Wind terms, A to Z.

The reference height of 10 metres above open, level ground at which surface wind is conventionally measured and forecast. Quoting wind at one agreed height makes stations and models comparable, but it is not the wind at a rotor, mast or sail.

Hub heightWind shear

Five small ticks under the instrument's headline number, each a pass or fail check: observation age, whether the models sit on the same side of your threshold, ensemble spread, height fidelity and lead time. A hollow or ember tick names the weakness in the number.

Model agreementSpread

An upslope flow driven by daytime heating of a sunlit slope, which warms the adjacent air and makes it rise along the surface. It is the daytime counterpart of the katabatic wind and is usually light and shallow.

Katabatic windDiurnal cycle

An instrument that measures wind speed, and in some designs direction. Common types are the cup anemometer, the propeller vane and the sonic anemometer. Accuracy depends on siting, exposure, height above ground and the averaging period used.

Cup anemometerSonic anemometerAveraging period

The prevailing flow from the west and south-west that crosses the North Atlantic and reaches Ireland, steered by the jet stream and a procession of depressions. Wind direction is where the wind blows from, so these are winds arriving from the Atlantic.

Jet streamDepression

A measure of whether a parcel of air displaced vertically tends to return, stay or keep moving. Stable conditions suppress vertical mixing and tend to steepen the wind profile; unstable conditions mix momentum downward and favour gusts.

Wind shearBoundary layerConvective gust

The length of time over which wind speed samples are averaged to produce a reported value. WMO practice for sustained wind is a 10-minute mean; US practice commonly uses 1 minute. Values on different periods are not directly comparable.

Sustained windGust

A change in wind direction in the anticlockwise sense, for example from north-west to west to south-west. Backing winds are commonly associated with approaching depressions in the Northern Hemisphere. It is the opposite of veering.

VeerDepression

The weight of the atmosphere above a point, usually reported in hectopascals reduced to mean sea level so stations at different altitudes can be compared. Spatial differences in this pressure, not its absolute value, are what drive the wind.

Pressure gradientIsobar

A 0 to 12 scale describing wind by its observed effects on land or sea. Force 8, a gale, spans 34 to 40 knots; force 10, storm, 48 to 55 knots; force 12, hurricane force, is 64 knots or more.

GaleStorm forceHurricane forceRead: The Beaufort scale

The lowest part of the atmosphere, in which the ground directly influences the flow through friction, heating and cooling. Its depth varies from a few hundred metres on a stable night to a kilometre or more on a convective day.

Friction layerAtmospheric stabilityRead: How wind works: pressure, rotation, friction and height

A floating platform that records sea-surface observations such as wind, pressure, wave height and period. Buoy winds are measured at low heights, often 3 to 5 metres, so they may be adjusted to 10 m before comparison with models.

NDBCSignificant wave height

The degree to which forecast probabilities match observed frequencies. A calibrated forecast that gives 30 percent for exceeding a threshold should see it exceeded about three times in ten. Raw ensemble output is often under-dispersive and needs adjustment.

EnsembleExceedance probability

A condition of negligible wind, reported as zero or below the starting threshold of the instrument. Calm is a measured value and is distinct from missing data; The Wind Agent shows missing data as UNKNOWN, never as calm or zero.

UNKNOWNBeaufort scale

A short burst of strong wind produced by showers or thunderstorms, as downdrafts bring faster upper-level air to the surface and spread outward. These gusts can be abrupt and much stronger than the surrounding mean wind.

SquallGust

The apparent deflection of moving air caused by the rotation of the Earth, to the right in the Northern Hemisphere. It is why wind around a depression circulates anticlockwise rather than flowing straight towards low pressure.

Geostrophic windPressure gradientRead: How wind works: pressure, rotation, friction and height

Wind blowing roughly parallel to the coastline, neither onshore nor offshore. Direction is where the wind comes from, so a cross-shore wind arrives along the shore rather than from the sea or the land.

Onshore windOffshore wind

The component of wind perpendicular to a chosen track, such as a runway, a road or a course. It equals wind speed multiplied by the sine of the angle between wind direction and the track, so direction matters as much as speed.

