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NOAA - Meteorology glossary
Category: Sciences > Meteorology
Date & country: 14/10/2013, US
Words: 671


Unstable Air
Air that rises easily and can form clouds and rain.

Updraft
A small-scale current of rising air. This is often associated with cumulus and cumulonimbus clouds.

Upper Level System
A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.

Upslope Flow
Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upstream
Toward the source of the flow, or located in the area from which the flow is coming.

UTC
Coordinated Universal Time. The time in the zero degree meridian time zone.

UVI
Ultraviolet Index

UVV
Upward Vertical Velocity.

Valley Breeze
System of winds that blow uphill during the day.

Vapor Pressure
The pressure exerted by water vapor molecules in a given volume of air

Variable Ceiling
A ceiling of less than 3,000 feet which rapidly increases or decreases in height by established criteria during the period of observation.

Veering Wind
Wind which changes 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). Veering winds with height are indicative of warm air advection (WAA).

Vertical Shear
The rate of change of wind speed or direction, with a given change in height.

Vertically-stacked System
A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere.

VHRR
Advanced Very High Resolution Radiometer. Main sensor on U.S. polar orbiting satellites.

Vicinity
A proximity qualifier used to indicate weather phenomena observed between 5 and 10 statute miles of the usual point of observation but not at the

VIL
Vertically-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.

Virga
Precipitation falling from the base of a cloud and evaporating before it reaches the ground.

Virtual temperature
The temperature a parcel of air would have if the moisture in it were removed and its specific heat was added to the parcel.

Visibility
The greatest horizontal distance an observer can see and identify a prominent object.

Volcanic Ash
Fine particles of rock powder that originate from a volcano and that may remain suspended in the atmosphere for long periods.

Vort Max
(Short for vorticity maximum), a center, or maximum, in the vorticity field of an airmass.

Vortex
An atmospheric feature that tends to rotate. It has vorticity and usually has closed streamlines.

Vorticity
A 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, positive values indicate cyclonic rotation.

Walker cell
A zonal circulation of the atmosphere confined to equatorial regions and driven principally by the oceanic temperature gradient. In the Pacific, air flows westward from the colder, eastern area to the warm, western ocean, where it acquires warmth and moisture and subsequently rises. A return flow aloft and subsidence over the eastern ocean complete the cell.

Wall Cloud
A local and often abrupt lowering of a rain-free cumulonimbus base into a low-hanging accessory cloud, from 1 to 4 miles in diameter. The wall cloud is usually situated in the southwest portion of the storm below an intense updraft marked by the main cumulonimbus cloud and associated with a very strong or severe thunderstorm. When seen from several miles away, many wall clouds exhibit rapid upward motion and rotation in the same sense as a tornado, except with considerably lower speed. A rotating wall cloud usually develops before tornadoes or funnel clouds by a time which can range from a few minutes up to possibly an hour.

Warm Advection
Transport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning.

Warm Front
A narrow transitions zone separating advancing warmer air from retreating cooler air. The air behind a warm front is warmer and typically more humid than the air it is replacing.

Warning Stage
The level of a river or stream which may cause minor flooding, and at which concerned interests should take action.

Watch
Forecast issued well in advance to alert the public of the possibility of a particular weather related hazard (e.g. tornado watch, flash flood watch). The occurrence, location and timing may still be uncertain.

Watch Box (or Box)
A severe thunderstorm or tornado watch.

Water
a transparent, odorless, tasteless liquid; composed of hydrogen and oxygen.

Water Equivalent
The liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow.

Water Vapor
Water substance in a gaseous state that comprises one of the most important of all the constituents of the atmosphere.

Waterspout
A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that reaches the surface of the water, (i.e. a tornado over water).

Wave
In meteorology any pattern identifiable on a weather map that has a cyclic pattern, or, a small cyclonic circulation in the early stages of development that moves along a cold front.

Wave Crest
The highest point in a wave.

Wave Trough
The lowest point in a wave.

Wavelength
Physical distance of one period (wave repeat).

Weather
State of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness. Also, weather is the meteorological day-to-day variations of the atmosphere and their effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.

