Loughborough Weather

Weather Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Adiabatic:

Changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere, with no exchange of heat with the surrounding air

Advection:

The transfer of heat by the horizontal movement of air or water, such as a warm wind or ocean current

Advection Fog:

Fog caused by the condensation of water vapour when warm, moist air crosses cold ground or sea

Air:

A naturally occurring mixture of gases, chiefly nitrogen and oxygen with small amounts of argon, carbon dioxide, and water vapour

Air Mass:

A large body of air with the fairly uniform temperature and humidity of its source region

Air Pressure:

The weight of the atmosphere pressing down on the Earth's surface. The effect is caused by gravity. Also called 'atmospheric pressure'

Albedo:

The measure of the reflectivity of a surface

Anabatic:

Rising, as when air moves up a hillside

Anemometer:

An instrument for measuring the force and speed of wind

Angular Momentum:

The energy of motion of a spinning body or mass of air or water. Its value depends on the mass of the body, its radius of spin, and its rate of spin

Angular Velocity:

The rate at which a spinning body, such as a planet, rotates. The Earth rotates at 15 degrees per hour, which is its angular velocity

Anticyclone:

A large area of high atmospheric pressure, characterised by outward-spiralling winds - a "high"

Atmosphere:

A layer of gases surrounding a planet - on the Earth, chiefly made up of the gases Oxygen, Hydrogen and Nitrogen

Atmospheric Pressure:

The weight of the atmosphere pressing down on the Earth's surface. The effect is caused by gravity. Also called 'air pressure'

Atoms:

The smallest parts of an element - such as hydrogen or oxygen

Barograph:

An instrument that provides a continuous record of air pressure on a paper strip.

Barometer:

An instrument for measuring atmospheric pressure and used to predict the weather.

Beaufort Scale:

A scale that indicates wind speed by the effect wind has on familiar objects:

Force 0
Strength: Calm
Speed: Less than 1 mile per hour (mph), less than 2 kilometres per hour (kph)
Observations: Tree leaves don't move, smoke rises vertically, sea is calm

Force 1
Strength: Light Air
Speed: 1-3 mph, 2-6 kph
Observations: Tree leaves don't move, smoke drifts slowly, sea is lightly rippled

Force 2
Strength: Slight Breeze
Speed: 4-7 mph, 7-11 kph
Observations: Tree leaves rustle, flags wave slightly, small wavelets or scale waves

Force 3
Strength: Gentle Breeze
Speed: 8-12 mph, 12-19 kph
Observations: Leaves and twigs in constant motion, small flags extended, long un-breaking waves

Force 4
Strength: Moderate Breeze
Speed: 13-18 mph, 20-29 kph
Observations: Small branches move, flags flap, waves with some whitecaps

Force 5
Strength: Fresh Breeze
Speed: 19-24 mph, 30-39 kph
Observations: Small trees sway, flags flap and ripple, moderate waves with many whitecaps

Force 6
Strength: Strong Breeze
Speed: 25-31 mph, 40-50 kph
Observations: Large branches sway, flags beat and pop, larger waves with regular whitecaps

Force 7
Strength: Moderate Gale
Speed: 32-38 mph, 51-61 kph
Observations: Whole trees sway, large waves ("heaping sea")

Force 8
Strength: Fresh Gale
Speed: 39-46 mph, 62-74
Observations: Twigs break off trees, moderately high sea with blowing foam

Force 9
Strength: Strong Gale
Speed: 47-54 mph, 75-87 kph
Observations: Branches break off trees, shingles blown from roofs, high crested waves

Force 10
Strength: Whole Gale
Speed: 55-63 mph, 88-101 kph
Observations: Some trees blown down, damage to buildings, high churning white sea

Force 11
Strength: Storm
Speed: 64-74 mph, 101 kph-119 kph
Observations: Widespread damage to trees and buildings, mountainous waves

Force 12
Strength: Hurricane
Speed: 75 mph or greater, 120 kph or greater
Observations: Severe and extensive damage

Black Ice:

Transparent ice that forms when liquid water on the ground freezes - for instance when the temperature falls sharply after rain.

Blizzard:

Snow blown by winds with an average speed of at least 52 km/h (32 mph).

