mudflow:

murus: See.

N
nacreous cloud:

A rare type of that forms at altitudes of 24 –, usually in high-latitude regions. These clouds are normally in form but may resemble, and often exhibit brilliant  similar to mother-of-pearl shortly after sunset or before sunrise.

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 that was located in Kansas City, Missouri.

National Severe Storms Laboratory (NSSL): A lab in Norman, Oklahoma tasked with researching.

National Tornado Database: The official NOAA record of all known 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, warnings of, 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 of minimum tidal range occurring semi-monthly near the times when the Moon is in quadrature, i.e. the first and third quarters.

needle ice:

negative tilt: The angular displacement of a line such that the axis of the trough is rotated clockwise from a north–south meridian (as opposed to the counterclockwise rotation of a 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.

nephoscope: A scientific instrument used to measure the altitude, direction, and velocity of atmospheric relative to a point on the ground directly below them.

NEXRAD:

nimbostratus (Ns): A of occurring at low or middle altitudes, typically between 0.5 and 5.5 km, 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  but no lightning or thunder. Low, ragged clouds frequently occur below nimbostratus and may or may not merge with it.

noctilucent cloud:

nonadiabatic process: See.

nor'easter:

A macro-scale, 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 winds.

Nor'west arch:

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 (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, most commonly three consecutive 10-year intervals totaling 30 years.

northern lights: See.

Novaya Zemlya effect:

nowcasting:

numerical weather prediction:

O
obscuring phenomena: Any atmospheric phenomenon exclusive of clouds that restricts, including various such as and  as well as  such as dust and sand.

occluded front: A type of formed during the process of when a  overtakes a. Occluded fronts usually form around mature when a warm  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. In, 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 or by forces related to seawater density gradients. They are major transporters of the heat introduced by solar radiation, usually moving warm water from the to higher latitudes and returning cold water in the opposite direction, by which they exert an important influence on  and weather phenomena across the world.

oceanic climate: See.

offshore current: Any that flows parallel to, or away from, the coastline of a landmass.

offshore wind: Any that blows from land out over a body of water, e.g. a. Contrast.

okta:

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

omega equation:

onshore wind: Any that blows from a body of water to land, e.g. a or. Contrast.

opacity:

orographic cloud: Any whose form and extent is determined by the effects of high-elevation terrain upon the passing flow of air, especially the forced of moist air as it passes over hills or mountains. As the rising air mass encounters reduced atmospheric pressures, commonly results in condensation and. Orographic clouds are usually very slow-moving or stationary; examples include and.

orographic lift:

The forced ascent of an 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, leading to saturation, condensation, and the formation of and often.

orographic precipitation:

overcast: The condition of wherein 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.

overrunning: The action of an aloft, often relatively warm, moving over another air mass of greater density at the surface, as occurs in a.

overshooting top: A distinct, bulging protuberance produced by a vigorous that rises above the top of the of a  cloud. Overshooting tops are generally short-lived, but those that persist may indicate the potential for strong and.

outflow: Air that flows outwards (away from) a system. Outflow typically radiates from in the form of a wedge of rain-cooled air, which is often delineated by a low, thick cloud preceded by a, apparent both from the ground and in  imagery. The altitude at which the outflow occurs is strongly correlated with the intensity and persistence of large storm systems such as.

outflow boundary:

The boundary between the cooled air from a and the air of the surrounding environment, similar to a. New thunderstorms often develop along outflow boundaries.

outflow jet:

ozone depletion:

ozone layer:

A region of the Earth's 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, between approximately 15 and 35 km in altitude, although its thickness varies seasonally and geographically.

P
paleoclimatology:

pampero:

pan evaporation:

pancake ice: A form of that consists of round, flat pieces of ice with elevated rims, with diameters ranging from 30 cm to 3 m, and thicknesses of up to 10 cm.

pannus:

parhelion:

An optical phenomenon in which a patch of bright light is visible along the main 22° 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 with diameters less than 30 um, e.g. those present in  or  clouds.

Particularly Dangerous Situation:

pascal (Pa): The SI derived unit of pressure, defined as one newton per square metre. In meteorology, measurements of are often given in hectopascals (hPa) or kilopascals (kPa).

Pascal's law:

A hydrostatic principle which states that pressure applied to a confined incompressible fluid (e.g. ) is transmitted equally and undiminished to every portion of the fluid and to the walls of the containing vessel.

