Weather Words - V | Weather | WGNTV.com | WGN TV

Weather Words - V

-- V --

Vapor: The gaseous state of a substance. When vapor is cooled sufficiently at constant pressure, it eventually reaches a state of saturation and any additional cooling causes condensation to the liquid phase. The gaseous state of water is known as water vapor.

Vapor trails: (1) Cloudlike streamers extending behind aircraft flying in clear, cold and humid air, formed when water vapor in engine exhaust gases condenses into visible ice crystals; condensation trails. (2) Long, narrow, ice-crystal clouds that form behind jet planes flying at high altitudes in below-freezing temperatures. They result from the condensation of water vapor in jet exhaust gases.

Vaporization: The process by which a substance (usually a liquid) evaporates. The resulting gas is known as a vapor. Water in the gaseous state is known as water vapor.

Veering: A gradual change in wind direction in a clockwise sense; for example, from south to southwest to west. Veering winds are the opposite of backing winds.

Veering winds: Winds that shift in a clockwise sense; for example, from southeast to south to southwest to west. Veering winds are the opposite of backing winds.

Velocity: The change of position with time, stipulated by speed and direction. A wind of 10 m.p.h. from the east and a wind of 10 m.p.h. to the east possess different velocities (but the same speed).

Ventilation: The process of causing "representative air" to be in contact with the sensing element of a weather instrument designed to measure a specific atmospheric quantity (such as air temperature).

Venus' atmospheric pressure: Air pressure at the surface of Venus is 90 times the atmospheric pressure at the Earth's surface. This is the same pressure found at a depth of 3,280 feet in the Earth's oceans.

Verkhoiansk, Siberia: With an average annual temperature of 3 degrees F, it is the coldest spot in the Northern Hemisphere -- the Hemisphere's so-called "cold pole." By comparison, Chicago's average annual temperature (O'Hare data) is 49 degrees.

Vernal: Pertaining to the spring season. The vernal equinox (the beginning of astronomical spring) occurs on about March 20 in the Northern Hemisphere and September 21 in the Southern Hemisphere.

Very light rain: The lightest classification of rain intensity, when scattered drops do not completely wet an exposed surface, regardless of duration. In increasing intensity: very light rain, light, moderate, heavy.

Very light snow: The lightest classification of snow intensity, when scattered flakes do not completely cover or wet an exposed surface, regardless of duration. In increasing intensity: very light snow, light, moderate, heavy.

Veteran storm chasers: They possess a thorough knowledge of, and respect for, severe weather. To be successful, they must forecast and intercept, yet not be caught in, the most violent weather on Earth.

VFR weather: In aviation terminology, weather conditions in which the bases of clouds covering six tenths of the sky or more are above 1,000 feet and horizontal visibility is greater than three miles.

VFR weather: In aviation terminology, weather conditions which are good enough to allow the operation of aircraft under visual flight rules: the bases of clouds covering six tenths of the sky or more are above 1,000 feet and horizontal visibility is greater than three miles.

VFR/IFR: Weather conditions good enough for aircraft operations under Visual Flight Rules (cloud ceiling 1,000 feet or higher and visibility at least 3 miles) or poor enough to require Instrument Flight Rules.

Vines, Father Benito: The Spanish Jesuit priest and scientist who, while meteorological director of the Royal College of Belen in Havana, achieved fame in the 1870s for his ability to predict hurricanes.

Violent tornado: A tornado with a wind speed of 207-318 m.p.h.; F4 or F5 on the tornado F scale developed by Dr. T. Fujita of the University of Chicago. Only 2 percent of all tornado occurrences.

Virga: Rain or snow that falls from the base of a cloud but evaporates completely before reaching the Earth's surface; resembles wisps or streaks extending downward, often at an angle, from cloud bases.

Visibility: The greatest distance at which it is just possible to identify with the unaided eye (1) by day, a prominent dark object against the sky at the horizon; (2) at night, a moderately intense unfocused light.

Vog: Volcanic smog. It occurs downwind of erupting volcanoes when sulfur dioxide in volcanic gases chemically reacts with moisture in the air to form an irritating smog.

Volcanic cooling: A decline of average global temperatures following volcanic eruptions. Global temperatures fell about one degree Fahrenheit in the few years following the eruptions of Krakatoa (between Sumatra and Java) in 1883 and Aging Volcano in Bali in 1963.

Volcanic winter: Severe global cooling following a volcanic eruption so massive and long lasting that it projects sufficient dust and aerosols into the atmosphere to lower worldwide temperatures.

Vorticity: A measure of the rotation or spin of a fluid about an axis within the fluid. Vorticity plays an important role in the development or dissipation of high and low pressure systems and is closely watched by weather forecasters.

Vostok, Antarctica: Location of the world's lowest recorded temperature, -129 degrees F (-128.6 degrees F), registered on July 21, 1983. Vostok is 11,220 feet above sea level.