T Tail wind A wind blowing in the same direction as the heading of a moving object, thus assisting the object's intended progress. The opposite of a head wind.     Teardrop balloon A sounding balloon which, when operationally inflated, resembles an inverted teardrop.     Telemeteorograph Any meteorological instrument, such as a radiosonde, in which the recording apparatus is located at some distance from the measuring apparatus.     Telemeter The measuring, transmitting, receiving, and indicating apparatus for obtaining the value of a quantity at a distance.     Telemetry The transmission of data collected at a remote location over communications channels to a central station.     Telephotometer A photometer that measures the received intensity of a distance light source.     Telethermoscope A temperature telemeter.     Temperature In thermodynamics, the integrating factor of the differential equation referred to as the first law of thermodynamics. In statistical mechanics, a measure of translational molecular kinetic energy (with three degrees of freedom). In general, the degree of hotness or coldness as measured on some definite temperature scale by means of any of various types of thermometers.     Temperature coefficient (1) The ratio of the speeds of a chemical reaction at two temperatures differing by 10°C. (2) A factor relating the response characteristics of a device with changes in the ambient temperature.     Temperature correction The correction applied to an instrument to account for the effect of temperature upon its response characteristics.     Temperature scale See approximate absolute temperature scale, Celsius temperature scale, centigrade temperature scale, Fahrenheit temperature scale, international practical temperature scale, international temperature scale, Kelvin temperature scale, Rankine temperature scale, Reaumur temperature scale.     Tercentesimal thermometric scale Sir Napier Shaw's name for the approximate absolute temperature scale.     Terminal A generic term for any machine that enables a human being to communicate with a computer.     Terrestrial radiation The total infrared radiation emitted from the earth's surface. To be carefully distinguished from atmospheric radiation, effective terrestrial radiation, and  insolation.      Thaw To free something from the binding action of ice by warming it to a temperature above the melting point of ice. Also, a warm spell when ice and snow melt.     Theodolite An optical instrument which consists of a sighting telescope mounted so that it is free to rotate around horizontal and vertical axes, with graduated scales so that the angles of rotation may be measured. Used to observe the motion of a pilot balloon.     Thermal shift The change in the measured transducer output caused by changes in ambient temperature. Usually expressed a percentage of full scale.     Thermistor A semiconductor which exhibits rapid and extremely large changes in resistance for relatively small changes in temperature.     Thermocouple A temperature-sensing element which converts thermal energy directly into electrical energy. In its basic form it consists of two dissimilar metallic conductors connected in a closed loop. Each junction forms a thermocouple. If one thermocouple is maintained at a temperature different from that of the other, an electrical current proportional to this temperature difference will flow in the circuit. The value varies with the materials used. Couples of copper and constantan, which generate approximately 40 microvolts per °C of couple temperature difference, are often used for meteorological purposes.     Thermodynamic temperature A measure, in kelvins (K), proportional to the thermal energy of a given body at equilibrium. A temperature of 0K is called "absolute zero" and coincides with the minimum molecular activity (i.e. thermal energy) of matter. Thermodynamic temperature was formerly called "absolute temperature." In practice, the International Temperature Scale of 1990 (ITS-90) serves as the basis for high-accuracy temperature measurements in science and technology.     Thermoelectric thermometer A type of electrical thermometer consisting of two thermocouples which are series-connected with a potentiometer and a constant-temperature bath. One couple, called the reference junction, is placed in a constant-temperature bath, while the other is used as the measuring junction.     Thermogram The record of a thermograph.      Thermograph A self-recording thermometer.     Thermo-integrator An apparatus, used in studying soil temperatures, for measuring the total supply of heat during a given period.      Thermometer An instrument for measuring temperature by utilizing the variation of the physical properties of substances according to their thermal states. Thermometers may be classified into types according to their construction; deformation thermometer, electrical thermometer, gas thermometer, liquid-in-glass thermometer, liquid-in-metal thermometer, sonic thermometer.     Thermometer screen Same as instrument shelter.     Thermometer shelter Same as instrument shelter.      Thermometer support A device used to hold liquid-in-glass maximum and minimum thermometers in the proper recording position inside an instrument shelter, and to permit them to be read and reset. See Townsend support.     