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Absolute drought

A period of at least 15 consecutive days, to none of which is credited 0.01 inches / 0.2 mm or more of precipitation. (See also drought; partial drought; dry spell)

Absolute vorticity

(or total vorticity) of air particles at any particular point is comprised of two elements: (i) on the rotating earth, air adopts the local vorticity due to the earth's solid-body rotation about its pole-to-pole axis, which is latitude dependent, and is known as the Coriolis parameter. This increases to a maximum over the poles and decreases to zero at the equator. The Coriolis rotation sense is always positive (or zero). (ii) the other element is known as the relative vorticity, the 'spin' tendency of air particles due to their motion relative to the earth - driven by atmospheric forces. Relative vorticity can be either positive (cyclonic sense) or negative (anticyclonic). (See also: Vorticity; Relative vorticity.)

AC

(abbr) Altocumulus (AC for METAR/SIGWX charts etc., Ac otherwise); a medium level, layer cloud type, but with varying levels of instability associated - the extreme event being ACCAST.

ACCAST

(" Alto-Q-Cast ")(Altocumulus castellanus or castellatus; the proper abbreviation for the Daily Register is 'Ac cas') These are medium level clouds (between circa 8000 and 18000 ft; 2.5 to 5.5 km) which exhibit, at least in their upper parts, marked cumuliform / turreted appearance - the convective towers are often taller than the width of the individual base; the bases are connected and often appear to be arranged in lines or streets. These clouds are a good indicator of medium level instability and high moisture content - and often the precursor for widespread thundery activity within the following 24 to 48 hours. Observers used to SYNOP coding may also refer to these (along with Altocumulus floccus/Ac flo), as CM8 clouds.

Adiabatic

A process where temperature changes occur in a 'system', without heat being supplied to, or lost from that system. In meteorology, used in connection with changes involving air parcels moving vertically in the atmosphere. [ If heat exchange is involved, the process is non-adiabatic (or diabatic)].

Adiabatic lapse rate

The change of temperature of an air parcel rising (cooling) or descending (warming) adiabatically. For dry (i.e. unsaturated) ascent or descent, the rate is ~10°C / km; for moist or saturated processes, it varies but a useful average is 5 or 6°C / km.

Advection

The transfer by horizontal air movement of heat, moisture (or humidity), momentum etc. The atmosphere at all levels is usually in some form of motion at most times, thus it is necessary to identify areas of significant advection. For the low/middle troposphere thickness products (e.g. 500-1000 hPa) are often used.

Aerosols

Solid particles suspended in the air. They include dust, salt particles, products of combustion etc. These are very important to the formation of water droplets or ice particles in the atmosphere, acting as nuclei for condensation / sublimation.

Ageostrophic effects

See "Why does the wind blow?"

Air mass

An extensive volume of air possessing uniform physical characteristics at similar heights in terms of humidity and temperature structure.

Albedo

The proportion of solar incoming visible light (after passage through the atmosphere) reflected by a surface (e.g. sea, cloud-tops, ice etc.), expressed as a fraction or percentage of the incident light falling on that surface. Clouds have highly varying albedo, dependent upon thickness and composition. Old snow is about 55% (or 0.55), new snow around 80% (or 0.8). Water surfaces vary from very low (about 5% or so) at high sun elevation, to at least 70% (0.7) at low sun-angles: very smooth water surfaces with a low sun-angle can give rise to the phenomenon known as 'sun glint' sometimes seen in visible satellite imagery where the albedo value is very high.

AMSL

(abbr) Above mean sea level.

Anabatic wind

A local-scale wind which blows up-slope, after strong heating of the hill/mountain-side by the sun. Such upslope winds can sometimes drag fog/stratus in the valley bottom (formed after a cold night) to upland areas that were previously clear of these phenomena. Airfields that were previously clear can suddenly 'fog-out' well after sunrise due to this effect.

Ana-front

When warm air ascends relative to the cold air at a frontal surface, the front is said to be an ana-front. Such fronts are normally 'active', in that thick/precipitation producing clouds (possibly with embedded instability), are usually located in the warm air associated with both a warm and cold front.

Anticyclones

A pressure feature where a maximum of pressure is surrounded by relatively lower values. On a synoptic chart, a system of closed isobars will be found, enclosing the central 'High'. The circulation (of wind) is clockwise in the northern hemisphere (anticlockwise in the southern hemisphere). The structure of such features in the vertical is that high-level convergence coupled to gentle divergent-outflow at the surface leads to descent (or subsidence) of air within the anticyclone: this in turn leads to a decrease in humidity and an increase in the stability of the air, often producing an inversion close to the surface. Two types are defined: cold and warm; see Cold anticyclone; Warm anticyclone.

Anticyclonic trough disruption

If the northern (southern in the southern hemisphere) portion of an upper trough moves forward and warms out, leaving a quasi-stationary cut-off low in the base of the trough, the process is described as anticyclonic trough disruption - because the net result is a strong build of pressure/new high cell formation behind the retreating trough. (See also cyclonic trough disruption)

Arctic Oscillation

The Northern Hemispheric broad-scale oscillation of long-wave type, of which the North Atlantic Oscillation (NAO) is our regional-scale component. (q.v. here)

AS

(abbr) Altostratus (AS for METAR/SIGWX charts etc., As otherwise); a medium level, layer cloud type, formed by wide-scale rising motion in the troposphere, varying from thin, non-precipitating type through which the sun/moon can be seen, to thick layer(s) associated with frontal development giving persistent, significant precipitation, & moderate in-flight turbulence & icing.

Asynoptic

Over the many years that operational meteorology has developed, certain hours have been designated 'synoptic' hours, and observation times standardised around these points; the MAIN synoptic hours currently being 00, 06, 12 and 18 UTC(formerly GMT), with intermediate hours at 03, 09, 15 and 21 UTC. Increasingly however, observing systems (e.g. satellite, radar-networks, drifting buoys etc.) provide data at times other than these 'fixed hours' - these are designated non, or 'asynoptic' observations. NWP models can assimilate these observations during the initialisation process.

AVN

Abbreviation used to stand for 'aviation'.