average speed or kinetic energy of the atoms and molecules in a substance.

thermograph — an instrument that measures and records air temperature.

thermometer — an instrument for measuring temperature.

thunder — the sound created by rapidly expanding gases along the channel of a lightning discharge.

thunderstorm — a local storm produced by cumulonimbus clouds and is always accompanied by lightning and thunder.

tornado — an intense, rotating column of air that protrudes from a cumulonimbus cloud in the shape of a funnel or a rope and touches the ground.

tornado outbreak — a series of tornadoes that form within a particular region that may include several states. Often associated with widespread damage and destruction.

tornado warning — a warning issued when a tornado has actually been observed either visually or on a radar screen.

tornado watch — a forecast issued to alert the public that tornadoes may develop within a specified area.

trace (of precipitation) — an amount of precipitation less than 0.01 inch (0.025 cm).

visibility — the distance an observer can see and identify prominent objects.

warm front — a front that moves in such a way that warm air replaces cold air.

wet-bulb temperature — the lowest temperature that can be obtained by evaporating water into the air.

wind — air in motion relative to the earth’s surface.

wind-chill factor — the cooling effect of any combination of temperature and wind, expressed as the loss of body heat. Also called wind-chill index.

wind direction — the direction from which the wind is blowing.

wind vane — an instrument used to indicate wind direction.

winter solstice — approximately December 22 in the Northern hemisphere when the sun is lowest in the sky and directly overhead at latitude 23.5° S, the Tropic of Capricorn.

Special Features

31