Meteopedia — Weather glossary

Ábrego

A warm, moist southwesterly wind on the Iberian Peninsula, associated with rainfall on the Atlantic slope.

Wind

Acid rain

Precipitation with pH below 5.6 caused by dissolution of atmospheric pollutants such as sulphur dioxide and nitrogen oxides.

Precipitation

Adiabatic lapse rate

Rate at which air changes temperature as it rises or descends without exchanging heat with its surroundings: ~10 °C/km for dry air and ~6 °C/km for saturated air.

General

Advection

Horizontal transport of heat, moisture, or other atmospheric properties by the wind, a key process in weather evolution.

General

Advection fog

Fog that forms when warm, moist air moves over a colder surface.

Atmospheric phenomena

Air mass

Extensive volume of air with uniform temperature and humidity characteristics acquired in its source region.

General

Airborne pollen

Pollen grains suspended in air from plants, trees, and grasses, the main cause of seasonal respiratory allergies such as hay fever and allergic asthma.

Air quality

Albedo

Fraction of solar radiation reflected by a surface, expressed from 0 (absorbs all) to 1 (reflects all).

Climatology

Altocumulus

Mid-level cloud in the form of white and grey patches, rolls or flakes, often in regular rows.

Clouds

Altostratus

Mid-level grey cloud that covers the sky like a veil, letting the sun show through diffusely.

Clouds

AMOC

Atlantic Meridional Overturning Circulation: a system of currents that transports warm water northward and cold water southward in the Atlantic Ocean.

Climatology

Anabatic wind

Wind that ascends mountain slopes heated by the sun during the day.

Wind

Anemometer

Instrument that measures wind speed, typically using spinning cups or ultrasonic sensors.

Instruments

Anti-trade winds

Upper-level winds blowing from the equator towards the poles above the trade winds, completing the return flow of the Hadley cell.

Wind

Anticyclone

High-pressure system where descending air produces stable weather and clear skies.

Atmospheric phenomena

Apparent temperature

Temperature perceived by the human body, differing from actual air temperature due to wind (wind chill) or humidity (heat index) effects.

Temperature

AQI

Air Quality Index that converts pollutant concentrations (PM2.5, PM10, O₃, NO₂, SO₂) into a numerical scale from 0 to 500 to inform the public.

Scales and measurements

Arctic amplification

Phenomenon whereby the Arctic warms 2-4 times faster than the global average, with profound consequences for atmospheric circulation.

Climatology

Arcus cloud

Accessory cloud in the form of a horizontal arch or roll appearing at the leading edge of intense storms.

Clouds

Aridity index

Numerical indicator classifying climatic zones according to the ratio between precipitation and potential evapotranspiration.

Climatology

AROME model

High-resolution (1.3 km) numerical prediction model from Météo-France, specialised in convection and mesoscale phenomena over western Europe.

Instruments

Asperitas

Cloud type officially recognised by the WMO in 2017, with a chaotic, wave-like underside resembling a rough sea viewed from below.

Clouds

Atmosphere

Gaseous layer surrounding the Earth, composed mainly of nitrogen (78%) and oxygen (21%), divided into concentric layers by their thermal profile.

General

Atmospheric aerosol

Solid or liquid particles suspended in air, of natural (dust, sea salt, pollen) or anthropogenic (combustion, industry) origin, affecting climate and air quality.

Air quality

Atmospheric corona

A luminous ring of colours appearing around the sun or moon, caused by diffraction of light by water droplets or ice crystals.

Atmospheric optics

Atmospheric pressure

Force exerted by the weight of the air column on the Earth's surface, measured in hectopascals (hPa).

General

Atmospheric sounding

A vertical profile of the atmosphere obtained by radiosonde, showing temperature, humidity, and wind at different altitudes.

Instruments

Atmospheric stability

Tendency of the atmosphere to resist or enhance vertical air motion: a stable atmosphere suppresses convection, an unstable one promotes it.

General

Aurora borealis

Luminous phenomenon in the night sky caused by the interaction of solar particles with Earth's magnetosphere, visible at high latitudes.

Atmospheric phenomena

Automatic weather station

Facility that measures and transmits meteorological data autonomously, without human intervention, forming the core observation network.

Instruments

Avalanche

Mass of snow that detaches and slides downhill at high speed, capable of burying people, destroying infrastructure, and reaching speeds of 200 km/h.

Hazards and safety

Banner cloud

Cloud forming attached to an isolated mountain summit and extending leeward like a flag, caused by pressure drop and condensation in the lee vortex.

Clouds

Barometer

Instrument that measures atmospheric pressure, essential for weather forecasting.

Instruments

Beaufort scale

Empirical scale from 0 to 12 that classifies wind intensity by its observable effects.

Scales and measurements

Belt of Venus

Pink band visible on the horizon opposite the sun during twilight, caused by backscattering of sunlight in the atmosphere.

Atmospheric optics

Black frost

A frost that destroys plant tissue without leaving visible ice deposits, caused by very dry air at sub-zero temperatures.

Temperature

Blizzard

Snowstorm with strong winds that drastically reduces visibility and produces significant accumulations.

Atmospheric phenomena

Bora

Violent, cold, gusty katabatic wind descending from the Balkan interior plateaux toward the Adriatic Sea coast.

Wind

Brocken spectre

Optical phenomenon in which the observer's shadow is projected onto fog or low clouds, surrounded by a rainbow-coloured halo.

Atmospheric optics

CAPE

Convective Available Potential Energy: a measure of the energy a rising air parcel can release, used to estimate the potential for severe thunderstorms.

General

Carbon footprint

Total amount of greenhouse gases emitted directly and indirectly by a person, organisation, event, or product, expressed in tonnes of CO₂ equivalent.

Climatology

Carbon monoxide

Toxic, odourless gas (CO) produced by incomplete combustion of fuels, reducing the blood's ability to carry oxygen.

Air quality

Castellanus cloud

Cloud variety with vertical turrets or crenellations rising from a common base, indicating instability in the mid-levels of the atmosphere.

