upstreamThe region extending in a 5' direction from a gene.
Upstream SlopeThe part of the dam which is in contact with the reservoir water. On earthen dams, this slope must be protected from the erosive action of waves by rock…
UpwellingIn ocean dynamics, the upward motion of sub-surface water toward the surface of the ocean. This is often a source of cold, nutrient-rich water.
upwelling longwave irradianceThe rate at which radiant energy, at a wavelength longer than approximately 4 µm, is being emitted upwards into a radiation field and transferred across…
upwelling shortwave radianceThe rate at which radiant energy, at a wavelength between 0.4 and 4 µm, is being emitted upwards into a radiation field and transferred across a surface…
Urban and Small Stream Flood AdvisoryThis advisory alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area.
Urban and Small Stream FloodingFlooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains.
Urban Flash Flood GuidanceA specific type of flash flood guidance which estimates the average amount of rain needed over an urban area during a specified period of time to initiate…
Urban FloodingFlooding of streets, underpasses, low lying areas, or storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening.
Urban Heat IslandThe increased air temperatures in urban areas in contrast to cooler surrounding rural areas.
UTCBy international agreement, the local time at the prime meridian, which passes through Greenwich, England.
UV IndexUltraviolet Index- This index provides important information to help you plan your outdoor activities in ways that prevent overexposure to the sun's rays.
UVMUpward Vertical Motion (also known as Upward Vertical Velocity)
VAD Wind ProfileA radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar.
Vadose ZoneThe locus of points just above the water table where soil pores may either contain air or water. This is also called the zone of aeration.
VALDRIFTAn air pollution transport and diffusion model developed to determine pesticide drift from aerial spraying operations in valleys.
Valid TimeThe period of time during which a forecast or warning, until it is updated or superseded by a new forecast issuance, is in effect.
Valid Time Event Code(VTEC) - The Valid Time Event Code (VTEC) always is used in conjunction with, and provides supplementary information to, the Universal Geographic Code…
Valley Exit JetA strong elevated down-valley air current issuing from a valley above its intersection with the adjacent plain.
Valley Volume EffectThe reduction in volume of a valley (or basin) as compared to an equal depth volume with a horizontal floor.
value-added product (VAP)(VAP)Physical models using ARM instrument data as inputs used to fulfill some of the unmet measurement needs of the ARM Program and to improve the quality of…
ValveA device used to control the flow in a conduit, pipe, or tunnel that permanently obstructs a portion of the waterway.
Vapor PressureThe partial pressure of water vapor in an air-water system.
Variable WindSame as Variable Wind Direction; a condition when (1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind…
Variable Wind DirectionA condition when (1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or…
VarianceA special exemption of a zoning law to allow the property to be used in a manner different from an existing law.
varveannual layer of sediment deposited in an ocean or lake.
VCPVolume Coverage Pattern - A volumetric sampling procedure designed for the surveillance of one or more particular meteorological phenomena.
VeeringA clockwise shift in wind direction (for example, south winds shifting to the west).
Veering WindsWinds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise…
Vehicle miles traveled (VMT)One vehicle traveling the distance of one mile. Thus, total vehicle miles is the total mileage traveled by all vehicles.
Velocity Azimuth DisplayA WSR 88-D product which shows the radar derived wind speeds at various heights.
Velocity Cross SectionThis WSR-88D radar product displays a vertical cross section of velocity on a grid with heights up to 70,000 feet on the vertical axis and distance up to…
Velocity ZonesAreas within the floodplain subject to potential high damage from waves. These sometimes appear on flood insurance rate maps.
Ventilation IndexProduct of the mixing depth and transport wind speed, a measure of the potential of the atmosphere to disperse airborne pollutants from a stationary…
Venturi EffectThe speedup of air through a constriction due to the pressure rise on the upwind side of the constriction and the pressure drop on the downwind side as…
Vertical VelocityThe component of velocity (motion) in the vertical. The evaluation of areas of upward vertical velocity is key to forecasting areas of active weather.