HeadwindWind direction

An anemometer with three or four cups on a vertical shaft that rotate at a rate proportional to wind speed. Robust and widely used, it responds more quickly to increases than decreases in speed and so tends to over-read in gusty air.

AnemometerSonic anemometer

The difference between dry-bulb and wet-bulb temperature, used in crop spraying as a guide to evaporation and droplet drift risk. Ranges of roughly 2 to 8 °C are commonly cited as suitable for spraying. It is an air-dryness measure, not a wind measure.

Site timeLead time

A region of low surface pressure around which air circulates anticlockwise in the Northern Hemisphere. Depressions bring cloud, rain and strong wind to Ireland, with the strongest winds usually where isobars are tightly packed.

IsobarFrontNamed storm

A single forecast run from one best-estimate starting state, giving one value for each time and place. It carries no information about how likely that outcome is, which is why it is usually read alongside an ensemble.

EnsembleNWP

The regular daily variation in wind driven by solar heating. Over land, surface wind is typically stronger in the afternoon when mixing is deepest and weaker at night, while winds aloft can strengthen overnight in a low-level jet.

Low-level jetSea breeze

A 0 to 9 scale that describes the state of the sea by wave height, from calm and glassy to phenomenal. It describes the sea surface, not the wind, although the two are related through fetch and duration.

Sea stateSignificant wave height

The ensemble forecast of the European Centre for Medium-Range Weather Forecasts, running 51 members: one control and 50 perturbed. It samples uncertainty in the initial state and model physics to give probabilities rather than a single outcome.

ECMWF IFSEnsembleMember

The Integrated Forecasting System, the global numerical weather prediction model of the European Centre for Medium-Range Weather Forecasts. Its high-resolution deterministic run is widely regarded as among the most skilful medium-range guidance.

ECMWF ENSNWPDeterministic model

A set of forecasts of the same event, each started from slightly different conditions or using varied model physics. The spread of members indicates uncertainty, and the fraction crossing a threshold gives an exceedance probability.

MemberSpreadExceedance probability

The fifth-generation global reanalysis from ECMWF, produced for the Copernicus Climate Change Service. It provides hourly gridded estimates of the atmosphere from 1940 onwards at roughly 31 km resolution, and is a common basis for wind climatology.

ReanalysisResolution

The stored set of inputs behind a displayed value: which sources, model runs and observations were used, and when. It lets any figure be traced back and checked rather than taken on trust.

ProvenanceUNKNOWN

The instrument's decision chart: ensemble P10 to P90 and P25 to P75 bands around the median, the primary model as a line and your threshold as a dashed line. Hours where the probability of exceeding the threshold passes your alert level are marked on the axis.

Exceedance probabilityPercentileMeteogram

The probability that a quantity, such as wind speed or gust, will be greater than a chosen threshold during a given period. In an ensemble it is estimated as the share of members that exceed the threshold.

ThresholdEnsemble

Rapid deepening of a depression, commonly defined as a central pressure fall of at least 24 hPa in 24 hours at mid-latitudes. Such storms, informally called weather bombs, can produce damaging winds over a short period.

DepressionSting jet

The distance over open water across which wind blows in a consistent direction. Along with wind speed and duration, fetch controls how large wind-generated waves can grow, so a longer fetch supports a higher sea.

Wind seaSignificant wave height

The lowest layer of the atmosphere, in which friction with the surface slows the wind and turns it towards lower pressure. Within it, wind speed typically increases and direction veers with height.

Boundary layerVeerWind shear

The boundary between two air masses of different temperature and density. Passage of a front is typically marked by a shift in wind direction, a change in pressure trend, and often a short spell of stronger or gustier wind.

OcclusionDepressionVeer

Acceleration of wind as it is channelled through a gap, valley or strait between higher ground. Speeds in the constriction can be well above those in the open, depending on how closely the channel aligns with the wind direction.

Venturi effect

Wind of Beaufort force 8, with a mean speed of 34 to 40 knots. Marine warnings commonly treat gale force as the point at which sustained mean wind is expected to reach this range, though gusts will be higher.

Beaufort scaleStorm forceShipping ForecastRead: The Beaufort scale

The Global Ensemble Forecast System run by NOAA, comprising a control and 30 perturbed members. It gives a freely available probabilistic view that can be compared with other ensembles.

