Weather Glossary Terms

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A (Back to Top)
Absolute zero The temperature at which all molecular motion ceases. Designated 0 degrees on the Kelvin or Absolute scale or -273.16 on the Celsius scale.
Acid Precipitation Rain or snow with a pH value of less than 5.6.
Adiabatic Temperature Change A process in which heat does not enter or leave a system. Adiabatic cooling takes place when a parcel of air moves upwards through the atmosphere. E.g. being forced over a mountain and adiabatic heating occurs when air descends as can happen in an area of high pressure
Advection Horizontal movement of air, moisture, or heat.
Advection Fog Fog formed by warm, humid air flowing over colder surface
Air Frost A temperature below 0 degrees Celsius measured in a calibrated instrument screen at a height of 1 to 2 metres above the ground.
Air Mass A large body of air with nearly uniform temperature and moisture content. There are six recognised airmasses that affect the British Isles denoted by their source region and then subsequent track. Polar maritime-Pm Arctic maritime-Am Tropical maritime-Tm Tropical continental-Tc Polar continental-Pc Returning Polar maritime-rPm. Returning Polar maritime is air that originates in the cold Polar Regions but is forced well south before returning to the British Isles from the southwest.
Airmass Analysis The identification of specific airmasses on a weather map. Fronts depict the boundaries between them.
Airstream A significant body of air flowing in the same general circulation.
Alpine Glow A sequence of colours seen on snow-covered mountaintops at sunset.
Altitude Height expressed as the distance above a reference point, which is normally sea level or ground level.
Anabatic wind a localised wind that blows up a hillside when the air is heated by the sun.
Ana-front An active weather front with large amounts of precipitation
Anemometer An instrument that measures wind speed
Aneroid Barometer An instrument for measuring atmospheric pressure that does not use a liquid. It is built around a metal structure that bends with changing air pressure.
Anticyclogenisis The formation of an anticyclone or area of high pressure.
Anticyclone A region of relatively high pressure where air is descending through the atmosphere normally brings dry and settled weather. Winds on the surface blow clockwise around it in the northern hemisphere and anti-clockwise in the southern hemisphere.
Anticyclonic Gloom Poor visibility, without precipitation, under an area of high pressure. The temperature inversion layer traps any pollution giving very dull conditions. Often seen in November.
Arctic Air A mass of very dry, very cold air that develops over the snow-and-ice-covered regions of the Far North or in the southern hemisphere the far south.
Arctic front The leading edge of very cold air moving out from Polar Regions.
Arctic Sea Smoke A shallow layer of fog formed when warmer water evaporates quickly into cold air and immediately condenses. e.g. "steam " coming out of a boiling kettle.
Atmosphere The mass of air surrounding the Earth.
Atmospheric Pressure The pressure asserted by the mass of the column of air directly above any specific point.
Azores high A semi-permanent area of high pressure often centred around the Azores in the mid-Atlantic. It is caused by the air at high levels moving out from the equatorial regions slowing down at about 30 degrees north and descending.
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Backing Wind Shifting of the wind in an anti-clockwise direction, often ahead of a low-pressure system.
Ball lightning Lightning that appears in a shape of a luminous sphere. Its formation is still not fully explained.
Banner cloud Cloud formed immediately downwind of an isolated mountain peak. Caused by a reduction in pressure. Gibraltar is a well-known place for banner clouds.
Barograph An instrument that provides a continuous record of atmospheric pressure.
Barometer An instrument for measuring atmospheric pressure.
Barometric Tendency The amount and direction of change in barometer readings over a three-hour period.
Beaufort Wind Scale A system used to classify wind speed, developed in 1805 by British Admiral Francis Beaufort. The scale was initially developed for ships at sea but has latterly been adapted for use on land. The scale ranges from 0, which is flat calm to force 12 for hurricane force winds.
Blizzard Heavy snow accompanied by a gale force wind.
Buys Ballots Law A physical law describing wind flow around high and low pressure.
