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T-bone

(strictly Frontal T-Bone) See "What is the Shapiro-Keyser cyclone model?"

TAF

Terminal Aerodrome Forecast (also known formerly as a TAFOR) A meteorological forecast for a specific airport/airfield for a period covering 9 to 24 hours in 'self-briefing' code.

TC

(abbr) Tropical cyclone; used in aviation reports, forecasts etc.

TCU

Abbreviation used in the METAR/TAF code (as defined by ICAO), for a cloud type that meteorologists would call cumulus congestus (Cu con) - cumulus clouds which are characterised by marked 'sprouting' limbs, and which often extend to a high altitude - certainly well above the zero degree level. The bulging upper part is sometimes said to resemble a cauliflower. The cloud type is important for aviation as they can generate significant icing and turbulence in flight, and heavy precipitation, reduced visibility on/near the surface. (see also Cb)

TEMPO

Temporarily; a temporary fluctuation in (aviation/airfield) conditions expected to last less than one hour in each instance and not occur in total during more than half of the period indicated.

Tephigram

Takes its name from Temperature and Entropy (formerly noted by the greek letter 'phi'). Used in the UK Met Office to plot upper air soundings, and assess such as instability, depth of moisture/cloud layers, fog points, temperature of free convection, condensation level etc. Designed by Napier Shaw early in the twentieth century. Other thermodynamic diagrams are used by other services (e.g. the pseudoadiabatic or Stüve chart [ common on web sites ]; skewT/log(p) diagram: ( see also the entry for Thermodynamic diagrams. )

Terrestrial radiation

Long-wave (infra-red) radiation emitted by the earth (and other components of the earth-atmosphere system).

Thermal steering

Mid-latitude, mobile surface features (frontal depressions, mobile highs etc.) are often found to 'follow' the pattern of the thickness field over them. In particular, classic warm-sector depressions can be forecast to move in the direction of the thermal wind over the wave tip, with a speed 4/5 of the average thermal wind within 300 nautical miles (5 deg. of latitude) of the centre. As development takes place, the thermal field becomes increasingly distorted and the rule is less applicable. [Strictly speaking, it is the surface centres of positive and negative vorticity forcing that are being steered, rather than the feature itself. Sutcliffe quantified this as one of the terms of his development equation. ]

Thermal wind

A theoretical (vector difference) wind that relates the magnitude of the horizontal temperature gradient in a defined layer to the real winds that blow at the top and base of that layer. The speed of the thermal wind is proportional to the temperature gradient. (see the thickness FAQ)

Thermodynamic diagram

(also known as an Aerological diagram)A graphical plot of the observations of temperature and humidity, against pressure, as obtained via a Radio-Sonde ascent, or derived by sensing returns of radiant energy using an artificial satellite. Many diagrams are in current use, the three most often found are described here.

Theta-W

From the greek letter 'theta' and subscript 'w', used to denote wet bulb potential temperature (q.v.) - one of a group of pseudo-conservative (q.v.) properties of air masses.

Thickness

The difference in height between two layers in the upper air. The most commonly used being the thickness between 500 mbar and 1000 mbar, and normally expressed in dekametres. The larger the value of thickness, the warmer the column of air (warm air expands). (See also "Thickness: what is it?")

Thunderstorm

The World Meteorological Organisation defines as follows: " One or more sudden electrical discharges, manifested by a flash of light (i.e. lightning) and a sharp or rumbling sound (i.e. thunder)". Cumulonimbus (Cb) clouds are required, providing the necessary 'factory' for the vigorous updraughts / downdraughts, which in turn generate the enhanced charge-separation mechanisms within the cloud, and thus the marked positive and negative areas found within a Cb. Positive charge is found concentrated in the upper parts of the cloud, and negative in the lower, but just above the zero degC level, with small, but important positive charge areas in the base area, just below the zero degC level. (for the meteorological conditions necessary for a thunderstorm to occur [ in mid-latitudes ], see "Conditions for thunderstorms".

Thundery rain

A phrase used in weather forecasts when some quite sharp bursts of rain are expected from large areas of unstable medium cloud - with some 'rumbles' of thunder; the thunder not the most significant feature of the situation, and no well developed thunderstorms with associated hail, squally winds etc., are expected.

Thundery showers

A phrase akin to "wintry showers" which has crept into the language because many forecasts have severe word constraints. If well-developed and 'vigorous' thunderstorms are expected, then the forecast should say so, but when you only have 4 words to play with, some short-hand phraseology is necessary on a showery day with perhaps some scattered 'rumbles' of thunder.

Time Standard

The international meteorological community use UTC (or GMT) as the standard 'clock' for observations and forecasts. The 'day' is based on 00UTC and 12UTC, with multiples of 6 hours used for main observations. However, some countries (e.g. Australia), will use slightly differing conventions. When making an observation, it is important to state what 'clock' you are using, e.g. local time, GMT, etc.

Tornado(TN)

A vortex extending upward from the surface (land or sea) at least as far as the cloud base (with that cloud base associated with deep moist convection), and is intense enough at the surface to do damage at one or more points along its path. (Doswell, C.A. III, 2001 / via: http://www.cimms.ou.edu/~doswell/ ... this will change in the near future)

TPC

Tropical Prediction Center (part of NCEP) also the National Hurricane Center (NHC) for the United States. For a wealth of information relating to tropical storm meteorology, visit the NHC site at:- http://www.nhc.noaa.gov/

TREND

A two-hour 'significant weather' forecast appended to a routine (or special) weather report intended for aviation purposes. (see also METAR).

Triple-point

The point where a cold, warm and occluded front all meet. Given the right upper-air structure, a discrete area of low-pressure can form here.

Tropopause

The (usually) abrupt change from falling temperatures with height in the troposphere, to near-uniform, or rising temperatures in the stratosphere. For coding purposes, defined as the lowest level at which the lapse rate decreases to 2 degC/km or less. (with caveats to rule out lower level inversions.) (See the FAQ entry on the various levels of the atmosphere, here)

Troposphere

(Greek, tropos=turn ) lowest layer of the atmosphere, with an average depth of 16 to 18 km around the equator, 9 to 12 km temperate latitudes and well below 9 km much of the time in arctic regions. There is a general fall of temperature with height (i.e. a positive lapse rate), with an average value of some 6.5 degC/km (or 2 degC/1000ft). (See the FAQ entry on the various levels of the atmosphere, here)

Trough

A feature on a weather map where mean sea level pressure (or upper contour heights) are lower than surrounding areas of the atmosphere, with a 'V' shape to the isobars/contours evident in the pattern. Often associated with unsettled/cloudy weather, but not always. (see also upper trough, and "What is a trough?".

Trough disruption

The process whereby part of an upper trough moves 'forward' (usually west to east), leaving a portion of the old trough behind. (See anticyclonic trough disruption and cyclonic trough disruption.)

Trough extension

If an upper trough or thermal trough develops a markedly increased amplitude it is said to have undergone meridional extension. The change is usually defined in terms of a latitude change of a defined contour or thickness isopleth.

TS

(abbr) Thunderstorm (as used in aviation weather reports, forecasts etc.)

TURB

(abbr) Turbulence (as used in aviation weather reports, forecasts etc: e.g. MOD TURB).

Turbulence

Atmospheric motion that shows irregular and random motion over very small distances and over short intervals of time.