Image glossary

A glossary explaining the definitions of some of the more complex words used in the Image Library.

In our Image Library you'll find a lot of scientific terminology, this glossary provides explanations for much of that terminology.

We also use a variety of keywords to categorise the entries, below you can find definitions of those keywords.

Our main glossary contains explanations of further terms used on this website.

The Product Navigator glossary provides definitions for items which are in the product catalogue.


A & B


Transport of an atmospheric property by wind.


Smoke, dust, ash and SO2 particles from events such as volcanic eruptions


A faint emission of light by a planetary atmosphere. Also called nightglow.

Air mass Boundary

The boundary between two air masses. Also includes: Frontal boundary.


Large-scale circulation of winds. Also includes: High pressure, Subtropical High Pressure Belt.


Also includes: Meteor, meteorite and meteorid

Atmospheric Pollution

Introduction into the atmosphere of chemicals, particulates, or biological materials that cause damage to humans, other living organisms and the environment. Also includes: SO2, air pollution, fossil fuel burning.

Atmospheric (Moisture) River

Horizontal flow of water vapour in the atmosphere, roughly in opposite direction of the big rivers. It can, under certain conditions, lead to heavy rainfall and flooding.


A northern to north-eastern katabatic wind in the Adriatic, Bosnia and Herzegovina, Croatia, Montenegro, Italy, Bulgaria, Greece, Slovenia, Poland, Novorossiysk and Turkey.

Burned Area

A area of land burned by fire which is visible to satellites.



High-level clouds (16,000 feet or higher), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands.

Cloud deck

Continuous cloud sheet at a certain altitude.

Cloud Street

Rows of cumulus or cumulus-type clouds aligned parallel to the low-level flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite images.


Areas on satellites images which don't have clouds, making the land or ocean below visible.

Cold Air Outbreak

When air travels from a cold region, such as the Arctic, over water and picks up moisture which is then deposited over land, often as snow.

Cold Front

A zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.

Cold pool

A region, or pool of relatively cold air surrounded by warmer air. This is usually applied to cold air of appreciable vertical extent that has been isolated in lower latitudes as part of the formation of a cut-off low.

Comma Cloud

A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on satellite images associated with large and intense low-pressure systems.


Short for 'condensation trails'. Long thin artificial clouds that sometimes form behind aircraft. They can sometimes be seen by satellites.


In meteorology, the term is used specifically to describe vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts in an unstable atmosphere. The terms convection and thunderstorms often are used interchangeably, although thunderstorms are only one form of convection. Also includes: Bénard Cell, Convective Outflow Boundary

Convergence Line

Any horizontal line along which horizontal convergence of the airflow is occurring.

Cut-off Low

Round-shaped, closed circulation high in the atmosphere, typically mesoscale in size, which provides instability and abundant precipitation underneath.


The formation or intensification of a cyclone or low-pressure storm system. Also includes: Rapid cyclogenesis, explosive cyclogenesis and 'weather bomb'.


A large-scale circulation of winds around a central region of low atmospheric pressure, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. Also includes: Genoa Cyclone, Lee cyclone, Medicane, Storm.

D & E

Deformation Zone

Cloud bands or lines which form within a deformation field and stretch or dissolve under the influence of elongation.


A process which occurs with the addition or loss of heat. The opposite of adiabatic. Meteorological examples include air parcels warming due to the absorption of infrared radiation or release of latent heat.


An area on a satellite image which shows a lack of soil moisture.


Particles, usually sand, carried in the atmopshere. Also includes: Haboob, Sandstorm.


(Astronomy) Time of the year (around 21 March and 23 September) when day and night have the same duration.

Extratropical transition

When a tropical cyclone enters the subtropics and mid-latitudes, it can interact with other extratropical weather systems, such as mature storm systems. A change takes place, resulting in a new weather system that is a cross between a tropical cyclone and an extratropical cyclone.



An ocean area where waves are generated by a wind having a constant direction and speed.

FoehN or Föhn

A warm, dry, downslope wind descending the lee side of the Alps as a result of synoptic-scale, cross-barrier flow over the mountain range. In other mountain ranges the foehn has a variety of local names, including chinook in the Rocky Mountains in North America; zonda for a westerly foehn from the Argentine Andes; ljuka in Carthinia (northwestern Croatia); halny wiatr in Poland; austru in Romania; and favogn in Switzerland. A northeasterly foehn descending the Massif Central in France and extending over the Garonne Plain is locally called aspre. A dry wind from the northwest descending the coastal hills in Majorca is named the sky sweeper. In New Zealand a foehn blowing from the New Zealand Alps onto the Canterbury Plains is the Canterbury northwester.


Thermal fire detection. Also includes; Smoke, Burned Area.


Any high flow, overflow, or inundation by water. Also includes: Flash Flood, Mudslide.


Water droplets suspended in the air at the Earth's surface. Also includes: Low Cloud.


A boundary or transition zone between two air masses of different density and, usually, of different temperature.