GFSEnsembleECMWF ENS

The theoretical wind that results when the pressure gradient force is exactly balanced by the Coriolis force, so that flow runs parallel to the isobars. It approximates the real wind above the friction layer.

Pressure gradientCoriolis effectIsobar

The Global Forecast System, the global deterministic weather model run by NOAA, with a resolution of about 13 km and four runs per day. Its output is freely available and widely used as model guidance.

GEFSNWPModel run

A location at which a numerical model calculates its values. Forecasts for a site are taken from nearby grid points and represent an area average, so local terrain and coastline finer than the grid are smoothed out.

ResolutionInterpolation

A brief increase in wind speed above the mean, typically lasting a few seconds. Gusts are generally reported as the highest speed over a short window, often 3 seconds, within the averaging period.

LullGust factorMETAR

The ratio of peak gust to mean wind speed over the same period. It is lower over open sea and higher over rough terrain or in unstable air, so a single commonly cited ratio should not be applied blindly.

GustRoughness lengthAtmospheric stability

A convection-permitting regional numerical weather prediction system, with a grid of about 2.5 km, used by several European weather services. Its finer grid resolves coastlines, showers and terrain better than global models.

NWPResolutionMet Éireann

The component of wind opposing a direction of travel. It equals wind speed multiplied by the cosine of the angle between the wind origin and the track, and is negative, a tailwind, when the wind is from behind.

CrosswindWind direction

A plot of the wind vector at successive heights, drawn as a curve from the surface upward. Its shape shows how speed and direction change with height, making veering and shear visible at a glance.

Wind shearVeer

The height of the centre of a wind turbine rotor above ground, commonly 80 to 150 metres for modern turbines. Wind speed at hub height is usually higher than at 10 m, so it must be estimated or measured at that level.

Power law10 m standard height

Beaufort force 12, a mean wind of 64 knots or more. In marine forecasts it denotes the top of the scale, and is separate from tropical hurricanes, which are a distinct type of storm.

Beaufort scaleStorm forceRead: The Beaufort scale

The Icosahedral Nonhydrostatic model run by the German weather service, DWD. It provides global and regional forecasts on an icosahedral grid and is a useful independent comparison against other global models.

NWPGFSECMWF IFS

The process of building the starting state of a forecast model from observations and a short previous forecast. Errors in the initial state grow with lead time, which is why uncertainty increases further ahead.

Model runLead time

Estimating a value at a point between known values, such as wind at 100 m from values at 80 m and 120 m, or at a site between grid points. It is only dependable when the quantity varies smoothly between the known points.

Grid pointPower lawLog wind profile

The span between the 25th and 75th percentiles of a set of values, containing the middle half of ensemble members. It is a robust measure of spread that is less affected by a few extreme members than the full range.

PercentileSpread

A line on a weather chart joining points of equal sea-level pressure. Closely spaced isobars indicate a steep pressure gradient and strong wind; widely spaced isobars indicate light wind.

Pressure gradientGeostrophic wind

A narrow band of very strong wind in the upper troposphere, commonly at 9 to 12 km, flowing broadly from west to east. It steers depressions across the Atlantic and its strength and position influence storm development.

DepressionAtlantic westerliesExplosive cyclogenesis

A downslope flow of air that has been cooled at the surface and becomes denser than its surroundings, typically at night or off ice and snow. It is the counterpart of the anabatic wind.

Anabatic windDiurnal cycle

A unit of speed used in road and some public weather contexts. One kilometre per hour equals about 0.540 knots or 0.278 metres per second, and one knot is exactly 1.852 km/h.

KnotMetres per secondMiles per hour

A unit of speed equal to one nautical mile per hour: exactly 1.852 km/h, or 0.5144 m/s. It is the standard unit for wind in marine, aviation and many meteorological contexts.

Metres per secondKilometres per hourMiles per hour

A light wind that blows from land towards the sea at night, when the land cools faster than the water. It is the nocturnal counterpart of the sea breeze and is usually weaker and shallower.

Sea breezeOffshore windDiurnal cycleRead: Sea breeze: the coastal wind that appears from nowhere

The interval between when a forecast was initialised and the time it predicts. Skill falls as lead time grows, and ensemble spread typically widens, so a value at 120 hours carries less confidence than one at 12 hours.