Celsius:

A scale of temperature based on one introduced in 1742 by Celsius, a Swedish astronomer and physicist, who divided the interval between the freezing and boiling points of water into 100 parts, the lower fixed point being marked 100. The present system, where the freezing point is marked 0 and the boiling point is marked 100, was introduced by Christin of Lyons, in 1743. This latter scale is now referred to as the Celsius scale; alternative names are the centigrade scale, and less commonly the centesimal scale.

Chinook:

See 'föhn'.

Climate:

The long-term average weather pattern of a region.

Cloud:

A structure formed in the lower atmosphere by condensed water vapour and ice particles.

Cold Front:

The boundary between two different air masses where cold air pushes warm air out of the way and brings colder weather.

Condensation:

The process by which water vapour becomes liquid water.

Condensation Nucleus:

A microscopic particle of dust or salt, upon which water vapour condenses in the atmosphere.

Conduction:

The process of heat transfer through materials and adjoining substances.

Continentality:

The tendancy for the middle regions of continents to have a wider temperature range than coastal areas.

Convection:

The process of heat transfer through fluids by means of rising currents.

Coriolis Force:

An effect caused by the Earth's rotation, which causes winds and currents to follow a curved path across the Earth's surface - to the right (clockwise) in the northern hemisphere, to the left (anti-clockwise) in the southern hemisphere.

Cyclone:

A large area of low atmospheric pressure, characterised by inward-spiralling winds. a "low" - also called a "depression". Also the name used for a hurricane in the Indian Ocean and Western Pacific.

Depression:

A low-pressure weather system; a "low". Also called a cyclone.

Desert:

An area, either hot or cold, where the annual precipitation is less than 25 cm (10 in).

Dew:

Liquid water that has condensed on to objects at or near the Earth's surface.

Dew Point:

The temperature at which water starts to condense out of a particular air mass.

Downburst:

A strong downdraught of short duration produced by some thunderstorms.

Downdraught:

An air current that is moving vertically downwards.

Drizzle:

Light rain with drops smaller than 0.5 mm (0.02 in).

Drought:

Prolonged and abnormal shortage of water caused by lack of rainfall.

Dust Bowl:

Area in the Great Plains region of the USA where soil erosion occurred during 1930-31 as the result of prolonged drought.

Evaporation:

The process where liquid water turns into vapour, apparently disappearing. e.g. a rain puddle may evaporate when the sun comes out - it is the opposite of 'condensation'.

Evapotranspiration:

The loss of water to the atmosphere as a result of the combined effect of evaporation and the transpiration of plants.

Exosphere:

The outermost layer of the Earth's atmosphere.

Fahrenheit:

A scale of temperature introduced in about 1709 by the German physicist, Fahrenheit, who was the first to use mercury as the thermometric substance. Primary fixed points were the temperatures of a mixture of common salt and ice and the temperature of the human body; with reference to these the freezing point of water was marked 32 degrees, and the boiling point of water was marked 212 degrees.

Fog:

Water droplets in the air that reduce visibility to less than 1,000 m (1,100 yards).

Föhn:

The European name for a dry, downslope wind. Also called "chinook" in North America.

Front:

The boundary between two air masses.

Frost:

White ice crystals deposited on the surface of objects that have a temperature below the freezing point of water.

Geostrophic Scale:

A scale relating to the spacing of isobars to the wind speed.

Hail:

Pieces of hard, solid ice falling from clouds.

Haze:

Impaired visibility as a result of smoke or dust.

Hemisphere:

One half of a sphere. The term is usually applied to regions north or south of the equator.

High:

Also known as an 'anticyclone' - an area of high air pressure with a system of winds rotating outwards. This usually means dry weather. It is the opposite of a 'low'.

Humidity:

The amount of water vapour in the air.

Hurricane:

A tropical revolving storm with sustained wind speeds of more than 118 km/h (73 mph). Also known as a typhoon or cyclone.

Hygrometer:

An instrument for measuring humidity.

Inversion:

A reversal of the normal lapse rate of decreasing temperature with increasing altitude.

Ionosphere:

A region of the upper atmosphere that reflects some radio waves.

Isobar:

A line on a map or chart that links points of equal atmospheric pressure.