Clouds

Cavum

Circular or elliptical hole in a mid or high cloud layer caused by local glaciation of supercooled droplets, also known as a fallstreak hole or hole-punch cloud.

Clouds

Ceilometer

Instrument measuring cloud base height using laser pulses, essential at airports and weather stations.

Instruments

Chilling hours

Accumulation of hours with temperature below 7 °C during winter dormancy, needed for temperate fruit trees to break dormancy and flower properly.

Agrometeorology

Chinook

Warm, dry wind descending the eastern slopes of the Rocky Mountains in North America, capable of raising temperatures by tens of degrees in hours.

Wind

Cierzo

Cold, dry northwesterly wind channelled through Spain's Ebro Valley.

Wind

Circumzenithal arc

A brightly coloured arc appearing in the sky above the sun, formed by refraction through ice crystals.

Atmospheric optics

Cirrocumulus

High cloud formed by small regular white tufts, giving a "mackerel sky" appearance.

Clouds

Cirrostratus

High, thin cloud forming a whitish, semi-transparent veil of ice crystals that partially or fully covers the sky and often produces solar or lunar halos.

Clouds

Cirrus

High cloud (6,000–12,000 m), thin and fibrous, composed entirely of ice crystals.

Clouds

Clear-air turbulence

Atmospheric turbulence occurring in clear skies with no visual cues, the principal invisible hazard for aviation.

Atmospheric phenomena

Climate change

Significant, lasting change in global climate patterns, currently driven by greenhouse gas emissions.

Climatology

Climate normal

Average of a meteorological variable calculated over a standard 30-year period (currently 1991–2020), used as a reference to compare current weather.

Climatology

Climate sensitivity

Global warming resulting from doubling atmospheric CO₂ concentration from pre-industrial levels, estimated at 2.5–4 °C.

Climatology

Climate tipping point

Critical threshold in the climate system beyond which a change becomes self-sustaining and irreversible, such as Greenland ice collapse or Amazon dieback.

Climatology

Cloud ceiling

Height of the base of the lowest cloud layer covering more than half the sky, critical aviation data determining visual or instrument flight conditions.

Clouds

Coastal upwelling

Rise of deep, cold, nutrient-rich water to the surface near the coast, driven by winds that push surface water offshore.

Marine meteorology

Cold front

Frontal boundary where cold air pushes under and lifts warm air, producing storms and a sharp temperature drop.

Atmospheric phenomena

Cold wave

Prolonged period of abnormally low temperatures for a given region and time of year, caused by the intrusion of polar or Arctic air masses.

Temperature

Condensation

The process by which water vapour changes to liquid state upon cooling below its saturation point.

Precipitation

Continental climate

Climate type with large temperature contrasts between summer and winter, moderate precipitation, and distance from oceanic influence.

Climatology

Contrail

An artificial line-shaped cloud created by aircraft exhaust gases at high altitude.

Clouds

Convection

Vertical transport of heat and moisture in the atmosphere through the rise of warm air and descent of cold air, driving cumuliform clouds and thunderstorms.

General

Convective precipitation

Precipitation generated by rapid ascent of warm, moist air in convective currents, typically intense, localised, and short-lived.

Precipitation

Coriolis effect

Apparent deflection of moving objects on Earth caused by the planet's rotation: to the right in the Northern Hemisphere and to the left in the Southern.

General

Crepuscular rays

Diverging beams of sunlight visible when rays filter between clouds or mountains, made visible by scattering off atmospheric particles.

Atmospheric optics

Cumuliform cloud

A type of vertically developing cloud with a puffy appearance and flat base, formed by convective updrafts.

Clouds

Cumulonimbus

Towering cloud of enormous vertical development, associated with thunderstorms, lightning, hail, and heavy rain.

Clouds

Cumulonimbus incus

Mature stage of the cumulonimbus with the characteristic anvil top, indicative of severe storms with hail, lightning, and strong winds.

Clouds

Cumulus

Vertically developing cloud with a flat base and cauliflower-shaped top, typical of fair weather or storm development.

Clouds

Cumulus congestus

Towering cumuliform cloud with cauliflower-shaped turrets that can produce showers but has not yet developed an anvil or electrical activity.

Clouds

Cumulus humilis

Small cumuliform cloud of limited vertical extent, wider than tall, indicating fair weather and atmospheric stability.

Clouds

Damaging hail

Hailstones large enough to damage crops, vehicles, and infrastructure, generally exceeding 1–2 cm in diameter.

Agrometeorology

DANA

Cut-off low: a cold air mass at upper levels detached from the jet stream, capable of triggering torrential rain.

Atmospheric phenomena

Depression

Low-pressure system associated with unstable weather, clouds, and precipitation.

Atmospheric phenomena

Derecho

Line of storms that produces a swath of destructive straight-line winds over hundreds of kilometres.

Cyclones and severe storms

Desert climate

Climate type with annual precipitation below 250 mm, large diurnal temperature range, and absent or very sparse vegetation.

Climatology

Dew

Water droplets that form on cold surfaces when the temperature drops to the dew point.

Precipitation

Dew point

Temperature at which air must cool for water vapour to condense into droplets (100 % relative humidity).

Temperature

Disdrometer

Instrument that measures the size, speed, and distribution of raindrops or precipitation particles, enabling hydrometeor classification.

Instruments

Diurnal temperature range

Difference between maximum and minimum temperatures recorded in a given period (usually a day), indicative of climate type and maritime influence.

Temperature

Double rainbow

Optical phenomenon with a second, fainter rainbow above the primary with inverted colours, caused by a double internal reflection of light inside raindrops.

Atmospheric optics

Douglas sea scale

Scale from 0 to 9 classifying sea state by wave height, from calm (0) to phenomenal (9, waves over 14 metres).

Marine meteorology

Downburst

A concentrated downdraft that produces destructive winds upon impacting the ground, with a damage area exceeding 4 km.

