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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.
- Warm Fronts
- See Front (Warm).
- Water cycle
Water is the lifeblood of the weather. It moves into the atmosphere as water vapour through evaporation from water surfaces or through transpiration from plants. The vapour condenses in the atmosphere to form clouds and returns to the surface as precipitation (rain, snow or hail) and in small quantities as fog, dew or frost.
Water falling on land is destined for one of many fates. If it falls as snow it can spend some time in a snow or ice-field before moving off as a glacier and perhaps eventually becoming part of an iceberg. Or it might melt and join a river. If it falls as liquid drops it might be retained in the soil as moisture for plants, or it might join a river, or be stored for a time in a lake or reservoir.
Ultimately rain or snow falling over land will either find its way back to the ocean or be evaporated back into the atmosphere, from where it will move along through various branches of the eternal water (hydrologic) cycle.- Water Resources
- Water available, or capable of being made available, for use in sufficient quantity and quality at a location and over a period of time appropriate for an identifiable demand.
- Water Resources Assessment
- Determination of the sources, extent, dependability and quality of water resources for their utilisation and control.
- Wave terms
- See Sea and Swell.
- Weather Words
- See Weather Words.
- Wet Bulb Global Temperature (WBGT)
- Human thermal comfort depends on many factors, only some of them environmental. The four environmental factors are air-flow (wind), air temperature, air humidity, and radiation (the sun and nearby hot surfaces). The exact response of a person to the environmental factors depends on other factors unique to themselves so there is no simple environmental index that can work for everyone under all conditions.
Nevertheless it can be useful to look at an index to help in determining if conditions are unfavourable to undertake strenuous sporting activity. Wet Bulb Global Temperature (WBGT) is a parameter that is often used for this purpose.
The WBGT values on the observation page is an approximation only and does not take into account variations in the intensity of solar radiation or of windspeed, and assumes a moderately high radiation level in light wind conditions.
Any decision regarding strenuous sporting activity should include a number of factors of which temperature and humidity only form part. Sports Medicine Australia (SA) has a booklet containing a Decision Checklist for sport in South Australia. The actual values might not be appropriate in all states but the methodology will be a useful guide. The booklet is available on the web as Hot Weather Guidelines. - Wet bulb temperature
- Wet-bulb temperature is measured using a standard mercury-in-glass thermometer, with the thermometer bulb wrapped in muslin, which is kept wet. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry-bulb (normal, unmodified) thermometer.
The rate of evaporation from the wet-bulb thermometer depends on the humidity of the air - evaporation is slower when the air is already full of water vapour For this reason, the difference in the temperatures indicated by the two thermometers gives a measure of atmospheric humidity. - Wind
- Moving air. See related Wind Terms below and a full description in Weather Words.
- Wind Barbs: How to interpret wind speed and direction?
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Wind fields can be displayed using a common meteorological representation known as wind barbs. The wind barbs indicate both direction and speed in a compact format. They have three components; a dot, a staff (or stem) and a flag/pennant set.
Wind speed is indicated by the flag/pennant set, to the nearest five knots (nautical miles per hour). The speed is represented by the sum of half-flags, flags and pennants on the barb, where:
- Half-flag = 5 knots
- Flag = 10 knots
- Pennant = 50 knots
Where there is no wind the barb is replaced with a simple black dot to indicate calm conditions.
Wind Direction is indicated by the angle of the barb (orientation).
- It is important to know that barbs are not like arrows, as the wind blows from the flag end towards the dot.
- Think of the dot as your location, with the wind barb representing the wind blowing towards you.
In the example below, the wind is coming from the southwest at 75 knots.
Another example of a wind coming from the northwest at 15 knots
- Wind terms
- The wind is a continuous succession of gusts and lulls (quiet intervals) associated with equally rapid changes of direction over a range which may exceed 30°. The mean wind speed over a period of time is therefore the mean of many gusts and lulls. Usually only the mean wind is forecast, unless the gusts are expected to be a significant feature. For instance, Fresh, gusty southwest winds indicates that the mean wind speed will be between 17 and 21 knots and the mean wind direction will be from the southwest, but that there will also be gusts to speeds significantly higher than the mean.
- Gust: a gust is any sudden increase of wind of short duration, usually a few seconds.
- Squall: a squall comprises a rather sudden increase of the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value. A squall may include many gusts.
- Surface Wind: wind speed and direction measured at 10 metres above the earth's surface. The surface wind drives wave generation locally, and is responsible for large swells generated by strong winds associated with intense storms.
- Winter
- The three coldest months June, July and August (in the southern hemisphere).
- Water vapour pressure
- The atmospheric pressure which is exerted by water vapour (water in its gaseous state). It is one way of measuring the humidity of the air. At a given temperature, an increase of water vapour in the air corresponds to an increase in the humidity of the air.
Water vapour is supplied to the atmosphere by evaporation of water from oceans, lakes, wet land surfaces or from vegetation (transpiration). Water vapour absorbs the Sun's radiation. As a result, the sunlight received at the Earth's surface will be more intense in a drier atmosphere.