31
Special Features
average speed or kinetic energy of the atoms
and molecules in a substance.
thermograph — an instrument that mea-
sures and records air temperature.
thermometer — an instrument for measur-
ing 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 accom-
panied 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 pre-
cipitation 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 temper-
ature 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.
