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V

VA

(abbr) Volcanic ash: as used in aviation reports, forecasts etc.

VC

(abbr) In vicinity: used in aviation reports, forecasts etc., e.g. VCFG, fog in vicinity but not at airfield.

Veering

See Backing

Vertical vorticity

Synoptic-scale vorticity about a vertical axis, caused by horizontal differences in wind speed (the 'shear' term), and / or horizontal 'curving' of the air-flow (the 'curvature' term). It is often assessed at the 500 hPa level (around 5.5 km). This property is linked to divergence / convergence at upper levels, which in turn will lead to enhancement of vertical air movement - key to synoptic developments. (See also Vorticity; Horizontal vorticity.)

Violent Storm

This term is only used in bulletins for shipping, and associated Gale/Storm Warnings. A 'Violent Storm/Force 11' is defined as the (10 minute) mean wind of between 56 and 63 knots. (Gusts not defined) (See also comments at Severe Gale).

VIS

Visible imagery ... see the FAQ entry on satellite imagery.

Visibility [ horizontal ]

(from the Observer's Handbook) " the greatest distance at which a black object of suitable dimensions can be seen and recognized against the horizon sky, or, in the case of night observations, could be seen and recognized if the general illumination were raised to the normal daylight level. With the advent of automatic sensors to detect this parameter, the WMO re-defined the above in terms of the 'meteorological optical range' (MOR), which is given by the path-length over which a light source of known intensity/colour is reduced to a certain percentage of the original flux.

Visibility [ reports ]

As used in SYNOP, SHIP and METAR observations: the value reported as the visibility in surface reports has for many years been the lowest visibility (as defined above) that an observer can determine. So, for example, if the general value were 6 to 9 km, but in one particular direction (say towards a major town or over adjacent coast) it was as low as 3000 m, then it would be this latter value that was recorded, e.g. 3000 m. Until 2003-2004, this was also the scheme adopted for METAR reports world-wide. However, the concept of 'prevailing' visibility has been introduced for these latter, and can be taken to be the dominant visibility (applicable to at least 50% of the local horizon, continuously or otherwise), with separate groups for significantly lower values reported as required. For more on this, see the page relating to the METAR code.

Visibility [ vertical ]

Used in aviation reports to assess how far upwards a surface-based observer can 'see' through fog or snow. (See VV).

Volcanic effects

The effect on the atmosphere due to a major volcanic eruption depend critically upon the plume maximum-altitude and its sulphur content. Unless the plume penetrates into the stratosphere (and does so for a substantial length of time), material only affects the troposphere, and will be washed-out by precipitation (rain, snow etc.): in such cases, the effects will be regional and short-lived. In broad terms, sulphate aerosols penetrating to the stratosphere in quantity warm the stratosphere (due to absorption) and cool the troposphere (by radiation back-scatter), but the effect is highly size-dependent; only the smallest particles lead to long-term near-surface cooling.

VOLMET

Voice weather broadcast for in-flight briefing, often using automated interpretation of weather bulletins.

Vorticity

A measure of the 'spin' of a portion of a fluid - in our case, of atmospheric particles. Vorticity in a cyclonic sense is designated 'positive', and in an anticyclonic sense is designated 'negative'. In synoptic meteorology, we often only consider vorticity in a horizontal plane - i.e. the 'spin' behaviour of air particles as they move along in the atmospheric flow as depicted on classical 'weather maps': vertical vorticity. However, the vertical component of vorticity is important in the study of tornadoes for example: this is horizontal vorticity. (See also: Absolute vorticity; Horizontal vorticity; Relative vorticity; Potential vorticity; Vertical vorticity.)

VRB

(abbr) Variable wind direction, as used in aviation weather reports, forecasts etc.

VSP

(abbr) Vertical speed, as used in aviation weather reports, forecasts etc. Particularly used with respect to forecasts of mountain wave motion.

VV

(abbr) Vertical visibility, as used in aviation weather reports, forecasts etc., e.g. VV002, vertical visibility 200ft. Where vertical visibility cannot be determined (or realistically forecast), then the group will be seen as VV///.