Subj : Weather Glossary (V)
To : All
From : Daryl Stout
Date : Tue Jul 23 2019 12:09 am
This weather glossary contains information on more than 2000 terms,
phrases and abbreviations used by the National Weather Service (NWS)...
the government agency who makes weather forecasts, and issues weather
advisories, watches, and warnings, for the United States, and its
territories.
Many of these terms and abbreviations are used by NWS forecasters to
communicate between each other and have been in use for many years and
before many NWS products were directly available to the public. It is the
purpose of this glossary to aid you in better understanding NWS products.
***
V Notch
A radar reflectivity signature seen as a V-shaped notch in the downwind
part of a thunderstorm echo. The V-notch often is seen on supercells,
and is thought to be a sign of diverging flow around the main storm
updraft (and hence a very strong updraft). This term should not be
confused with inflow notch or with enhanced V, although the latter is
believed to form by a similar process.
VAAC
Volcanic Ash Advisory Centers
VAD
Velocity Azimuth Display
VAD Wind Profile
A radar plot of horizontal winds, derived from VAD data, as a function
of height above a Doppler Radar. The display is plotted with height as
the vertical axis and time as the horizontal axis (a so-called
time-height display), which then depicts the change in wind with time at
various heights. This display is useful for observing local changes in
vertical wind shear, such as backing of low-level winds, increases in
speed shear, and development or evolution of nearby jet streams
(including low-level jets). This product often is referred to erroneously
as a VAD.
Vadose Zone
The locus of points just above the water table where soil pores may
either contain air or water. This is also called the zone of aeration.
VALDRIFT
An air pollution transport and diffusion model developed to determine
pesticide drift from aerial spraying operations in valleys.
Valid Time
The period of time during which a forecast or warning, until it is
updated or superseded by a new forecast issuance, is in effect.
Valid Time Event Code
(VTEC) - The Valid Time Event Code (VTEC) always is used in conjunction
with, and provides supplementary information to, the Universal
Geographic Code (UGC), to further aid in the automated delivery of
National Weather Service text products to users. The VTEC is included
in many event driven or non-routine products and in some routine Marine
forecasts. The VTEC provides information on the event, while the UGC
describes the affected geographic area.
Valley Exit Jet
A strong elevated down-valley air current issuing from a valley above
its intersection with the adjacent plain.
Valley Volume Effect
The reduction in volume of a valley (or basin) as compared to an equal
depth volume with a horizontal floor. Because the valley volume is
smaller, equivalent heat fluxes will cause larger changes in temperature
in the valley volume than in the flat-floor volume.
Valve
In hydrologic terms, a device fitted to a pipeline or orifice in which
the closure member is either rotated or moved in some way as to control
or stop flow.
Vapor Pressure
The partial pressure of water vapor in an air-water system.
Variable Wind
Same as Variable Wind Direction; a condition when
(1) the wind direction fluctuates by 60� or more during the 2-minute
evaluation period and the wind speed is greater than 6 knots; or
(2) the direction is variable and the wind speed is less than 6 knots.
Variable Wind Direction
A condition when
(1) the wind direction fluctuates by 60� or more during the 2-minute
evaluation period and the wind speed is greater than 6 knots; or
(2) the direction is variable and the wind speed is less than 6 knots.
Variance
A measure of variability.
VCNTY
Vicinity
Veering
A clockwise shift in wind direction (for example, south winds shifting
to the west).
Veering Winds
Winds which shift in a clockwise direction with time at a given location
(e.g., from southerly to westerly), or which change direction in a
clockwise sense with height (e.g., southeasterly at the surface turning
to southwesterly aloft). The latter example is a form of directional
shear which is important for tornado formation. Compare with backing
winds.
Velocity Azimuth Display
A WSR 88-D product which shows the radar derived wind speeds at various
heights. This radar product shows the wind speeds from 2,000 to 55,000
feet above the ground. VAD and EVAD (Extended VAD) are methods of
guessing the large scale two-dimensional winds from one-dimensional
radial velocity data. They are essentially multivariate regressions
which fit a simple, large scale wind model to the observed winds. EVAD
also estimates the large scale horizontal divergence and particle fall
speed. See VWP.
