CYCLOIDAL WAVE . A very steep, symmetrical wave 

 whose crest forms an angle of 120 degrees. The 

 wave form is that of a cycloid. A trocholdal wave 

 of maximum steepness. See also TROCHOIDAL WAVE. 

 (U) 



CYCLONE . An atmospheric cyclonic circulation, a 

 closed circulation. A cyclone's direction of rota- 

 tion (counterclockwise in the Northern Hemisphere) 

 is opposite to that of an anticyclone. While 

 modern meteorology restricts the use of the term 

 cyclone to the so-called cyclonic-scale circulations, 

 it is popularly still applied to the more or less 

 violent, small-scale circulations such as tornadoes, 

 WATERSPOUTS, dust devils, etc. (which may in fact 

 exhibit anticyclonic rotation), and even, very 

 loosely, to any strong wind. The first use of this 

 term was in the very general sense as the generic 

 term for all circular or highly curved wind systems. 



Because cyclonic circulation and relative low 

 atmospheric pressure usually co-exist, in common 

 practice the terms cyclone and low are used inter- 

 changeably. Also, because cyclones nearly always 

 are accompanied by inclement (often destructive) 

 weather, they are frequently referred to simply as 

 storms. (24) 



CYLINDRICAL WAVE . A cylindrical wave is a wave in 

 which the wave fronts are coaxial cylinders. (1) 



CYTOLOGY. The branch of the biological sciences 

 that deals with the structure and processes of 

 protoplasm and the cell. (18) 



CYTOPLASM . That part of a cell outside the nucleus 

 and within the cell membrane. (19) 



D 



DDI. 



Depth deviation indicator. (6) 



DAILY RETARDATION (OF TIDES ) . The amount of time 

 by which corresponding tidal phases grow later day 

 by day (averages approximately 50 minutes). (11) 



"DAILY WATER" . See REFERENCE PLANE. 



DALTON'S LAW . States that the total pressure 

 exerted by a mixture of gases may be considered to 

 be the sum of the pressures that would be exerted 

 by each of the gases if it alone were present and 

 occupied the total volume. (37) 



DAMPING . Damping is the dissipation of energy with 

 time or distance. (2) 



DAMPING RATIO . The damping ratio for a system with 

 viscous damping is the ratio of actual damping co- 

 efficient to the critical damping coefficient. (2) 



DAN BUOY . A buoy consisting of a ballasted float 

 carrying a staff which supports a flag or light. 

 Dan buoys are used principally in minesweeping, and 

 by British fishermen to mark the position of deep- 

 sea fishing lines or the place for trawling. (17) 



DANGER BUOY . A buoy marking an isolated danger to 

 navigation, such as a rock, shoal, or sunken wreck. 

 (17) 



DASH . A drone antisubmarine helicopter for de- 

 livery of a weapon to an enemy submarine. 



DATUM PLANE . A horizontal plane used as a refer- 

 ence from which to reckon heights or depths. See 

 also REFERENCE PLANE. (27) 



DAVIDSON CURRENT . A countercurrent of the Pacific 

 Ocean running north along the west coast of the 

 United States (from northern California to Washing- 

 ton to at least latitude 48°N) during the winter 

 months . (24) 



DP . U. S. Navy ship designation for a Destroyer. 



DDC. Defense Documentation Center. 



DDE . U. S. Navy ship designation for an Escort 

 Destroyer. 



DDG . U. S. Navy ship designation for a Guided 

 Missile Destroyer. 



DDK . U. S. Navy ship designation for a Hunter- 

 Killer Destroyer. 



DDR . U. S. Navy ship designation for a Radar 

 Picket Destroyer. 



DE . U. S. Navy designation for an Escort Vessel. 



DEAD AHEAD . Bearing 000° relative. If the bearing 

 is approximate, the term ahead should be used. (17) 



DEAD ASTERN . Bearing 180° relative. If the bear- 

 ing is approximate, the term astern should be used. 

 Also called right astern. (17) 



DEAD RECKONING . Determination of position by ad- 

 vancing a previous position for courses and dis- 

 tances. A position so determined is called a dead 

 reckoning position. In air and land navigation, 

 the best estimate of course and speed over the 

 ground is used. In marine navigation, dead reckon- 

 ing course and speed are generally reckoned without 

 allowance for wind or current, although this prac- 

 tice is not universal. Dead reckoning performed 

 automatically by a device which gives a continuous 

 indication of position by double integration of 

 accelerations since leaving a starting point is 

 called inertial navigation, and when performed by 

 integrating the speed derived from measurement of 

 the Doppler effect of echoes from directed beams 

 of radiant energy transmitted from the craft is 

 called Doppler navigation, (17) 



DEBRIS LINE . A line near the limit of storm wave 

 uprush marking the landward limit of debris 

 deposits. (11) 



DEC . U. S. Navy designation for a Control Escort 

 Vessel. 



DECADE BAND . In accordance with accepted usage a 

 decade band is one having a band width of f, - f 

 = 10. (4) " 



DECAY DISTANCE . The distance through which waves 

 travel after leaving the generating area. (11) 



DECAY OF WAVES . The change that waves undergo 

 after they leave a generating area (FETCH) and pass 

 through a calm, or region of lighter winds. In the 

 process of decay, the significant wave height de- 

 creases and the significant wave length increases. 

 (11) 



DECCA . An electronic navigational system by which 

 hyperbolic lines of position are determined by 

 measuring the phase difference of continuous wave 



31 



