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Weather Glossary - T

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T LAPS

Tropical Limited Area Prediction System. A modified version of the LAPS numerical model, covering tropical latitudes in the western Pacific (over Australia).

T Rolls

[Slang] same as transverse rolls.

Tail cloud

A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation area and towards the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds. See supercell.

Temperature

A measure of heat in an object. It is a physical quantity characterising the random motion of molecules within a body. The most common scale is the Celsius scale with 0C the freezing point of water at sea level. This corresponds to 273.15 K in the Kelvin scale which has its zero at absolute zero, the lowest physically possible temperature.

Thermocline

A vertical temperature gradient in a water body appreciably greater than gradients above or below.

Thermodynamic chart

(or Aerological diagram) A chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart is typically used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability.

Thermodynamics

In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Thermometer

An instrument used to measure temperature.

Theta e

See Equivalent Potential Temperature

Thickness

Thickness usually refers to the depth of the 1000-500 hPa layer in the atmosphere. However charts are also produced for thicknesses of other layers in the atmosphere as well. The thickness gives an indication of the mean temperature within a layer; lower thicknesses indicate colder air, higher thicknesses warmer air.

Forecast MSL charts will also often show the 1000-500 hPa thickness as dashed lines. The thickness on these charts is usually given in decametres. So the 540 line indicates the 1000-500 hPa layer is 5400 metres deep.

Thickness charts in general and the 1000-500 hPa thinkness charts in particular are also very useful for determining baroclinic zones and development. Thickness lines can also indicate the steering direction of surface highs and lows.

Thunder

The loud booming sound associated with a lightning strike. A lightning strike rapidly heats a column of air (to conduct the charge) which expands outwards. This expansion results in a compression wave in the air which we hear as thunder. The "rolling thunder" sound is a result of the thunder echoing off surrounding hills and buildings, and also the thunder originating from sections of the lightning stroke at different distances from the observer.

A good rule of thumb to tell how far away a storm is to count the seconds between seeing a lightning strike and hearing the thunder from it. Divide this number by three to tell approximately how many kilometres away the storm is.

Thunderstorm

Convective showers which produce lightning. Heavy rain, strong winds and hail are all possible in thunderstorms.

Tilted storm

(or tilted updraft) A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favourable condition for severe storm development.

Tornado

A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.

Tornado family

A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

Total-Totals index

A stability index and severe weather forecast tool, equal to T850 + Td850 - 2 T500.

The total-totals index is the arithmetic sum of two other indices: the Vertical Totals Index (T850 - T500) and the Cross Totals Index (Td850 - T500). As with all stability indices there are no magic threshold values, but in general, values of less than 50 or greater than 55 are considered weak and strong indicators, respectively, of potential severe storm development.

Tower

(Short for towering cumulus) A cloud element showing appreciable upward vertical development.

Towering cumulus

Same as cumulus congestus. Often shortened to "towering cu" abbreviated TCu.

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 southeast winds (in the southern hemisphere) which affect tropical and sub-tropical regions, including the northern half of Australia.

Translucidus

Latin - to shine through, transparent
Clouds that cover a large part of the sky and are sufficiently thin to reveal the position of the sun or moon.

Transpiration

The process by which water in plants is transferred as water vapour to the atmosphere.

Transverse bands

Bands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence. Transverse bands observed at low levels (called transverse rolls or T rolls) often indicate the presence of a temperature inversion (or cap) as well as directional shear in the low- to mid-level winds. These conditions often favour the development of strong to severe thunderstorms.

Transverse rolls

Elongated low-level clouds arranged in parallel bands and aligned parallel to the low-level winds but perpendicular to the mid-level flow. Transverse rolls are one type of transverse band, and often indicate an environment favourable for the subsequent development of supercells. Since they are aligned parallel to the low-level inflow, they may point toward the region most likely for later storm development.

Triple Point

The intersection point between two boundaries (dry line, outflow boundary, cold front, etc.), often a focus for thunderstorm development.

Triple point also may refer to a point on the gustfront of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favoured location for tornado development (or redevelopment).

In thermodynamics, triple point refers to the temperature and air pressure at which a material (e.g. water) can exist in all three phases (liquid, solid and gas) in equilibrium. The triple point for water is 0.01C (273.16K) and 607.8 Pa.