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Campbell-Stokes recorder A sunshine recorder that focuses the sun?s rays through a glass sphere burning to a card.
Carbon Dioxide A colourless odourless gas. One of the greenhouse gases
Ceiling The height above the ground of the base of the lowest layer of clouds, when at least clouds cover 60 percent of the sky.
Celsius Temperature Scale A temperature scale in which 0 degrees is the melting point of ice and 100 degrees is the boiling point of water.
Central England Temperature The average temperature for a selection of sites in central England. There is a continuous monthly record since 1659.
Chaos Theory A theory that weather systems behave unpredictably over time.
Chlorofluorocarbon (CFC) Compounds of carbon, chlorine and flourine, which have been responsible for destruction of high-level ozone.
Circulation The pattern of the movement of air. General circulation is the flow of air of large, semi-permanent weather systems, while secondary circulation is the flow of air of more temporary weather systems.
Climate A statistical portrait of weather conditions in a specific place over a long period.
Climatology The study of climate.
Cloud Amount The portion of the sky in eighths covered either by one specific cloud or by all of them
Cloud classification The way of dividing clouds into various categories.There are two main types of cloud at three levels in the atmosphere. Cumulus, which is a heaped cloud Stratus a sheet cloud Low cloud from the surface to 8000 Feet mainly made up of water droplets Medium cloud 8000-20000 Feet made from supercooled water droplets and ice crystals High cloud 20000 Feet and above virtually all ice crystals
Cloudburst A sudden, intense rainfall that is normally of short duration.
Cloudy Day At least six eighths of the sky covered with cloud all day
Coalescence The growth of liquid cloud droplets grow by collision.
Col On the weather map an area between two highs and two lows.
Cold Front The leading edge of a colder mass of air that displaces a warmer mass of air.
Condensation The change of water vapour to liquid water.
Condensation Level The level in the atmosphere at which a parcel of air reaches saturation.
Condensation Nuclei Small particles in the air onto which water vapour condenses.
Conduction The transfer of heat by molecular action within a substance or when two substances are in direct contact.
Continental Air Mass An air mass that forms over land. It is usually dry, but may be cold or warm.
Contrail A cloud-like stream formed in cold, clear air behind the engines of an aeroplane.
Convection The transfer and mixing of heat by movement, normally in the vertical.
Convective Cloud Cloud formed as a result of convection. Typical clouds are cumulus and cumulonimbus.
Convergence In meteorology the act of a net inflowing of air. An example is at the base of a developing depression.
Coriolis Effect The apparent force due to the rotating Earth which deflects winds to the right in the northern hemisphere and to the left in the southern hemisphere.
Cumuliform Cloud cloud that forms by convection. Typically a heap cloud such as cumulus.
Cumulonimbus sometimes called thundercloud. The largest single cloud in the sky and can have a base of 2000 feet and tops of 40000 feet in temperate latitudes and even larger in the tropics.
Cumulus A heaped cloud associated with showers. Often described as looking like a cauliflower.
Cyclone A low-pressure system in which winds spin inward in a counterclockwise direction in the northern hemisphere. Tropical cyclone is used to describe a hurricane or typhoon in the Indian Ocean.
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Deepening A decrease of the central pressure of a depression.
Depression An area of low atmospheric pressure.
Dew Water that condenses onto grass and other objects near the ground.
Dew Point The temperature at which air becomes saturated with water vapour. I.e. the relative humidity is 100 %.
Diurnal Variation The fluctuation in a weather element over a period of 24 hours.
Divergence Outflow of air as in an anticyclone.
Doldrums The area of light winds within 10 degrees of the equator.
Doppler Radar Sophisticated radar that can measure the speed and direction of moving objects, such as wind.
Downburst A sudden, strong, downward blast of air, usually from a thundercloud.
Drizzle Precipitation featuring tiny water droplets 200-500 microns in diameter. Usually associated with Stratus cloud or rain, which has nearly evaporated before it reaches the ground.
Dropsonde An instrument, usually dropped from an aircraft, which measures weather elements 9 such as temperature, wind, humidity and pressure) as it descends
Drought Abnormally dry weather in a region over an extended period.
