Weather Glossary - U V
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Ultraviolet Radiation - radiation having a wavelength shorter than wavelengths of visible light and longer than those of X rays
Universal Coordinated Time (UTC) - a global time scale based on the local time observed on the Prime Meridian; also called Greenwich
Mean Time (GMT) or Zulu (Z) time
Unstable Air - generally, air which will continue to rise and accelerate when briefly forced upward
Updraft - a relatively small-scale current of air with marked upward motion
Upslope Flow - air that flows toward higher terrain and, hence, is forced to rise
Upslope Precipitation - precipitation that forms due to moist, stable air gradually forced upward along an elevated plain (e.g., the Rocky Mountains)
Upstream - toward the source of the flow, or located in the area from which the flow is coming
Upwelling Radiation - the component of radiation (either reflected solar or emitted terrestrial) directed upward from the earth's surface; opposite of downwelling radiation
V
V Notch - a distinct feature in the radar reflectivity field, seen as a V-shaped notch in the downwind part of a thunderstorm echo. The V-notch 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.
VAD (Velocity Azimuth Display) - a radar display on which the mean radial velocity is plotted as a function of azimuth
Vapor - a substance in a gaseous state at a temperature below that necessary for condensation to occur (i.e., the boiling point)
Vapor Pressure - in meteorology, the pressure exerted only by molecules of water vapor in the air
Vault - same as BWER
VCP - an acronym for Volume Coverage Pattern. The VCP is the sequence of elevation angles that a NEXRAD radar is programmed to use to scan the atmosphere. The NEXRAD operator can choose to scan using one of four possible VCPs.
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.
Velocity Folding - a limitation of pulsed Doppler radar that occurs when the actual target velocity exceeds the maximum unambiguous velocity (or Nyquist velocity) that can be measured by the radar. When velocity folding occurs, for example, a large negative velocity would appear as a large positive velocity.
Vernal Equinox - the equinox when the sun approaches the Northern Hemisphere; typically called the first day of spring
Vertical Motion - in meteorology, refers to air moving upward or downward; vertical motion is important in determining if clouds will form
Vertical Visibility - the distance that an observer can see vertically into a surface-based obscuring phenomenon, such as fog, rain, or snow
Vertically Integrated Liquid - the total liquid water equivalent throughout a vertical column of radar reflectivities. VIL is computed using an empirical equation by assuming that all the reflectivity values are from liquid water. VIL has been shown to be seasonally and geographically dependent upon hail size.
Viewing Angle - the angle that the radar uses to sample a given storm. Certain features of a storm may be invisible to a certain radar (they may be aligned along a radial) but be clearly visible by a second radar. In addition, the viewing angle of the radar is important in velocity interpretation. A wind blowing perpendicular to a radial results in zero radial velocity.
VIL - an acronym for Vertically Integrated Liquid
VIP (Video Integrator and Processor) - software which contours radar reflectivity (in dBZ) into six 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 - water or ice particles falling from a cloud but evaporating before reaching the earth's surface
Visibility - the greatest distance toward the horizon that prominent objects can be identified visually with the naked eye
Visible Radiation - the type of electromagnetic radiation that the human eye can detect
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
Vort Max - short for vorticity maximum; a maximum in the vorticity field of a fluid
Vorticity - a 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.
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