Weather Balloon
Large balloon filled with helium or hydrogen that carries a radiosonde (weather instrument) aloft to measure temperature pressure and humidity as the balloon rises through the air. It is attached to a small parachute so that when the balloon inevitably breaks, the radiosonde doesn't hurtle back to earth dangerously quickly.

Weather Synopsis
a description of weather patterns affecting a large area.

Wedge (or Wedge Tornado)
A large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base.

Whiteout
A condition caused by falling and/or blowing snow that reduces visibility to nothing or zero miles; typically only a few feet. Whiteouts can occur rapidly often blinding motorists and creating chain-reaction crashes involving multiple vehicles. Whiteouts are most frequent during blizzards.

Wind
Air in motion relative to the surface of the earth.

Wind Advisory
Issued for sustained winds 31 to 39 mph for at least 1 hour or any gusts 46 to 57 mph. However, winds of this magnitude occurring over an area that frequently experiences such winds would not require the issuance a wind advisory.

Wind Aloft
The wind speeds and wind directions at various levels in the atmosphere above the area of surface.

Wind Chill Advisory
Issued when the wind chill index is expected to be between -25F and -39F for at least 3 hours. This is using the wind chill of the sustained wind, not gusts.

Wind Chill Factor
The apparent temperature which describes the cooling effect on exposed skin by the combination of temperature and wind, expressed as the loss of body heat. Increased wind speed will accelerate the loss of body heat. The formula to calculate wind chill is: WC=.0817(3.71 V^.5 + 5.81

Wind Chill Warning
Issued when life-threatening wind chills of -40F or colder are expected for at least 3 hours. This is using the wind chill of the sustained wind, not gusts.

Wind Direction
The direction from which the wind is blowing.

Wind Shear
Variation in wind speed and/or direction over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Wind Speed
The rate at which air is moving horizontally past a given point. It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, or gust).

Wind Vane
An instrument that determines the direction from which a wind is blowing.

Wind Wave
A wave that is caused by the action of wind on the surface of water.

Windward
Upwind, or the direction from which the wind is blowing; the opposite of leeward.

Winter Storm
A heavy snow event. A snow accumulation of more than 6 inches in 12 hrs or more than 12 inches in 24 hrs.

Winter Storm Warning
Issued when 7 or more inches of snow or sleet is expected in the next 24 hours, or 1/2 inch or more of accretion of freezing rain is expected. A warning is used for winter weather conditions posing a threat to life and property.

Winter Storm Watch
A significant winter storm may affect your area, but its occurrence, location and timing are still uncertain. A winter storm watch is issued to provide 12 to 36 hours notice of the possibility of severe winter weather. A watch will often be issued when neither the path of a developing winter storm nor the consequences of the weather

Winter Weather Advisory
Issued when 4, 5, or 6 inches of snow or sleet is expected in 24 hours; or any accretion of freezing rain or freezing drizzle is expected on road surfaces; or when blowing or drifting snow is expected to occasionally reduce visibility to 1/4 mile or less.

Wiresonde
an atmospheric sounding instrument that is used to obtain temperature and humidity information between ground level and height of a few thousand feet; this instrument is supported by a captive balloon while traveling from the ground level.

Wrapping Gust Front
A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.

WSR-88D
Weather Surveillance Radar

Yellow Wind
a strong, cold, dry west wind of eastern Asia that blows across the plains during winter and carries a yellow dust from the desert.

Youg
a hot wind during unsettled summer weather in the Mediterranean.

Young Ice
newly formed flat, sea or lake ice generally between two and eight inches thick.

Zigzag Lightning
ordinary lightning of a cloud-to-ground discharge that appears to have a single lightning channel.

Zodiac
the position of the sun throughout a year as it appears to move through successive star groups or constellations.

Zonal Flow (Zonal Wind)
Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant.

Zone Of Maximum Precipitation
The belt of elevation at which the annual precipitation is greatest in a mountain region.

Zulu time
Same as UTC, Universal Coordinated Time. Is is called Zulu because Z is often appended to the time to distinguish it from local time.