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UK Extremes

High Temp
14.1° C
Topcliffe
Low Temp
-4.9° C
Aviemore
Precipitation
2.2 mm
Scilly, Saint Mary'S
Wind Gust
41.6 mph
Warcop Range

Weather Glossary

A plain-English reference for the weather terms you'll come across on this site and in general forecasts. Where a term has its own full page here, the definition links straight through to it.

 

A

Adiabatic
Describes a temperature change caused purely by a parcel of air expanding or compressing as it rises or sinks, with no heat added or removed from outside. It's the reason rising air cools and sinking air warms even on a day with no weather front anywhere nearby.
Advection
The horizontal transport of a property of the air — usually heat or moisture — simply by the wind carrying it from one place to another, as opposed to a local change caused by heating or cooling in place.
Air Mass
A huge body of air, often hundreds of miles across, that has taken on a fairly uniform temperature and moisture profile from sitting for days over a particular source region such as the Arctic, the tropics, or the open Atlantic.
Anticyclone
An area of high atmospheric pressure, with air sinking and spiralling outward (clockwise in the Northern Hemisphere). Usually brings settled, calmer weather than the low-pressure systems either side of it.
Aurora
A natural light display in the night sky, most often seen at high latitudes, caused by charged particles from the sun colliding with gases in the upper atmosphere. See our Aurora Activity page for the current UK forecast.

B

Backing
A wind direction change in an anticlockwise sense (e.g. from westerly to southerly). The opposite of veering.
Barometer
An instrument that measures atmospheric pressure, traditionally using a column of mercury or an evacuated metal capsule. A falling reading generally signals approaching unsettled weather.
Barometric Pressure
The weight of the atmosphere pressing down at a given point, usually quoted in hectopascals (hPa) or millibars (mb) — the two units are numerically identical.
Beaufort Scale
A scale from 0 (calm) to 12 (hurricane) describing wind strength by its observed effects on the sea or on land, devised by Francis Beaufort in 1805. See our full Beaufort Scale page.

C

Chinook / Föhn Effect
A warm, dry wind that develops on the downwind (lee) side of a mountain range, as air that has already dropped its moisture on the windward slopes descends and warms by compression.
Cirrus
Thin, wispy, high-altitude cloud made of ice crystals, often the first visible sign of an approaching warm front.
Cold Front
The leading edge of an advancing cold air mass, which undercuts warmer air ahead of it steeply and tends to bring a short, sharp band of rain or showers. See our Weather Fronts page.
Convection
The upward movement of warmer, less dense air, driven by surface heating — the basic engine behind cumulus cloud and thunderstorm development on a sunny day.
Coriolis Effect
The apparent deflection of moving air (and anything else moving freely across the Earth's surface) caused by the planet's rotation — to the right in the Northern Hemisphere, to the left in the Southern. It's why wind spirals around pressure systems rather than flowing straight from high to low pressure.
Cumulonimbus
The towering, anvil-topped cloud associated with thunderstorms, capable of producing heavy rain, hail, lightning and sometimes tornadoes.
Cyclone
In general meteorological use, simply another word for an area of low pressure (a depression). In the tropics specifically, "cyclone" is also the regional name for what's called a hurricane in the Atlantic or a typhoon in the Northwest Pacific.

D

Depression
An area of low atmospheric pressure, with air rising and spiralling inward (anticlockwise in the Northern Hemisphere), generally bringing cloud, wind and rain — the system responsible for most of the UK's unsettled weather.
Dew Point
The temperature air would need to cool to, at its current pressure, to become fully saturated and start condensing moisture out as dew. See our full Dew Point page.
Doldrums
A belt of light, unreliable winds near the equator where the trade winds of each hemisphere converge, historically notorious for becalming sailing ships.
Dry Line
A boundary between a moist air mass and a dry air mass, common across the central United States, along which severe thunderstorms often develop.

E

El Niño / La Niña
The warm (El Niño) and cool (La Niña) phases of a recurring, multi-year shift in Pacific Ocean surface temperatures that influences weather patterns worldwide, including a real but comparatively modest effect on European winters.
Evapotranspiration
The combined total of water evaporating from soil and open water plus water released by plants through their leaves — a key figure for gardeners and farmers working out irrigation needs. See our Evapotranspiration page.

F

Fog
Cloud that has formed at ground level, reducing visibility to below 1km, most often on calm, clear nights when the ground cools quickly and the air above it reaches its dew point.
Front
The boundary between two air masses of different temperature and moisture characteristics — see our full Weather Fronts page for the cold, warm, occluded and stationary types.
Frost
Ice crystals that form directly on surfaces when the air temperature at ground level falls below freezing and the dew point is also at or below 0°C, so moisture deposits straight as ice rather than condensing as liquid first.

