Glossary

Pulse of wind, cloud and rainfall that cycles eastwards around the Earth near the equator. It typically takes 30–60 days to make it all the way around the globe.

Any water storage that has a storage capacity of 1GL or more.

An air mass that has originated over an extensive water surface.

Region between the Indian and Pacific oceans, including the archipelagos of Indonesia, Borneo, New Guinea, the Philippine islands, the Malay Peninsula and surrounding seas. The region is made up of thousands of islands, mountainous terrain and many shallow seas.

The terms 'maritime' and 'continent' usually describe 2 opposite climate types. Used together, they describe the extensive interaction between ocean and land that happens across this region.

Average value. It's calculated by adding all the values in a dataset together and then dividing by the number of values.
For example, to calculate Hobart's average daily July rainfall, add together the daily rainfall totals and divide by 31 (the number of days in July).

A tidal level. The average of all high waters observed over a sufficiently long period.

Average level of bodies of water (the sea, lakes and so on) from which elevation can be measured.

A tidal datum. The arithmetic mean of hourly heights of the sea at the tidal station observed over a period of time (preferably 19 years). Also referred to as MSL.

Air pressure converted to equivalent pressure at mean sea level.

This is calculated from the air pressure recorded at various altitudes. As air pressure varies at different altitudes the readings must be converted to equivalent mean sea level pressures. This allows us to compare and analyse the pressures across an area. If they weren't converted, horizontal changes in pressure would be overridden by vertical variations, due to differences in altitude between observing stations.

A mean sea level pressure (MSLP) weather map therefore shows pressures affected by changing weather conditions, not distorted by the different altitudes where air pressure was recorded.

Disrupted air-flow resulting from wind flowing over or around terrain or manmade obstructions, whereby normal horizontal wind flow is disturbed and transformed into eddies and other irregular movements.

The middle value in a dataset when all values are ordered from lowest to highest. It exceeds half of all the values in the dataset. In the case of an even number, it's the mean of the 2 middle values.

For example, to find Hobart's median July rainfall:

• rank the rainfall amounts received each day in July from lowest to highest
• the middle value is the median.

Atmospheric circulation in which the north and south component of motion is pronounced.

Periodical or quasi-periodical changes in water level caused by the daily or seasonal variations in local meteorological conditions. See also radiational tides.

Science concerned with the processes and phenomena of the atmosphere, especially as a way to forecast the weather.

A severe downburst of wind, usually from a thunderstorm, over an area less than 4 km in diameter. If the area is larger, we use 'downburst' rather than 'microburst'.

In the southern hemisphere, areas between about 30° and 55° latitude.

For Australia, the mid-latitudes are south of a line from halfway between Geraldton and Perth in Western Australia to Bourke in New South Wales. This part of Australia generally has a temperate climate.

Any water storage in which water is stored for taking and that:

• is not a major storage
• has a storage capacity of 100 ML or more
• is not used primarily for the precipitation of ash
• is not a mine tailings dam
• is not managed by an urban infrastructure operator primarily for the purposes of flood management or pollution abatement.

Optical illusion caused by refraction (bending) of light in the atmosphere. This can happen when light passes through 2 air masses of different temperature.

For example, on a sunny day the ground can warm the air directly above it, while the next layer of air remains cold. In these conditions you may see the illusion of water over the road or in the desert.

Similar to fog, but you can see further than 1 km.

The type of tide characterised by large diurnal inequalities in heights and/or times of successive high and/or low waters. In general, a type of tide intermediate between predominantly semi-diurnal and predominantly diurnal. See also form factor.

The ratio of the mass of a variable atmospheric constituent to the mass of dry air. In meteorology, the term normally refers to water vapour.

Computer simulation of the atmosphere, ocean, land surface and sea ice. A model uses mathematical equations to predict future weather or climate. The predictions are based on the laws of physics and chemistry.

Wind blowing at speeds of 20–29 km/h.

Place where observations of the environment are made to a specific standard. This is typically a location where sensors measure properties of the environment (for example, river depth or air temperature).

Seasonal reversal of winds that happens in parts of the tropics.

In northern Australia, the prevailing wind is from the east or southeast for most of the year. During active monsoon periods (any time from November to April) the winds shift to northwesterly.

As the Australian summer approaches, the continent heats up. This creates low pressure, which draws the monsoon trough – a zone of low pressure and rising air – over northern Australia. The trough pulls in moist air from the warm oceans to our north and causes widespread cloud and rain. This is referred to as the monsoon.

Mostly more clear sky than cloud.

Cloud expected to cover about 15–30% of the sky for most the day. For example, during the day the sun is behind cloud for short periods of time.

An oscillation above and downwind of a mountain caused by a disturbance in the horizontal air flow due to the high ground. They usually occur in groups, and aircraft can expect to encounter moderate to severe turbulence, uncontrolled altitude loss and possible icing if flying through them. In aviation forecasts it is coded as MTW.

A storm system composed of two or more convective cells at various stages of their life-cycle.

Large, agriculturally important region in southeastern Australia. It is defined by the catchment areas of the Murray and Darling rivers.