Cyclones and severe storms

Downdraft

Column of rapidly descending air within or beneath a convective storm, capable of producing damaging winds when it hits the ground.

Atmospheric phenomena

Drizzle

Uniform precipitation of very fine droplets (< 0.5 mm diameter) that fall slowly.

Precipitation

Drop in cold air

Popular Spanish term for torrential rain events in the Mediterranean, associated with upper-level cold air over warm seas.

Atmospheric phenomena

Drought

A prolonged period of below-normal precipitation causing water scarcity and affecting ecosystems, agriculture, and populations.

Climatology

Dry haze

Suspension of mineral dust particles in the atmosphere reducing visibility without significant moisture, distinct from mist or fog.

Atmospheric phenomena

Dry line

A boundary between a dry air mass and a moist air mass at the surface, frequently associated with severe thunderstorm formation.

Atmospheric phenomena

Dust devil

Rotating column of rising air that picks up dust and debris from the ground, formed by intense local heating without connection to thunderstorms.

Atmospheric phenomena

Early warning

System combining weather monitoring, forecasting, and communication to warn the population before a dangerous phenomenon strikes, minimising casualties and damage.

Hazards and safety

ECMWF model

Global numerical prediction model from the European Centre (ECMWF), considered the world's most accurate, with 9 km resolution and forecasts up to 15 days.

Instruments

El Niño

Warm phase of the ENSO cycle: anomalous warming of the equatorial Pacific that disrupts global climate.

Climatology

Enhanced Fujita scale

Scale from EF0 to EF5 that classifies tornado intensity based on observed damage.

Scales and measurements

Ensemble forecast

A forecasting technique that runs multiple weather model simulations with slightly different initial conditions to estimate uncertainty.

Instruments

Equinox

Moment of the year when day and night are approximately equal in duration, marking the start of spring or autumn.

Meteorological astronomy

Etesian winds

Dry, persistent northerly winds blowing over the Aegean Sea and eastern Mediterranean during summer, also known as meltemi.

Wind

Evaporation

The process by which liquid water transforms into water vapour by absorbing heat energy.

Precipitation

Evapotranspiration

The total amount of water transferred to the atmosphere by soil evaporation and plant transpiration combined.

Precipitation

Exosphere

Outermost layer of Earth's atmosphere, above 600 km, where gas particles are so sparse they can escape to space without colliding.

General

Explosive cyclogenesis

Rapid deepening of an extratropical cyclone whose central pressure drops at least 24 hPa in 24 hours, generating hurricane-force winds. Also called a bomb cyclone.

Atmospheric phenomena

Extratropical cyclone

Low-pressure system forming outside the tropics from the contrast between cold and warm air masses, the main weather driver in mid-latitudes.

Atmospheric phenomena

Eye of the hurricane

Circular zone of calm at the centre of a tropical cyclone, with partly clear skies, light winds, and the lowest pressure in the system.

Cyclones and severe storms

Fetch

Uninterrupted distance of open water over which wind blows in a constant direction, determining the maximum wave size that can develop.

Marine meteorology

Field capacity

Maximum amount of water a soil can retain against gravity after excess has drained, the optimal moisture point for plant growth.

Agrometeorology

Fire whirl

Fire vortex generated when a wildfire creates a convective column intense enough to produce rotation, with winds that can exceed 200 km/h.

Atmospheric phenomena

Flash flood

Sudden flood caused by intense rainfall in a short period, overwhelming watercourses, dry riverbeds, and drainage systems within minutes or hours, with great destructive power.

Hazards and safety

Flood zone

Geographic area susceptible to flooding from river or coastal surges with a given return period, regulated by urban planning and water legislation.

Hazards and safety

Foehn effect

Warming and drying of air as it descends the leeward slope of a mountain after losing moisture as precipitation on the windward side.

Wind

Fog

Cloud at ground level that reduces visibility to less than 1,000 metres.

Atmospheric phenomena

Fogbow

An arc similar to a rainbow but white in colour, formed by refraction and reflection of light in very small fog droplets.

Atmospheric optics

Fractus cloud

Irregular, ragged cloud fragments forming beneath rain clouds or in strong winds, with constantly changing edges.

Clouds

Freezing fog

Fog consisting of supercooled water droplets that freeze on contact with cold surfaces, depositing a layer of crystalline ice (rime).

Atmospheric phenomena

Freezing level

Altitude at which air temperature drops to 0 °C, determining the snow line and the type of precipitation reaching the ground.

Precipitation

Freezing rain

Rain that freezes on contact with surfaces below 0 °C, forming a transparent ice layer.

Precipitation

Frontal precipitation

Rain or snow generated by the lifting of warm air over cold air along a weather front, generally widespread and persistent.

Precipitation

Frost

Drop in air or surface temperature below 0 °C, with possible ice formation.

Temperature

FWI index

Fire weather index combining temperature, relative humidity, wind speed, and accumulated drought to assess wildfire spread risk.

Hazards and safety

Gale

Very strong wind with sustained speed of 62–117 km/h (Beaufort force 8–11), capable of damaging trees, roofs, and power lines.

Atmospheric phenomena

Galerna

A sudden, violent northwesterly squall occurring over the Bay of Biscay, with abrupt temperature drops and hurricane-force winds.

Wind

Garbí

South-westerly breeze along the Spanish Mediterranean coast, especially in Catalonia and Valencia, associated with warm, humid summer afternoons.

Wind

Geopotential height

Altitude of a constant atmospheric pressure surface (500 hPa, 850 hPa, etc.) used in upper-air charts to identify troughs, ridges, and the jet stream.

General

Geostrophic wind

Theoretical wind that would blow parallel to isobars when the pressure gradient force and Coriolis force are in perfect balance.

Wind

GFS model

Global numerical weather prediction model from the US weather service (NOAA), generating forecasts up to 16 days at 13 km resolution, publicly available.

Instruments

Glory

Optical phenomenon producing concentric coloured rings around the observer's shadow projected onto clouds or fog.