Dry Adiabatic Lapse Rate The rate at which the temperature of unsaturated air changes as it ascends or descends through the atmosphere. Approximates to some 10 degrees Celsius per kilometre.
Dust Devil A small, whirling column of wind that picks up dirt and other loose material as it travels.
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ECMWF European Centre for Medium Range Forecasting based near Reading
El Ni€o- (or Southern Oscillation) Warming of the water in the tropical eastern Pacific Ocean which has world-wide implication on weather patterns especially in tropical regions.
Ensemble Forecasts A means of forecasting where the computers are given slightly different initial conditions.
Environmental Lapse Rate The actual change of temperature with height through the atmosphere.
Equinox The twice-yearly occurrence (about March 21 and September 21) when the sun at its highest point crosses the equator.
Evaporation The change in a substance from a liquid to a gaseous state. I.e. Water into water vapour.
Extra-tropical Cyclone A cyclone that forms in the tropical and then moves to more temperate latitudes but can still retain some tropical characteristics such as heavy rainfall.
Eye The roughly circular area of relatively calm weather at the centre of a hurricane, typhoon or Tropical Cyclone
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Fahrenheit Temperature Scale A temperature scale that uses 32 degrees as the melting point of ice and 212 degrees as the boiling point of water.
Flash Flood Flooding caused by a rapid rise in the water level of rivers, streams, or lakes, usually as a result of heavy rains.
Fog A cloud of water droplets suspended in the air that touches the ground giving a horizontal visibility of less than 200 metres.
Fohn A warm dry wind that descends on the lee side of a mountain.
Forked Lightning Lightning that moves from cloud to ground in a series of jagged movements.
Freezing The change in a substance from a liquid to a solid state.
Freezing Fog Fog with a temperature below freezing that deposits rime on contact with objects.
Freezing Level The level in the atmosphere where the temperature is 0 degrees Celsius.
Freezing Nuclei Particles suspended in the air around which ice crystals form.
Freezing Rain Liquid water droplets with a temperature below 0 deg. C (supercooled) that turn to ice when they hit a cold surface.
Front The boundary between two different air masses.
Frost Ice crystals that form on grass and other objects when the temperature and dew point fall below freezing.
Frost Hollow A valley bottom where cold dense air below 0 degrees Celsius collects at night
Fujita Scale A scale for estimating damage caused by the winds of a tornado, developed by Theodore Fujita. Similar to the Torro scale.
Funnel Cloud A rotating column of cloud descending from the base of a large cumulus or cumulonimbus but which does not reach the surface.
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Gale A wind reaching a mean speed of 39 miles per hour or gusts 49 miles per hour. Described as force 8 on the Beaufort scale.
General Circulation The overall circulation of the entire atmosphere.
Geostationary Satellite A satellite placed at nearly 36000 Km above the earth on the equator. It orbits the earth once a day so stays approximately stationary above a specific point. Five such satellites give a continuous cloud picture of virtually the entire globe
Glaze A smooth coating of ice formed when water droplets freeze on contact with a cold surface. Sometimes described as black ice.
Global Forecast System (GFS) A global weather model which is compiled and run by NCEP in the USA and made available for distribution around the world. The GFS is made up of two seperate models, the AVN (AViatioN model) which is for 0-180 hours in 3 hour intervals and the MRF (Medium Range Forecast) which is for 192 - 384 hours in 12 hour intervals.
Global Warming An increase in the average temperature of the Earth caused mainly by the increase in greenhouse gases.
Grass Minimum The minimum temperature given by a thermometer resting on the grass
Graupel Precipitation formed when super cooled water droplets collide and freeze.
Greenhouse Effect The warming that takes place when molecules in the atmosphere trap heat radiating away from the surface and redirect it back toward Earth.
Ground Frost A temperature of 0 degrees Celsius or below recorded on a thermometer placed on short grass
Gulf Stream A warm ocean current that flows from the Gulf of Mexico across the Atlantic to the coast of western Europe.
