Weather terms

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.

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

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.

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.

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.

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.

Atmospheric Pressure (also called air pressure or barometric 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.

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.

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.

Buys Ballots Law - A physical law describing wind flow around high and low pressure.

Campbell-Stokes recorder - A sunshine recorder that focuses the sun’s rays through a glass sphere burning to a card.

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 (also known as Centigrade) - A temperature scale in which 0 degrees is the melting point of ice and 100 degrees is the boiling point of water.

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.

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 .

Medium cloud 8000-20000 Feet made from supercooled water droplets and ice crystals

Cold Front - The leading edge of a colder mass of air that displaces a warmer mass of air.

Cold Front - A boundary between two airmasses where the colder one is advancing.

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 .

Dew Point - The temperature at which just 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.

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

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.

El Niño- (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.

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.

Evaporation - The change in a substance from a liquid to a gaseous state. I.e. Water into water vapour.

Eye - The roughly circular area of relatively calm weather at the centre of a hurricane, typhoon or Tropical Cyclone

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.

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.

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.

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.

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.

(GFS) Global Forecast System - 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.

Haar - A term used in northeast England and Scotland for fog and low cloud along the northsea coasts.

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.

Hectapascal - Has taken over from the millibar as a unit used to express atmospheric pressure. Fortunately 1 millibar is equal to 1 hectapascal.

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.

Hurricane - A tropical cyclone in the western hemisphere that has sustained wind speeds of 74 miles per hour or greater.

Hurricane - A tropical revolving storm which occurs in the south Atlantic, Caribbean or eastern Pacific.

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.

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.

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.

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- (Also know as the Absolute 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.

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.

Long Range Forecast - A weather forecast extending past 10 days ahead.

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.

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.

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.

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.

Numerical Forecasting - forecasting the weather through digital computations carried out by supercomputers.

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

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.

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.

Radar - Stands for "radio detection and ranging." An instrument that detects and ranges distant objects by measuring the scattering and reflection of radio beams.

Radiosonde - A package of instruments to measure temperature, humidity and pressure carried aloft by a helium filled balloon.

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.

Category		Wind Speed in mph	Storm Surge in m
1	Weak	74 – 95	1.2 – 1.5
2	Moderate	96 – 110	1.8 – 2.5
3	Strong	111 – 130	2.8 – 3.7
4	Very Strong	131 – 155	4 – 5.5
5	Devastating	> 155	> 5.5

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.

Shower - Precipitation falling from convective or cumuliform cloud. Normally has some sunshine or clear spells between the rain or snow.

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.

Sublimation - The change ice crystals directly into water vapour without passing through the water stage.

Supercell Thunderstorm - An unusually violent thunderstorm that is capable of generating tornadoes.

Subsidence - The descent of a body of air, usually in a high-pressure area, that warms the lower levels of the atmosphere.

Supercooled Water - Water droplets cooled to a temperature below 0 degrees Celsius without freezing. This state is quite common in the atmosphere.

Temperate Zone - The area of the globe between the tropics and the Polar Regions .

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.

Tropics - The area of the globe from latitudes 23.5 degrees north to 23.5 degrees south.

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.

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 .

Unstable Air - Air with temperature differences that encourage the formation of convection currents.

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.

Virga - Water droplets or ice crystals that fall from high clouds but that evaporate before hitting the ground.

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.

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. Often wrong.

Wind - Air in motion that moves relatively horizontally in relationship to the surface of the Earth.

Wind-chill Factor- A measure of the effect of wind in increasing the heat loss from exposed flesh.

Wind Direction - The direction from which the wind is blowing.