Glossary
Wall Cloud
a generally rain-free region of rotating clouds that extends beneath a severe thunderstorm and from which a funnel cloud may form
Warm Core Low
a low-pressure area that is warmer at its center than at its edges. Tropical cyclones are warm core lows. Unlike cold core lows, these lows produce much of their cloud cover and precipitation during the nighttime.
Warm Front (WRMFNT)
the boundary between the advancing edge of a warm air mass and a retreating cool air mass
Warm Sector
a region of warm surface air between a cold front and a warm front. Severe weather can occur in this area of warm, moist air.
Warning (WRNG)
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).
Warning Coordination Meteorologist (WCM)
the primary connection between a local NWS office and the public. This meteorologist is in charge of spotter training and teaching meteorology to the public. This is also a senior forecaster and a back-up Meteorologist-in-Charge.
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. The Storm Prediction Center in Norman, OK issues Severe Thunderstorm and Tornado watches.
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 Imagery (WV)
satellite imagery that detects moisture between 700 and 200 mb; therefore, it is good for determining mid and upper level moisture in the atmosphere. Abundant water vapor appears white in this imagery. Meanwhile, dry air appears black in this satellite imagery. This satellite imagery can be used both day and night.
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. It is usually a non-supercell tornado.
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).
Weak Echo Region (WER)
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.
Weather (WX)
the state of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness
Weather Forecast Office (WFO)
this type of National Weather Service office (a local forecast office) is responsible for issuing advisories, warnings, statements, and short-term forecasts for its county warning area. There are approximately 120 offices across the U.S.
Weather Map
a map or chart showing the principal meteorological elements at a given time and over an extended region
Wedge Tornado
slang for 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. "Wedge" is often used somewhat loosely to describe any large tornado. However, not every large tornado is a wedge, nor is every violent tornado a wedge. Wedges can produce damage anywhere from EF0 to EF5.
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. For winter weather, wet bulb temperature can be used as a poor man’s indicator as to whether or not temperatures may cool to below freezing.
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. In other words, the hotter an object is, the shorter the wavelength at which it will emit most of its radiation.
Wind Barb
an "arrow" plotted on a map to depict the wind speed and direction (velocity) at a station. The "feathers" of the arrow indicate the wind speed (a half barb is 5 mph, a full barb is 10 mph, and a flag is 50 mph). The arrow flying with the wind shows the direction from which the wind is coming. For example, if the arrow, from feathers to point, is moving from the south to the north, the wind is from the south.
Wind Rose
a diagram, for a given location or area, showing the frequency and strength of the wind from various directions.
Wind Shear
the local variation of the wind speed and/or 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 development. See Bulk Richardson Number.
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
Winter (WINT)
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 (December 21)
Work
an energy form arising from the displacement of an object by a force; the conversion of potential energy to kinetic energy.
World Meteorological Organization (WMO)
Established in 1950, WMO became the specialized agency of the United Nations in 1951 for meteorology (weather and climate). WMO promotes international cooperation in the establishment of networks for making meteorological and climatological observations. The headquarters are in Geneva, Switzerland.