Gust A sudden, brief increase in wind speed lasting for at least 20 seconds.
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Haar A term used in northeast England and Scotland for fog and low cloud along the northsea coasts.
Hadley Cell A simple explanation of hemispherical circulation.
Hail Pieces of ice that form in layers in the updrafts of thunderstorms.
Halo A ring or arc of light around the sun or moon that is caused by the refraction of sunlight through dense ice crystals in the atmosphere.
Haze Particles or fine dust suspended in the air that produce limited visibility.
Heat Island A local pool of warmer air in an urban area.
Hectapascal Has taken over from the millibar as a unit used to express atmospheric pressure. Fortunately 1 millibar is equal to 1 hectapascal.
Helm Wind A blustery wind that descends the lee side of a mountain range.
High An area of relatively high atmospheric pressure.
Hill Fog Low cloud covering hills and giving a visibility below 200 metres.
Hoar Frost Ice crystals deposited on objects below 0 degrees Celsius or dew that freezes after formation.
Humidity A measure of the water vapour content of the air.
Hurricane A tropical revolving storm which occurs in the south Atlantic, Caribbean or eastern Pacific.
Hurricane A tropical cyclone in the western hemisphere that has sustained wind speeds of 74 miles per hour or greater
Hydrological Cycle The cyclic process whereby water is evaporated from the oceans as water vapour; condenses to form clouds which then drop their rain into the sea completing the cycle.
Hydrosphere The Earth's water.
Hygrometer An instrument that measures the water vapour content of the air.
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Ice Crystals Crystalline form of ice. When they collide or coalesce with others form snowflakes, hail or graupel.
Indian Summer A term used for a period of warm settled weather in mid to late autumn.
Instability A state of the atmosphere in which rising air is warmer than its surroundings and continues to rise.
Intertropical Convergence Zone The area in tropical regions where the trade winds from the northern and southern hemispheres meet. Characterised by cloud and rain. The zone moves north and south with the northern and southern summers.
Inversion A condition in which air near the ground is cooler than air above it; a condition opposite the normal decrease in temperature with height.
Isobar A line on a weather map that joins places with the same atmospheric pressure.
Isotherm A line on a weather map joins places with the same temperature.
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Jet Stream A narrow band of winds blowing high in the troposphere at speeds in excess of 60 miles per hour but can reach speeds in excess of 300 miles per hour. Typically thousands of kilometres long, hundreds of kilometres wide and a few kilometres deep. They are closely associated with the weather systems in temperate latitudes.
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Katabatic Wind A valley or drainage wind formed as cold dense air drains down a valley. Examples are the Mistral in southern France and the strong winds blowing of the high Antarctic plateaux.
Kelvin Temperature Scale A temperature scale in which 0 degrees is the point at which all molecular motion ceases (absolute zero). To convert degrees Kelvin into degrees Celsius add 273.
Knot A speed of one nautical mile per hour. Miles per hour equals knots plus one seventh.
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La Nina A cooling of the eastern Pacific-opposite to El Nino.
Latent Heat The heat absorbed or released by a substance that undergoes a change of phase i.e. ice to water and water to water vapour or vice versa.
Layer Cloud Clouds that form in sheets or layers often associated with weather fronts.
Levanter a humid easterly wind that affects the Straits of Gibraltar mainly in summer or early autumn.
Lightning An electrical discharge produced by a thunderstorm.
Long Range Forecast A weather forecast extending past 10 days ahead.
Low An area of relatively low atmospheric pressure.
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Maritime Air Mass An air mass that forms over water. It is usually humid, and may be cold or warm.
Mean Temperature The average of a series of temperatures taken over a period of time, such as a day or a month.
Medium Range Forecast A weather forecast for 3 to 10 days ahead.
Mercury Barometer An instrument that measures barometric pressure by measuring the level of mercury in a column.
Meteosat The Geostationary satellite 36000 Km above the equator and on the Greenwich meridian.
Microburst A downburst from a thunderstorm that is confined to a small area.