Pearson scale:

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

pedestal cloud: See.

pentad: A period of five consecutive days sometimes used in preference to the seven-day week in the analysis of meteorological data because it divides conveniently into the number of days (365) in a standard year.

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.

perlucidus: A 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 and.

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,, , , and.

physical meteorology: A branch of concerned with the structure and composition of the and the various optical, electrical, acoustical, and thermodynamic phenomena that characterize it, including  and,, and electromagnetic radiation.

Phi_DP ($\Phi_{DP}$):

pileus:

A small, appearing as a smooth, shallow, "cap", that forms above or attached to the top of a or  cloud. Pileus clouds are formed when moist air above the parent cloud is cooled to its by a strong, and are good predictors of ; a pileus atop a cumulus cloud often foreshadows its transformation into a cumulonimbus cloud.

pilot balloon: See.

pilot report (PIREP): An inflight report by an aircraft pilot or crew member of the 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.

polar high:

An extensive across the polar latitudes of either the Northern or Southern Hemisphere which acts as a source of very cold and generally dry air. The over the Arctic, known as the Arctic high, is generally seasonal, while that over Antarctica, known as the Antarctic high, is semi-permanent.

polar low:

A relatively small-scale, non-frontal, migratory that occurs in the polar latitudes of either the Northern or Southern Hemisphere. Such systems are that form over oceans poleward of the, most commonly during the local, and can produce blustery, snowy conditions.

polar front: Either of the two semi-permanent, semi-continuous 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. It is the most significant front in terms of air mass contrast and susceptibility to disturbance.

polar mesospheric cloud (PMC):

polar stratospheric cloud (PSC):

polar vortex: Either of the two very large, persistent, rotating, upper-level 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,, and masses of cold, dry polar air into the mid-latitudes, which can cause sudden, dramatic drops in temperature known as.

potential temperature ($\theta$):

potential vorticity:

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

precipitable water:

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

precipitation: Any product of the condensation of atmospheric that falls by gravity, the main forms of which include,, , , and. Precipitation occurs when a portion of the atmosphere becomes locally saturated with water vapor such that the water condenses into liquid or solid droplets and thus "precipitates" out of the atmosphere.

pressure gradient: The horizontal or vertical rate of change of in the, usually expressed in  (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.

pressure gradient force (PGF): The force experienced by a unit mass of in response to differences in in either the horizontal or vertical plane, i.e. a, such that  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.

pressure system: A relative peak or lull in the spatial distribution of sea-level. and systems evolve by the interactions of,, and  in the atmosphere, and are directly responsible for most local  phenomena.

prevailing winds: The predominant encountered at a particular point or region of the Earth's surface, identified by their source and. 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 and by local topography.

psychrometer:

psychrometrics:

The field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures, especially the mixture of air and.

Pulse-Doppler radar:

pulse storm: A that produces brief but strong, common in humid areas of the continental United States during the. These storms are often associated with, particularly sudden and intense wind , very large which grow continuously as they are repeatedly moved up and down within the storm, and.

pyranometer:

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

pyrgeometer:

pyrheliometer:

Q
Q vector: In and semi-geostrophic theory, a horizontal vector which appears in the and tends to point in the direction of rising air. If $Q$ points toward warm air, the is ; if it points toward cold air, the geostrophic flow is.

quantitative precipitation estimation (QPE): A method of estimating the approximate amount or rate of 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 that will fall at a given location within a given time period, expressed in units of depth (e.g. inches).

quasi-biennial oscillation (QBO):

A marked oscillation in the in the lower part of the equatorial, in which the  changes gradually from westerly to easterly and back to westerly with a period that fluctuates between approximately 24 and 30 months.

quasi-geostrophic approximation:

A form of the equations of motion in which the, an idealized approximation to the actual, 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, which is presumed small. The quasi-geostrophic approximation is useful in the analysis of  systems, but less accurate in situations in which the  plays an important  role, e.g. near.

quasi-geostrophic motion: The flow of a fluid in which an approximate between the Coriolis force and the 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.

quasi-geostrophic theory: A theory of atmospheric dynamics that involves the in the derivation of the quasi-geostrophic equations. This theory is relatively accurate for atmospheric motions in which the  is less than unity, but it cannot accurately describe some local atmospheric structures such as  or small, strong  cells as well as other theories.

quasi-linear convective system (QLCS): See.

quasi-stationary front: A that is or nearly so; conventionally, a front that is moving at a speed less than about 5 knots.

