Glossary and Acronyms: W X Y Z

Warm low:

any depression which is warmer than its surroundings at corresponding atmospheric levels.

Warning system:

(at La Réunion Island): The Prefect only is authorized to activate the cyclone warnings provided for in the special emergency plan "cyclone". According to the meteorological forecasts about a cyclone development, only the interregional director of Météo-France at La Réunion (or its representative) is competent to inform and suggest decisions of warnings activation. Warnings levels are as follows :

Cyclone pre-warning:: warning against the cyclonic hazard. A tropical disturbance develops in the area and is a potential threat for La Reunion Island in the next days (in more than 24 hours).
Orange warning: the cyclonic threat grows. The territory may be in danger within the next 24 hours. Schools and créches shut up, but economical life continues.
Red warning: the cyclonic threat is imminent. The phenomenon will affect La Reunion Island within the next hours. Traffic is forbidden. The red warning is given with a 3 hours notice to allow the population to take shelter.
Caution stage: the cyclonic threat is apart from the Island but dangers remain. Schools and créches remain shut up, but economical life carries on as before.

Wave number:

The reciprocal of wavelength; the number of waves per unit distance in the direction of propagation; or, frequently, 2π times this quantity.
In the tropical regions, the zonal wave numberk gives an order of size of the space-time scale of a meteorological phenomenon.
For exemple :

k=1, only one wave around the Earth, called planetary wave (ex : Kelvin wave).
k=6, six waves are propagated around the Earth. In the tropical regions, the wave length and the period associated vary between 5000 and 10000 km and between 10 and 60 days and are called intra-seasonal scale waves (ex : MJO, equatorial Rossby waves)
k=10, ten waves are propagated around the Earth. In the tropical regions, the wave length and the period associated vary between 3000 and 5000 km and between 2 and 10 days, and are called synoptic scale waves (Rossby waves).

Wavelength:

it is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings (in simple terms, the horizontal distance between successive wave crests measured perpendicular to the crests). A long wavelength radiation is when the wave length is above 4m.

Wave period :

Time between the passage of two successive wave crests past a fixed point. (It is equal to the wavelength divided by the wave speed).

Wind:

air motion relative to the Earth's surface. Unless otherwise specified, only the horizontal component is considered. The vertical wind refers to the vertical atmospheric motions in the upward or downward direction generated by turbulence, convection, an air flow over a mountain or a colder air mass... The (horizontal) wind is characterized by : the wind direction, direction from which the wind blows, measured by a wind vane; the wind speed : measured by an anemometer. At sea, the wind force is a number on a progressive scale (Beaufort scale) corresponding to the effects produced by winds within a particular range of speeds.

Wind direction:

direction from where the wind blows.

Wind shear:

local variation of the wind vector or any of its component in a specific direction.
The vertical wind shear is the variation of the horizontal wind vector (in speed and direction) between two different atmospheric levels.
In the advisories issued by the Tropical Cyclone Centres, this wind shear is the difference between the upper level horizontal vector wind and the low level one. The wind shear given by the often mentionned CIMSS data is the difference between the mean vector wind of the layer 150-300hPa and the one of the layer 700-925hPa.
The wind shear is one of the crucial parameter to favour or not the cyclolysis or the cyclogenesis of a tropical system, which will be more or less concerned by the wind shear depending on its size and/or its intensity. Windshear can be weak (5-10kt), moderate (10-20kt) or strong (plus de 20kt).

Windward side:

portion of the side of a hill, mountain, or region, which faces the wind.

Wind sea:

Waves raised by the wind blowing in the immediate neighbourhood of an observation site at the time of observation.

Wind speed:

Ratio of the distance covered by the air to the time taken to cover it. The instantaneous speed or, more briefly, the speed, corresponds to the case of an infinitely small time interval. The mean speed corresponds to the case of a finite time interval. For the cyclones specialists, the sustained wind corresponds to the 1-min average surface wind speed (used by the American meteorologists), and the average wind is the 10-min average surface wind speed (used in most of the countries, according to the WMO's recommendations), all are measured at a height of 10 m above ground. However mean speed and sustained speed are often confused.
The apparent wind or relative wind, is the wind vector relative to a moving object. The apparent wind velocity is the vector difference between the velocities of the true wind and the object.
The gradient wind is the theoretical wind resulting from the equilibrium between the horizontal pressure force, the horizontal component of the deviating force due to the Earth's rotation, and the centrifugal force due to air motion an a curved path, only these forces being supposed to act on the air. It blows parallel to curved isobars or contours.

The wind speed is generally measured in metres per second (m/s) or in kilometres per hour (km/h), or in knots (kt) or nautical mile per hour in marine and aeronautical meteorology. A nautical mile (NM) corresponds to the distance between two points of a same meridian of which latitudes are distant of one minute of angle (1° of latitude = 60 nautical miles = 111.12 kilometres; for 1° of longitude, multiply by the cosinus of the latitude).

1 NM (nautical mile) = 1,852 km. Be careful to not confuse a nautical mile with the Anglo-Saxon terrestrial mile (1,609 km).
1 m/s = 3,6 km/h = 1,9438 knots,
1 km/h = 0,2778 m/s= 0,5400 knots,
1 knot (kt)= 0,5144 m/s = 1,852 km/h.

WMO:

World Meteorological Organization. Specialized agency of the United Nations Organization for coordinating, standardizing and improving meteorological activities throughout the world and for encouraging the efficient exchange of information between countries, in the interest of various human activities.

World Weather Watch, WWW:

The world-wide, coordinated, developing system of meteorological facilities and services provided by WMO Members for the purpose of ensuring that all Members obtain the meteorological information required both for operational work and for research. The essential elements of the WWW are the Global Observing System; the Global Data-processing System; the Global Telecommunication System.

X

Y

Z:

abbreviation for "Zulu", to designate Zulu Time, equivalent to the Universal Time Coordinated (UTC). See also FAQ A10).