White frost can be seen on RGB imagery.

Funnel cloud

A cone-shaped cloud which extends from the the base of a cloud towards the ground without reaching the ground.

G & H

Gap Wind

Intense wind blowing through a breach in a mountain range or similar geological formation.

Geopotential height

The height of a given point in the atmosphere in units proportional to the potential energy of unit mass (geopotential) at this height relative to sea level.

Gravity Wave

A wave created by the action of gravity on density variations in the stratified atmosphere. A generic classification for lee waves, mountains waves, and many other waves that form in the atmosphere.

Gust Front

The leading edge of gusty surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud. Also includes: Outflow boundary.


Named after the Arabic word 'habūb' meaning strong wind) it is a dust storm caused by convective downbursts.


Showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud. Also includes: Hailstorm.


An aggregation in the atmosphere of very fine, widely dispersed, solid or liquid particles, or both, giving the air an opalescent appearance that subdues colours.

Homogeneous layer

Layer with the same characteristics for a wide area or many satellite pixels.

Hot Spot

In a satellite image, pixel or small group of pixels with brightness temperatures significantly above those in the neighbourhood.


A measure of the water vapor content of the air.

I, J & K


A large floating mass of ice, sometimes of immense size, typically detached from a glacier or ice sheet and drifting in the sea.

Inter-Tropical Conversion Zone

The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator. Also known as ITCZ.


A fast-moving wind current surrounded by slower moving air. Also includes: Sting Jet.

Jet stream

The fast-moving ribbon of air 10 km up in the atmosphere that drives the weather at the surface. The jet stream helps create deep areas of low pressure and transports them from one place to another.

Katabatic wind

From the Greek word katabatikos meaning 'to flow downhill', it is the technical name for a drainage wind, a wind that carries high density air from a higher elevation down a slope under the force of gravity.


Colloquial Arabic term for the southeastern monsoon, used in southern Oman, southeastern Yemen, southwestern Saudi Arabia and Sudan.

L & M


A visible electrical discharge produced by a thunderstorm.

Local wind

Winds that blow over short distances caused by unequal heating of the earths surface in a small area. Also includes: Katabatic Wind, Bora, Foehn, Mistral.


A Medicane (MEDIterranean hurriCANE) or Tropical-Like Cyclone (TLC) is a cyclone observed in the Mediterranean Sea.

Meridional flow

A type of atmospheric circulation pattern in which the meridional (north and south) component of motion is unusually pronounced.

Mesoscale Convective System

A group of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones and squall lines.


A time cross-section of data for a specific surface reporting station. The data plotted include temperatures, winds, pressure, clouds and present weather.


A unit of length equal to one-millionth of a meter or one-thousandth of a millimeter. A convenient length for measuring wavelengths of infrared radiation, diameters of atmospheric particles, etc. Also known as Micrometer or µ


A strong, cold and northwesterly wind that blows from southern France into the Gulf of Lion in the northern Mediterranean, with sustained winds often exceeding 40 km/h, and sometimes reaching 100 km/h. Most common in the winter and spring, and is the strongest in the transition between the two seasons.

Moisture Transport

Moisture carried by the atmospheric circulation.


A thermally driven wind arising from differential heating between a land mass and the adjacent ocean that reverses its direction seasonally.


Noctilucent clouds

Thin silvery-blue cirrus-like clouds frequently seen during summer twilight conditions at high latitudes (above 50°) in both hemispheres.

Ocean Parameter

Also includes: Ocean Circulation, Ocean Colour, Ocean Current, Ocean Eddy, Ocean Surge, Plankton, River Sediment, Sea Height Anomaly, Sea Level, Sea Level Height, Sea Wave, Storm Surge, Wave, White Caps, Ship Wave.

Ocean Wind

Wind close to the surface of the ocean.


The process by which the cold front of a rotating low-pressure system catches up the warm front, so that the body of warm air between the fronts is forced upwards away from the ground between wedges of cold air. Also includes: Instant Occlusion.

Oil Slick

A layer or film of oil on the surface of water.

Orographic Cloud

Orographic clouds develop in response to the forced lifting of air by the earth's topography, e.g. mountains. Also includes: Lee cloudiness, Mountain Wave, Stau, Stau Cloudiness, Von Karman Vortex Street.

Overshooting Top

A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell.


Colourless gas in the atsmopshetre which absorbs the harmful components of sunlight, ultraviolet B. Also includes: Ozone Hole, Total Ozone.

P & R

Polar Low

A small-scale, short-lived atmospheric low pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres.

Polar vortex

A large pocket of very cold air, typically the coldest air in the Northern Hemisphere, which sits over the polar region during the winter season.

Potential vorticity anomaly

Deviation from the standard convection conditions in the atmosphere.


The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth. Also includes: Rain, Snow ad Sleet.


Referring to an aerosol or ash cloud, the elongated shape of its presence in the atmosphere as a result of dispersion by wind.