R
radar echo: 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. The term may also refer to the produced by these objects.

radar imaging: 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 by. 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, or the height and orientation of wind-driven ocean waves.

radar meteorology: A branch of concerned with the use of primarily ground-based radar technologies for the analysis and of atmospheric phenomena across a wide variety of spatial scales.

radar winds: Atmospheric motion detected by using radar to track a target attached to a, or by.

radiation 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. 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.

radiosonde:

A battery-powered scientific instrument released into the, usually by a, which measures various and transmits them by radio telemetry to a ground receiver. Radiosondes are essential sources of meteorological data.

radius of maximum wind (RMW): The distance between the center of a and its band of strongest, often used as a metric for determining a cyclone's potential intensity.

rain: A type of that occurs when liquid water in the form of droplets condenses from atmospheric, 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 and structure associated with an elongated area of  and generated by differences in temperature. Rainbands may develop as ahead of ;  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, 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.

raindrop size distribution (DSD):

rainy season:

An annually recurring period of one or more months during which, particularly, is at or near its average annual maximum for a certain region. The term is used especially in climates, where the rainy season contrasts with the.

rain and snow mixed: A class of composed of both and, the latter usually partially melted, that is reported in some weather observation formats. It usually occurs only briefly at any one location as a transition phase from rain to snow or vice versa.

rain gauge:

An instrument used to collect and measure the amount of liquid that occurs within a certain area over a certain period of time.

rain of animals:

rain shadow: A relatively and consistently dry area on the 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 -producing weather systems: moist air masses crossing high elevations are forced upward by, which causes the moisture to condense and precipitate on the side, leaving the air depleted of moisture by the time it reaches the leeward side.

rain showers:

Short, intense periods of, especially when occurring in widely scattered locations.

rapid intensification:

ravine wind: A generated as a result of a 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.

rawinsonde: A type of -borne that is tracked using position change as determined by radar or in order to specifically measure  and  aloft, and sometimes also other meteorological variables.

rear flank downdraft (RFD):

regional forecast: A for a specified geographic region, usually a wider area than that covered by a.

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: A strong northwesterly that blows across the Caucasus Mountains from the Black Sea in the west to the Caspian Sea in the east. A, cold in winter and hot in summer, that affects northern Syria, northern Iraq, western Iran, and southeastern Turkey.

retrogression:

Any motion of an atmospheric wave or that opposes, or occurs in a direction opposite to, the normal or typical flow in which it is embedded, e.g. a situation in which move westward, contrary to the generally westerly winds flowing through the pattern.

Rho_hv ($\rho_{hv}$):

ridge:

An elongated region of relatively, almost always associated with an area of maximum curvature of wind flow. Ridges may exist at the surface or aloft or both; they may contain the closed circulation of a distinct high-pressure area, and a high may have one or more distinct ridges. Under certain conditions, ridges may alternate with in a high-amplitude pattern.

rime: A coating of on the surface of an object. See ' and '.

rocketsonde: A type of that is transported into the upper atmosphere, e.g. the, by rocket propulsion before being ejected and descending to the Earth's surface by parachute. Rocketsondes are used to make at altitudes much higher than can usually be obtained by  or aircraft. They can provide instantaneous vertical profiles for a number of meteorological variables (temperature, pressure, ozone concentration, wind speed and direction, etc.) as they descend through the layers of the atmosphere.

rogue wave:

roll cloud: An elongated, low-level in the shape of a horizontal tube that appears to rotate slowly about its horizontal axis, and is associated with but completely detached from the of a  cloud above it. Though rare, roll clouds typically occur behind the along the leading edge of a  or ; they are also sometimes associated with.

Rossby number:

Rossby wave:

A very large-scale atmospheric wave appearing on an upper-air isobaric analysis of the middle and upper. Rossby waves consist of a series of and with very long wavelengths (typically a few thousand kilometres) stretching around the Earth, principally in the middle latitudes. They are strongly linked to surface weather patterns.

rotation: See.

S
saddle point: See.