Model runInitialisationSpread

A stationary oscillation in the airflow downwind of a hill or mountain ridge, produced when stable air is forced over the barrier. It can bring alternating zones of strong and light wind in the lee.

Atmospheric stabilityVenturi effect

A remote-sensing instrument that infers wind speed and direction at several heights from laser light scattered by aerosols, using the Doppler shift. It can profile wind to hub height without erecting a tall mast.

SodarMet mastHub height

A model in which wind speed increases with the logarithm of height above ground, governed by surface roughness. It is most appropriate in neutral stability near the surface, and less reliable in strongly stable or unstable air.

Roughness lengthPower lawAtmospheric stability

A band of fast wind a few hundred metres above the ground, commonly forming at night as surface friction decouples from the air above. It can make wind at hub height much stronger than a surface reading suggests.

Wind shearDiurnal cycleHub height

A brief drop in wind speed below the mean, the counterpart of a gust. The difference between peak gust and lowest lull over a period is a measure of how gusty the wind is.

GustGust factor

An atmospheric research station on the Atlantic coast of Connemara, County Galway, exposed to air arriving directly from the ocean. Its long records make it a reference site for clean-sector Atlantic air and wind conditions.

Malin HeadAtlantic westerlies

The most northerly point of Ireland, in County Donegal, and the site of a long-running Met Éireann weather station. Its exposed position makes it a useful indicator of conditions off the north Atlantic coast.

Mace HeadMet ÉireannShipping Forecast

One individual forecast within an ensemble, run from a slightly different starting state or with perturbed model physics. Members are treated as equally plausible, and the distribution across them describes the forecast uncertainty.

EnsembleSpread

The Irish national meteorological service. It issues forecasts, observations, marine bulletins and colour-coded weather warnings, and names storms in cooperation with the UK Met Office and the Dutch service KNMI.

Status Orange and Red warningsNamed stormHARMONIE-AROME

A tall mast carrying anemometers and vanes at several heights, used to measure wind for site assessment. Measurements at hub height or close to it are more reliable than extrapolating from a 10 m reading.

AnemometerLidarHub height

A time-series chart of forecast variables at one location, such as wind speed, gust and direction against time. Shown with ensemble members or percentile bands, it conveys both the expected evolution and its uncertainty.

Exceedance fanEnsemble

A routine aviation weather report, usually issued half-hourly or hourly, with wind as a mean over 10 minutes. A gust group is added when peak wind exceeds the mean by 10 knots or more.

SYNOPGustAveraging period

The SI unit of speed, widely used in science and in many national forecasts. One metre per second equals about 1.944 knots or 3.6 km/h; one knot is 0.5144 m/s.

KnotKilometres per hourMiles per hour

A unit of speed used in everyday British and American contexts. One mile per hour equals about 0.869 knots, 1.609 km/h or 0.447 m/s. It is not the same as a knot, a nautical mile per hour.

KnotKilometres per hourMetres per second

The extent to which independent forecast models or ensembles predict similar outcomes. High agreement raises confidence, though models share some assumptions and data, so agreement is evidence of consistency rather than proof of correctness.

Agreement SpineSpread

One complete execution of a forecast model from a given initialisation time, commonly 00, 06, 12 or 18 UTC. A newer run incorporates more recent observations, but successive runs can differ, so the run time should be stated.

InitialisationLead timeGFS

A storm given a name in advance of its impact to improve public awareness of the warning. The names are chosen by meteorological services, and for Ireland this is done jointly by Met Éireann, the Met Office and KNMI.

Met ÉireannDepressionStatus Orange and Red warnings

The National Data Buoy Center, part of the US National Oceanic and Atmospheric Administration. It operates moored and drifting buoys and coastal stations, and publishes their observations of wind, pressure and waves.

BuoySignificant wave height

Numerical weather prediction: forecasting by solving the equations of atmospheric motion on a computer grid, starting from observed conditions. Models differ in resolution, physics and data assimilation, and so give different forecasts.

Deterministic modelEnsembleGrid point

A stage in the life of a depression when the cold front overtakes the warm front, lifting the warm sector off the ground. It generally marks a mature system, often with persistent cloud and rain.