Jet Stream:

A strong, high-level wind that can reach speeds in excess of 320 km/h (200 mph) - it is usually around 5-10 km up, in the atmosphere.

Katabatic Wind:

A wind which blows down a slope.

Lapse Rate:

The rate at which air temperature decreases with existing altitude.

Latitude:

Position on the Earth's surface north or south of the equator.

Lee:

The side of a mountain, hillside, or island that is facing away from the prevailing wind.

Lightning:

Discharge of static electricity in the atmosphere, usually between the ground and a storm cloud.

Longitude:

Position on the Earth's surface east or west of the Greenwich meridian.

Low:

Also called a 'depression' - this region of low pressure can mean wet weather - it is the opposite of 'high' pressure or 'anticyclone'.

Mesosphere:

The layer of the Earth's atmosphere above the stratosphere.

Meteorologist:

Someone who makes a scientific study of weather and weather processes.

Meteorology:

The science of the atmosphere - meteorology embraces both weather and climate and is concerned with all aspects of the Earth's atmosphere (and those of the planets) and with the interaction between the atmosphere and the surface. The term was first used by Aristotle.

Millibar:

International unit for measuring air pressure, now sometimes called a hectopascal.

Mist:

Slight impairment of visibility resulting from water droplets suspended in the air.

Monsoon:

The seasonal shift in wind direction that brings alternate very wet and very dry seasons to India and much of Southeast Asia.

Numerical Models:

Modern meterology uses a number of mathematical models made up of systems of mathematical equations. These equations require complex and powerful computers which constantly process huge amounts of data to help us analyse the environment and make predictions about its future behaviour.

Orographic Rainfall:

Enhanced rainfall as a result of moist air cooling as it crosses a mountain range.

Occluded Front:

The combination of warm and cold fronts as a cold front overtakes a warm front. The front develops during the later stages of the life cycle of a frontal depression and is so called because of the associated occluding (shutting off) the warm air from the Earth's surface.

Ozone:

A form of oxygen which has three atoms instead of the usual two. In the troposphere, ozone is a pollutant. In the stratosphere, ozone filters out harmful ultraviolet radiation.

Polar Front:

The surface between polar and tropical air masses, along which cyclonic disturbances form.

Precipitation:

Moisture that is released from the atmosphere as rain, drizzle, hail, sleet or snow, as well as dew and fog.

Q-Code:

A letter code used by aircraft in requests for information and used in the supply of information to aircraft. Items in the code relate to meteorological information, for example, QFE relates to station-level pressure.

Quasi-Stationary Front:

Front: A front whose position is (almost) unchanged on successive pressure charts. There is a strong tendency in such cases for wave-like disturbances of the front to form.

Radar:

A system of detecting and locating targets which are capable of reflecting high frequency radio waves (microwaves), generally in the wavelength range from a fraction of a centimetre to some tens of centimetres. Radar is used in meteorology to detect and measure cloud and precipitaion elements. Radar images are regularly used in television weather broadcasts to show the movement of rain bearing cloud formations across the country.

Radiation:

Process by which energy travels across space.

Radiosonde:

A small radio transmitter which is attached to a balloon and released to measure pressure, temperature and humidity in the upper atmosphere.

Rain:

Drops of liquid water falling from clouds.

Rain Shadow:

An area of decreased rainfall on the lee side of a hill or mountain.

Refraction:

The bending of light as it passes from one medium to another e.g., from air to water.

Ridge:

An elongated area of high air pressure.

Sea-Level:

The normal level of high-tide, used as a baseline for measuring height or depth.

Seeder-Feeder:

The process of orographic rainfall by which high-level clouds 'seed' rain clouds at a lower level.

Seeding:

The process by which condensation nuclei are artificially released in the atmosphere to encourage precipitation.

SIGWX:

The abbreviation for Significant Weather Effects - weather that affects flying conditions.

Smog:

Dirty fog produced by air pollution in cities, and often occurring beneath a temperature inversion. The action of sunlight can produce photo-chemical smog.

Snow:

Ice crystals that fall from clouds and which may stick together to form snowflakes.

Snowline:

The vertical limit of snow lying on mountain sides throughout the year.

