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Glossary - Meteorology

Absolute Humidity A statement of the amount of water vapour in the air at a place by the expression of it in grams per cubic metre. It is now being referred to as vapour concentration.
Accretion The growth of a precipitation element such as a snow flake, or an ice deposit by the collisions with it by supercooled water droplets.
Adiabatic Process An adiabatic process is one in which the temperature of a body of gas is changed without the transmission of heat energy into or out of the sample undergoing the change.
Advection The transfer of air over the earth horizontally. This is simply another term for horizontal wind.
Advection Fog Fog that is produced by the action of relatively warm humid air being transported over a cool surface.
Aggregation The process by which snow flakes or ice crystals are permitted to grow by collisions and resulting adherence to each other.
Air Mass A large body of air containing approximately homogeneous properties throughout. This applies especially to temperature and humidity. To qualify as an air mass, the body of air must be several thousands of kilometres across and also cover several degrees of latitude.
AIREP Aircraft weather report, provided by the pilot of an aircraft in flight.
Altocumulus White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed of laminae, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not be merged; most of the regularly arranged small elements usually have an apparent width of between one and five degrees.
Altostratus Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Altostratus does not show halo phenomena.
Anabatic Wind A local wind which results when air adjacent to sloping ground is heated by radiation from the earth’s surface during the day.
Anemometer A device designed to permit the measurement of wind speed.
Anticyclone A pressure system consisting of a series of closed isobars surrounding an area of high pressure, with the highest pressure at the centre of the system. In most cases the resulting closed curves are in the form of a series of ovals.
Anvil Cloud The upper portion of a cumulonimbus cloud which has spread out under a stable layer of air, usually the tropopause. This shape occurs as the result of the cloud billowing from beneath.
Atmosphere The envelope of gases which surrounds the earth and is held near it by the force of gravity.
Backing A change of wind direction which occurs in an anticlockwise sense.
Beaufort Scale A scale of wind strengths adopted by Commodore (later Admiral) Francis Beaufort in 1805 determined by the state of the sea and divided into 12 stages (or forces). During the first half of the twentieth century, the scale was expanded to 17 forces.
Blocking The process which sometimes occurs in the middle latitudes when a large slow-moving anticyclone teams up with a mid-latitude depression to create a strong easterly wind flow between them. The result is to slow or even prevent their eastward migration
for a period of several days.
Blowing Dust Dust that is raised locally by the wind in such a way as to reduce the visibility (i.e. to a height of more than 2 metres above the ground), but in such a way as not to qualify as a dust storm.
Blowing Snow Fine powdery snow raised locally by the wind to a height of more than two metres above the ground.
Breeze A wind of light to moderate strength.
Buys Ballots Law The rule attributed to the Dutch meteorologist C.H. Buys Ballot, who was the first person to achieve a synoptic analysis over Europe in 1857. From it he made the observation that the wind flows at right angles to the pressure gradient in such a way that higher pressure is to the right in the Northern hemisphere. The reverse is true in the southern hemisphere.
Calm A condition in which the surface wind speed is zero.
Carbon Dioxide CO2 The fourth largest gas by volume in the atmosphere, accounting for approximately 0.03%.
Carbon Monoxide CO A noxious gas produced by the incomplete burning of substances containing carbon. It is present in the atmosphere only in minute amounts.
Castellanus A species of cloud genera which exhibits a formation of protuberances along its top and which appear in the form of cumuliform turrets. It is most commonly found with Altocumulus but may also be found with Cirrus, Cirrocumulus and Stratocumulus. It indicates the presence of convection and hence instability within the cloud.
Ceilometer An electronic device for measuring the height of the cloud base.
Celsius Scale A scale of temperature attributed to the Swedish astronomer Anders Celsius in 1742. It is based on the principle that water freezes at 0 degrees and boils at 100 degrees. Also known as the Centigrade Scale.
Cirrocumulus Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripples, etc., merged or separate, and more or less regularly arranged; most of the elements have an apparent width of less than one degree.
Cirrostratus Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena.
Cirrus From the latin, cirrus ‘lock or tuft of hair’. Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous (hair-like) appearance, or a silky sheen, or both.
Clear Air Turbulence CAT CAT is turbulence which is normally associated with cloud. It is used to describe patches of turbulence which are associated with windshears at the edges of upper troposphere jetstreams. It may be severe and highly dangerous to high-flying jet aircraft.
Clear Ice Generally, a uniform and transparent or translucent deposit of ice formed by the freezing of supercooled drizzle droplets or raindrops on objects, the surface temperature of which is below or at 0°C. Sometimes referred to as glaze ice.
Cloud A collection of minute water droplets or ice crystals which are not individually visible but as a collection are visible floating in the air above the earth’s surface, normally within the troposphere or occasionally in the lower stratosphere.
Cloud Condensation Nuclei Minute solid particles in suspension in the air, upon which water vapour condenses to form a cloud particle.
Coalescence The growth of raindrops by collisions with other water drops or the growth of ice pellets by collision with water drops or droplets.
Col A synoptic system which lies between two anticyclones (or ridges) and two depressions (or troughs) arranged at opposite sides of the system. Characterised by light variable winds.
Cold Front The boundary between a warm air mass and an advancing cold air mass.
Cold Occlusion A variety of occluded front involving air masses such that the air behind the cold front associated with a frontal depression, is colder than the air ahead of the warm front. The result is that the cold front appears to remain at the surface, while the warm sector and warm front are lifted from the surface.
Condensation The process of formation of liquid water from water vapour.
Conditional Stability Stability of a sample of air which depends upon the sample remaining unsaturated.
Conduction The process by which heat energy is transferred by direct contact from one body (or mass) to another without the transfer of matter.
Contour (Contour Line) In synoptic meteorology, a line of constant height of an isobaric surface.
Contour Analysis A method of analyzing the pressure field over a region by drawing contour lines for the height of a given pressure surface.
Contrail A trail of water droplets or ice crystals produced by an aircraft engine exhaust when products of fuel combustion add humidity to the air in suitable conditions. To be distinguished from aerodynamic trail.
Convection The process by which samples of air move vertically in the atmosphere by virtue of the fact that they are either warmer or cooler than their environment.
Convection Condensation Level CCL The level determined from an aerological diagram, at which the base of convective cloud would occur if moist air at the surface was to be warmed to the convection temperature.
Convergence The drawing together of air, usually along a line, as in a trough, or near the surface in a depression.
Coriolis Effect (Force) An apparent acceleration that air possesses by virtue of the earth’s rotation, with respect to axes fixed in the earth. It is sometimes