which is a photocell to measure ambient light; and 

 a pressure transducer. Filters are used to make 

 the photocell response similar to that of the human 

 eye. The deck unit consists of a photocell for 

 measuring surface Illumination, an electronic 

 unit, and a recorder. Any of the four parameters 

 may be recorded Individually or all In succession 

 by switching. (35) 



HATER COLOR . The apparent color of the surface 

 layers of the sea caused by the reflection of cer- 

 tain components of the visible light spectrum 

 coupled with the effects of dissolved material, 

 concentration of PLANKTON, DETRITUS, or other matter 

 Color of oceanic water varies from deep blue to 

 yellow and is expressed by number values which are 

 a variation of the FOREL SCALE. Plankton concen- 

 trations may cause a temporary appearance of red, 

 white, green, or other colors. (15) 



WATER-COMPRESSIBILITY . Water can be compressed 

 only to a slight degree, the compressibility is so 

 slight that even at the depth of a mile, a cubic 

 foot of water weighs only about 1/2 pound more 

 than that at the surface. 



WATER CONTENT . Of a bottom sediment Is a ratio 

 obtained by multiplying the weight of the water in 

 the sample by 100 and dividing the results by the 

 weight of the dried sample; expressed as a percen- 

 tage. (17) 



WATERLINE ATTACK . CORROSION that takes place at 

 the Interface of the atmosphere and water on metal- 

 lic structures partly immersed in the water. 



WATER NOISE . The minimum ambient noise of practi- 

 cal significance is that due to movement of the 

 water itself. The term water noise is used to 

 designate acoustic energy resulting primarily from 

 this cause. (4) 



WATER SKY . Dark streaks, patches, or grayness on 

 the underside of extensive cloud areas due to the 

 absence of reflected light from open water areas. 

 Water sky is darker than LAND SKY. (25) 



WATERSPOUT . A small whirling storm over water, 

 the chief characteristic of which is a funnel- 

 shaped cloud extending, in a fully developed water- 

 spout, from the surface of the water to the base of 

 a cumulus type cloud. Only the lower part ordinar- 

 ily has water, which may be drawn up by the action 

 of the vortex or may be produced by condensation. 

 Waterspouts usually rotate in the same direction 

 as CYCLONES and are most frequent in the tropics . 

 (17) 



WAVE . A wave is a disturbance which is propagated 

 In a medium in such a manner that at any point in 

 the medium the quantity serving as measure of dis- 

 turbance is a function of the time, while at any 

 Instant the displacement at a point is a function 

 of the position of the point. Any physical quantity 

 that has the same relationship to some independent 

 variable (usually time) that a propagated distur- 

 bance has, at a particular instant, with respect 

 to space, may be called a wave. (1) 



WAVE , CAPILLARY . A wave whose velocity of propaga- 

 tion is controlled primarily by the surface tension 

 of the liquid In which the wave Is travelling. 

 Water waves of a length less than one inch are 

 considered to be capillary waves. (11) 



WAVE CELERITY. See PHASE SPEED. 



WAVE, CYCLOIDAL . A very steep, symmetrical wave 

 whose crest forms an angle of 120°. The wave form 

 is that of a cycloid. A TROCHOIDAL WAVE of maxi- 

 mum steepness. (11) 



WAVE DIRECTION . The direction, in degrees true, 

 from which the waves come. (35) 



WAVE FILTER (FILTER) . A wave filter Is a trans- 

 ducer for separating waves on the basis of their 

 frequency. It Introduces relatively small inser- 

 tion loss to waves In one or more frequency bands 

 and relatively large insertion loss to waves of 

 other frequencies. (9) 



WAVE FORECASTING . The theoretical determination 

 of future wave characteristics, usually from ob- 

 served or predicted meteorological phenomena. (11) 



WAVE FRONT . Wave front is an expression which is 

 applied to a progressive wave In space at any given 

 instant, and is the line or surface over which the 

 phase is everywhere the same at the given instant. 

 A wave front is particularly obvious in the case of 

 surface waves on water, for all the points lying at 

 the crest of the wave at any given instant are In 

 the same phase. These points form a line and are 

 an example of a case where the wave front is a line 

 rather than a surface. (9) 



WAVE GAGES . Ocean wave measurements may be made 

 in several various ways. Capillary waves or those 

 having a 10"^ WAVE PERIOD (In seconds) may be 

 measured optically while GRAVITY (type) WAVES with 

 a wave period characterized by 7 seconds are 

 measured by electromechanical methods. For long 

 (10 seconds) wave periods, pneumatic methods are 

 employed; numerical methods on TRANSTIDAL (10^ 

 seconds) waves and Astronomical methods for waves 

 having a wave period in the order of 10° seconds. 



In general, the most common class of devices 

 used are - Bottom Pressure Instruments, Floating 

 Wave Gages, Fixed Wave Gages and Airborne Systems. 



In obtaining wave height records from pressure 

 transducers placed on the ocean floor, the measured 

 pressure fluctuations due to the overlying wave 

 are translated by appropriate theoretical equations 

 into corresponding wave heights. The method is 

 restricted however to so-called shallow water waves 

 where the depth is less than half a wave length, 

 and suffers in addition from the fact that high 

 frequency components of the wave system become 

 greatly attenuated with depth and may not even be 

 sensed by the transducer. 



A promising method for measuring waves at sea 

 lies in the use of the SHIP-BORNE WAVE RECORDER. 

 The device makes use of pressure sensors located 

 on the ship's hull below the water surface. The 

 heave of the vessel, determined from an accelero- 

 meter pickup, is effectively factored out of the 

 total response, leaving only a record of wave 

 height. A minor disadvantage of this method is 

 that the presence and motion of the ship alters 

 the wave pattern to some degree. 



Another scheme of wave measurement Is the wave 

 pole, a spar-like floating buoy which uses a ver- 

 tical wire whose resistance changes as a wave passes 

 by it. The resulting voltage change, proportional 

 to the wave height, is telemetered to shore or a 

 nearby vessel and recorded. Although attempts are 

 made to restrict the pole's motion In the seaway 

 by means of damping devices, its excursions from a 

 fixed position in space do lead to errors of 

 measuremant in varying degrees. (35) 



WAVE GROUP . A series of waves in which the wave 

 direction, WAVE LENGTH, and wave height vary only 

 slightly. (11) 



WAVE HEIGHT COEFFICIENT . The ratio of the wave 

 height at a selected point to the deep water wave 

 height. The REFRACTION COEFFICIENT multiplied by 

 the SHOALING COEFFICIENT. (11) 



WAVE INTERFERENCE . Wave interference Is the phe- 

 nomenon which results when waves of the same or 

 nearly the same frequency are superposed and is 



127 