Saffir–Simpson hurricane wind scale (SSHWS):

A rating system used to classify ( in the Western Hemisphere) into one of five categories according to the intensity of their, measured as the speed averaged over a one-minute interval at an altitude of 10 meters above the surface. Category 1, the lowest rating on the scale, indicates average sustained wind speeds of 33 –, where the lower limit is also used to define the distinction between a and a hurricane; Category 5, the highest rating, indicates wind speeds of 70 m/s or more.

sandstorm: See.

sastrugi:

Sharp, irregular grooves or ridges formed on a surface by wind erosion, saltation of snow particles, and deposition, usually parallel to the prevailing winds. They are often found in the polar regions and in large, open areas such as frozen lakes in cold temperate regions.

satellite sounding: An obtained from instruments on a meteorological satellite in orbit around the Earth.

satellite tornado: An independent that revolves around a larger, primary tornado (typically a very large and intense one) and interacts with the same. Satellite tornadoes are distinct from the subvortices of a, though they may still merge into their companion tornado.

saturated adiabat:

A drawn on a thermodynamic diagram that traces the path of a  as it moves through the atmosphere. Saturated parcels tend to behave very differently from dry parcels; the latter are instead described by a.

saturated adiabatic lapse rate (SALR):

saturation vapor pressure: The maximum possible partial pressure exerted by a quantity of in the atmosphere at a given temperature. Saturation vapor pressure increases non-linearly with air temperature according to the Clausius–Clapeyron relation, such that the vapor pressure in millibars at 32 C is approximately double the value at 21 C.

scarf cloud: See.

scavenging: The process by which particulate matter in the atmosphere is captured and removed by.

scud: See.

sea breeze: An onshore that blows from sea to land, a result of the more rapid warming of the land surface relative to the sea during the day. It blows in the opposite direction of a, its nighttime counterpart in a cycle of coastal winds caused by lateral differences in surface temperature between land and sea.

sea spray: particles formed directly by the ocean, mostly by ejection into the atmosphere by bursting bubbles at the air-sea interface.

sea state:

sea surface temperature (SST):

The water of the surface layer of a sea or ocean, usually measured at a depth between 1 mm and 20 m beneath the surface. in the atmosphere are strongly influenced by sea surface temperatures within a short distance of the shore.

season: Any division of the year marked by changes in, ecology, and the duration of daylight. Seasons result from the Earth's orbit around the Sun and its axial tilt relative to the ecliptic plane. In temperate and polar regions, four calendar-based seasons –, , , and – are generally marked by significant changes in the intensity of sunlight that reaches the Earth's surface; these changes become less dramatic as one approaches the Equator, and so many tropical regions have only two or three seasons, such as a and a. In certain parts of the world, the term is also used to describe the timing of important ecological events, such as, flood seasons, and seasons.

secular trend: The slow change (either an increase or a decrease) in the values of one or more climatic elements (e.g. temperature) that takes place over a long period of time, after fluctuations that occur over comparatively short periods have been eliminated.

seiche: A stationary or standing wave (i.e. a wave that oscillates in time without moving through space) that occurs in an enclosed or semi-enclosed body of water, such as a lake or bay, or in the atmosphere, continuing to oscillate for some time after the force initiating its formation has ceased (occasionally as long as several days). Seiches may be caused by a variety of forces, including strong, earthquakes, landslides, and sudden changes in.

sensible heat: The heat absorbed or transmitted by a substance during a change in temperature that is not accompanied by a change of phase (i.e. enthalpy) and which can be measured or "sensed", e.g. with a thermometer. Contrast.

sensible temperature: The of the air or an object as it is felt or experienced by an individual. This may differ from the actual measured temperature for any of a number of reasons, e.g. as a result of (as with a ) or  (as with ). Compare.

severe thunderstorm: A type of consisting of an especially strong or intense accompanied by locally damaging  winds exceeding 50 kn, heavy, frequent, and/or large  with a diameter of at least 20 mm. Severe thunderstorms are often capable of producing  as well.

severe weather: Any dangerous meteorological phenomena with the potential to cause damage on the ground surface, serious social disruption, or loss of human life. There are many types of severe weather, including strong, excessive, , , , , and. Some severe weather may be more or less typical of a given region during a given ; other phenomena may be.

sferics: See.

shade temperature: The air as measured by a thermometer housed inside an instrument shelter, which allows air to circulate freely around the thermometer while sheltering it from the potentially confounding effects of direct, precipitation, and thermal energy emitted from the ground and surrounding objects. Shade temperature is a standard meteorological method for measuring air temperature.