G

Gale
Officially, sustained winds of Beaufort force 8 or above (39mph/34kts and upwards). Met Office gale warnings for shipping are issued specifically in these Beaufort terms — see our Beaufort Scale page.
Glaze
A smooth, clear coating of ice that forms when supercooled rain freezes on contact with a surface at or below 0°C — the cause of the most dangerous icy-road conditions.
Gust
A brief, sharp increase in wind speed lasting only a few seconds, as distinct from the sustained "mean" wind speed measured over a longer period.

H

Haar
A cold sea fog that rolls in off the North Sea onto the coasts of eastern Scotland and northeast England, typically in spring and summer when warm air passes over much colder water.
Halo
A ring of light around the sun or moon, caused by sunlight or moonlight refracting through ice crystals in high cirrostratus cloud — often taken as a sign that rain is on the way within a day or so.
Heat Index
A combined "feels like" figure that factors humidity into the effect of high temperature on the human body, since humidity slows sweat evaporation. See our Heat Index page.
Hoar Frost
Feathery ice crystals that form directly from water vapour on cold, exposed surfaces such as grass, car windscreens and branches, usually on clear, calm, cold nights.
Humidity
A general term for how much water vapour is in the air; almost always used to mean relative humidity, the percentage of the maximum the air could hold at its current temperature. See our full Humidity Explained page.
Hurricane / Tropical Cyclone
An intense, rotating tropical storm system with sustained winds of 74mph or more, fuelled by warm ocean water. Called a hurricane in the Atlantic and Northeast Pacific, a typhoon in the Northwest Pacific, and a cyclone elsewhere.
Hygrometer
Any instrument used to measure humidity directly, from a simple hair-tension dial to an electronic capacitive sensor.

I

Ice Pellets / Sleet
Small, translucent pellets of ice, formed when falling snowflakes partially melt and then refreeze on the way down through a shallow layer of sub-freezing air near the surface.
Inversion
A layer of the atmosphere where temperature increases with height, the reverse of the normal pattern. Inversions act like a lid, trapping fog, low cloud, and pollution close to the ground until the layer breaks up.
Isobar
A line on a weather map joining points of equal atmospheric pressure. Tightly packed isobars mean a steep pressure gradient and strong winds; widely spaced isobars mean light winds.
Isotherm
A line on a weather map joining points of equal temperature.

J

Jet Stream
A narrow, fast-moving ribbon of wind high in the atmosphere, formed where cold and warm air masses meet, which steers the succession of weather systems across the UK. See our Jet Streams Explained and Live Jet Stream Tracker pages.

K

Katabatic Wind
A wind caused by dense, cold air draining downhill under gravity, most noticeable on clear, calm nights in valleys and on hillsides.
Knot
A unit of speed equal to one nautical mile per hour (about 1.15mph), the standard unit for wind speed and boat speed in marine and aviation forecasting.

L

Lapse Rate
The rate at which air temperature falls with increasing height. The "dry" adiabatic lapse rate for unsaturated air is about 9.8°C per kilometre.
Lightning
A giant electrical discharge inside a thunderstorm, or between the storm and the ground, caused by charge separation within the cloud — see our UK Lightning page for the current live strike map.
Low Pressure
See Depression.

M

METAR
A standardised, coded weather report issued every hour (or half-hour) by airports worldwide, covering wind, visibility, cloud, temperature, dew point and pressure. See our Metars Lookup page.
Microclimate
A small, localised area where the climate differs noticeably from its immediate surroundings — a sheltered south-facing garden, an urban heat island, or a frost-prone valley bottom are all examples.
Millibar / Hectopascal
The standard unit of atmospheric pressure used on weather charts; one millibar (mb) equals one hectopascal (hPa), and average sea-level pressure is around 1013hPa.
Mist
Like fog, but by definition slightly less dense — official visibility is 1km or more, but still noticeably reduced by suspended water droplets.
Monsoon
A seasonal reversal in prevailing wind direction, most famously over South Asia, that brings a pronounced wet season as moisture-laden wind blows in from the ocean for months at a time.

O

Occluded Front
The combined boundary formed when a faster-moving cold front catches up with the warm front ahead of it, lifting the warm air between them clear off the ground. See our Weather Fronts page.
Orographic Rainfall
Rain produced when moist air is forced to rise over high ground, cooling and condensing as it goes — the reason hills and mountains are so often wetter than the surrounding lowlands.
Ozone
A gas made of three oxygen atoms; in the stratosphere it forms a protective layer that absorbs harmful ultraviolet radiation, while at ground level it's a pollutant and a component of summer smog.

P

Precipitation
The general meteorological term for any water falling from the sky, in whatever form — rain, drizzle, snow, sleet or hail.
Pressure Gradient
The rate of change of atmospheric pressure over distance; a steep gradient (isobars packed close together) drives strong wind, while a gentle gradient produces light wind.
Psychrometer
A humidity-measuring instrument using two thermometers side by side, one with a wet wick; the cooling effect of evaporation from the wet bulb, compared with the dry bulb, gives relative humidity from a standard table.

