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Height of sea level relative to the centre of the Earth. See also relative sea level.

The process of supercooled water droplets freezing on impact with snowflakes, ice particles or other cold objects including aircraft.

Cooling of a parcel of gas by expansion, with no heat exchange between the parcel and the surrounding air.

Warming of a parcel of gas by compression, with no heat exchange between the parcel and the surrounding air.

A process where a parcel of air cools or warms due to a change in pressure and volume (expansion or compression), with no heat exchange between the parcel and the surrounding air.

Fog transported by local winds from one locality to another.

Horizontal movement of air in the lower atmosphere due to wind.

Fog resulting from the movement of moist air over a colder surface and the consequent cooling of the air to saturation.

The delay in time between the transit of the moon and the highest spring tide. Normally one or two days, but it varies widely. In many places the maximum tidal range is one or two days after the new or full moon, and the minimum range is a day or two after first and third quarter.

The mass of air per unit volume.

Large body of air with approximately uniform temperature and water vapour throughout.

Any deposit of ice forming on an aircraft.

Instrument that determines altitude using atmospheric pressure.

The vertical distance from mean sea level to an object aloft.

A principal cloud type, forming in the middle levels of the troposphere, and appearing as a white and/or grey layer or patch with a waved aspect. In aviation forecasts and reports it is coded as AC.

A principal cloud type, forming in the middle levels of the troposphere, and appearing as a grey or bluish sheet. In aviation forecasts and reports it is coded as AS.

A point in the sea which is a node with little or no tidal amplitude around which the tide rotates, for a particular tidal constituent.

One half of the range of a constituent, tide or wave in general.

An uphill wind generated by the heating of a sloping surface.

Instrument used to measure wind speed and, in some cases, wind direction.

Rate of rotation, usually expressed in radians per unit of time (as compared to cycles per second).

The non-standard propagation of a beam of energy (radio or radar) under certain atmospheric conditions, which results in false echoes (i.e. non-precipitation) on a radar image. Usually caused by unusual rates of refraction in the atmosphere.

Difference from the average value.

For example, if the maximum temperature for June in Melbourne was 1 °C higher than the long-term average for this month, the anomaly would be +1 °C.

The international standard is to use the 30-year average from 1961 to 1990 as the long-term average. However, more recent periods (for example 1981 to 2010) will sometimes be used to calculate an anomaly that is more descriptive of the current climate. Anomalies calculated from periods of less than 30 years may not accurately describe all the variation that can be expected.

Aircraft equipment, such as heating elements and flexible rubber strips, used to prevent or clear structural icing.

Also called high pressure systems, anticyclones are circulations of high air pressure that rotate anticlockwise in the southern hemisphere. They generally bring lighter winds and fine, settled conditions.

The upper portion of a cumulonimbus cloud that spreads out under the tropopause, often in the shape of a blacksmith’s anvil, sometimes for hundreds of kilometres downstream from the parent cloud. It indicates the mature or decaying stage of a thunderstorm.

The point in the elliptical orbit of Earth or other planet when it is furthest from the Sun.

The point in the Moon’s elliptical orbit when it is furthest from Earth. The tidal range tends to be reduced at this time – this is referred to as an apogean tide.

Apparent (‘feels like’) temperature measures how warm or cool you might feel, which can be different from the measured temperature. Our instruments measure ‘ambient temperature’ – how warm the air is in the shade and sheltered from the wind. However, apparent temperature also takes into account the effect of wind and humidity. It is based on a mathematical model of an adult walking outdoors in the shade.

The points in the orbit of a planet or moon which are the nearest and farthest from the centre of gravity. In the Earth's orbit these are called perihelion and aphelion, and in the Moon's orbit, perigee and apogee. The line passing through the apsides of an orbit is called the line of apsides.

An angle, which is a function of the longitudes of the Moon, Sun, lunar perigee, lunar node and solar perihelion, that is used in the determination of the tide-generating potential of a particular constituent at a particular time.

The gaseous portion of the physical environment that encircles the earth. The divisions of the atmosphere are the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere.

The total weight of the atmosphere above the point of measurement.

Periodic variations of the atmosphere caused by gravitational and radiational effects, analogous to ocean tides. See also radiational tides.

Mapping system that sets mean sea level (measured from 1966 to 1968) as zero elevation. Australian mapping measures the height of features from this level.

A perpetual or ongoing entitlement, by or under a law of a State, to exclusive access to a share of the water resources of a water resource plan area or other water management area.

The specific volume of water allocated to an Australian water access entitlement in a given water accounting period.

Weatherproof enclosure that usually includes a thermometer, barometer and humidity probe. It may also have a rain gauge and anemometer attached, and communications equipment to transmit the data it collects in real time. An AWS takes automated measurements of weather elements, including some or all of these:

  • air temperature
  • humidity
  • wind speed and direction
  • atmospheric pressure
  • precipitation
  • soil temperature at various levels below ground
  • sea level
  • cloud base height
  • horizontal visibility.

One of the 4 seasons in a year. In the southern hemisphere it’s March, April and May.

A horizontal angle calculated clockwise from the meridian, especially the horizontal direction of a celestial point from a terrestrial point, expressed as the angular distance from a reference direction (hence often designated as true, magnetic, compass or relative, depending on the reference).

When applied to current or stream, it is a direction toward which the current or stream is flowing, and usually calculated from the north point. See also celestial sphere.

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