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Weather Glossary: W's

An acronym for Warm Air Advection.
Wall Cloud

It is formed in a supercell thunderstorm. A 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, and normally are found on the south or southwest (inflow) side of the thunderstorm.

Even though this cloud is lowering, it remains attached to the rain free cloud base of the thunderstorm. It is usually located south or southwest of the visible precipitation area, and marks the strongest updraft in the thunderstorm. The wall cloud develops as the strong updraft draws in surface moisture from several miles away.

Eventually, this updraft will pull air from the rain cooled area of the thunderstorm. Since the rain cooled air is very humid, it will quickly condense in the updraft at a lower altitude than the rain free cloud base. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation.

However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion.

"Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.

Warm Air Advection
Transport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.
Warm Core Low
A low-pressure area which is warmer at its center than at its periphery. Tropical cyclones exhibit this temperature pattern. Unlike cold core lows, these lows produce much of their cloud cover and precipitation during the nighttime.
Warm Front
A front that moves in such a way that warm air replaces cold air.
A product issued by NWS local offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to act to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning). See short-fuse warning.
An NWS product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).
Watch Box (or Box)
Slang for a severe thunderstorm or tornado watch.
Watch Cancellation (SEL)
This product will be issued to let the public know when either a Tornado Watch or Severe Thunderstorm Watch has been canceled early. It is issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. In the text of the statement it will specify the severe weather watch number and the area which the watch covered.
Watch Redefining Statement (SLS)
This product tells the public which counties/parishes are included in the watch. This is done not only by writing them all out, but by using the county FIPS codes in the Header of the product. It is issued by the local National Weather Service Forecast Office (NWFO).
Watch Status Reports (WWA)

This product lets the NWFO know of the status of the current severe weather watch (Tornado or Severe Thunderstorm). During the severe weather watch, the Storm Prediction Center (SPC) will issue these reports periodically.

These reports will describe, in plain language, the current evaluation of the severe weather situation and whether the watch will expire or be reissued. A status report is not issued if a cancellation or replacement has been issued at least 1 hour prior to the expiration time of the original watch.

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.
The total area drained by a river and its tributaries. Sometimes called a basin.
Water Supply Outlook
A seasonal volume forecast, generally for a period centered around the time of spring snowmelt (e.g., April-July). The outlooks are in units of acre-feet and represent the expected volume of water to pass by a given point during a snowmelt season. The outlook categories include Most Probable, Reasonable Maximum, and Reasonable Minimum.
Water Supply Outlook (ESS) Product
A public product issued by a Forecast Office which contains narrative and numeric information on current and extended water supply conditions.
Water Table
The level below the earth's surface at which the ground becomes saturated with water. The water table is set where hydrostatic pressure equals atmospheric pressure.
Water Vapor (WV) Satellite Imagery
This satellite imagery uses 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.
Water Vapor Plume

This appear in the water vapor satellite imagery. It is a plume-like object that extends from the Intertropical Convergence Zone (ITCZ) northward or southward into the higher latitudes. It is usually located over a 850 to 700 mb theta-e ridge axis.

As a result, it is a favored location for the development of a Mesoscale Convective Complex. Researchers have found it to be a favored region for very heavy rain. It is thought that the ice crystals located in this plume help thunderstorms to become highly efficient rainfall producers.

In North America, this is sometimes called the "Mexican Connection" for moisture moving into the continental US from Mexico or the "Pineapple Connection" for water moisture moving into the west coast from the tropics.

Water Year
The time Period from October 1 through September 30.
Any surface flow such as a river, stream, tributary.
Land area from which water drains toward a common watercourse in a natural basin. They range in size from a few acres to large areas of the country.

A violently rotating column of air, usually a pendant to a cumulus or cumulonimbus cloud, over a body of water with its circulation reaching the water. In the summer and spring, these phenomena are usually "tornadoes over water" that have been generated by thunderstorms.

In the fall months, these most often begin as cold air funnels, being generated by a cold air mass passing over much warmer waters. Such waterspouts are generally much less intense than tornadoes and usually dissipate upon approaching shore.

An identifiable, periodic disturbance or motion in a medium that shows displacement. The most commonly referred medium is water, followed by the atmosphere. The forecasted heights of waves on the Great Lakes or in the oceans are those heights expected at the end of the fetch for that body of water.
Wave Crest
The highest point in a wave.
Wave Trough
The lowest point in a wave.
The distance a wave will travel in the time required to generate one cycle. The distance between two consecutive wave peaks (or other reference points) in space. Weather radar wavelengths typically range from 1 mm to 50 cm.
Wave Spectrum
The distribution of wave energy with respect to wave frequency or period. Wave spectra assist in differentiating between wind waves and swells.
Weather Forecast Office (WFO)
This National Weather Service office is responsible for issuing advisories, warnings, statements, and short-term forecasts for its county warning area.
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 does not return to earth dangerously quickly.
Weather synopsis
A description of weather patterns affecting a large area.
Wedge (or 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. The term "wedge" often is used somewhat loosely to describe any large tornado.

