Glossary


A

The terms and definitions found in the glossary relate to the science of meteorology.

For descriptions and definitions of words used in the Bureau's forecasts and warnings, please refer to Weather Words.

Advection
The sideways movement of air in the lower atmosphere due to the differences in air pressure (commonly called wind). Process of transfer of air mass properties by the velocity field of the atmosphere.
Air
The mixture of gases and particles that make up our atmosphere.
Amounts
Amounts shows the quantity of the element requested. For example, choosing the amounts column under Rainfall Maps will give a rainfall map of how much rainfall (in millimetres) has fallen for the time period and area chosen.

If you choose deciles, this gives an element a ranking, thus demonstrating how it compares with the surrounds. For example, a decile rainfall map will show whether the rainfall is above average, average or below average for the time period and area you have already chosen.
Analogue
A past situation, which resembles the current situation, is called an analogue, which can then be used as a guide for what is to come. For example, by finding similar SOI patterns to the current year, we can then examine rainfall patterns from that time to use as a guide as to what will happen this year. The Bureau of Meteorology's Seasonal Climate Outlook includes a listing of several analogue years in which the SOI behaved similarly to the current year.
Anemometer
A device used to measure wind speed.
Anomaly
The departure of an element from its long-period average value for the location concerned. For example, if the maximum temperature for June in Melbourne was 1 degree Celsius higher than the long-term average for this month, the anomaly would be +1 degrees Celsius. The current international standard is to use the 30 year average from 1961 to 1990 as the long-term average.
Anticyclones
Atmospheric circulations that rotate anti-clockwise in the southern hemisphere. Anticyclones are areas of higher pressure and are generally associated with lighter winds and fine and settled conditions.
The Apparent Temperature (AT) - Heat Index
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 by the Bureau is based on a mathematical model of an adult, walking outdoors, in the shade. 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.

The Apparent Temperature is available for each state and territory:

Atmosphere
The mixture of gases surrounding a planet.
Australian Height Datum (AHD)
In 1971 the mean sea level for 1966-1968 was assigned the value of zero on the Australian Height Datum at thirty tide gauges around the coast of the Australian continent. The resulting datum surface, with minor modifications in two metropolitan areas, has been termed the Australian Height Datum (AHD) and was adopted by the National Mapping Council as the datum to which all vertical control for mapping is to be referred. Elevations quoted using this datum are normally followed with the acronym (AHD). See Australian Height Datum - Geoscience Australia for further information.
Autumn
One of the four seasons of the year. The transition months March, April and May (southern hemisphere).
AWS
Automatic Weather Station.