Atmospheric optics

Graupel

Precipitation in the form of soft, opaque pellets of rimed snow, intermediate between snow and hail, 2-5 mm in diameter.

Precipitation

Green flash

A brief flash of green light appearing at the upper rim of the solar disc during sunrise or sunset.

Atmospheric optics

Greenhouse effect

Natural process by which certain atmospheric gases trap part of the heat radiated by Earth's surface.

Climatology

Gregal

North-easterly wind in the western Mediterranean, common in the Balearic Islands and Catalonia, bringing cold, moist air with rain and swell.

Wind

Growing degree days

Unit of thermal accumulation measuring heat available for organism growth, calculated as the daily difference between mean temperature and a base temperature.

Agrometeorology

Growing season

Part of the year with temperatures favourable for plant growth, generally defined as days with mean temperature above 5 or 10 °C.

Agrometeorology

Gust front

The surface boundary between the cold outflow air from a thunderstorm downdraft and the surrounding warm air.

Atmospheric phenomena

Gustnado

Weak, short-lived vortex forming along a thunderstorm gust front, without direct connection to the cloud base, weaker than a tornado.

Cyclones and severe storms

Haboob

Intense sandstorm or dust storm driven by the outflow of a convective thunderstorm, forming a wall of dust up to 1,500 metres high.

Wind

Hadley cell

Tropical atmospheric circulation cell where air rises at the equator, moves poleward aloft, and descends at subtropical latitudes (~30°).

Atmospheric phenomena

Hail

Solid precipitation in the form of ice balls or irregular lumps that fall from intense convective clouds.

Precipitation

Hailstorm

An episode of hail precipitation, usually associated with severe thunderstorms with strong updrafts.

Atmospheric phenomena

Harmattan

Dry, dusty wind from the Sahara blowing towards the Gulf of Guinea between November and March, reducing visibility and cooling coastal temperatures.

Wind

Haze

Suspension of dust, sand or organic particles in the atmosphere that reduces visibility and gives the sky a yellowish tone.

Atmospheric phenomena

Heat action plan

Health action plan activated when maximum temperature thresholds are exceeded for several days, with protective measures for vulnerable groups.

Hazards and safety

Heat index

A measure of apparent temperature combining air temperature and relative humidity to indicate how the human body perceives heat.

Temperature

Heatstroke

Medical emergency caused by failure of the body's thermoregulation when core temperature exceeds 40 °C, potentially fatal.

Temperature

Heatwave

Extended period of abnormally high temperatures for the season and region.

Temperature

Hoarfrost

Deposit of ice crystals forming on cold surfaces when water vapour sublimates directly to solid, typical of clear, calm nights.

Atmospheric phenomena

Humidex

Canadian index combining temperature and humidity to express perceived thermal sensation: values above 40 indicate considerable discomfort.

Scales and measurements

Hurricane

Tropical cyclone with sustained winds of 119 km/h or more, formed over warm tropical oceans.

Cyclones and severe storms

Hydrometeor

Any water particle, liquid or solid, in the atmosphere: includes rain, snow, hail, fog, dew, frost, and all forms of precipitation.

Precipitation

Hygrometer

Instrument that measures air humidity (relative or absolute).

Instruments

Ice age

Prolonged geological period of low global temperatures with expansion of ice caps and glaciers, regulated by Milankovitch cycles.

Climatology

Ice pellets

Precipitation as transparent ice pellets 1–5 mm in diameter that bounce on impact, formed when snow partially melts then refreezes during its fall.

Precipitation

Ice-albedo feedback

Positive feedback loop where melting ice reduces surface albedo, increases solar radiation absorption, and accelerates warming and further melting.

Climatology

Icing

Accumulation of ice on surfaces exposed to wind when supercooled water droplets or freezing rain freeze on contact, hazardous for aviation and infrastructure.

Atmospheric phenomena

Intertropical Convergence Zone

An equatorial low-pressure belt where the northern and southern trade winds converge, generating intense convection and rainfall.

Climatology

Iridescent cloud

Cloud displaying bright rainbow-like colours at its edges, produced by diffraction of sunlight passing through uniformly sized water droplets or ice crystals.

Clouds

Isobar

Line on a weather map connecting points of equal atmospheric pressure.

General

Isohyet

Line on a map connecting points of equal accumulated precipitation.

General

Isotherm

Line on a map connecting points of equal temperature.

General

Jet stream

Very strong wind current in the upper troposphere (9–12 km) that steers depressions and anticyclones.

Wind

Katabatic wind

Cold wind that flows downhill driven by gravity, common in mountain areas and polar regions.

Wind

Kelvin

SI unit of temperature. 0 K is absolute zero (−273.15 °C).

Scales and measurements

Kelvin-Helmholtz wave cloud

Cloud formation with rolling wave crests resembling breaking ocean waves, caused by Kelvin-Helmholtz instability between air layers of different velocities.

Clouds

Köppen classification

A worldwide climate classification system based on monthly temperature and precipitation, created by Wladimir Köppen.

Climatology

La Niña

Cold phase of the ENSO cycle: anomalous cooling of the equatorial Pacific that disrupts global climate.

Climatology

Lake-effect snow

Phenomenon whereby cold air masses crossing large warm lakes or seas generate intense bands of snow or showers on the downwind shore.

Atmospheric phenomena

Landslide

Downhill movement of a mass of soil, rock, or debris along a slope, triggered by heavy rainfall, earthquakes, or human activity.

Hazards and safety

Landspout

Weak tornado forming without a mesocyclone, through vertical stretching of surface vorticity beneath a developing convective cloud.

Atmospheric phenomena

Late frost

Frost occurring after the start of the growing season in spring, causing serious damage to fruit trees, vineyards, and vegetables that have already budded.

Agrometeorology

Lebeche

Warm, moist south-westerly wind along the Spanish Mediterranean coast, associated with unstable weather, rising temperatures, and swell.

Wind

Lenticular cloud

Stationary lens-shaped cloud that forms downwind of mountains due to orographic waves.

Clouds

Levante

Easterly wind in the western Mediterranean, moist and associated with coastal rainfall.

