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VALLEY BREEZE

An anabatic wind, it is formed during the day by the heating of the valley floor. As the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up the mountainsides. It blows in the opposite direction of a mountain breeze.

VAPOR PRESSURE

The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapor content. It is independent of other gases or vapors.

VAPOR TRAIL

A cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. Also called a contrail, for condensation trail.

VARIABLE CEILING

Occurs when the height of a ceiling layer increases and decreases rapidly, The ascribed height is the average of all the varying values.

VEERING

A clockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is counterclockwise. This can either happen horizontally or vertically (with height). For example, the wind shifts from the north to the northeast to the east. It is the opposite of backing.

VERNAL EQUINOX

Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20 and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is the astronomical opposite of the autumnal equinox.

VERTICAL TEMPERATURE PROFILE

A series of temperature measurements taken at various levels in the atmosphere that show the thermal structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

VERTICAL VISIBILITY

The distance an observer can see vertically into an undefined ceiling, or the height corresponding to the top of a ceiling light projector beam, or the height at which a ceiling balloon disappears during the presence of an indefinite ceiling.

VERTICAL WIND PROFILE

A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

VIRGA

Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground. From a distance, the event sometimes may be mistaken for a funnel cloud or tornado. Typically, it may fall from altocumulus, altostratus, or high based cumuonimbus.

VISIBILITY

A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. The National Weather Service has various terms for visibility. Surface visibility is the prevailing visibility determined from the usual point of observation. Prevailing visibility is considered representative of visibility conditions at the station. Sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. Tower visibility is the prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.

VISIBLE LIGHT

The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 mile per second. It has a wave length longer than ultraviolet light and shorter than x-rays.

Related term: light waves

VISUAL FLIGHT RULES (VFR)

Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceiling greater than 3,000 feet and greater than 5 miles.

Related terms: MVFR and IFR

VORTEX

Any circular or rotary flow in the atmosphere that possesses vorticity.

VORTICITY

The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.

Related term: relative vorticity

VORTICITY MAXIMUM

A center of vorticity, or the maximum of the vorticity field of a fluid.