sheet lightning: A diffuse illumination of the sky caused by a discharge in which the form of the discharge is not visible to an observer because of the presence of an obfuscating.

shelf cloud:

A low, elongated, wedge-shaped that occurs along a, often masking the boundary between and. Shelf clouds are associated with and attached to the base of a cloud, unlike, which are not attached.

short wave: Any relatively small, short-wavelength ripple (i.e. a or a ) superimposed upon a pattern in the planetary-scale movement of  within the middle and upper. Short-wave troughs in particular are frequently associated with major cyclonic developments.

shower: A brief downpour of (especially, but also or ) that starts and ends abruptly and typically lasts less than 10 minutes. Showers are characterized by rapid changes in intensity and are usually associated with (e.g. ) which do not completely cover the sky, such that brightness is frequently evident during showers.

SIGMET:

significant level: In a observation, an altitude or elevation (other than a ) for which temperature, pressure, and humidity are reported because temperature and/or data at that level are sufficiently important or unusual to warrant the attention of the, or because they are required for the accurate portrayal of the observation.

simoom:

single cell thunderstorm: See.

sirocco:

skew-T log-P diagram:

sky:

Skywarn:

The program of the U.S. . Skywarn organizations have also been formed in Europe and Canada.

skipping tornado:

sleet:

slush: A slurry mixture of small ice crystals (such as ) and liquid water. Slush forms when ice or snow melts.

snow: A type of solid in the form of which precipitate from the atmosphere and subsequently undergo changes on the Earth's surface. Snow occurs when particles in the atmosphere attract supercooled water droplets, which nucleate and freeze into hexagonal crystals known as ; upon reaching the ground it may then accumulate into snowpack or and, over time, metamorphose by sintering, sublimation, and freeze-thaw mechanisms. Unless the local climate is cold enough to maintain persistent snow cover on the ground, snow typically melts seasonally.

snow gauge:

snow grains:

snow roller:

A phenomenon in which large snowballs form naturally as clumps of snow are blown along the ground by strong winds, growing larger as they accumulate material along the way.

Snowbelt: A region near the Great Lakes of North America where heavy snowfall in the form of is particularly common.

snowdrift: A deposit of sculpted by into a mound during a.

snowflake:

snowspout: See.

snowsquall: A sudden, moderately heavy snowfall characterized by strong surface wind and. It is similar to a but is more local in scale, and snow accumulations may or may not be significant.

snowstorm:

A type of accompanied particularly by heavy precipitation in the form of. Very large snowstorms with strong winds and meeting certain other criteria are called.

SODAR:

soft hail: See.

solar irradiance:

solarimeter: See.

sounding: See.

sounding balloon: See.

sounding rocket:

A sub-orbital rocket carrying scientific instruments designed to record measurements and perform experiments in the upper atmosphere while in flight, usually reaching altitudes ranging from 48 to 145 km above the surface of the Earth, i.e. higher than but lower than.

specific humidity:

spindrift:

blown from cresting waves during a. This spray "drifts" in the direction of the gale and is distinct enough that it is sometimes used to judge at sea.

spring:

sprite:

squall:

squall line:

St. Elmo's fire: A weather phenomenon in which luminous plasma is created by a corona discharge at the tips of long, sharply pointed objects in a strong atmospheric electrical field, such as that generated by a.

standard atmosphere:

standing cloud: See.

static atmospheric model:

station model:

stationary front:

steam devil:

steering: Any influence upon the direction of movement of an atmospheric disturbance that is exerted by another aspect of the state of the atmosphere.

Stevenson screen:

storm: Any disturbed state of an environment or especially affecting the ground surface and strongly implying. Storms are characterized by significant disruptions to normal atmospheric conditions, which can result in strong, heavy, and/or and  (as with a ), among other phenomena. They are created when a center of develops within a system of  surrounding it.

storm cell: An which contains up and down in convective loops and which moves and reacts as a single entity. It functions as the smallest unit of a -producing weather system.

storm chasing:

Storm Data and Unusual Weather Phenomena (SD):

A National Climatic Data Center (NCDC) publication beginning in 1959 which details quality-controlled tornado and other severe weather summaries as the official NOAA record of such events.