Vertical Wind Shearthe change in the wind's direction and speed with height. This is a critical factor in determining whether severe thunderstorms will develop.
Vertically Stacked SystemA 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.
VILVertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D units that takes into account the three-dimensional reflectivity of an…
VIPthis is another name for a high roller and stands for a Very Important Person.
VirgaStreaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.
Virtual Potential TemperatureThe virtual potential temperature is the temperature a parcel at a specific pressure level and virtual temperature would have if it were lowered or raised…
Virtual TemperatureThe virtual temperature is the temperature a parcel which contains no moisture would have to equal the density of a parcel at a specific temperature and…
VIS1. Visible satellite imagery 2. Visibile or Visibility
VisibilityThe distance at which a given standard object can be seen and identified with the unaided eye
Visibility Protection ProgramThe program specified by the Clean Air Act to achieve a national goal of remedying existing impairments to visibility and preventing future visibility…
Visible LightElectromagnetic radiation from about 400 to 700 nm, which is detectable by the human eye.
Visible Satellite ImageryThis type of satellite imagery uses reflected sunlight (this is actually reflected solar radiation) to see things in the atmosphere and on the Earth's…
Visual SpectrumThe portion of the electromagnetic spectrum to which the eye is sensitive, i.e., light with wavelengths between 0.4 and 0.7 micrometers.
visual stratigraphyprocess of identifying different layers in ice, rock, sediment, or other deposit, based on visually apparent distinctions such as color or other…
VMDVolume median diameter. A statistical measure of the average droplet size in a spray cloud, such that fifty percent of the volume of sprayed material is…
Volcanic AshConsists of rock, mineral, and volcanic glass fragments smaller than 2 millimeters (mm) (0.
Volume ScanA radar scanning strategy in which sweeps are made at successive antenna elevations (i.e.
Volume Velocity ProcessingA way to guess the large-scale 2-dimensional winds, divergence and fall speeds from one-dimensional radial velocity data.
Voluntary Observing Ship Program(VOS) - An international voluntary marine observation program under the auspices of the World Meteorological Organization (WMO).
Vort MaxCommon slang reference to Vorticity Maximum; a center, or maximum, in the vorticity field of a fluid.
VortexA revolving mass of water (whirlpool) in which the streamlines are concentric circles and in which the total head is the same.
VorticityA measure of the rotation of air in a horizontal plane. Positive (counter-clockwise or cyclonic) vorticity can be correlated with surface low development…
VOS(Voluntary Observing Ship Program) - An international voluntary marine observation program under the auspices of the World Meteorological Organization…
Vostoksite in Antarctica (78°S, 106°E) where a 400,000 year long ice core was collected.
VTEC(Valid Time Event Code) - The Valid Time Event Code (VTEC) always is used in conjunction with, and provides supplementary information to, the Universal…
VVPVolume Velocity Processing - a way to guess the large-scale 2-dimensional winds, divergence and fall speeds from one-dimensional radial velocity data.
WAAWarm Air Advection - the advection (movement) of warm air into a region.
WakeThe region of turbulence immediately to the rear of a solid body caused by the flow of air over or around the body.
Wall CloudA localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter…
Warm AdvectionTransport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning.
Warm Core LowA low pressure area which is warmer at its center than at its periphery. Tropical cyclones exhibit this temperature pattern.
Warm FrontA transition zone between a mass of warm air and the colder air it is replacing.
Warm OcclusionA frontal zone formed when a cold front overtakes a warm front and, finding colder air ahead of the warm front, leaves the ground and rises up and over…
Warm SectorA region of warm surface air between a cold front and a warm front.
WarningThe fourth of five Early Warning System components consisting of the processes (including the media) and equipment necessary to make the public aware of…
Wasatch WindA strong easterly wind blowing out of the mouths of the canyons of the Wasatch Mountains onto the plains of Utah. Also called canyon wind.