Snowpack:

Snow lying on the ground for any significant period of time.

Solstice:

The time of the year when the Sun appears to be directly overhead at either the Tropic of Cancer or the Tropic of Capricorn.

Stratosphere:

The layer of the Earth's atmosphere above the troposphere.

Streamline:

A line used on some weather maps to indicate wind flow.

Synoptic Chart:

A map showing the weather in an area at a given moment

Thermometer:

An instrument for measuring temperature. In normal meterological practice, mercury-in-glass thermometers are used. For extreme conditions alcohol (which has a freezing point of -114 degrees C) thermometers are used.

Troposphere:

The lowest level of the atmosphere, from the ground upward to about 16 km at the equator and 11 km at the polar regions.

Typhoons:

A name of Chinese origin (meaning 'great wind' applied to the intense tropical cyclones which occur in the western Pacific Ocean. They are basically the same as the hurricanes of the Atlantic Ocean and the cyclones of the Bay of Bengal.

Uncinus:

A cloud species, from the Latin for 'hooked'. A cirrus cloud often shaped like a comma, and with a hook or tuft at the top.

Undersun:

A halo phenomenon produced by reflection of sunlight on ice crystals in clouds. It can be seen from aircraft or mountains and looks like a brilliant white spot on the clouds below the sun.

Undulatus:

Clouds in patches, sheets or layers, which show undulations like waves. Comes from the Latin for 'waved'.

Upbank Thaw:

A thaw, or marked rise in temperature at mountain level, which happens before the thaw takes place in a valley, sometimes many hours earlier.

Upslope Fog:

Fog which is formed on the windward slopes of high ground by the forced uplift of stable, moist air until saturation is reached by adiabatic expansion.

UV Index:

UV Index Description, Media Graphic Colour and Recommended Protection:

UV Index 0–2 Low danger to the average person Green Wear sunglasses; wear sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.

UV Index 3–5 Moderate risk of harm from unprotected sun exposure Yellow Wear sunglasses and sunscreen, cover the body with clothing and a hat, and seek shade around midday when the sun is most intense.

UV Index 6–7 High risk of harm from unprotected sun exposure Orange Wear sunglasses and sunscreen having SPF 15 or higher, cover the body with clothing and a wide-brim hat, and reduce time in the sun from two hours before to three hours after solar noon (roughly 11:00 to 16:00 during summer in zones that observe daylight saving time).

UV Index 8–10 Very high risk of harm from unprotected sun exposure Reddish-purple Same precautions as above, but take extra care — unprotected skin can burn quickly.

UV Index 11+ Extreme risk of harm from unprotected sun exposure Violet Take all precautions, including: wear sunglasses and sunscreen, cover the body with a long-sleeve shirt and pants, wear a broad hat, and avoid the sun from two hours before to three hours after solar noon.

Valley Wind:

A wind which blows up a valley during the day in quiet, clear conditions (see anabatic), the opposite at night being the mountain or katabatic wind which blows down into the valley.

Veering:

The action of the wind changing in a clockwise direction.

Warm Front:

The boundary between two different air masses where warm air pushes cold air away to bring warmer weather.

Water Vapour:

The boundary between two different air masses where warm air pushes cold air away to bring warmer weather. : Water in its gaseous form. One of the most important constituents of the atmosphere and also the most variable in space and time. Caused by evaporation at the Earth's surface the concentration of water vapour tends to decrease fairly steadily with height.

Wave Cyclone:

A storm or low-pressure centre that moves along a front.

Weather:

The changing atmospheric conditions at a place or time, including temperature, humidity, wind, cloud and precipitation as they affect man.

Xenon:

One of the inert gases that can be found in the air in minute quantities. The others are Helium, Neon, Argon, Krypton and Radon, and are called inert gases or noble gases because of their chemical inactivity.

Year:

In meteorological terms, the full cycle of the seasons, that is, the time taken by the sun to transverse the Ecliptic completely - or Tropical Year. It is equal to 365.242194 mean solar days.

Zephyr:

A westerly breeze with pleasant warm weather supposed to prevail at the summer solstice.

Zonal Flow:

West to East airflow, as opposed to Negative Zonal Flow which is East to West.