Weather Glossary - W X Y Z


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Wall Cloud - a generally rain-free region of rotating clouds which extends beneath a severe thunderstorm and from which a funnel cloud may form

Warm Advection - the transport of warm air into an area by horizontal winds

Warm Cloud - a cloud comprised of only water droplets (i.e., no ice particles)

Warm Front - the advancing edge of a warm air mass

Warning - a product issued by the local National Weather Service office when a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning).

Watch - a National Weather Service product indicating that conditions are favorable for the occurrence of a particular hazard. A watch is a recommendation for planning, preparation, and increased awareness.

Water Vapor - water in a gaseous (vapor) form

Waterspout - a funnel-shaped or tubular column of rotating cloud-filled wind usually extending from the underside of a cumulus or cumulonimbus cloud down to the surface of an ocean or lake

Watt (W) - the derived unit for power. Named for James Watt (1736-1819), a Scottish engineer. 1 watt is equal to 1 joule per second

Wavelength - the distance between two corresponding points of two consecutive waves (e.g., crest to crest or trough to trough).

Weather - the state of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness

Weather Map - a map or chart showing the principal meteorological elements at a given time and over an extended region

Weather Radar - any radar which can be used to detect precipitation or clouds

WER (Weak Echo Region) - a radar term for a region of relatively weak reflectivity at low levels on the inflow side of a thunderstorm echo, topped by stronger reflectivity in the form of an echo overhang directly above it. The WER is a sign of a strong updraft on the inflow side of a storm, within which precipitation is held aloft. When the area of low reflectivity extends upward into, and is surrounded by, the higher reflectivity aloft, it becomes a BWER.

Wet Bulb Temperature - The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation requires heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity.

Wet Microburst - A microburst accompanied by heavy precipitation at the surface

Whitebody - a hypothetical body whose surface absorbs no electromagnetic radiation of any wavelength; opposite of a blackbody.

Wideband Data - the high-resolution, base products (reflectivity, radial velocity, and spectrum width) in polar-coordinate form that are transmitted from the RDA to the RPG for further processing

Wien's Displacement Law - the radiation law that states that the wavelength of maximum radiation intensity for a blackbody is inversely proportional to the absolute temperature of the radiating blackbody

Wind - a natural movement of air at a velocity relative to the surface of the earth

Wind-Chill Factor - a still-air temperature that would have the same cooling effect on exposed human flesh as a given combination of temperature and wind speed

Wind Direction - the direction from which the wind is blowing

Wind Shear - the local variation of the wind speed and/or direction in a given direction. Shear usually refers to vertical wind shear (i.e., the change in wind with height) but the term also is used in Doppler radar meteorology to describe changes in radial velocity over short horizontal distances. Moderate to strong wind shear is required for supercell evelopment. See Bulk Richardson Number.

Wind Speed - the ratio of the distance traveled by the air to the time taken to cover the distance

Wind Vane - an instrument used to indicate wind direction

Windward Side - the side of an object facing the direction from which the wind is blowing

Winter - the period extending from the winter solstice, about 22 December, to the vernal equinox, about 21 March

Winter Solstice - the solstice when the sun is lowest in the sky; the first day of winter

Work - an energy form arising from the displacement of an object by a force; the conversion of potential energy to kinetic energy.

WSR-57, WSR-74 - NWS Weather Surveillance Radar units, replaced by WSR-88D units

WSR-88D (Weather Surveillance Radar - 1988 Doppler) - a NEXRAD unit



X Rays - a type of electromagnetic radiation with wavelengths between gamma rays and ultraviolet radiation. Nearly all X-rays from space are absorbed by the earth's upper atmosphere



Year - The interval required for the earth to complete one revolution around the sun. A sidereal year, which is the time it take for the earth to make one absolute revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds. The calendar year begins at 12 o'clock midnight local time on the night of December 31st-January 1st. Currently, the Gregorian calendar of 365 days is used, with 366 days every four years, a leap year. The tropical year, also called the mean solar year, is dependent on the seasons. It is the interval between two consecutive returns of the sun to the vernal equinox. In 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds, and it is decreasing at the rate of 0.53 second per century.

Yellow Snow - Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.


Z-R Relationship - a mathematical equation that converts radar-measured reflectivity to an estimated rainfall rate. The reflectivity factor (Z) is related to both the dropsize distribution and the 6th power of the drop diameters. However, the radar only measures reflectivity, not the dropsize distribution. Hence, the dropsize distribution must be assumed and can be wrong. Unfortunately, different dropsize distributions can produce the same reflectivity factor but have markedly different rainfall rates. For example, many small drops can produce the same reflectivity as one large drop; however, the rainfall rate from the small drops is very different from the one large drop.

Zulu Time (Z) - global time scale based on the local time observed on the Prime Meridian; also called Greenwich Mean Time (GMT) or

Universal Coordinated Time (UTC)


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