storm shelter: A type of underground bunker designed to protect the occupants from violent, particularly.

storm spotting: A type of in which observers watch for the approach of and  and actively relay their findings to local meteorological authorities.

storm surge:

Storm Prediction Center (SPC):

Storm Track:

straight-line wind:

Any very strong and potentially damaging that lacks the rotational damage pattern associated with the winds of a and hence is said to blow in a "straight line". Straight-line winds commonly accompany the of a  or originate with a  and may  as high as 130 mph.

stratocumulus:

stratocumuliform:

stratosphere: The second major layer of the Earth's, above the and below the. The lower boundary of the stratosphere varies between 7 and 20 km above the Earth's surface, depending on latitude.

stratospheric oscillation: See.

stratus:

subtropical high:

summer:

Stüve diagram:

sun dog: See.

sunshine recorder:

sunshower: A meteorological phenomenon in which falls while the sun is shining.

supercell:

subtropical cyclone:

surface weather analysis:

surface weather observation:

sustained wind:

synoptic scale meteorology:

T
tail cloud:

A ragged band of and/or extending from a  toward the precipitation core.

temperature: A physical quantity expressing the thermal motion of a substance, such as a mass of air in the, and proportional to the average kinetic energy of the random microscopic motions of the substance's constituent particles. Temperature is measured with a calibrated in one or more temperature scales: the Kelvin scale is the standard used in scientific contexts, but the Celsius and Fahrenheit scales are more commonly used in everyday contexts and for.

temperature gradient: A physical quantity that describes in which direction and at what rate the changes within or across a particular system or location. It is typically expressed in units of degrees (on a particular temperature scale) per unit length; the SI unit is per meter (K/m).

temperature inversion:

tephigram:

terminal aerodrome forecast (TAF): A format for reporting current and forecast weather conditions, particularly as such information relates to aviation. Standard TAFs are issued by major civil airfields at least four times a day (every six hours) and generally apply to a 24- or 30-hour period and an area within approximately 8 km from the center of an airport runway complex. TAFs complement and use similar encoding to reports, but also take into account local geographic influences on weather.

Terminal Doppler Weather Radar (TDWR):

thermal:

A column of rising air in the lower altitudes of the Earth's atmosphere. It is a form of atmospheric created by the uneven heating of the Earth's surface by, and an example of.

thermal wind:

thermo-hygrograph:

thermodynamic diagrams:

thermometer: An instrument used to measure or a.

thermosphere:

thunder: The sound produced as a result of the sudden thermal expansion of air within and surrounding the channel of a discharge. This expansion creates an audible supersonic shock wave that, depending on the listener's distance from the source, can range from a sharp, loud crack (sometimes called a thunderclap or peal of thunder) to a deep, sustained rumble. Thunder is a defining feature of.

thundershower: A relatively weak.

thundersnow:

thunderstorm:

A characterized by the presence of and its acoustic effect on the Earth's atmosphere, known as. Thunderstorms result from the rapid upward movement of warm, moist air, often along a. They can develop in any geographic location but are most common in the mid-latitudes. They are usually accompanied by strong and heavy ; especially strong or  can produce some of the most dangerous weather phenomena, including large,, and.

thunderstorm asthma:

tilted updraft:

tornado:

A rapidly rotating column of air that is in contact with both a parent and the surface of the Earth. Tornadoes come in many shapes and sizes, and they are often visible in the form of a originating from the base of a  cloud, usually during a, with a cloud of rotating dust and debris beneath it. The most extreme tornadoes can achieve wind speeds of more than 480 kph, span more than 3.2 km in diameter, and stay on the ground for more than 100 km (dozens of miles) before dissipating.

Tornado Alley:

tornado climatology:

tornado debris signature (TDS):

An area of high reflectivity detected by that is caused by large amounts of debris being lofted into the air, which is often indicative of a.

tornado emergency:

tornado family:

tornadogenesis:

tornado outbreak: The occurrence of multiple (typically at least six to ten) spawned by the same weather system, usually within the same day and in the same region.

tornado outbreak sequence:

A period of continuous or nearly continuous activity consisting of a series of spanning multiple days, with very few or no days lacking outbreaks.

tornado preparedness:

tornado vortex signature (TVS): A rotation algorithm detected by that indicates the likely presence of a strong such as a. Such signatures can be used to track the location and development of a tornadic rotation within a larger storm.

