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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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• VAD - Abbreviation for velocity-azimuth display.

• vapor - Any substance existing in the gaseous state at a temperature lower than that of its critical point; that is, a gas cool enough to be liquefied if sufficient pressure were applied to it. If any vapor is cooled sufficiently, say at constant pressure, it ultimately reaches a state of saturation such that further removal of heat is accompanied by condensation to the liquid phase. Except for states quite close to that of saturation, vapors exhibit the general properties of all gases. Quantitatively, however, vapors exhibit measurable departures from perfect-gas laws even in states well removed from that of saturation. Since the critical temperature for water (374°C) is far above any atmospheric temperatures (except for the extreme upper air), all water substance found in the atmosphere in the gaseous state is appropriately called water vapor.

• variability - Mathematically, same as spread.

• variable - Something that can assume different values or states. See dependent variable, independent variable, random variable.

• variable wind - Wind that changes direction frequently.

• variation - 1. The range within which values of a variable lie, as in the diurnal or annual variation. 2. Same as declination.

• vario - Same as baguio.

• veering - 1. According to general international usage, a change in wind direction in a clockwise sense (e.g., south to southwest to west) in either hemisphere of the earth; the opposite of backing. 2. According to widespread usage among U.S. meteorologists, a change in wind direction in a clockwise sense in the Northern Hemisphere, counterclockwise in the Southern Hemisphere; the opposite of backing.

• vertical wind shear - The condition produced by a change in wind velocity (speed and/or direction) with height.

• VIL - Abbreviation for vertically integrated liquid.

• VIS - Abbreviation for visible.

• visibility - 1. The greatest distance in a given direction at which it is just possible to see and identify with the unaided eye 1) in the daytime, a prominent dark object against the sky at the horizon, and 2) at night, a known, preferably unfocused, moderately intense light source. After visibilities have been determined around the entire horizon circle, they are resolved into a single value of prevailing visibility for reporting purposes. There are inherent difficulties with the conventional requirement that visibility markers be both detected and recognized. The more rigorously defined concept of the visual range avoids reference to recognition; thus, if the recognition requirement were dropped, the visibility could be defined as a subjective estimate of visual range. For most practical purposes, it can be defined that way now. Daytime estimates of visibility are subjective evaluations of atmospheric attenuation of contrast, while nighttime estimates represent attempts to evaluate something quite different, namely, attenuation of flux density. Thus, visibility data must be regarded as falling into two distinct classes, those obtained by day, and those by night. In U.S weather observing practice, it is the value as obtained and reported by an observer or by an automatic weather station. See surface visibility, control-tower visibility, runway visual range, night visual range. 2. The clarity with which an object can be seen.

• volcanic aerosol - The cloud of particles injected into the stratosphere by explosive volcanic eruptions. The particles consist mainly of sulfuric acid droplets, and their influence on incoming solar radiation gives rise to cooling at the earth's surface during major events. The effects can persist for years.

• vortex - In its most general use, any flow possessing vorticity. More often the term refers to a flow with closed streamlines or to the idealized case in which all vorticity is concentrated in a vortex filament.

• vorticity - A vector measure of local rotation in a fluid flow, defined mathematically as the curl of the velocity vector, z[&zgr;] = ´[×] u, where z[&zgr;] is the vorticity, u the velocity, and the del operator. The vorticity component normal to a small plane element is the limit of the circulation per unit area as the area of the element approaches zero (see Stokes's theorem). The vorticity of a solid rotation is twice the angular velocity vector. In meteorology, "the vorticity" usually refers to the vertical component of the vorticity. See also relative vorticity, absolute vorticity, geostrophic vorticity, thermal vorticity, vorticity equation, curl; compare deformation, divergence.