FrontDepression

Wind blowing from land towards the sea. Because direction is where the wind comes from, an offshore wind arrives off the land; it can be gusty near the coast and can drive sea state in the opposite sense to an onshore wind.

Onshore windLand breezeFetch

Wind blowing from the sea towards land. Direction is where the wind comes from, so an onshore wind arrives off the water; it brings marine air with generally smoother flow and a steadier gust factor.

Offshore windSea breezeGust factor

An open weather data service that provides programmatic access to forecasts from several national and international models, as well as historical reanalysis data. It is used here as a source, with each value attributed to its model.

NWPERA5Provenance

The 10th, 50th and 90th percentiles of a distribution such as an ensemble. P50 is the median, while P10 and P90 bound the central 80 percent of outcomes and describe a plausible low and high case.

PercentileInterquartile range

The value below which a given percentage of a set of values falls. In an ensemble, the 90th percentile of wind speed is the value that 90 percent of members do not exceed.

P10, P50 and P90Interquartile rangeEnsemble

A simple relation for extrapolating wind speed with height, in which speed scales with the ratio of heights raised to a shear exponent. It is a practical approximation, but its accuracy depends on terrain and stability.

Shear exponent (alpha)Log wind profileHub height

The rate at which atmospheric pressure changes with distance. It is the force that sets air in motion: the steeper the gradient, shown by closer isobars, the stronger the wind.

IsobarGeostrophic windBarometric pressure

The recorded origin of a value: its source, model or station, run time and processing steps. Showing provenance lets a reader judge a figure by where it came from rather than only by its number.

Evidence recordUNKNOWN

A consistent historical reconstruction of the atmosphere produced by running a fixed model over past observations. It gives gridded wind and pressure for places and times without measurements, though local detail is smoothed.

ERA5Resolution

The spacing of a model grid, for example 9 km or 2.5 km. Finer resolution can represent coastlines, hills and showers more faithfully, but is not automatically more accurate at a specific point.

Grid pointHARMONIE-AROME

The average interval between events at least as severe as a stated level, for example a 50-year gust. It is a statistical expectation, not a schedule: a 50-year event may occur in consecutive years.

Weibull distributionExceedance probability

An elongated area of high pressure extending from an anticyclone. Ridges are generally associated with settled weather and lighter winds, though the flank facing a deepening depression can still have strong gradients.

TroughIsobar

A parameter describing how rough the surface is for airflow, being the height at which the logarithmic wind profile extrapolates to zero speed. Values are very small over open sea and larger over forest or towns.

Log wind profileGust factor

A daytime wind from the sea towards the land, caused by land heating faster than water. It typically begins late morning, strengthens in the afternoon and may be modest in strength, but can alter wind direction sharply near the coast.

Land breezeOnshore windDiurnal cycleRead: Sea breeze: the coastal wind that appears from nowhere

The general condition of the sea surface, described by wave height, period and direction, and combining local wind sea and any swell. It reflects recent and distant wind history, not only the wind at the moment.

Wind seaSwellDouglas sea scale

The exponent in the power law, describing how rapidly wind speed increases with height. A value near 0.14 is commonly cited for open, neutral terrain, but real values vary widely with surface and stability.

Power lawWind shearAtmospheric stability

The instrument's side-on cylinder pinned to one location, with a filament for the wind at 10, 80, 120 and 180 m and a ring at your working height. Filament length is speed and its angle shows where the wind blows; a violet smear shows ensemble spread.

Wind shearHodographHub height

The BBC Radio 4 broadcast of the Met Office marine forecasts for named sea areas around the British Isles, which include Irish areas such as Shannon, Malin and Fastnet. It uses Beaufort force, and warns of gales at force 8.

Beaufort scaleGaleMalin Head

A standard measure of sea state, defined as the mean height of the highest third of waves. Individual waves can be considerably larger, with the largest in a long record often about twice the significant height.

Wave periodSea stateBuoy

The local clock time at the place being forecast, as opposed to UTC or the viewer's own time zone. Showing site time avoids misreading the hour of a wind event, particularly around summer time changes.