However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least as wide at the ground as it is tall, is very rare. Whether or not a tornado achieves "wedge" status depends on several factors other than intensity - in particular, the height of the environmental cloud base and the availability of moisture below cloud base.

Weighing-Type Precipitation Gage
A rain gage that weighs the rain or snow which falls into a bucket set on a platform of a spring or lever balance. The increasing weight of its contents plus the bucket are recorded on a chart. The record thus shows the accumulation of precipitation.
a) A low dam built across a stream to raise the upstream water level (fixed-crest weir when uncontrolled); b) A structure built across a stream or channel for the purpose of measuring flow (measuring or gauging weir).
WER - Weak Echo Region
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.
West African Disturbance Line (WADL)
It is a line of convection about 300 miles (483 km) long, similar to a squall line. It forms over west Africa north of the equator and south of 15 degrees North latitude. It moves faster than an Easterly Wave between 20 and 40 mph (32 to 64 km/h). They move off the African coast every 4 to 5 days mainly in the summer. Some reach the American tropics and a few develop into tropical cyclones.
Wet-Bulb Temperature
The lowest temperature that can be obtained by evaporating water into air.
Wet-Bulb Zero (WBZ)

The height where the wet-bulb temperature goes below 0°F (-17°C). WBZ heights between 7,000 ft. and 10,500 ft. (2,100 m and 3,200 m) (above ground level) correlate well with large hail at the surface when storms develop in an airmass primed for strong convection.

Higher values infer mid- and upper-level stability and also indicate a large melting area for falling hail. Lower WBZ heights indicate that the low-level atmosphere is often too cool and stable to support large hail.

Wet Floodproofing

An approach to floodproofing which usually is a last resort. Flood waters are intentionally allowed into the building to minimize water pressure on the structure. Wet Floodproofing can include moving a few valuable items to a higher place or completely rebuilding the floodable area.

Wet floodproofing has an advantage over other approaches: not matter how little is done, flood damage will be reduced. Thousands of dollars in damage can be avoided just by moving furniture and appliances out of the flood-prone area.

Wet Microburst
A microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst. See dry microburst.
An area that is regularly wet or flooded and has a water table that stands at or above the land surface for at least part of the year.
A National Weather Service Weather Forecast Office.
A small, rotating column of air; may be visible as a dust devil.
Any free burning uncontainable wildland fire not prescribed for the area which consumes the natural fuels and spreads in response to its environment.
Any non-urbanized land not under extensive agricultural cultivation, e.g., forests, grasslands, rangelands.
In Australia, a dust devil.

The horizontal motion of the air past a given point. Winds begin with differences in air pressures. Pressure that's higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force.

The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the "pressure gradient force."

High and low pressure are relative. There's no set number that divides high and low pressure. Wind is used to describe the prevailing direction from which the wind is blowing with the speed given usually in miles per hour or knots. The following table gives descriptions of winds used in National Weather Service forecasts.

Descriptions of winds used forecasts
Descriptive Term Sustained Wind Speed
mph km/h
Strong, dangerous, or damaging ≥40 ≥64
Very Windy 30-40 48-64
Windy 20-30 32-48
Breezy, Brisk, or Blustery 15-25 24-40
None used 5-15 or 10-20 8-24 or 16-32
Light or light and variable wind 0-5 0-8
Wind Advisory
Issued for sustained winds between 24 to 39 mph (39 to 63 km/h), or sustained winds of less than 25 mph (40 km/h) but frequent gusts between 25 and 39 mph (39 to 63 km/h).
Wind Aloft
The wind speeds and wind directions at various levels in the atmosphere above the area of surface.
Wind Chill
The wind chill is the effect of the wind on people and animals. The wind chill temperature is based on the rate of heat loss from exposed skin caused by wind and cold and is to give you an approximation of how cold the air feels on your body.

As the wind increases, it removes heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it FEEL much colder. If the temperature is 0°F (-18°C) and the wind is blowing at 15 mph (24 km/h), the wind chill temperature is -19°F (-28°C). At this level, exposed skin can freeze in just a few minutes.