tornado warning:

tornado watch:

Tornado and Storm Research Organisation (TORRO):

TORRO scale:

Totable Tornado Observatory (TOTO):

trace: An amount of that is too small to reliably or accurately measure.

training:

tropical cyclone:

A very large, system characterized by a   surrounded by a closed low-level, strong, and continuous spiral bands of  that produce heavy. Tropical cyclones almost exclusively over and derive their strength from warm  seas. The strongest systems can last for more than a week, span more than 1600 km in diameter, and cause significant damage to coastal regions with powerful winds,, and concentrated precipitation that leads to. Depending on its location and strength, a tropical cyclone may be referred to by different names and categorized within a variety of classes.

tropical cyclone scales:

tropical cyclogenesis: The process by which a develops and strengthens within the atmosphere. The mechanisms governing cyclone formation in the tropics are distinct from those that govern the development of and.

tropical depression:

tropical disturbance:

tropical storm:

tropical wave:

tropics: The region of the Earth surrounding the Equator, generally delimited in latitude between the Tropic of Cancer (23°26' N) in the Northern Hemisphere and the Tropic of Capricorn (23°26' S) in the Southern Hemisphere.

tropopause: The boundary in the Earth's atmosphere between the and the, on average situated approximately 17 km above equatorial regions and 9 km above the polar regions.

troposphere: The lowest layer of the Earth's, within which nearly all phenomena occur. The troposphere contains approximately 75% of the atmosphere's total mass and 99% of its and. The average height of the troposphere above the Earth's surface varies between 6 and 18 km depending on latitude.

trough: An elongated region of relatively, often associated with a. Troughs may exist at the surface or aloft or both; the lifting of moist air by usually causes clouds and precipitation to follow immediately behind a trough. Under certain conditions, troughs may alternate with in a high-amplitude pattern.

trowal:

tsunami:

turbulence: Fluid motion characterized by chaotic changes in pressure and flow velocity, caused by excessive kinetic energy in parts of the fluid flow.

twilight: The indirect illumination of the lower atmosphere caused by the scattering of sunlight when the Sun itself is not directly visible because it is below the horizon. The time period during which such illumination occurs, either between astronomical and sunrise or between sunset and astronomical.

TWISTEX: An acronym for Tactical Weather-Instrumented Sampling in/near Tornadoes EXperiment.

typhoon: The local name for a that occurs in the northwestern Pacific Ocean, between 180° and 100°E in the Northern Hemisphere.

U
unstable air mass: Any with high, characterized by dramatic vertical.

updraft:

upper-air chart:

upper-air sounding:

upper-level low:

upper-level outflow:

upslope fog:

urban heat island (UHI): An urban or metropolitan area within which air temperatures are significantly warmer than in surrounding rural or uninhabited areas as a result of human activities, especially the artificial modification of land surfaces and the generation of waste heat by energy usage. Urban heat islands can greatly influence precipitation, air quality, and the likelihood of certain weather phenomena in the vicinity of large cities, though not all cities have a distinct urban heat island.

US Standard Atmosphere:

University Corporation for Atmospheric Research (UCAR):

V
valley breeze:

valley exit jet:

vertical draft: See.

vertically integrated liquid (VIL): An estimate of the total mass of contained in a, obtained by measuring the intensity of returned from the atmosphere.

vertical wind shear:

virga:

virtual temperature ($T_v$): The temperature of a moist at which a theoretical dry air parcel would have a total and  equal to those of the moist parcel.

visibility:

visual flight rules (VFR): A set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going, as opposed to, under which operation of the aircraft primarily occurs through referencing the onboard instruments rather than through visual reference to the ground and environs.

Von Kármán constant:

Von Kármán vortex street:

Von Kármán wind turbulence model:

vortex:

A region within a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices are a major component of and may be observed in many types of meteorological phenomena, including the winds surrounding a,, or.

vorticity:

W
wall cloud:

A large, localized, persistent, and often abrupt lowering of that develops beneath the surrounding base of a cloud and from which  sometimes form.

warm front: A type of located at the leading edge of a warmer as it overtakes a cooler air mass that is moving more slowly in the same direction. Warm fronts lie within broader of  than, which sometimes follow them, and the temperature difference between the air masses they separate is often greater. clouds,, and steady with occasional  often precede the boundary as it moves. In, warm fronts are symbolized by a red line with semicircles pointing in the direction of travel.

