NOAA's National Weather Service Weather Forecast Office

 
V

VAD (Velocity-Azimuth Display): It is 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.

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.

Valley Winds:  Valley winds encompass several effects, the first of which is the tendency of wind wind to funnel down a pronounced valley.  The term also refers to the movement of airdown the slopes of a valley at night (katabatic winds) or up the slopes of valley during the day (anabatic winds).

Valve:  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 Direction: A condition when (1) the wind direction fluctuates by 60^o 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.

Vault:  Same as BWER.

Veering Wind: It is the clockwise turning of the wind direction as we move up through the atmosphere. For example, a wind changing in height from south near the ground to southwest at 5,000 feet and finally to west at 10,000 feet. This is indicative of warm air advection. Compare with backing winds.

Velocity Aliasing (folding):  Ambiguous detection of radial velocities outside the Nyquist co-interval. On a radial velocity display, this shows up as regions where the radial velocity suddenly switches from one extreme to its opposite (e.g., max towards to max away).

Velocity Azimuth Display (VAD):  This WSR-88D radar product displays a graphical plot of mean radial velocity versus azimuth angle for a particular altitude.  A best fit sine wave is overlaid on the plot of velocity points if a sufficient number (25) of data points exists.  This wave is used to compute wind speed and direction for a specific height is symmetry and root mean square error thresholds are not exceeded.  It is used to:  1) Check suspicious or missing wind data on the VAD Wind Profile (VWP); 2) Determine the potential strength of wind gusts; 3) Identify jets (low/mid/high level); and 4) Identify thermal advection patterns, vertical wind shear, depths of frontal surfaces, and the development of isentropic lift situations.  The usefulness of this radar product is sometimes limited by lack of scatters at times.

Velocity Azimuth Display Wind Profile (VWP):  This WSR-88D radar product displays mean horizontal winds on a time versus height chart.  Wind data is available at a maximum of 30 altitudes up to 70,000 feet.  The latest wind/height displayed with the 10 most recent profiles (5 to 10 minute intervals depending on the Volume Coverage Pattern).  It is used to:  1) Identify jets (low/mid/high level);  and 2) Identify thermal advection patterns, vertical wind shear, depths of frontal surfaces, and the development of isentropic lift situations.  The usefulness of this radar product is sometimes limited by lack of scatters at times.

Velocity Cross Section (VCS):  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).

Very Severe Thunderstorm Warning:  It is issued for sustained winds over 75 mph in the 7-county Minneapolis/St. Paul area.

Very Windy:  30 to 40 mph winds

Vicinity (VC): A proximity qualifier used to indicate weather phenomena observed between 5 and 10 statue miles of the usual point of observation, but not at the station.

VIL (Vertically Integrated Liquid Water): This WSR-88D product displays reflectivity data converted into liquid water equivalent via an empirically-derived relationship which assumes that all reflectivity returns are from liquid water.  VIL values are derived for each 2.2 nm by 2 nm grid box for each elevation angle within 124 nm radius of the radar then vertically integrated.  It is used to: 1) indicate presence and approximate size of hail (used in conjunction with spotter reports); 2) locate the most significant thunderstorms or areas of possible heavy rainfall; and 3) rapid decrease in VIL values may signify the onset of wind damage.

VIL Of the Moment In Time (VOMIT): This computer program is used by the forecaster to predict the VIL in which hail will be produced by a thunderstorm. This computer program was designed by several forecasters at the National Weather Service Office in Detroit/Pontiac Michigan. It uses the freezing levels of various sites and interpolates between them the critical VIL for hail for each grid point.

Velocity Zones:  Areas within the floodplain subject to potential high damage from waves. These sometimes appear on flood insurance rate maps.

Vertical Wind Shear: It is 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.

VIL - Vertically-Integrated Liquid water: A property computed by the WSR-88D 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:  An acronym for Video Integrator and Processor.  This processor was used on the WSR-57 and WSR-74C radars to indicate rainfall rates.  It is still used occasionally on WSR-88D radar products.  This processor contours radar reflectivity (in dBZ) into six VIP levels.
 
VIP Levels: Categorized intervals of reflectivity which are computer processed by a Digital Video Integrator Processor (D/VIP). These intervals were very important before the installation of the 88-D Radar network. Some of the 88-D Radar products still have these intervals on them.  The following table illustrates the various rainfall rates associated with VIPs:
 

 
The following table shows how the VIP Levels were related to Equivalent Reflectivity (dBZ) and Rainfall Rate:
 

Virga: It is a patch of precipitation that evaporates before it reaches the ground. It appears as wisps or streaks of rain or snow falling out of a cloud. As the precipitation evaporates, it cools the air and starts a down draft.  In certain cases, shafts of virga may precede a microburst; see dry microburst.

Visible (VIS) 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.

Visibility: The greatest distance an observer can see and identify prominent objects.

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. See supercell.

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 scans are necessary to determine thunderstorm type, and to detect features such as WERs, BWERs, and overhang.

Vortex:  In its most general use, any flow possessing vorticity. More often the term refers to a flow with closed streamlines.

Vorticity:  A vector measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.

Vort Max:  This short for vorticity maximum. It is a center, or maximum, in the vorticity field of a fluid.

VSB:  An acronym for visible satellite imagery.

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.

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.