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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.
- Temperature
- A physical quantity characterising the mean random motion of molecules in a physical body.
- Temperature inversion
- A temperature inversion occurs when the temperature of air increases with increasing height. Generally the temperature decreases with height in the lower atmosphere, called the troposphere. Low-level inversions generally form on clear calm nights due to cooling of the ground through loss of heat by radiation. The warm air on the ground is replaced by cooler air at the surface resulting in a temperature inversion. The inversion creates a boundary layer that restricts vertical motion and mixing of air between the two air masses either side. Low-level inversions act like a lid to trap pollutants resulting in smog over our cities. For further information please read http://www.bom.gov.au/info/ftweather/page_16.shtml.
- Thickness
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Meteorologists often consider the atmosphere in terms of layers, that is, from one height to another, or from one pressure level to another. The vertical distance between two pressure levels is called that layer's thickness. A common layer used for forecasting the weather is the 1000-500 hPa thickness, the vertical difference in decametres (dam) between the 1000 hPa isobaric surface, which is usually close to mean sea level (MSL), and the 500 hPa surface, which is about 5.5 km above MSL.
Due to the way in which air molecules behave at different temperatures, thickness is related to the temperature of the air in that layer. Larger thickness values correspond to higher average temperatures and smaller thickness values correspond to lower temperatures. Thus, thickness is a useful way to determine the locations of warm and cold air masses in the atmosphere.
A commonly used general guide for meteorologists when forecasting the rain/snow boundary during a precipitation event is that snow is likely at an elevation of 1500 m where the 1000-500 hPa thickness is less than 540 dam. - Thunder day
- A thunder day at a given location is a calendar day which thunder is heard at least once.
- Thunderstorm
- Sudden electrical discharges manifested by a flash of light (lightning) and a sharp rumbling sound. Thunderstorms are associated with convective clouds (Cumulonimbus) and are more often accompanied by precipitation. They are usually shortlived and hit on only a small area.
- Time descriptions used in forecasts and warnings
- Refer to the time tab under Weather Words
- Tornado
- A whirlwind or mass of rotating air with high wind speeds at its center.
- Total-totals
- Measure or Index of atmospheric instability used to assess the strength of storms and for severe weather forecasts. A combination of calculations relating to the air temperature and dew point temperature at different levels of the atmosphere.
- Trace
- A trace of rain is reported by rainfall observers when a little precipitation can be seen in the rain gauge, but there is less than 0.1 mm in total. The precipitation could be from any source such as rain, drizzle, dew, melted frost, melted hail or melted snow. It is quite often reported as "tce" or "tr" in rainfall bulletins. Rainfall amounts between 0.1 mm and 0.2 mm are reported as 0.2 mm in rainfall bulletins.
- Trade winds
- East to southeasterly winds (in the southern hemisphere) which affect tropical and subtropical regions, including the northern areas of Australia. During the monsoon season in northern Australia, the easterly trade winds are replaced by moist northwesterly (monsoonal) winds from the Indian Ocean and southern Asian ocean waters.
As mentioned above, the trade winds in the southern hemisphere are east to southeasterly in direction. In the northern hemisphere however, the trade winds are east to northeasterly in direction. It means that in both hemispheres, they tend to blow from the east to the west and towards the equator. Sometimes the trade winds will just be called "easterly" to avoid having to specify the hemisphere. - Tropical cyclones
- Tropical cyclones are intense low pressure systems which form over warm ocean waters at low latitudes. Tropical cyclones are associated with strong winds, torrential rain and storm surges (in coastal areas). Tropical cyclones can cause extensive damage as a result of the strong wind, flooding (caused by either heavy rainfall or ocean storm surges) and landslides in mountainous areas as a result of heavy rainfall and saturated soil. Tropical cyclones are also known (in other parts of the world) as tropical storms. If they attain maximum mean winds above 117 km/h (63 knots) they are called severe tropical cyclones. In the northwestern Pacific severe tropical cyclones are known as typhoons and in the northeast Pacific and Atlantic/Caribbean they are called hurricanes.
See also Cyclones for more information. - Tropical storm
- Term used in the northern hemisphere for a tropical cyclone.
- Trough
- A trough of low pressure is an elongated area where atmospheric pressure is low relative to its immediate surroundings. A trough of low pressure is sometimes indicated on the synoptic chart by a centre line or trough line denoted by a dashed line e.g. - - - - -. The trough line often extends outward from a low pressure centre, or an enclosed area of relatively low pressure.
When moving across a trough from one side of a trough line to another, atmospheric pressure decreases as you approach the trough line. The atmospheric pressure increases again after you cross the trough line and move away. A change in wind direction will generally be observed as you cross from one side of the trough to the other. Compare with the definition for a Ridge and a Low. - Typhoon
- Term used in the northwestern Pacific for a tropical cyclone with maximum winds above 117 km/h (63 knots).