Weather Glossary - A

Absolute humidity

The mass of water vapour in a given volume of air. It represents the density of water vapour in the air. See also Specific humidity, Relative humidity.

Absolute zero

The temperature at which (theoretically) there is no molecular/atomic motion. (-273°C or -460°F). Absolute zero is the lowest possibly attainable temperature.

Absorption

The process in which incident radiant energy is retained by a substance.

ACCAS

(Pronounced Ack-kas) AltoCumulus CAStellanus Latin - castle
Mid-level clouds (bases generally 2000 - 8000m), of which at least a fraction of their upper parts show cumulus-type development. These clouds are often taller than they are wide, giving them a turret-shaped appearance. AcCas clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.

access-g

Australian Community Climate and Earth-System Simulator (ACCESS). Based upon the UK Met Office Unified Model system. ACCESS G is the global numerical forecast model operated by the BoM. It is run twice daily (00Z and 12Z) and provides forecast data out to 240 hours with a horizontal resolution of 80km and 50 vertical levels.

access-r

Australian Community Climate and Earth-System Simulator (ACCESS). Based upon the UK Met Office Unified Model system. ACCESS R is the regional numerical forecast model operated by the BoM. It is run twice daily (00Z and 12Z) and provides forecast data out to 72 hours with a horizontal resolution of 37.5km and 50 vertical layers.

Accessory cloud

A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds and wall clouds are examples of accessory clouds.

Accretion

The growth of a precipitation particle by the collision of ice crystals or snow-flakes with supercooled liquid droplets that freeze upon impact.

Adiabatic

A thermodynamic process in which no heat is transferred to the surrounding air.

In an adiabatic process, compression of an air parcel results in an increase in temperature, while expansion results in a decrease in temperature.

Advection

Transport of an atmospheric property, e.g. heat or moisture, by the wind. See cold advection, moisture advection, and warm advection.

Advection fog

Fog which develops when a mass of relatively warm, moist air moves over a cooler surface and cools the air below its dew point temperature. Advection fog requires movement of air to form (hence the name). Advection fog is common at sea where it is called sea fog.

Air

The mixture of gases and particles which make up the Earth's atmosphere.

Air mass

A large body of air throughout which the horizontal temperature and moisture characteristics are similar.

Air mass thunderstorm

Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to solar insolation and dissipate rather quickly after sunset.

Air mass thunderstorms are generally less likely to be severe than other types of thunderstorms, but they are still capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 2cm in diameter.

Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable. Therefore the term is somewhat controversial and should be used with discretion.

Air pressure

A measure of the mass of air above a given point. Usually expressed in millibars (mb) or hectopascals (hPa). Also known as atmospheric or barometric pressure (pressure measured by a barometer).

Albedo

The reflectivity of a surface. It is the percent of radiation reflected from a surface compared to the radiation striking it. A perfectly reflective surface would have an albedo of 100. The Earth’s albedo is around 30, snow has a higher albedo, grasslands and forests a lower albedo.

Altimeter

An instrument to measure the altitude of an object above a fixed level. Generally, mean sea level is used for the reference level.

Altocumulus

(Ac) Mid-level cloud (bases generally 2000 - 8000m), made up of grey, puffy masses, sometimes appearing in parallel waves or bands. An indicator of mid-level instability. Altocumulus can take on various forms such as Ac Lenticularis, Ac Undulatus, Ac Castellanus, Altocumulus 'mackerel sky'.

Altocumulus Castellanus

A middle level cloud with vertical development that forms from altocumulus clouds. It is composed primarily of ice crystals in its higher portions and characterised by its turrets, protuberances or crenulated tops.

Altostratus

(As) Mid-level cloud composed of water droplets and ice crystals. Usually gives the sun a watery or dimly visible appearance.

Anabatic winds

A local wind that flows up the side of valleys due to increased heating along the valley walls. Often the anabatic wind results in cumulus clouds along the ridges either side of the valley. See also Katabatic winds.