Wind

Lifting condensation level

Altitude at which a rising air parcel reaches saturation and begins forming clouds, visible as the flat base of cumulus clouds.

General

Lightning

Electrical discharge between a cloud and the ground, between clouds, or within a cloud, reaching temperatures of 30,000 °C.

Atmospheric phenomena

Longshore drift

Transport of sediment along the coast by the combined action of oblique waves and coastal currents, shaping beaches and coastal formations.

Marine meteorology

Madden-Julian Oscillation

Pattern of tropical convection that propagates eastward every 30-60 days, modulating rainfall and storm activity on a global scale.

Climatology

Mammatus

Rounded protuberances hanging from a cloud base, associated with severe storms.

Clouds

Medicane

Quasi-tropical cyclone that forms over the Mediterranean Sea.

Cyclones and severe storms

Mediterranean climate

Climate type with hot, dry summers and mild, wet winters, found on Mediterranean coasts, California, Chile, South Africa, and western Australia.

Climatology

Mesocyclone

Region of organised rotation within a convective storm, 2 to 10 km in diameter, a precursor to tornadoes in supercells.

Cyclones and severe storms

Mesoscale convective system

An organised complex of thunderstorms spanning 100 km or more, lasting several hours and capable of producing torrential rainfall.

Cyclones and severe storms

Mesosphere

The atmospheric layer between 50 and 85 km altitude, the coldest layer of Earth's atmosphere.

General

METAR

Coded aeronautical weather report describing current conditions at an aerodrome: wind, visibility, clouds, temperature, pressure, and significant phenomena.

Instruments

Meteorological lidar

Instrument using laser pulses to measure vertical profiles of aerosols, clouds, water vapour, and wind in the atmosphere.

Instruments

Meteorological tide

Sea level variation caused by atmospheric pressure and wind, superimposed on the astronomical tide, which can raise or lower coastal levels by several decimetres.

Marine meteorology

Meteotsunami

Tsunami-like wave generated by rapid atmospheric disturbances (storms, gust fronts) over shallow waters, capable of raising sea level 1–3 metres within minutes.

Marine meteorology

Microburst

Intense, localised downdraft that produces destructive winds upon hitting the ground.

Cyclones and severe storms

Microclimate

Particular climatic conditions of a very small area that differ from the general climate of the region, shaped by topography, vegetation, water, or urbanisation.

Climatology

Milankovitch cycles

Periodic variations in Earth's orbit and axial tilt that regulate glacial and interglacial cycles over tens of thousands of years.

Climatology

Mirage

An optical illusion caused by the refraction of light through layers of air with different temperature and density.

Atmospheric optics

Mist

Suspension of microscopic water droplets in the atmosphere reducing visibility to between 1 and 5 km — thinner than fog but enough to blur the landscape.

Atmospheric phenomena

Mistral

Cold, dry, and violent wind channelled down the Rhone valley toward the western Mediterranean, typical of southern France.

Wind

Mixing layer

Lowest atmospheric layer where pollutants are dispersed by turbulence, varying in height from 100 m on stable nights to 2–3 km on convective afternoons.

Air quality

Monsoon

Seasonal wind system that reverses direction between summer and winter, creating wet and dry seasons.

Atmospheric phenomena

Mountain wave

Oscillation of airflow passing over a mountain, generating turbulence, lenticular clouds, and vertical currents on the lee side of the terrain.

Atmospheric phenomena

Mud rain

Precipitation carrying suspended Saharan dust that leaves a reddish or orange deposit on surfaces as it evaporates.

Atmospheric phenomena

Nimbostratus

Dark grey cloud of great horizontal extent that produces continuous rain or snow.

Clouds

Nitrogen dioxide

Pollutant gas (NO₂) emitted mainly by traffic and combustion, irritating airways and contributing to tropospheric ozone and fine particle formation.

Air quality

Noctilucent cloud

Extremely high clouds (80-85 km) formed by ice crystals in the mesosphere, visible only during twilight.

Clouds

Nortada

Northerly wind along the Atlantic coast of Portugal and western Iberia during summer, driven by the contrast between the Azores High and the heated interior plateau.

Wind

North Atlantic Oscillation

A climate pattern describing the variation in atmospheric pressure difference between Iceland and the Azores.

Climatology

Nowcasting

Very short-range weather prediction (0–6 hours) based on radar, satellite, and real-time observations, with high spatial and temporal resolution.

Instruments

Occluded front

Front formed when a cold front catches up with a warm front, combining characteristics of both.

Atmospheric phenomena

Ocean current

Continuous flow of seawater in a given direction, driven by wind, temperature and salinity differences, and Earth's rotation.

Marine meteorology

Ocean salinity

Concentration of dissolved salts in seawater, typically 35 g/L (35‰), which together with temperature determines water density and deep ocean circulation.

Marine meteorology

Ocean waves

Surface waves on the sea generated by wind, characterised by height, period, and direction. Wind sea (local) and swell (remote) are distinguished.

Marine meteorology

Oceanic climate

Climate type with mild temperatures year-round, frequent, well-distributed rainfall, and small temperature range, typical of the European Atlantic coast.

Climatology

Orographic clouds

Clouds formed by the forced ascent of air over a mountain barrier, remaining stationary above peaks while wind flows through them.

Clouds

Orographic precipitation

Rain or snow caused when moist air is forced to rise over a mountain.

Precipitation

Ozone layer

Stratospheric zone (15–35 km) with high ozone concentration that filters harmful solar UV radiation.

General

Pacific Decadal Oscillation

Climate pattern of North Pacific sea surface temperature variation with 20–30-year cycles, modulating fisheries, rainfall, and El Niño frequency.

Climatology

Pampero

Strong, cold southerly or south-westerly wind sweeping across the Argentine Pampas and Uruguay after a cold front passage, causing a sharp temperature drop.

Wind

Pannus

Layer of low, irregular, dark cloud fragments forming beneath precipitating clouds such as cumulonimbus or nimbostratus, also called accessory clouds.

