Weather Glossary - W

Walker Circulation

The Walker circulation is an atmospheric circulation of air over the equatorial Pacific Ocean. It is caused by the pressure gradient force that results from high pressure over the eastern pacific and low pressure over Indonesia. The Walker circulation is seen at the surface as easterly trade winds which move water and air towards the west. The ocean is some 60 cm higher in the eastern Pacific as the result of this motion. The water is returned to the east at the Ocean floor while the air is returned in the upper atmosphere. An El Niņo episode is characterised by a breakdown of this water and air cycle, resulting in relatively warm water and moist air in the eastern Pacific. A La Nina episode is characterised by an intensification of the circulation

Walker Cycle

Transition and movement of water around the Earth involving evaporation, transpiration, condensation, precipitation, percolation, runoff, and storage. It moves into the atmosphere as water vapour through evaporation from water surfaces or through transpiration from plants. The vapour condenses in the atmosphere to form clouds and returns to the surface as precipitation. Water falling on land is destined for one of many fates. If it falls as snow it can spend some time in a snow or ice-field before moving off as a glacier and perhaps eventually becoming part of an iceberg. Or it might melt and join a river. If it falls as liquid drops it might be retained in the soil as moisture for plants, or it might join a river, or be stored for a time in a lake or reservoir. Ultimately it will either find its way back to the ocean or be evaporated back into the atmosphere, from where it will move along through various branches of the eternal water cycle.

Wall cloud

A localized, persistent, often abrupt lowering from a rain-free thunderstorm base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the north or northwest (inflow) side of the thunderstorm. When seen from within several kilometres, 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" is also 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.

A wall cloud attached to a supercell thunderstorm.

Warm advection

Transport of warm air into an area by horizontal winds.

Low-level warm advection sometimes is referred to (incorrectly) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.

Warm front

A moving boundary that separates warmer air from cooler air. A warm front is named as such because the warm air is advancing on the cold air. These are not common in mainland Australia but cross TAS regularly during winter.

Water vapour

Water vapour is gaseous water present in the atmosphere. The amount of water vapour present in the atmosphere depends on the temperature and pressure of the air, however it rarely exceeds a few grams of water vapour per kilogram of air, even in clouds.

Water vapour pressure

The atmospheric pressure which is exerted by water vapour.

Water vapour satellite images

Water vapour (WV) satellite images indicate the amount of moisture present in the mid-to-upper atmosphere (from 500 hPa upwards). Areas of high humidity will show as bright white, while dry areas will be dark. WV images are useful for forecasting where heavy rain is possible, where the jet streams lie, or identifying dynamical features such as upper level highs or lows.

Waterspout

In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters.

The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.

Wave Height

Generally taken as the height difference between the wave crest and the preceding trough.

Wave Length

The mean horizontal distance between successive crests (or troughs) of a wave pattern.

Wave period

The average time interval between passages of successive crests (or troughs) of waves.

Weather warnings

In Australia, state by state warnings of impending or actual weather that has the potential to cause loss of life or damage to property. These are issued by the BoM.

Wedge

(or Wedge Tornado) [Slang] 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.

Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many documented wedges have been rated lower. Some violent tornadoes may not appear as wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series of suction vortices without a central condensation funnel). 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. Therefore, spotters should not estimate wind speeds or F-scale ratings based on visual appearance alone. However, it generally is safe to assume that most (if not all) wedges have the potential to produce strong (F2/F3) or violent (F4/F5) damage.

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. This requires three-dimensional radar imagery.

The WER is a sign of a strong updraft onthe 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 Coast trough

A persistent trough of low pressure near the West Australian coastline that separates hot east to northeast winds from cooler south to southeast winds. It is most noticable during the warmer months, strengthened by diurnal heating of the land.

The position of the west coast trough can have a marked effect on local winds near the west coast, mainly on the development of the sea breeze.

Wet bulb temperature

Wet-bulb temperature is measured using a standard mercury-in-glass thermometer, with the thermometer bulb wrapped in muslin, which is kept wet. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry-bulb (normal, unmodified) thermometer.

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.

Wind

The movement of air across the Earth's surface. The wind is a continuous succession of gusts and lulls (quiet intervals) associated with equally rapid changes of direction over a range which may exceed 30°. The mean wind speed over a period of time is therefore the mean of many gusts and lulls. The standard measure of 'mean' wind is the 10 minute mean.

Usually only the mean wind is forecast, unless the gusts are expected to be a significant feature. For instance, 'Fresh, gusty southwest winds' indicates the mean wind speed will be between 16 and 24 knots and the mean wind direction will be from the southwest, but there will also be gusts to speeds significantly higher than the mean.
Common wind terms:

• Light: Below 10 knots
• Moderate: 10 - 15 knots
• Fresh: 16 - 24 knots
• Strong: 25 - 33 knots
• Gales: 34 - 47 knots
• Storm winds: Above 48 knots
• Gust: a sudden increase of wind of short duration, usually a few seconds.
• Squall: comprises a rather sudden increaseof the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value. A squall may include many gusts.

Wind chill

Based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. As the wind speed increases, heat is carried away from the body at an accelerated rate, driving down the body temperature. The wind chill temperature is an 'apparent' temperature and gives a better estimate of how cold it really feels outside.

More information is available from the US National Weather Service wind chill site.

The wind chill is combined with the heat index for Weatherzone's 'Feels Like' temperature shown on the Current Observations page.

Wind shear

See shear.

Winter

The three coldest months June, July and August (in the southern hemisphere). Winter in Australia marks the middle of the southern wet season and northern dry season.

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.

ww3

WAVEWATCH III? (Tolman 1997, 1999a, 2009) is a third generation wave model developed at NOAA/NCEP in the spirit of the WAM model (WAMDIG 1988, Komen et al. 1994). It is a further development of the model WAVEWATCH, as developed at Delft University of Technology (Tolman 1989, 1991a) and WAVEWATCH II, developed at NASA, Goddard Space Flight Center (e.g., Tolman 1992). WAVEWATCH III?, however, differs from its predecessors in many important points such as the governing equations, the model structure, the numerical methods and the physical parameterizations.

WAVEWATCH III? solves the random phase spectral action density balance equation for wavenumber-direction spectra. The implicit assumption of this equation is that properties of medium (water depth and current) as well as the wave field itself vary on time and space scales that are much larger than the variation scales of a single wave.