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 Warning stage The stage on a fixed river gauge at which it is necessary to begin issuing warnings or river forecasts if adequate precautionary measures are to be taken before flood stage is reached. 
Water Dihydrogen oxide, molecular formula H2O.
Water balance Balance of the water resources of a region, comparing precipitation and inflow with outflow, evaporation, and accumulation.
Water budget See hydrologic accounting.
Water equivalent The depth of water that would result from the melting of snow or ice, assuming measurement on a horizontal surface and no infiltration or evaporation.
Water-flow pyrheliometer An absolute pyhrliometer, developed by C.G. Abbott, in which the radiation-sensing element is a blackened water-calorimeter.
Watershed The total area drained by a river and its tributaries. Same as river basin.
Water-stage recorder A device for obtaining a continuous record of stage at a point on a stream. The most common recorders consist of a float-actuated pen which traces a record on a clock driven chart.
Water table The depth below which the ground is saturated with water. No water table exists if the ground water is confined by an overlying impermeable stratum, as in the case of artesian ground water.
Water year Any twelve-month period, usually selected to begin and end during a relative dry season. Used a basis for processing streamflow and other hydrologic data. The period from October 1 to September 30 is widely used in the U.S.
Watt A unit of power equal to one joule per second or 10' ergs per second.
Wave pole A device for measuring sea-surface waves. It consists of a weighted pole below which a disk is suspended at a depth sufficiently deep for the wave motion associated with deepwater waves to be negligible. The pole will then remain nearly as if anchored to the bottom, and wave height and period can be acertained by observing or recording the length of the pole that extends above the surface.
Wave recorder An instrument for recording ocean waves. Most recorders are designed for recording wind waves, that is waves of periods up to about 25 seconds, but some are designed to record waves of longer periods such as tsunamis or tides.
Wave staff Same as wave pole.
Weather The state of the atmosphere, mainly with respect to its effects upon life and human activities. As distinguished from climate, weather consists of the short-term (minutes to months) variations of the atmosphere.
Weather glass An old nautical term for mercury barometer.
Weger aspirator An instrument of the aspiration condenser type which measures the concentration and mobility of small ions.
Weighing rain gauge A precipitation gauge consisting of a receiver in the shape of a funnel which empties into a bucket mounted upon a weighing mechanism. The weight of the catch is recorded as inches of precipitation.
Weight barograph A recording weight barometer.
Weight barometer A mercury barometer which measures atmospheric pressure by weighing the mercury in the column or cistern.
Wet-bulb depression Difference between the temperatures of the dry-bulb and the wet-bulb thermometers of a psychrometer.
Wet-bulb temperature The lowest temperature that can be obtained on a wet-bulb thermometer in any given sample of air. Obtained by evaporation of water (or ice) from the muslin wick. Used in computing dew point and relative humidity.
Wet-bulb thermometer A thermometer with a muslin-covered bulb which is moistened. used to measure wet-bulb temperature.
White body A hypothetical "body" whose surface absorbs no electromagnetic radiation of any wavelength. An idealization exactly opposite to that of the black body. In nature, no true white bodies are known. Moist white pigments exhibiting high reflectivity for visible radiation are fairly good absorbers in the infrared range, hence they are not white bodies in the sense of radiation theory. However, one does speak of a white body with respect to a particular wavelength interval. Compare to black body, gray body.
White rainbow Same as fogbow.
Whole gale Wind with a speed between 48 and 55 knots (55 and 63 mph), Beaufort scale number 10.
Wien's law One of the radiation laws which states that the wavelength of maximum radiation intensity for a black body is inversely proportional to the absolute temperature of the radiating black body.
Wild fence A wooden enclosure about sixteen feet square and eight feet high with a precipitation gauge at its center. The function of the fence is to minimize eddies around the gauge and thus insure a catch that is representative of the actual rainfall or snowfall.
Wilting point Value of soil moisture, expressed as a percentage of the mass of dry soil, below which a plant living in the soil dies by wilting.
Wind Air in motion relative to the surface of the earth. Almost exclusively used to denote the horizontal component.
Wind cone Same as wind sock.
Wind direction The direction from which the wind is blowing, measured in points of the compass or in azimuth degrees.
Wind gust See gust and peak gust.
Windmill anemometer A rotation anemometer in which the axis of rotation is horizontal. The instrument has either flat vanes (as in the air meter) or helicoidal vanes (as in the propeller anemometer). The relation between wind speed and angular rotation is almost linear.
Wind passage The distance or length of flow of the air past a point during a given interval of time.
Wind rose A flower-like diagram indicating the relative frequencies of different wind directions for a given station and period of time.
Wind run The distance or length of flow of the air past a point during a given interval of time.
Winds aloft The wind speed and direction at various levels in the atmosphere above the level reached by surface weather observations.
Winds-aloft observation The measurement and computation of wind speeds and directions at various levels above the surface of the earth. Methods include pilot balloon observations, rabals, rawin or rawinsonde observations, radar tracking, or acoustic sounding.
Winds-aloft plotting board A graphical aid used in the reduction of data from a winds aloft observation.
Wind shear A local variation of the wind vector or any of its components in a given direction.
Wind sleeve Same as windsock.
Wind sock A fabric cone attached to a metal ring and used to indicate wind direction. often at airfields.
Wind speed Rate of wind movement in distance per unit time.
Wind vane An instrument used to indicate wind direction.
Wind vector A component of the wind (often using Cartesian coordinates. i.e. X and Y wind vectors). Also, an arrow representing wind velocity, drawn to point in the direction of the wind and with a length proportional to wind speed.
Wind velocity A vector term which includes both wind speed and wind direction.
Windward Situated on the side from which the wind blows.
Windwave A wave resulting from the action of wind on a water surface.
Wiresonde An atmospheric sounding instrument which is supported by a captive balloon and used to obtain temperature and humidity data from the ground level to a height of a few thousand feet. The data is telemetered to the ground through a wire cable.
Wireweight gauge A river gauge in which a weight suspended on a wire is lowered to the water surface from a bridge or other overhead structure to measure the distance from a point of known elevation to the water surface.
Word A fixed-length group of bits representing the large data element handled as a unit by a computer. Word length is determined by the capacity of the CPU registers.
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