Pyro Cb

Cumulonimbus cloud generated around intense fires, after condensation of the water originally in the biological mass.


Energy transport through electromagnetic waves.


A battery-powered telemetry instrument package carried into the atmosphere, usually by a weather balloon, that measures various atmospheric parameters and transmits them by radio to a ground receiver.



South American convergence zone.

Sandwich product

Colour enhancement method that blends an RGB image with a colour enhanced Brightness Temperature image.


A type of wind that arises from a warm, dry, tropical airmass that is pulled northward by low-pressure cells moving eastward across the Mediterranean Sea.


A coastal local wind that blows from sea to land, caused by the temperature difference when the sea surface is colder than the adjacent land, its opposite is called a land-breeze. It has a cooling effect.

Sea Ice

Any form of ice found at sea which has originated from the freezing of sea water (does NOT include superstructure icing).

Sea Surface Temperature

The mean temperature of the ocean in the upper few metres. Also known as SST.

Ship Trail

White trails, which look similar to contrails, seen in satellite imagery. They result from ship exhaust.


Plumes of smoke from fires. Also includes: Fire.


Precipitation in the form of ice crystals.

Soil Moisture

Degree of water content in the land surface, measurable from satellites.

Solar Eclipse

When the Moon passes between the Sun and Earth, and the Moon fully or partially blocks the Sun.

Solar radiation

The total electromagnetic radiation emitted by the sun. Also includes: direct solar radiation, diffuse sky radiation, global radiation, extraterrestrial radiation, solar constant, total solar irradiance.

Solar Wind

A fast outflow of hot gas in all directions from the upper atmosphere of the Sun ('solar corona').


(Astronomy) Time of the year (around 21 December and 21 June) when the Sun reaches its highest or lowest altitude above the horizon at noon, and daylight is maximum or minimum.

Space Weather

Conditions on the Sun and in the solar wind, magnetosphere, ionosphere and thermosphere that can influence the performance and reliability of satellites.

Split-window difference

Difference in brightness temperatures as given by the satellite for two channels where gas absorption is small, for instance a channel around 11 micron and another channel at about 12 micron.


A strong wind characterised by a sudden onset, a duration of the order of minutes, and then a sudden decrease in speed


A low, generally grey cloud layer with a fairly uniform base. Also includes: Stratocumulus, Stratocumulus Clouds.

Stratospheric air

Air originating in the stratosphere.


Occurs when the sun reflects off the surface of the ocean at the same angle that a satellite viewing the surface.


A thunderstorm characterised by the presence of a mesocyclone: a deep, persistently rotating updraft. Sometimes referred to as a rotating thunderstorm.

Supercooled cloud

A cloud containing pure water droplets at temperatures considerably below the nominal freezing temperature of 0°C. Also includes: Icing.

Surface Moisture

See Soil Moisture



A tool for plotting the atmospheric temperature profile.

Thickness line

A line drawn through all geographic points at which the thickness of a given atmospheric layer is the same.


A local storm produced by a cumulonimbus cloud and accompanied by lightning and thunder.


A compound of the words ‘thunder’ and ‘snow’ used, informally, to describe an observation of snow at the surface that occurs with lightning and thunder.

Trace Gases

Rare gases in the atmosphere, like Argon, ozone or sulphur and nitrogen oxides.


Any substance in the atmosphere that can be used to track the history of an airmass.

Tropical Cyclone

A warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters with organized deep convection and a closed surface wind circulation about a well-defined center. Also includes: Hurricane, Tropical Depression, Tropical Low, Tropical Storm, Typhoon.

Tropical Wave

A trough or cyclonic curvature maximum in the trade wind easterlies.


An elongated region of relatively low atmospheric pressure, often associated with fronts.


A series of long-period waves (tens of minutes) usually generated by disturbance that displaces massive amounts of water, such as an earthquake occurring on or near the sea floor, underwater volcanic eruptions and landslides.


Irregular motion of the atmosphere, as indicated by gusts and lulls in the wind.

U, V & W

Undular Bore

A wave disturbance in the Earth's atmosphere and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or a cold front moves through.

Upper Level Low

Cut off low in upper levels without corresponding surface low, accompanied by cloud bands at the boundaries and convective cloud in the low centre.

Urban Heat Island

Heat generated and trapped in towns, typically persistent during the night, which shows as hot spots in thermal satellite images, for instance.


Areas of vegetation as seen on satellite imagery.

Volcanic Eruption

Ash plumes, SO2 emissions and hot lava seen by satellites.

Vorticity Centre

An area of the atmosphere with a local maximum of cyclonic circulation.

Warm Front

A transition zone between a mass of warm air and the colder air it is replacing. Also includes: Detached Warm Front.

Water Vapour Vortex

In water vapour images, an area of cyclonic rotation.

Wind shear

A difference in wind speed and direction over a relatively short distance in the atmosphere. Horizontal wind shear seen across fronts and near the coast, and vertical shear is, typically, near the surface.

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