Q

QNH / QFE
Aviation pressure settings: QNH is atmospheric pressure adjusted to mean sea level, so an altimeter reads true altitude above sea level; QFE is the actual pressure at the airfield itself, so an altimeter set to it reads height above the airfield.

R

Radiation Fog
Fog that forms overnight as the ground loses heat rapidly under clear skies and light winds, cooling the air just above it to its dew point — the classic cause of valley and coastal fog around Southampton on still winter mornings.
Rain Shadow
A drier region on the lee (downwind) side of high ground, where descending air has already lost most of its moisture on the windward slopes.
Ridge
An elongated extension of high pressure, generally bringing a spell of settled, drier weather as it passes over an area, even without a fully separate anticyclone forming.

S

Sea Breeze
A local wind that blows from sea to land on sunny days, as the land heats up faster than the sea and the rising warm air over land draws in cooler air off the water — a regular feature along the Solent coast in summer.
Shipping Forecast
The Met Office's specialist marine forecast for UK and near-continental waters, broadcast on BBC Radio 4 and divided into named sea areas such as Wight, Portland and Plymouth. See our Shipping Forecast page.
Squall
A sudden, sharp increase in wind speed, usually accompanied by a shower or thunderstorm, that arrives and passes within a matter of minutes.
Stratosphere
The layer of the atmosphere above the troposphere, roughly 10-50km up, where temperature increases with height and the ozone layer sits — also where the polar-front jet stream has its roots.
Sublimation
The direct change of a substance from solid to gas (or gas to solid) without passing through a liquid phase — how frost forms directly from water vapour, and how snow can slowly vanish on a cold, dry, sunny day without ever melting.
Sundog
A bright spot of light appearing to one or both sides of the sun, caused by sunlight refracting through flat, hexagonal ice crystals in high cirrus cloud.
Synoptic Chart
A weather map showing the large-scale pattern of pressure systems, fronts and wind at a single point in time — the standard chart used in daily forecasting.

T

Thermal
A rising column or bubble of warm air, generated by uneven surface heating, that gliders and birds of prey use to gain height without flapping.
Thunder
The sound produced by air rapidly expanding along a lightning channel's superheated path; counting the seconds between a lightning flash and the thunder that follows gives a rough distance to the strike, about one mile for every five seconds.
Tornado
A violently rotating column of air extending from a thunderstorm base to the ground, capable of severe localised damage; the UK sees a surprising number each year, though almost all are far weaker than the strongest American tornadoes.
Trade Winds
The steady easterly winds blowing towards the equator in the tropics of both hemispheres, historically used by sailing ships crossing the Atlantic and Pacific.
Tropopause
The boundary between the troposphere and stratosphere, where the temperature stops falling with height — roughly 10-12km up in mid-latitudes, and the rough upper limit of most weather and cloud.
Troposphere
The lowest layer of the atmosphere, extending up to the tropopause, where almost all everyday weather — cloud, rain, wind, storms — actually happens.
Trough
An elongated area of lower pressure extending from a larger low, typically bringing a band of cloud, wind and rain as it passes through, even without a fully closed depression.

U

UV Index
A scale, typically 0-11+, describing the strength of ultraviolet radiation reaching the ground at a given time and place, used to judge sunburn risk and the need for sun protection.

V

Veering
A wind direction change in a clockwise sense (e.g. from southerly to westerly). The opposite of backing.
Virga
Streaks of rain or snow visible falling from a cloud base that evaporate completely before reaching the ground, often seen beneath high-based clouds over warm, dry air.
Visibility
The greatest horizontal distance at which an object can still be clearly seen against the sky, reduced by fog, mist, haze, heavy rain or snow.

W

Warm Front
The leading edge of an advancing warm air mass, which slides gradually up and over retreating cold air ahead of it, bringing thickening cloud and prolonged, steadier rain. See our Weather Fronts page.
Warm Sector
The wedge of relatively mild, moist air sitting between the warm front and cold front of a mature depression.
Waterspout
A rotating column of air over water, functionally similar to a tornado, most common over warm coastal waters in late summer.
Wind Chill
A "feels like" figure combining air temperature and wind speed, reflecting how much faster the body loses heat in windy conditions compared with still air at the same temperature.
Wind Rose
A circular diagram showing how often wind blows from each compass direction at a location, often combined with typical speed for each direction.
Wind Shear
A change in wind speed and/or direction over a short distance, either horizontally or with height — a key ingredient for severe, rotating thunderstorms, and a hazard pilots watch for on approach and departure.

Z

Zephyr
A traditional poetic term for a gentle, mild breeze, originally named after Zephyrus, the Greek god of the west wind.
Zonal Flow
A weather pattern where the jet stream and its embedded systems run in a fairly straight, unbuckled line from west to east, typically bringing a fast-moving, changeable sequence of Atlantic weather to the UK. Compare with a blocked or "meridional" pattern — see our Jet Stream Tracker page.

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