Weather Glossary: V's

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.

VAD

An acronym for Velocity Azimuth Display.

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 to funnel down a pronounced valley. The term also refers to the movement of air down the slopes of a valley at night (katabatic winds) or up the slopes of valley during the day (anabatic winds).

Vapor Pressure

The pressure exerted by water vapor molecules in a given volume of air.

Variable Ceiling

A ceiling of less than 3,000 feet (900 meters) which rapidly increases or decreases in height by established criteria during the period of observation.

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 (11 km/h); or
2. The direction is variable and the wind speed is less than 6 knots (11 km/h).

Variance

A measure of variability.

Vault

Same as BWER.

Veering Wind

Wind which changes 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). Veering winds with height are indicative of warm air advection (WAA).

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:

• Check suspicious or missing wind data on the VAD Wind Profile (VWP),
• Determine the potential strength of wind gusts,
• Identify jets (low/mid/high level), and
• 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:

• Identify jets (low/mid/high level); and
• 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 Zones

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

Vertical Wind Shear

The rate of change of wind speed or direction, with a given change in 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 Windy

30 to 40 mph (48 to 64 km/h) winds.

Vicinity (VC)

A proximity qualifier used to indicate weather phenomena observed between 5 and 10 statute miles (8 and 16 kilometers) 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 (4.0 km by 3.7 km) grid box for each elevation angle within 124 nm (230 km) radius of the radar then vertically integrated. It is used to:

• Indicate presence and approximate size of hail (used in conjunction with spotter reports),
• Locate the most significant thunderstorms or areas of possible heavy rainfall; and
• Rapid decrease in VIL values may signify the onset of wind damage.

Virga

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.

Virtual Temperature

The temperature a parcel of air would have if the moisture in it were removed and its specific heat was added to the parcel.

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.

Visual Flight Rules (VFR)

Refers to the general weather conditions pilots can expect at the surface. VFR is ceiling greater than or equal to 1,000 feet (300 meters) and visibility greater than or equal to 3 miles (4.8 km). Marginal VFR (MVFR) is a sub-category of VFR (ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 miles // ceiling 305 to 914 meters and/or visibility 5 to 8 kilometers).

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.

These volume scan strategies tell the radar how many elevation angles will be used during a single volume scan (a volume scan is the completion of a sequence of elevation angles), and the amount of time it will take to complete that sequence of elevation cuts, each one being a single rotation of the antenna's 1° beam at selected elevation angles.

Vort Max

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

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.

VSB

An acronym for visible satellite imagery.

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).