Mid-Latitudes The areas in the northern and southern hemispheres between the tropics and the Arctic and Antarctic circles.
Millibar A metric unit of air pressure measurement equal to one hectapascal.
Minimum temperature The lowest temperature in a given period.
Mist A horizontal visibility of more than 200 metres but less than 6 miles caused by water droplets in suspension in the atmosphere.
Mistral A strong cold dry wind that blows down the Rhone valley in southern France.
Monsoon a persistent wind throughout a season but often refers to the heavy rains that accompany them. Best known is the summer southwesterly monsoon over India followed by a northeasterly monsoon over Southeast Asia in winter. The major cause is the unequal heating between the land and sea.
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Nacreous Cloud A rare, often coloured, cloud at heights between 15 and 30 km. sometimes called mother of pearl clouds.
Nautical Mile A distance of 1852 metres. One nautical mile per hour equals one knot.
Nimbo-Stratus A thick grey cloud associated with rain.
Noctilucent Cloud The highest clouds in the sky at 80-85 Km.
Numerical Forecasting forecasting the weather through digital computations carried out by supercomputers.
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Occluded Front Formed in the latter stages of the life of a depression. The cold front associated with the depression moves more quickly than the preceding warming front and after catching it up lifts the warm air above the surface. The amalgamation of the cold and warm fronts produce the Occlusion or Occluded front
Orographic Lifting Lifting of air over hills and mountains
Ozone An unstable oxygen compound that is a pollutant at ground level, but that absorbs deadly ultraviolet rays in the stratosphere.
Ozone Hole A thinning of the ozone layer over Antarctica and beyond which occurs each spring.
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Polar Air A mass of very cold, very dry air that forms in Polar Regions.
Precipitation Any liquid or solid form of water that falls from the atmosphere and reaches the surface of the Earth.
Pressure The force exerted by the column of air above a point to the top of the atmosphere.
Pressure Gradient Force Force acting on air that causes it to move from areas of higher pressure to areas of lower pressure.
Prevailing Wind The direction from which the wind blows most frequently in any location.
Prognosis A forecast
Psychrometer An instrument that measures relative humidity of the air.
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Quasi-stationary front A stationary weather front.
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Radar Stands for "radio detection and ranging." An instrument that detects and ranges distant objects by measuring the scattering and reflection of radio beams.
Radiation The transferring of energy through electromagnetic waves.
Radiation Fog Fog which forms over the land in still conditions.
Radiosonde A package of instruments to measure temperature, humidity and pressure carried aloft by a helium filled balloon.
Rain Liquid precipitation with drops larger than 500 microns in diameter.
Rain Day A day on which at least 0.2 mm of rain fell.
Rainbow An arc or circle of coloured light caused by the refraction of light by water droplets in the air.
Refraction The bending of light as it passes through areas of different density, such as from air through ice crystals.
Relative Humidity A measure of the amount of water vapour actually held by a specific volume of air in comparison to the maximum water vapour that air could hold at a constant temperature; given as a percentage
Ridge An elongated area of high pressure, often seen between two areas of low pressure.
Rime Tiny balls of ice that form when tiny drops of super-cooled water freeze on contact with the surface.
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Saffir-Simson Scale A scale to describe severity of Tropical Storms. Category - Wind speed - Storm surge 1 - Weak 74 to 95 mph - 1.2 to 1.5 m 2 - Moderate 96 to 110 mph - 1.8 to 2.5 m 3 - Strong 111 to 130 mph - 2.8 to 3.7 m 4 - Very Strong 131 to 155 mph - 4.0 to 5.5 m 5 - Devastating greater than 155 - Over 5.5 m.
Saturated Adiabatic Lapse Rate The rate at which saturated air cools as it ascends through the atmosphere. Generally between 4 and 7 degrees Celsius per kilometre, about half that of dry air.
Saturation A condition of the atmosphere in which a certain volume of air holds the maximum water vapour it can hold at a specific temperature. The higher the temperature the more it can hold and vice versa.