Delta-TLead time

A ground-based remote sensor that uses sound pulses and the Doppler shift of their echoes to estimate wind speed and direction at several heights. It is generally limited to lower heights than lidar.

LidarMet mast

An anemometer with no moving parts that measures wind from the transit time of ultrasonic pulses between fixed transducers. It samples many times per second, so it resolves gusts and turbulence in fine detail.

AnemometerCup anemometerGust

The degree of disagreement among ensemble members, often measured as a standard deviation or a percentile range. Larger spread indicates greater forecast uncertainty and typically grows with lead time.

EnsembleInterquartile rangeLead time

A sudden, sharp increase in wind speed lasting minutes rather than seconds, often with a shift in direction and associated with showers or a front. Formal definitions require the rise to be large, commonly at least 16 knots, and sustained for about a minute.

Convective gustGustFront

The upper two levels of Met Éireann's colour-coded warning scheme, after Yellow. Orange signals potentially dangerous weather and Red signals very dangerous weather. They reflect expected impact as well as the forecast wind speed.

Met ÉireannNamed storm

A narrow core of very strong wind that can descend from the cloud head of a rapidly deepening depression into the boundary layer. It is rare, short-lived and difficult to forecast, but linked with some damaging storms.

Explosive cyclogenesisDepression

Beaufort force 10, a mean wind of 48 to 55 knots. Marine bulletins treat it as a distinct warning level above gale and severe gale, with gusts expected to be considerably higher.

Beaufort scaleGaleHurricane forceRead: The Beaufort scale

The mean wind speed over a standard averaging period. WMO practice is a 10-minute mean; US practice commonly uses 1 minute, which tends to give a higher figure. Gusts are reported separately.

Averaging periodGust

Waves that have travelled out of the area where the wind generated them, becoming smoother, longer in period and more regular. Atlantic swell can reach the Irish coast days after the distant storm that created it.

Wind seaWave periodSea state

A coded surface observation exchanged internationally by weather stations, typically at 3 or 6 hourly intervals. It reports wind as a mean, usually over the 10 minutes before the observation, with direction and speed.

METARAveraging period

A wind speed or gust value chosen as the line to be crossed, set by the user for their own purpose. A threshold is not a legal or regulatory limit; any commonly cited figure is a rule of thumb that depends on the activity and equipment.

Exceedance probabilityExceedance fan

An elongated area of relatively low pressure, extending from a depression. Troughs are often marked by a wind shift, cloud and showers as they pass, and may be sharper than a front.

RidgeDepressionFront

The state shown by The Wind Agent when a value is missing, stale or cannot be sourced. It is never displayed as zero, because zero wind is a real measurement and an absent one is not.

CalmProvenanceEvidence record

A change in wind direction in the clockwise sense, for example from south-west to west to north-west. Veering is typical after a cold front passes in the Northern Hemisphere, and with height in the friction layer.

BackingFrontFriction layer

The acceleration of a fluid as it is forced through a constriction. In wind terms it describes speed-up through gaps, passes and between buildings, although real terrain flow is more complex than the simple textbook case.

FunnellingLee wave

The time between successive wave crests passing a fixed point, usually in seconds. Short periods indicate local wind sea; long periods, often 10 seconds or more, indicate swell with greater energy.

SwellWind seaSignificant wave height

A two-parameter probability distribution, with scale and shape parameters, that commonly fits the frequency of wind speeds at a site. It is widely used to estimate energy yield and the likelihood of extremes.

Return periodWind rose

The direction the wind blows from, in degrees clockwise from true north, so a westerly is 270 degrees. This is the opposite of how water currents are named, and is the convention used throughout The Wind Agent.

VeerBackingWind rose

A circular diagram showing how often wind blows from each direction sector, often subdivided by speed. Each petal points to where the wind comes from, so a long petal to the west means frequent westerlies.

Wind directionWeibull distribution

Waves being generated by the local wind, which are short-crested, steep and irregular, and respond quickly when the wind changes. They contrast with swell, which arrives from distant storms.

SwellFetchSea state

The change in wind speed or direction over a distance, most often with height. Strong shear means conditions at 10 m can differ markedly from those at hub height or at the top of a mast.

Power lawLow-level jetShear Glass