Velocity Cross Section
This WSR-88D radar product displays a vertical cross section of velocity
on a grid with heights up to 70,000 feet on the vertical axis and distance
up to 124 nm on the horizontal axis. The two end points to create cross
section are radar operator selected along a radial or from one AZRAN to
another AZRAN within 124 nm of the radar that are less than 124 nm apart.
It is used to:
1) Examine storm structure features such as location of
updrafts/downdrafts, strength of storm top divergence, and the depth of
mesocyclones;
2) Locate areas of convergence/divergence (when generated along a radial;
and
3) Analyze areas of rotation (when generated from one AZRAN to another).
Velocity Zones
In hydrologic terms, areas within the floodplain subject to potential high
damage from waves. These sometimes appear on flood insurance rate maps.
Ventilation Index
Product of the mixing depth and transport wind speed, a measure of the
potential of the atmosphere to disperse airborne pollutants from a
stationary source. Sometimes referred to as a Clearing Index.
Venturi Effect
The speedup of air through a constriction due to the pressure rise on the
upwind side of the constriction and the pressure drop on the downwind
side as the air diverges to leave the constriction.
Ver High Frequency (VHF)
That portion of the radio frequency spectrum from 30 to 300 MHz
Vertical Velocity
The component of velocity (motion) in the vertical. The evaluation of
areas of upward vertical velocity is key to forecasting areas of active
weather.
Vertical Wind Shear
The change in the wind's direction and speed with height. This is a
critical factor in determining whether severe thunderstorms will develop.
Vertically Stacked System
A low-pressure system, usually a closed low or cutoff low, which is not
tilted with height, i.e., located similarly at all levels of the
atmosphere. Such systems typically are weakening and are slow-moving,
and are less likely to produce severe weather than tilted systems.
However, cold pools aloft associated with vertically-stacked systems
may enhance instability enough to produce severe weather.
Very Low Frequency (VLF)
That portion of the radio frequency spectrum from 3 to 30 kHz.
Very Windy
30 to 40 mph winds.
VFR
Visual Flight Rules
VIL
Vertically-Integrated Liquid water. A property computed by RADAP II and
WSR-88D units that takes into account the three-dimensional reflectivity
of an echo. The maximum VIL of a storm is useful in determining its
potential severity, especially in terms of maximum hail size.
VIP
Video Integrator and Processor, which contours radar reflectivity (in
dBZ) into six VIP levels:
VIP 1 (Level 1, 18-30 dBZ) - Light precipitation.
VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.
VIP 3 (Level 3, 38-44 dBZ) - Moderate to heavy rain.
VIP 4 (Level 4, 44-50 dBZ) - Heavy rain.
VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.
VIP 6 (Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible.
Virga
Streaks or wisps of precipitation falling from a cloud but evaporating
before reaching the ground. In certain cases, shafts of virga may precede
a microburst.
Virtual Potential Temperature
The virtual potential temperature is the temperature a parcel at a
specific pressure level and virtual temperature would have if it were
lowered or raised to 1000 mb. This is defined by Poisson's equation.
Virtual Temperature
The virtual temperature is the temperature a parcel which contains no
moisture would have to equal the density of a parcel at a specific
temperature and humidity.
VIS
1. Visible satellite imagery
2. Visibile or Visibility
Visibility
The distance at which a given standard object can be seen and identified
with the unaided eye.
Visibility Protection Program
The program specified by the Clean Air Act to achieve a national goal of
remedying existing impairments to visibility and preventing future
visibility impairment throughout the United States.
Visible Satellite Imagery
This type of satellite imagery uses reflected sunlight (this is actually
reflected solar radiation) to see things in the atmosphere and on the
Earth's surface. Clouds and fresh snow are excellent reflectors, so they
appear white on the imagery. Clouds can be distinguished from snow,
because clouds move and snow does not move. Meanwhile, the ground
reflects less sunlight, so it appears black on the imagery. The
satellite uses its 0.55 to 0.75 micrometer (um) channel to detect this
reflected sunlight. Since this imagery relies on reflected imagery, it
cannot be used during night.