Clouds

Permafrost

Ground remaining frozen for at least two consecutive years, found in polar and high-mountain regions, containing enormous reserves of organic carbon.

Climatology

Phenology

Study of periodic biological events (flowering, migration, harvest) and their relationship with climate and seasons, used as a climate change indicator.

Agrometeorology

Photochemical smog

Mixture of secondary pollutants formed by exhaust gases reacting with sunlight, producing a visible brown haze over cities with heavy traffic and intense sunshine.

Air quality

Photoperiod

Duration of the sunlight period in a day, varying with latitude and season, influencing the biological cycles of plants and animals.

Meteorological astronomy

Pileus

A smooth cap-shaped accessory cloud that forms briefly above the top of a growing cumulus or cumulonimbus.

Clouds

Planetary boundary layer

Lowest layer of the atmosphere (0-2 km) directly influenced by Earth's surface, key for pollutant dispersion and local weather.

General

Pluviograph

Self-recording rain gauge measuring precipitation intensity over time, generating a chart (pluviogram) of hourly accumulation.

Instruments

PM10

Airborne particles smaller than 10 micrometres, from dust, construction, traffic, and industry, penetrating the upper respiratory system.

Air quality

PM2.5

Airborne particles smaller than 2.5 micrometres, able to penetrate lung alveoli and the bloodstream, considered the most harmful pollutant for health.

Air quality

Polar climate

Climate type with warmest month mean temperature below 10 °C, with permanently frozen ground or ice cover throughout the year.

Climatology

Polar vortex

Mass of cold, low-pressure air spinning over each pole in the stratosphere and troposphere, whose weakening can trigger extreme cold spells at mid-latitudes.

General

Poniente

A westerly wind blowing in the Mediterranean and various regions of Spain, generally warm and dry.

Wind

Potential temperature

Temperature an air parcel would have if brought adiabatically to the standard pressure level of 1,000 hPa, used to compare air masses at different altitudes.

Temperature

Powder snow

Light, dry snow with low density and high air content, ideal for winter sports, formed at very low temperatures.

Precipitation

Precipitable water

Total amount of water vapour in a vertical atmospheric column, expressed in millimetres if it were all condensed.

Precipitation

Pressure gradient

Difference in atmospheric pressure between two points divided by the distance between them, the primary driving force of wind.

General

Pyranometer

Instrument measuring total (direct plus diffuse) solar radiation reaching a horizontal surface, expressed in watts per square metre.

Instruments

Pyrocumulonimbus

Cumulonimbus generated by intense heat from a wildfire or volcano, capable of producing lightning, hail, and fire tornadoes, and injecting smoke into the stratosphere.

Clouds

Radiation fog

Fog formed by radiative cooling of the ground during clear, calm nights.

Atmospheric phenomena

Radiative forcing

Difference between solar energy absorbed by Earth and energy radiated back to space, caused by changes in greenhouse gases, aerosols, or albedo.

Climatology

Radiosonde

Instrument lifted by a balloon that measures temperature, humidity, and pressure as it ascends to 30 km.

Instruments

Rain

Precipitation in the form of liquid water drops with diameter exceeding 0.5 mm falling from clouds.

Precipitation

Rain gauge

Instrument that collects and measures the amount of precipitation fallen on a surface.

Instruments

Rain shadow

Arid zone on the leeward side of a mountain range caused by moisture loss as air ascends and precipitates on the windward slope.

Atmospheric phenomena

Rainbow

Multicoloured arc produced by refraction, reflection, and dispersion of sunlight in water droplets.

Atmospheric optics

Rainfall intensity

Amount of rain fallen per unit time, expressed in mm/hour, determining the potential for runoff, flooding, and soil erosion.

Precipitation

Reanalysis

Retrospective reconstruction of atmospheric state by combining historical observations with a modern numerical model, creating a coherent climate record.

Instruments

Red sky

Reddish colouring of the sky at sunrise or sunset caused by Rayleigh scattering, which filters out blue wavelengths and lets reds pass through the atmosphere.

Atmospheric optics

Relative humidity

Percentage of water vapour in the air relative to the maximum it could hold at that temperature.

General

Return period

Average time interval between occurrences of extreme events of equal or greater magnitude, used in engineering and planning.

Climatology

Ridge

Elongated extension of a high-pressure area on a synoptic chart, where isobars form an inverted "U", associated with stable weather and clear skies.

Atmospheric phenomena

Rime ice

Ice deposit formed by the freezing of supercooled fog or cloud droplets upon contact with solid surfaces.

Precipitation

Rip current

Narrow, strong current flowing from the shore out to sea, formed when water from breaking waves seeks to return to the ocean through a channel.

Marine meteorology

River flood

Sudden, violent rise in a river or stream caused by heavy rainfall, rapid snowmelt, or dam failure, with great destructive capacity.

Hazards and safety

Rossby waves

Large-scale undulations of the jet stream and westerly winds caused by Earth's rotation, determining mid-latitude weather patterns.

Atmospheric phenomena

Saffir-Simpson scale

Scale from 1 to 5 that classifies hurricane intensity by sustained wind speed.

Scales and measurements

Sandstorm

Phenomenon in which strong winds lift large quantities of sand and dust from the ground, reducing visibility below 1 km.

Atmospheric phenomena

Santa Ana wind

Hot, dry, strong wind descending from interior plateaus to the coast of southern California, associated with high wildfire risk.

Wind

Sea breeze

Local wind that blows from sea to land during the day, caused by differential heating of both surfaces.

Wind

Sea breeze front

Convergence line where the sea breeze collides with hot continental air, capable of triggering convective storms as it advances inland.

Atmospheric phenomena

Sea surface temperature

Temperature of the ocean's topmost layer (first centimetres to metres), a key factor in tropical cyclone formation and climate regulation.

Marine meteorology

Severe weather

Dangerous weather conditions including thunderstorms with large hail, destructive winds, tornadoes, torrential rain, or heavy snowfall.