Thermopile A transducer for converting thermal energy directly into electrical energy. It is composed of pairs of thermocouples which are connected either in series or in parallel. Thermopiles are used in thermoelectric radiation instruments when the output of a single pair of termocouples is not large enough. See Moll thermopile, Eppley pyrheliometer.      Thermoscreen Same as instrument shelter.     Thermostat A device used to switch electrical current at a selectable setpoint temperature.     Threshold (starting speed) The lowest value of a measured quality at which a sensor responds. Compare to tracking.     Tide gauge A device for measuring the height of tide. It may be simply a graduated staff in a sheltered location where visual observations can be made, or it may consist of an elaborate recording instrument (sometimes called a marigraph) making a continuous graphic record of tide height against time. Such an instrument is usually actuated by a float in a pipe communicating with the sea through a small hole which filters out shorter waves.     Time constant The time required for an instrument to register 63.2% of a step change in the variable being measured.     Tipping-bucket rain gauge A rain gauge where the precipitation collected by the receiver empties into one side of a chamber which is partioned transversely at its center and is balanced bistably upon a horizontal axis. When a predetermined amount of water has been collected, the chamber tips, spilling out the water and placing the other half of the chamber under the receiver. Each tip of the bucket generates a signal.     Torricelli's tube An early, and once universal name for the mercury barometer.     Torsion hygrometer A hygrometer in which the rotation of the hygrometric element is a function of humidity.     Totalizing anemometer An anemometer in which the sensor rotation is transmitted to a mechanical counter which directly integrates the air movement past the sensor. Used to determine total air passage (wind run). Average wind speed can be calculated from the difference between successive counter readings divided by the time interval between readings.     Total lift The upward force produced by the gas in a balloon. It is equal to the free lift plus the weight of the balloon and the attached equipment.      Total radiation The sum of solar and terrestrial radiation.     Townsend support A fixed support for mounting maximum and minimum thermometers of the liquid-in-glass type. The support holds the thermometers at the correct operating attitude and also permits their rotation for resetting when desired.     Trace A precipitation amount of less than 0.005 inches. Also, the record made by any self-registering instrument.     Trace recorder Same as ombrometer.      Tracking The lowest value of a measured quality at which a sensor meets its accuracy specification.     Transducer A device which converts energy from one form into another, i.e. an ac generator transducer which converts the mechanical motion of anemometer cups into an electrical signal. Transmissivity A measure of luminous flux remaining in a light beam after it has passed through a specified distance of the atmosphere.     Transmissometer An instrument which measures the transmissivity of the atmosphere between two points for the determination of visual range.     Transpiration The process by which water in plants is transferred as water vapor to the atmosphere. Also, the amount of water so transferred.      Transponder ranging An addition to a rawinsonde system which allows determination of the slant range to the radiosonde.      Triple-point temperature The temperature at which all three phases of a substance can exist in equilibrium. This temperature occurs at only one pressure. The triple-point of water is 273.16°K and is the basis of the Kelvin scale.     Tropopause The boundary between the troposphere and stratosphere, usually characterized by an abrupt change in lapse rate. Its height varies from 10 to 20 km. Regions above the tropopause have greater atmospheric stability than regions below.      True freezing point The temperature at which the liquid and solid forms of a substance may exist in equilibrium at a given pressure (usually one standard atmosphere). The true freezing point of water is known as the ice point.     True wind direction The direction, with respect to true north, from which the wind is blowing. Distinguish from magnetic wind direction. In all standard upper-air and surface weather observations, it is true wind direction that is reported.     Trough An elongated area of relatively low atmospheric pressure. Usually associated with and most clearly identified as an area of maximum cyclonic curvature of the wind flow. The opposite of a ridge.     T-sonde A radiosonde equipped to measure temperature only.     TTL Transistor-Transistor Logic.     Tulipan radiometer A calorimetric radiation instrument of historic interest used for the measurement of outgoing heat radiation from the earth during an interval of time. The time integration is performed by allowing the radiation to fall on an uninsulated vessel containing a volatile liquid. The amount of liquid distilled into a connected insulated vessel is a measure of the incident radiation.