THIN LINE ECHO—A narrow, elongated non-
precipitating echo usually associated with
thunderstorm outflow, fronts, or other density
discontinuities. Also called a fine line.
THRESHOLD VALUE—An adaptable parameter
which serves as a minimum or maximum value
that phenomena must or must not exceed to be
accepted by an algorithm or displayed on a
product.
TIROS—Television InfraRed Operational Satellite.
U
UHF—Ultra high frequency. (300 MHz to 3 GHz.)
V
VAS—Vertical Atmospheric Sounder.
VEERING—A change in the wind direction in a
clockwise manner in the Northern Hemisphere, or
a counterclockwise manner in the Southern
Hemisphere.
VELOCITY ALIASING—Ambiguous detection
and display of radial velocities.
VHF—Very high frequency. (30 MHz to 300 MHz.)
VOLUME COVERAGE PATTERN—A vol-
umetric sampling procedure. A combination of
elevation slices designed to provide radar
coverage over a specific volume of the
atmosphere.
VOLUME SCAN—The completion of a sequence of
360° scans at prescribed elevation angles to obtain
a three-dimensional view around the radar site.
VOLUMETRIC PRODUCT—Any product the
WSR-88D produces which requires data from the
entire volume coverage pattern before processing
can be completed.
VORTEX— In its most general use, any flow
possessing vorticity. More often the term refers to
converging flow producing rotation. A tornado is a
vortex.
W
WATER VAPOR IMAGE—A satellite image
derived from the thermal energy emitted by water
vapor, usually at the 6.7 micrometerwavelength.
WEAK ECHO REGION (WER)—Within a
convective echo, a localized minimum of
equivalent reflectivity associated with a strong
updraft region.
WEFAX—An acronym for weather facsimile,
specifically the NWS service providing satellite
imagery and graphic products via a geostationary
satellite data broadcast.
AI-6