Atmospheric phenomena

Simoom

Strong, very hot, dry wind of the Sahara and Arabian deserts, raising sandstorms with temperatures that can exceed 54 °C.

Wind

Sirocco

Hot wind originating in the Sahara that crosses the Mediterranean, picking up moisture and carrying African dust toward southern Europe.

Wind

Sleet

Mixed precipitation of rain and partially melted snow, or translucent ice pellets.

Precipitation

Snow

Solid precipitation in the form of hexagonal ice crystals that cluster into snowflakes.

Precipitation

Snowstorm

Storm with heavy snowfall, strong winds, and very reduced visibility that can paralyse transport and produce significant snow accumulation in just a few hours.

Atmospheric phenomena

Solano

A warm, dry east or southeasterly wind blowing in central and southern Spain, associated with heat waves.

Wind

Solar halo

Luminous ring around the sun produced by refraction of light in ice crystals of high clouds.

Atmospheric optics

Solar radiation

Electromagnetic energy emitted by the sun reaching the Earth, the engine of the entire climate system.

Meteorological astronomy

Solstice

Moment of the year when the sun reaches its highest or lowest position in the sky, marking the start of summer or winter.

Meteorological astronomy

SPI

Standardised Precipitation Index quantifying rainfall deficit or surplus relative to the mean at different time scales, used as a drought indicator.

Scales and measurements

Squall line

An organised line of severe thunderstorms that moves as a front, producing destructive winds along a narrow band.

Cyclones and severe storms

Stationary front

Frontal boundary between two air masses that barely moves, causing persistent rain in the same area.

Atmospheric phenomena

Storm surge

Abnormal rise in sea level caused by a tropical cyclone's wind and low pressure, potentially exceeding 5 metres and the leading cause of death in hurricanes.

Marine meteorology

Stratocumulus

Low cloud formed by rounded masses or rolls grouped in layers, partially grey and partially white.

Clouds

Stratosphere

The layer of the atmosphere between approximately 12 and 50 km altitude, where the ozone layer is located.

General

Stratus

Low, grey, uniform layer cloud that often covers the entire sky, producing drizzle.

Clouds

Sublimation

The direct phase change from ice or snow to water vapour without passing through the liquid phase.

Precipitation

Subsidence

Slow, widespread sinking of air in the atmosphere, which warms and dries by compression, inhibiting cloud formation and producing stable weather.

General

Sulphur dioxide

Pollutant gas (SO₂) emitted by power plants, refineries, and volcanoes, a precursor of acid rain and fine sulphate particles.

Air quality

Sun photometer

Instrument measuring atmospheric transparency at different wavelengths to determine the amount of aerosols, water vapour, and ozone in the atmospheric column.

Instruments

Sun pillar

Vertical column of light extending above or below the sun, caused by reflection of sunlight off horizontally oriented ice crystals suspended in the atmosphere.

Atmospheric optics

Sundog

A bright spot of light appearing on either side of the sun, caused by refraction through atmospheric ice crystals.

Atmospheric optics

Sunshine recorder

Instrument that records hours of effective sunshine by burning a mark on a cardboard strip through a glass sphere that focuses sunlight.

Instruments

Supercell

Thunderstorm with persistent rotation (mesocyclone), the most severe of all storms.

Cyclones and severe storms

Supernumerary bow

Additional narrower, fainter colour arcs appearing just inside the primary rainbow, caused by light wave interference.

Atmospheric optics

Surf zone

Coastal strip where ocean waves lose stability and break upon encountering the shallow bottom, generating foam, turbulence, and littoral currents.

Marine meteorology

Swell

Regular, long-period ocean waves generated by distant storms that travel thousands of kilometres without local wind, arriving at shores under clear skies.

Marine meteorology

Synoptic chart

A weather map showing simultaneous atmospheric conditions over a wide region at a specific time.

Instruments

TAF

Aeronautical weather forecast predicting conditions at an aerodrome for the next 24–30 hours, including expected changes in wind, visibility, and clouds.

Instruments

Teleconnection

Statistical correlation between climate anomalies in geographically distant regions, linked through atmospheric circulation patterns.

Climatology

Temperature anomaly

Deviation of temperature from the climate normal for a given date and location, expressed in positive degrees (warmer) or negative degrees (cooler).

Climatology

Temperature inversion

Anomalous situation where air temperature increases with altitude, trapping pollution and fog.

Temperature

Terral

A nocturnal wind blowing from land toward the sea, particularly notable on the Mediterranean coast of Andalusia.

Wind

Thermal low

Shallow low-pressure area generated by intense surface heating, without associated fronts, typical of summer over continental regions.

Atmospheric phenomena

Thermocline

Ocean layer where temperature drops sharply with depth, separating warm surface waters from cold deep waters.

Marine meteorology

Thermohaline circulation

The global system of deep ocean currents driven by differences in water temperature and salinity.

Climatology

Thermometer

Instrument that measures air temperature, typically in degrees Celsius (°C) or Fahrenheit (°F).

Instruments

Thermosphere

Atmospheric layer between 85 and 600 km altitude where temperature rises rapidly with height due to UV radiation absorption, home to auroras and the ISS orbit.

General

Thunderstorm

Weather phenomenon with lightning and thunder, produced by cumulonimbus clouds with strong convection.

Atmospheric phenomena

Tornado

Violently rotating column of air extending from a thunderstorm cloud to the ground.

Cyclones and severe storms

Torrential rain

Rainfall exceeding 60 mm/hour that can trigger flash floods, river surges, and landslides within a few hours.

Atmospheric phenomena

TORRO scale

Scale from T0 to T11 that classifies tornado intensity in Europe, complementary to the Fujita scale.

Scales and measurements

Trade winds

Steady winds blowing from the subtropical high-pressure belts toward the equator — from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.

Wind

Tramontana

A strong, cold north or northeasterly wind blowing in Catalonia, the Balearic Islands, and southern France.

Wind

Tropical climate

Climate type with monthly mean temperatures above 18 °C year-round and abundant precipitation, with no defined cold season.