Visual Spectrum
The portion of the electromagnetic spectrum to which the eye is
sensitive, i.e., light with wavelengths between 0.4 and 0.7
micrometers. Compare shortwave radiation and longwave radiation.
VLCTY
Velocity
VLY
Valley
VMD
Volume median diameter. A statistical measure of the average droplet
size in a spray cloud, such that fifty percent of the volume of
sprayed material is composed of droplets smaller in diameter than the
VMD.
Volcanic Ash
Fine particles of mineral matter from a volcanic eruption which can be
dispersed long distances by winds aloft. The chemical composition and
abrasiveness of the particles can seriously affect aircraft and also
machinery on the ground. If it is blown into the stratosphere and it
is thick enough, it can decrease the global temperature.
Volume Scan
A radar scanning strategy in which sweeps are made at successive antenna
elevations (i.e., a tilt sequence), and then combined to obtain the
three-dimensional structure of the echoes.
Volume Velocity Processing
A way to guess the large-scale 2-dimensional winds, divergence and fall
speeds from one-dimensional radial velocity data. Essentially a
multivariate regression which fits a simple wind model to the observed
radial velocities. Very similar to VAD and EVAD, except it uses different
functions for the fit.
Voluntary Observing Ship Program
(VOS) - An international voluntary marine observation program under the
auspices of the World Meteorological Organization (WMO). Observations
are coded in a special format known as the ships synoptic code, or
"BBXX" format. They are then distributed for use by meteorologists in
weather forecasting, by oceanographers, ship routing services, fishermen,
and many others.
Vort Max
Common slang reference to Vorticity Maximum; a center, or maximum, in
the vorticity field of a fluid.
Vortex
A whirling mass of air in the form of a column or spiral. It need not be
oriented vertically but, for example, could be rotating around a
horizontal axis.
Vorticity
A measure of the rotation of air in a horizontal plane. Positive
(counter-clockwise or cyclonic) vorticity can be correlated with surface
low development and upward vertical motion (in areas of positive
vorticity advection).
VOS
(Voluntary Observing Ship Program) - An international voluntary marine
observation program under the auspices of the World Meteorological
Organization (WMO). Observations are coded in a special format known as
the ships synoptic code, or "BBXX" format. They are then distributed for
use by meteorologists in weather forecasting, by oceanographers, ship
routing services, fishermen, and many others.
VOT
Vorticity
VR
Veer
VRT MOTN
Vertical motion
VSB
Visible Satellite Imagery
VSBY
Visibility
VTEC
(Valid Time Event Code) - The Valid Time Event Code (VTEC) always is used
in conjunction with, and provides supplementary information to, the
Universal Geographic Code (UGC), to further aid in the automated delivery
of National Weather Service text products to users. The VTEC is included
in many event driven or non-routine products and in some routine Marine
forecasts. The VTEC provides information on the event, while the UGC
describes the affected geographic area.
VVP
Volume Velocity Processing - a way to guess the large-scale 2-dimensional
winds, divergence and fall speeds from one-dimensional radial velocity
data. Essentially a multivariate regression which fits a simple wind
model to the observed radial velocities. Very similar to VAD and EVAD,
except it uses different functions for the fit.
VVSTORM
Model-based convection algorithm.
VWP
VAD Wind Profile - a radar plot of horizontal winds, derived from VAD
data, as a function of height above a Doppler Radar. The display is
plotted with height as the vertical axis and time as the horizontal axis
(a so-called time-height display), which then depicts the change in wind
with time at various heights. This display is useful for observing local
changes in vertical wind shear, such as backing of low-level winds,
increases in speed shear, and development or evolution of nearby jet
streams (including low-level jets). This product often is referred to
erroneously as a VAD.
--- SBBSecho 3.07-Win32
* Origin: The Thunderbolt BBS - tbolt.synchro.net (801:1/2)
� Synchronet � Temple of Doom BBS - tod.eothnet.com