water vapor: Water in its gaseous state. Water vapor is ubiquitous in the atmosphere, being continuously generated by evaporation and removed by condensation, and plays a major role in numerous meteorological processes.

waterspout:

weak echo region (WER):

weather: The state of the at a given time and location. Weather is driven by a diverse set of naturally occurring phenomena, especially,, and differences between one place and another, most of which occur in the.

weather balloon:

A high-altitude balloon used to carry scientific instruments into the atmosphere, which then measure, record, and transmit information about meteorological variables such as,, , and by means of a or other measurement device, often one which is expendable. Weather balloons are only feasible in the lower atmosphere and typically do not exceed 40 km in altitude; higher parts of the atmosphere are generally studied with or.

weather bomb: See.

weather forecasting: The application of science and technology to predict the conditions of the at a given time and location. Weather forecasts are made by collecting quantitative data about the current state of the atmosphere at a given place and then using to project how the atmosphere will change. Forecasting is important to a wide variety of human activities, including business, agriculture, transportation, recreation and general health and safety, because it can be used to protect life and property.

weather front: See.

weather map: A map which displays various meteorological features across a particular area for a particular point or range of time. Weather maps often use symbols such as to conveniently present complicated meteorological data. They are used for both research and purposes.

weather modification:

Weather Prediction Center (WPC):

Weather Surveillance Radar (WSR): In the United States, WSR-1, WSR-57, WSR-74, and. In Canada, the Canadian weather radar network (WKR and CWMN).

weather reconnaissance:

weather satellite:

weather spotting: The act of observing, often on the ground, for the purpose of reporting to a larger group or organization, such as the U.S..

weather station: Any facility, either on land or at sea, with instruments and equipment for measuring atmospheric conditions in order to provide information for and to study the and/or.

weather vane:

An instrument (often an architectural ornament) used to indicate the of the.

Weatherwise: A photographically adorned general interest weather magazine that frequently publishes articles on and other.

wet-bulb temperature:

wet-bulb globe temperature:

wet season:

whirlwind: Any vertically oriented rotating of air that develops as a result of  created by heating and flow gradients. Examples include major whirlwinds such as,, and and minor whirlwinds such as  and.

wildfire:

willy-willy: See.

wind: The bulk movement of air within the Earth's. Wind occurs on a wide range of scales, from very strong flows lasting tens of minutes to milder local lasting a few hours to global  caused by the differential heating of the Equator and the poles and the Earth's rotation. Winds are often referred to by their strength and ; the many types of wind are classified according to their spatial scale, their, the types of forces that cause them, the regions in which they occur, and their effects.

wind chill:

A meteorological index that estimates the effect of on the perceived by humans, particularly the decrease in human body temperature attributable to the movement of cold air. There is no universally agreed-upon formula for measuring or calculating wind chill, though it is commonly reported as a. It is usually defined only for air temperatures at or below 10 C and wind speeds above 4.8 km/h.

wind direction: The direction from which a originates; e.g. a northerly wind blows from the north to the south. Wind direction is usually reported using cardinal directions or in azimuth degrees measured clockwise from due north. Instruments such as,, and are commonly used to indicate wind direction.

wind gradient:

wind gust: A brief increase in the of the, usually lasting less than 20 seconds. Gusts are more transient than. They are usually only reported by weather stations when the maximum or peak wind speed exceeds the average wind speed by 10 –.

wind profiler:

wind shear:

Any difference in and/or over a relatively short distance in the. Atmospheric wind shear is normally described as either or.

wind speed: The measured speed of the air comprising a. Changes in wind speed are often caused by being exposed to and  in the atmosphere. Wind speed is measured with an, but may also be less precisely classified using the.

windstorm: Any that produces or is characterized by very strong.

windsock:

winter:

winter storm:

Any which occurs during the local. Any meteorological event in which varieties of which can only occur at low temperatures are formed, such as,, or. Such events are not necessarily restricted to the winter season but may occur in late or early, or very rarely in the , as well.

winter waterspout:

World Meteorological Organization (WMO):

X
X band:

Y
yellow wind:

Younger Dryas:

Z
Z-R relation:

Zdr:

zastrugi: See.

zephyr:

zonal flow:

Zonda wind:

zud: