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 Tropical storm — winds of 35 to 64 knots

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Hurricane or typhoon — winds of 65 knots or higher.

tropopause. The division between the troposphere, the lower level of the earth’s atmosphere, and the stratosphere, the layer of the atmosphere above the troposphere.
troposphere. The lower level of the earth’s atmosphere. The troposphere extends upward from the surface of the earth to an altitude of about 36,000 feet (11,000 meters). The temperature drops steadily in the troposphere, and it is in the troposphere that all of the clouds and weather conditions exist.
troubleshooting. A procedure used in aircraft maintenance in which the operation of a malfunctioning system is analyzed to find the reason for the malfunction and to find a method for returning the system to its condition of normal operation.
trough (meteorology). An elongated area of relatively low atmospheric pressure. Troughs are usually associated with and most clearly identified as an area of maximum cyclonic curvature of the wind flow.
A trough is the opposite of a ridge.
TRSA (terminal radar service area) (air traffic control). Airspace surrounding designated airports in which ATC provides radar vectoring, sequencing, and separation on a full-time basis for all IFR and participating VFR aircraft. Service provided in a TRSA is called Stage III service.
The Aeronautical Information Manual contains an explanation of TRSA, and TRSAs are depicted on VFR aeronautical charts. Pilot participation is urged, but not mandatory.
true airspeed. The airspeed shown on the airspeed indicator after it has been corrected for nonstandard temperature and pressure. 14 CFR Part 1: “The airspeed of an aircraft relative to undisturbed air. True
airspeed is equal to equivalent airspeed multiplied by (p0/p)1/2.”
true airspeed indicator (aircraft instrument). A special airspeed indicator that modifies the differential pressure measured by its diaphragm to take into consideration free-air temperature and barometric pressure. The indication of the instrument is in units of true airspeed, rather than indicated airspeed.
true altitude. The exact height of an aircraft above mean sea level.
true course (navigation). The navigational course for an aircraft measured from the geographic north pole.
true north. True direction on the earth’s surface measured from the geographic north pole.
true power (electrical power). The power, measured in watts, actually available in an alternating current circuit. True power is found by multiplying the circuit voltage by the current in phase with this voltage. It may also be found by multiplying the circuit voltage by the total current and then multiplying this by the power factor of the circuit.