Tropical Cyclone

A tropical depression of sufficient intensity to produce sustained gale force winds (sustained winds of 63 km/h or greater with gusts in excess of 90 km/h). The typical structure consists of bands of cumulonimbus clouds which spiral towards a clear central eye, though this can be obscured by high cirrus cloud.Tropical cyclones are associated with extremely strong winds, torrential rain, storm surges (in coastal areas) and huge seas.The name Tropical cyclone is usually used for systems in the southwest Pacific and Indian Oceans, while other names such as tropical storms, hurricanes and typhoons are used in other parts of the world. If they attain maximum mean winds above 117 km/h (63 knots) they are called Severe Tropical Cyclones (Category 3 or above).

Tropical Cyclone categories

CATEGORY 1
Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings.A Category 1 cyclone's stongest winds are GALES with gusts to 125 km/h.These winds correspond to Beaufort 8 and 9 (Gales and strong gales).

CATEGORY 2
Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.A Category 2 cyclone's strongest winds are DESTRUCTIVE winds with gusts of 125 -170 km/h.These winds correspond to Beaufort 10 and 11 (Storm and violent storm).

CATEGORY 3
Some roof and structural damage. Some caravans destroyed. Power failures likely.A Category 3 cyclone's strongest winds are VERY DESTRUCTIVE winds with gusts of 170 - 225 km/h.These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

CATEGORY 4
Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.A Category 4 cyclone's strongest winds are VERY DESTRUCTIVE winds with gusts of 225 - 280 km/h.These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

CATEGORY 5
Extremely dangerous with widespread destruction.A Category 5 cyclone's strongest winds are VERY DESTRUCTIVE winds with gusts of more than 280 km/h.These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

Tropical disturbance

An area of organized convection, originating in the tropics, that maintains its identity for more than 24 hours. It is often the first developmental stage of a tropical depression or tropical cyclone.

Tropical Storm

Term used in the northern hemisphere for a tropical cyclone.

Tropics

The region of the earth located between 23.5 North and 23.5 South.

Tropopause

The upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height). In mid-latitudes, it is around 10km high, but can be much higher in tropical latitudes, and lower in polar regions.

Troposphere

The lowest (major) layer of the atmosphere from the earth's surface up to the tropopause (at around 10 km in mid-latitudes) where most of the Earth?s weather occurs. It is characterized by decreasing temperature with height (except, perhaps, in thin layers - see inversion, cap), vertical wind motion, appreciable water vapour content, and sensible weather (clouds, rain, etc.).

Trough

An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. Usually marks a sharp boundary in wind direction and is associated with cloudiness and shower or storm development. A trough can often denote an airmass boundary. This may be an area of sharp temperature or humidity gradients. Convergence along these troughs can result in showers or thunderstorms. In more southern latitudes in Australia, troughs forming along strong temperature gradients are often responsible for summertime cool changes. A cold front is a trough that exhibits particular characteristics that warrant special identification. In the tropics, troughs (particularly over the ocean) are often driven by troughs or disturbances in the upper levels that causes convergence in the lower levels. These are also known are tropical waves and are often a precursor tropical cyclone development.

Turbulence

A state of fluid flow in which the instantaneous velocities exhibit irregular and apparently random fluctuations. Theses fluctuations are capable of transporting atmospheric properties.

Turkey tower

[Slang] A narrow, individual cloud tower that develops and falls apart rapidly, leaving cloud fragments suspended in elevated regions. The sudden development of turkey towers from small cumulus may signify the breaking of a cap.

TVS

Tornadic Vortex Signature. Doppler radar signature in the radial velocity field indicating intense, concentrated rotation - more so than a mesocyclone. Like the mesocyclone, specific criteria involving strength, vertical depth, and time continuity must be met in order for a signature to become a TVS. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it. A TVS is not a visually observable feature.

TWC

Abbreviation for The Weather Company.

TWC Mesocast

TWC Meteocast is based on the WRF model, and tuned for Australian conditions. The Weather Research and Forecasting (WRF) Model is a next-generation mesoscale numerical weather prediction system designed to serve both operational forecasting and atmospheric research needs. It features multiple dynamical cores, a 3-dimensional variational (3DVAR) data assimilation system, and a software architecture allowing for computational parallelism and system extensibility.

Twister

[Slang] A tornado.

Typhoon

Term used in the northwestern Pacific for a tropical cyclone with maximum winds above 117 km/h (63 knots).

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