The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to shorten the amount of time for the object to cool. The inanimate object will not cool below the actual air temperature. For example, if the temperature outside is -5°F (-21°C) and the wind chill temperature is -31°F (-35°C), then your car's radiator temperature will be no lower than the air temperature of -5°F (-21°C).
Wind Chill Advisory
The National Weather Service issues this product when the wind chill could be life threatening if action is not taken. The criteria for this warning vary from state to state.
Wind Chill Warning
The National Weather Service issues this product when the wind chill is life threatening. The criteria for this warning vary from state to state.
Wind Couplet
It is an area on the radar display where you have two maximum wind speeds which are blowing in opposite directions.
Wind Direction
The true direction FROM which the wind is moving at a given location. It is normally measured in tens of degrees from 10° to 360°.
Wind Gust
They are rapid fluctuations in the wind speed with a variation of 10 knots (12 mph / 19 km/h) or more between peaks and lulls. The speed of the gust will be the maximum instantaneous wind speed.
Wind Rose
A diagram that shows the percent of time that the wind blows from different directions at a given location over a given time.
Wind Shear
The rate of change of wind speed and/or direction over a given distance. Also, see shear.
Wind Shift
A change in wind direction of 45° or more in less than 15 minutes with sustained wind speeds of 10 knots (12 mph / 19 km/h) or more throughout the wind shift.
Wind Shift Line
A long, but narrow axis across which the winds change direction (usually veer).
Wind Sock
A tapered fabric shaped like a cone that indicates wind direction by pointing away from the wind. It is also called a wind cone.
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, wind gust, or squall). The following table is a way of estimating wind speed:
Wind speeds and effects
Force Speed Name Conditions at Sea/Land
knots mph km/h
12 ≥64 ≥74 ≥119 Hurricane Sea: 46 ft. (14 m) waves, air filled with foam and spray, visibility bad.
Land: Severe structural damage to buildings, wide spread devastation.
11 56 to 63 64 to 73 103 to 118 Violent storm Sea: Exceptionally high waves (36 ft. / 11 m), visibility poor.
Land: Widespread damage to structures.
10 48 to 55 55 to 63 86 to 102 Storm Sea: Very high waves (30 ft. / 9 m), heavy sea roll, visibility impaired. Surface generally white.
Land: Trees uprooted, structural damage likely.
9 41 to 47 47 to 54 76 to 85 Strong gale Sea: High waves (23 ft. / 7 m), dense foam, visibility affected.
Land: Minor structural damage may occur (shingles blown off roofs).
8 34 to 40 39 to 46 63 to 75 Gale Sea: Moderately high waves (18 ft. / 5.5 m), crests break into spindrift.
Land: Difficult to walk against wind. Twigs and small branches blown off trees.
7 28 to 33 32 to 38 51 to 62 Near gale Sea: Mounting sea (13 ft. / 4 m) with foam blown in streaks downwind.
Land: Whole trees in motion, inconvenience in walking.
6 22 to 27 25 to 31 40 to 50 Strong breeze Sea: Large waves (10 ft. / 3 m), probably some spray.
Land: Large branches move, wires whistle, umbrellas are difficult to control.
5 17 to 21 19 to 24 31 to 39 Fresh breeze Sea: Moderate waves (6 ft. / 1.8 m), many whitecaps.
Land: Small trees begin to sway.
4 11 to 16 12 to 18 19 to 30 Moderate breeze Sea: Small waves (3 ft. / 1.0 m), some whitecaps.
Land: Dust and small branches move.
3 7 to 10 8 to 11 12 to 18 Gentle breeze Sea: Large wavelets (2 ft. / 0.6 m), crests begin to break.
Land: Flags extended, leaves move. 
2 4 to 6 5 to 7 7 to 11 Light breeze Sea: Small wavelets (0.7 ft / 0.2 m). Crests have a glassy appearance.
Land: Wind felt on face.
1 1 to 3 1 to 4 1 to 1 Light air Sea: Ripples only.
Land: Smoke drifts and leaves rustle.
0 < 1 < 1 < 1 Calm Sea: Like a mirror.
Land: Smoke rises vertically.

Wind Vane
An instrument that determines the direction from which a wind is blowing.
Wind Waves
Local, short period waves generated from the action of wind on the water surface (as opposed to swell). Commonly referred to as waves. In a National Weather Service Coastal Marine Forecast or Offshore Forecast, wind waves are used when swells are described in the forecast.
Winds and Temperatures Aloft
This NWS aviation product contains winds aloft which are computer prepared and contain forecast wind direction and speed as well at forecast temperatures. Forecast winds and temperatures aloft are prepared for: 6,000 feet (1,829 meters), 9,000 feet (2,743 meters), 12,000 feet (3,656 meters), 18,000 feet (5,486 meters), 24,000 feet (7,315 meters), 30,000 feet (9,144 meters), 34,000 feet (10,363 meters), 39,000 feet (11,887 meters). All heights are above Mean Sea Level. Forecast winds are also prepared for 3,000 feet (914 meters). Wind directions are true directions.
20 to 30 mph (32 to 48 km/h) winds.
Winter Storm Warning
Issued when more than one type of hazardous winter weather is occurring, imminent, or highly likely over part or all of the forecast area. Winter storm warnings are normally issued for the first period of the forecast but can be extended into the second period. They are reissued whenever there is a change to the timing, areal extent, or expected condition.
Winter Storm Watch
Issued when conditions are favorable for hazardous winter weather conditions to develop over part or all of the forecast area in the next 6-36 hours, but the occurrence is still uncertain. Watches will be reissued whenever there is a change in the timing, areal extent, or expected conditions. Winter storm watches either evolve into winter storm warnings or advisories, or they are canceled.
Wire Weight Gage
A river gage comprised of a weight which is lowered to the water level. The weight is attached to a cable; and as the weight is lowered, a counter indicates the length of cable released. The stage is determined from the length of cable required to reach the water 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.
A NWS Weather Surveillance Radar designed in 1957. It used to be part of weather radar network. It was replaced by WSR-88D units.
A NWS Weather Surveillance Radar designed in 1974. It used to be part of weather radar network. It was replaced by WSR-88D units.
Weather Surveillance Radar - 1988 Doppler; NEXRAD unit.
Severe Thunderstorm Watch or Tornado Watch.