Weather Glossary - V
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V NotchA 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. |
VapourGaseous form of a substance. |
Variability indexAn index for assessing the variability of annual rainfall. |
VaultSame as BWER. |
Veering windsWinds 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. |
Vertical velocityA measure of the upward motion of air in the atmosphere. Vertical velocities are in general around 1 cm.s-1 (compared to 10 m.s-1 for horizontal velocities), but can reach several m.s-1 in thunderstorm updrafts. Often the vertical velocity is displayed in hectopascals per hour. Since pressure decreases with height, negative values of the vertical velocity indicate rising motion in the atmosphere, and positive values indicate sinking air). Large values of the vertical velocity on these charts can (when combined with high moisture level) indicate the potential for heavy rainfall. |
Vertically stacked systemA low-pressure system, usually a closed low or cut-off 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. |
VILVertically-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. |
VIPVideo Integrator and Processor, which contours radar reflectivity (in dBZ) into six VIP levels:
This corresponds to the 6 levels of shading on the radars for the different intensity rainfall rates. For 15 level radar images the radar reflectivity is split into smaller bands. The 15 level weather radar bands are: Level 1, 12-23 dBZ Level 2, 23-28 dBZ Level 3, 28-31 dBZ Level 4, 31-34 dBZ Level 5, 34-37 dBZ Level 6, 37-40 dBZ Level 7, 40-43 dBZ Level 8, 43-46 dBZ Level 9, 46-49 dBZ Level 10, 49-52 dBZ Level 11, 52-55 dBZ Level 12, 55-58 dBZ Level 13, 58-61 dBZ Level 14, 61-64 dBZ Level 15, 64+ dBZ |
VirgaStreaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst; see dry microburst. |
Visible satellite imageVisible satellite images can only be viewed during the day, since clouds reflecting sunlight are used to create the images. On visible images, clouds show up as white or light grey, the land is normally grey and the oceans dark (some features of the land can be identified). Visible images can reveal low cloud features that infrared images do not show well, such as fog and small cumulus. Visible images are useful to identify developing thunderstorms, before they are detected on radar. |
Volume scanA 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. |
Vort Max[Slang] Short for vorticity maximum. A centre, or maximum, in the vorticity field of a fluid. |
VorticityA 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 in the Southern Hemisphere, negative values indicate cyclonic (clockwise) rotation. |
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