_Thermometer._--An instrument for measuring temperature; in meteorology, generally the temperature of the air. _Maximum_ and _minimum thermometers_ indicate, respectively, the highest and lowest temperatures occurring between the times of setting the instruments. A _wet-bulb thermometer_ is used in measuring humidity. (See _psychrometer_.) _Thermometer Screen._--A cage, or sometimes merely a roof, for protecting thermometers from direct sunshine and other radiation that would cause the readings to indicate a temperature different from that of the air. In the United States generally called an _instrument shelter_. _Thunder._--The sound produced by a lightning discharge. _Thunderstorm._--A storm attended by thunder and lightning. Thunderstorms are local disturbances, often occurring as episodes of cyclones, and, in common with squalls, are marked by abrupt variations in pressure, temperature, and wind. _Thunderstorm Recorder._--Any device that furnishes a record of thunderstorms, either near or distant. Most thunderstorm recorders register, by radiotelegraphy, the strays set up by lightning discharges. _Tornado._--1. A violent vortex in the atmosphere, usually attended by a pendulous, more or less funnel-shaped cloud. 2. In West Africa: A violent thundersquall. _Totalizer._--A form of snow gauge used chiefly in the Alps, designed to be read only once a year. (French, _totalisateur_.) _Trade Winds._--Two belts of winds, one on either side of the equatorial doldrums, in which the winds blow almost constantly from easterly quadrants. _Troposphere._--The part of the atmosphere lying below the stratosphere. _Trough._--1. A line drawn at right angles to the path of a cyclonic area through all points at which the pressure has reached a minimum and is about to rise. 2. An elongated area of low barometric pressure. _Twilight._--_Astronomical twilight_ is the interval between sunrise or sunset and the total darkness of night. _Civil twilight_ is the period of time before sunrise and after sunset during which there is enough daylight for ordinary outdoor occupations. “_Twister._”--A tornado; also, a small whirling dust storm. _Typhoon._--The name applied in the Far East to a tropical cyclone. _Ulloa’s Ring._--1. A glory. 2. A halo (also called _Bouguer’s halo_) surrounding a point in the sky diametrically opposite the sun; sometimes described as a “white rainbow.” _Vane._--A device that shows which way the wind blows; also called _weather vane_ or _wind vane_. _Variability._--_Interdiurnal variability_ is the mean difference between successive daily means of a meteorological element. _Verdant Zone._--(See _thermal belt_.)