Secondary Cold Front A front that follows a primary cold front and ushers in even colder air.
Sheet Lightning Cloud to cloud lightning.
Shower Precipitation falling from convective or cumuliform cloud. Normally has some sunshine or clear spells between the rain or snow.
Sleet Precipitation consisting of rain and snow mixed.
Smog Air pollution caused by a mixture of smoke and fog.
Snow Precipitation consisting of clumps of ice crystals.
Solar Energy The energy produced by the sun.
Solstice The time of year when the sun is the farthest north or the farthest south (about June 21 and December 21).
Spanish Plume A layer of warm air that originates over Spain and moves north towards the British Isles.
Squall Line A line of heavy showers or thunderstorms often accompanied by strong and gusty winds.
Stable Air Air in which temperature and humidity at various levels discourage the formation of convection currents.
Stationary Front The border between cold and warm air masses that are not moving.
Stevenson Screen A shelter used by all Meteorological Offices to house thermometers and hygrometers.
Storm Track The path that storms generally follow in a given area.
Stratus A grey low level cloud often giving drizzle.
Sublimation The change ice crystals directly into water vapour without passing through the water stage.
Subsidence The descent of a body of air, usually in a high-pressure area, that warms the lower levels of the atmosphere.
Supercell Thunderstorm An unusually violent thunderstorm that is capable of generating tornadoes.
Supercooled Water Water droplets cooled to a temperature below 0 degrees Celsius without freezing. This state is quite common in the atmosphere.
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Temperate Zone The area of the globe between the tropics and the Polar Regions.
Thermal A bubble of warm buoyant air rising through the atmosphere.
Thermometer An instrument for measuring temperature.
Thunder The sound produced by lightning discharges.
Thunderstorms Storms that produce lightning and thunder.
Tornado A violently rotating column of air that reaches from the base of a cloud to the ground.
Torro Scale A scale of tornado intensity ranging from TO with winds 17-24 mph to T6 winds 73-83 mph to super tornadoes T10 with winds above 121 mph.
Tropical Air Warm, humid air masses that form in tropical regions.
Tropical Cyclone A low-pressure system that forms in the tropics. Another name for Hurricane or Typhoon in the Indian Ocean.
Tropical Depression A tropical cyclone with winds that do not exceed 38 miles per hour.
Tropical Storm A tropical depression with winds of 39-73 miles per hour.
Tropics The area of the globe from latitudes 23.5 degrees north to 23.5 degrees south.
Trough An elongated low-pressure system that enhances showers into longer spells of rain or snow.
Tsunami A destructive wave often formed by the winds around a Typhoon, Hurricane or Tropical Cyclone.
Typhoon A tropical cyclone with winds of 74 miles per hour or greater that occurs west of the International Date Line to the Indian Ocean.
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Unstable Air Air with temperature differences that encourage the formation of convection currents.
Updraft An upward current of air, usually within a thundercloud.
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Veering Winds Winds that shift in a clockwise direction.
Virga Precipitation falling from a cloud in streaks but evaporating before reaching the ground
Visibility The greatest distance at which one can see and identify objects in the horizontal.
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Warm Front The leading edge of a mass of warmer air that displaces a mass of colder air.
Warm Sector The area of tropical origin air between a warm and cold front.
Water Vapour The invisible gaseous form of water.
Waterspout A tornado-like formation over water, usually much smaller and less vigorous than a true tornado. Waterspouts tend to occur most frequently in tropical waters.
Wave A small cyclonic circulation in the early stages of development that moves along a cold front.
Wave cloud A stationary cloud formed by the air "bouncing" over a mountain range.
Weather The conditions in the atmosphere at any given time.
Weather Lore Empirical sayings and rhymes based on the experience and superstitions.
Wind Air in motion that moves relatively horizontally in relationship to the surface of the Earth.
Wind Direction The direction from which the wind is blowing.
Wind Shear A sudden shift in wind direction.
Wind-chill Factor A measure of the effect of wind in increasing the heat loss from exposed flesh.


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