Climatology

Tropical night

Night in which the minimum temperature does not fall below 20 °C, preventing restorative rest and increasing mortality during heat waves.

Temperature

Tropical storm

Tropical cyclone with sustained winds between 63 and 118 km/h, the intermediate stage between a tropical depression and a hurricane, with closed circulation and organised rain bands.

Atmospheric phenomena

Tropopause

The boundary between the troposphere and stratosphere, where temperature stops decreasing with altitude.

General

Troposphere

Lowest layer of the atmosphere (0–12 km) where all weather phenomena occur.

General

Tropospheric ozone

Ozone in the lower atmosphere, a secondary pollutant formed by photochemical reactions between nitrogen oxides and volatile organic compounds under intense solar radiation.

Air quality

Trough

Elongated extension of a low-pressure area on a synoptic chart, where isobars form a "V" shape, channelling instability and precipitation.

Atmospheric phenomena

Twilight

Period of diffused light between day and night, when the sun is below the horizon but its light is scattered by the atmosphere. Civil, nautical, and astronomical types are distinguished.

Meteorological astronomy

Typhoon

Tropical cyclone with sustained winds exceeding 118 km/h forming in the western Pacific, equivalent to a hurricane in the Atlantic.

Cyclones and severe storms

Undulatus cloud

Cloud variety displaying regular parallel undulations resembling sea waves, caused by atmospheric gravity waves.

Clouds

Updraft

Column of rising air within a convective cloud, responsible for the vertical growth of cumulus and cumulonimbus and for suspending hail and rain.

Atmospheric phenomena

Urban heat island

Phenomenon whereby cities record temperatures several degrees higher than surrounding rural areas, due to heat absorption by buildings, asphalt, and human activity.

Temperature

UTCI

Universal Thermal Climate Index evaluating thermal stress on the human body by combining temperature, wind, humidity, and solar radiation.

Scales and measurements

UV index

International scale from 0 to 11+ measuring the intensity of solar ultraviolet radiation at the surface.

Scales and measurements

Valley breeze

Daytime wind blowing up valley slopes due to solar heating of the hillsides, reversing direction at night as a mountain breeze.

Wind

Vernalisation

Physiological process by which certain plants require a period of winter cold to flower and bear fruit, such as winter wheat or stone fruit trees.

Agrometeorology

Virga

Precipitation that falls from a cloud but evaporates before reaching the ground.

Clouds

Volcanic ash cloud

Cloud of pulverised rock particles and volcanic glass emitted by an eruption, extremely dangerous for aviation due to its ability to melt jet engines.

Clouds

Volutus

Long, solitary, tube-shaped horizontal cloud that rolls about its axis, completely detached from other clouds. Includes the Australian Morning Glory phenomenon.

Clouds

Vorticity

Measure of the local rotation of the air, a fundamental concept in dynamic meteorology for understanding cyclones, depressions, and fronts.

General

Wall cloud

A low, often rotating cloud formation descending from the base of a cumulonimbus, frequently associated with severe thunderstorms and tornadoes.

Clouds

Warm front

Frontal boundary where warm air glides over retreating cold air, producing stratiform clouds and prolonged rain.

Atmospheric phenomena

Water balance

Relationship between water inputs (precipitation, irrigation) and outputs (evapotranspiration, runoff, percolation) in soil or a basin over a given period.

Agrometeorology

Water cycle

Continuous circulation of water between the atmosphere, land surface, and oceans through evaporation, condensation, and precipitation.

General

Water stress

Condition where a plant loses more water through transpiration than it can absorb through its roots, causing wilting, stomatal closure, and yield reduction.

Agrometeorology

Waterspout

Rotating column of air forming over the sea, similar to an aquatic tornado.

Cyclones and severe storms

Weather buoy

Instrumented floating platform measuring meteorological and oceanographic variables at sea: wind, waves, water temperature, pressure, and humidity.

Instruments

Weather model

A computer program that simulates the atmosphere using physical equations to predict future weather.

Instruments

Weather radar

Instrument that emits microwave pulses to detect, locate, and measure precipitation in real time.

Instruments

Weather satellite

Artificial satellite that observes the atmosphere and Earth's surface for weather forecasting and monitoring.

Instruments

Weather warning

Official communication issued by the national weather service when dangerous atmospheric conditions are expected, classified by severity levels (yellow, orange, red).

Hazards and safety

Weather-driven wildfire

Forest fire whose spread is dominated by meteorological conditions: strong wind, low relative humidity, high temperature, and accumulated drought.

Hazards and safety

Wet snow

Snow with high liquid water content that readily adheres to surfaces, weighs more than powder snow, and can damage infrastructure with its weight.

Precipitation

Wet-bulb temperature

Lowest temperature achievable by water evaporation in the air, a key indicator of the human survival limit under heat.

Temperature

Wind chill

Perceived temperature when wind accelerates heat loss from exposed skin, making cold feel more intense than the actual air temperature.

Temperature

Wind gust

Brief, sudden increase in wind speed significantly exceeding the mean speed, generally lasting less than 20 seconds.

Atmospheric phenomena

Wind profiler

Vertical radar that continuously measures wind speed and direction at multiple altitudes, providing a real-time vertical wind profile.

Instruments

Wind shear

Abrupt change in wind speed or direction over a short distance, either vertically or horizontally, hazardous for aviation and associated with severe storms.

Wind

Wind vane

Instrument indicating wind direction by rotating freely on a vertical axis, with an arrow or pointer showing where the wind blows from.

Instruments

WMO

World Meteorological Organization. United Nations agency coordinating international cooperation in meteorology, climatology, hydrology, and related geophysical sciences.

General

Xaloc

Warm south-easterly wind in the western Mediterranean, a regional variant of the sirocco bringing hot African air to the Balearics, Catalonia, and Valencia.

Wind

Zonda

Hot, dry wind descending the eastern slopes of the Andes in Argentina, the South American equivalent of the Fohn.

Wind