Anemometer

A device used to measure wind speed.

Anomaly

The departure of an element from its long-term average for the location concerned. For example, if the average maximum temperature for Melbourne in June is 14 degrees and on one particular day the temperature only reaches 10 degrees, than the anomaly for that day is -4.

Anticyclone

A large scale atmospheric circulation system in which the winds rotate anti clockwise in the Southern Hemisphere (clockwise in Northern Hemisphere). Anticyclones are areas of high atmospheric pressure and are generally associated with light winds and stable weather conditions. Interchangeable with High pressure system.

Anticyclonic rotation

Rotation in the opposite sense as the Earth's rotation, i.e., anticlockwise in the Southern Hemisphere as seen from above. The opposite of cyclonic rotation.

Anvil

The flat, spreading top of a Cb (cumulonimbus) often shaped like an anvil. Thunderstorm anvils may spread hundreds of kilometres downwind from the thunderstorm itself, and sometimes may spread upwind (see back sheared anvil).

Anvil crawler

[Slang] A lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active mesoscale convective system (MCS). Also called sheet lightning.

Anvil dome

A large overshooting top or penetrating top on a thunderstorm.

Apparent Temperature

The apparent temperature (AT), invented in the late 1970s, was designed to measure thermal sensation in indoor conditions. It was extended in the early 1980s to include the effect of sun and wind. Only the modification due to wind is taken into account on this site. The AT index used here is based on a mathematical model of an adult, walking outdoors, in the shade (Steadman 1994). The AT is defined as; the temperature, at the reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity.

Basically the ATis an adjustment to the ambient temperature (T) based on the level of humidity. An absolute humidity with a dewpoint of 14°C is chosen as a reference (this reference is adjusted a little with temperature). If the humidity is higher than the reference then the AT will be higher than T; and, if the humidity is lower than the reference, then AT will be lower than T. The amount of deviation is controlled by the assumptions of the Steadman human model. In practice the AT is more intuitive to use than the WBGT, as it is an adjustment to the actual air temperature based on the perceived effect of the extra elements such as humidity and wind. AT is valid over a wide range of temperature, and it includes the chilling effect of the wind at lower temperatures.

Version including the effects of temperature, humidity, and wind:

Version including the effects of temperature, humidity, wind, and radiation:

Arcus

A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e. the gustfront). Roll clouds and shelf clouds both are types of arcus clouds.

Arid

An extremely dry climate - often referred to as a "desert" climate. Much of Australia is considered arid.

ASWA

Australian Severe Weather Association.

Atmosphere

The mixture of gases and particles surrounding the Earth where weather occurs.

Attenuation

A reduction in the strength of a radar echo when there is widespread rainfall. The second problem tends to occur in sitations where there is widespread rainfall or convectve activity, such as when thunderstorms are occurring. In this situation, some of the energy from the radar transmission is reflected by the closer rainfall in a given direction leaving less energy to be reflected by other rainfall in that same direction. Also, if there are mountains near the radar site, there may not be enough signal for the detection of precipitation behind the mountains.

Attenuation often results in the appearance of rainfall 'intensifying' as it nears the radar site. One should check other radar sites in the vicinity (if possible) to see if the rainfall is actually intensifying, or whether it is an extensive rainband.

Aurora

A glowing light display in the night-time sky caused by excited gases in the upper atmosphere. In the Southern Hemisphere it is called the Aurora Australis or Southern Lights, in the Northern Hemisphere Aurora Borealis (Northern Lights).

Autumn

One of the four seasons, indicating the transition between summer and winter during the months March, April and May (southern hemisphere). Autumn in Australia is characterised by the end of the northern wet season and the beginning of the southern wet season. Autumn also brings and end to the convective thunderstorm season in eastern states.

AVN

AViatioN model; one of the operational forecast previously models run at NCEP. The AVN has been superseded by the Global Forecast System (GFS).

AWS

Abbreviation for Automatic Weather Station.