GNP . Gross National Product. 



COIN. State Institute of Oceanography (USSR). 



GONG BUOY . A buoy with one or more gongs. In the 

 United States a gong buoy is a flat-topped float 

 with a skeleton superstructure supporting a series 

 of three or four gongs of varied tone. (17) 



GONIMOBLAST . One of the sporogenous filaments 

 which arise from the fertilized CARPOGONIUM in 

 most red algae. (20) 



GPRC . Geophysical and Polar Research Center. 



GRAB-CAMERA . An ocean floor sampling system in- 

 corporating a large sediment grab with a deep sea 

 camera. (35) 



GRADED BEDDING . Beds whose grain size of particles 

 decreases systematically upward; the finer material 

 overlies the coarser, owing to the more rapid set- 

 tling out of coarse material from a mixture of 

 grain sizes . (27) 



GRADIENT . 1. The space rate of decrease of a func- 

 tion. The gradient of a function in three space 

 dimensions is the vector normal to surfaces of 

 constant value of the function and directed toward 

 decreasing values, with magnitude equal to the rate 

 of decrease of the function in this direction. The 

 ascendent is the negative of the gradient. 



2. Often loosely used to denote the 

 magnitude of the gradient or ascendant (i.e. with- 

 out regard to sign) of a horizontal pressure field. 

 See SLOPE. (24) 



GRADIENT HYDROPHONE . 

 PHONE) . 



(See PRESSURE GRADIENT HYDRO- 



GRADIENT WIND . A wind that blows parallel to 

 curved isobars because of a balance of forces. 

 These forces are the pressure force (high to low). 

 Coriolis (apparent deflecting force due to the 

 rotation of the earth), and the centrifugal force. 

 A GEOSTROPHIC WIND blows parallel to straight 

 isobars. (17) 



GRAF SEA GRAVIMETER . A balance type gravity meter 

 (heavily overdamped to attenuate shipboard verti- 

 cal accelerations) which consists of a mass at the 

 end of a horizontal arm that is supported by a 

 torsion spring rotational axis. The mass rises and 

 falls with gravity variation but is restored to 

 near its null position by a horizontal reading 

 spring, tensioned with a micrometer screw. Differ- 

 ence between actual beam position and null position 

 gives an indication of gravity value after micro- 

 meter screw position has been taken into account. 

 (37) 



GRAVEL . Loose detrital material which consists of 

 fragments ranging in size from approximately 0.08 

 to 10.08 inches (2 to 256 millimeters) (16) 



GRAVIMETER (GRAVITY METER) . An instrument to mea- 

 sure the value of gravity or for measuring varia- 

 tions in the magnitude of the earth's gravitation- 

 al field. 



Measurements of gravity are accomplished gen- 

 erally by one of three methods; dropped ball, pen- 

 dulum or spring gravimeter. The latter type of 

 gravimeter based upon the principle of the weighted 

 spring and where the length or measured variations 

 in the length of this spring are a function of the 

 gravitational field at different locations are the 

 type widely used today. See GRAF SEA GRAVIMETER. 

 (35) 



GRAVING DOCK. See DOCK. 



or below sea level. The unit in m.t.s. system is 

 one dynamic decimeter. 



GRAVITY WAVE . (Also called gravitational wave.) 

 A wave disturbance in which buoyancy (or reduced 

 gravity) acts as the restoring force on parcels 

 displaced from hydrostatic equilibrium. There is 

 a direct oscillatory conversion between potential 

 and kinetic energy in the wave motion. 



Pure gravity waves are stable for fluid sys- 

 tems which have static stability. This static 

 stability may be (a) concentrated in an interface 

 or (b) continuously distributed along the axis of 

 gravity. The following remarks apply to the two 

 types, respectively. 



(a) A wave generated at an interface is simi- 

 lar to a surface wave, having maximum amplitude at 

 the interface. A plane gravity wave is character- 

 istically composed of a pair of waves, the two 

 moving in opposite directions with equal speed 

 relative to the fluid itself. In the case where 

 the upper fluid has zero density, the interface 

 is a free surface and the two gravity waves move 

 with speeds . .„ 



fe -^ ^ 



-fe 



where U is the current speed of the fluid, g the 

 acceleration of gravity, L the wave length, and 

 H the depth of the fluid. For deep-water waves 

 (or Stokesian waves or short waves), H»L and 

 the wave speed reduces to 



u*\/IF- 



For shallow-water waves (or Lagrangian waves or 

 long waves ) , H«L , and 



All waves of consequence on the ocean surface or 

 interfaces are gravity waves, for the surface ten- 

 sion of the water becomes negligible at wave 

 lengths of greater than about one inch. 



(b) Heterogeneous fluids, such as the atmos- 

 phere, have static stability arising from a strat- 

 ification in which the environmental lapse rate is 

 less than the process lapse rate. The atmosphere 

 can support short internal gravity waves and long 

 external gravity waves. The short waves (of the 

 order of 10 km) have been associated, for example, 

 with lee waves and billow waves. Such waves have 

 vertical accelerations which cannot be neglected 

 in the vertical equation of perturbation motion. 

 The long gravity waves, moving relative to the at- 

 mosphere with speed ±ygH , where H is the 

 height of the corresponding homogeneous atmosphere, 

 have small vertical accelerations, and are there- 

 fore, consistent with the quasi-hydrostatic ap- 

 proximation. In neither type of gravity wave, 

 however, is the horizontal divergence negligible. 

 For meteorological purposes in which neither type 

 is desired as a solution, e.g., numerical fore- 

 casting, they may be eliminated by some restriction 

 on the magnitude of the horizontal divergence. 



The above discussion is based upon the method 

 of small perturbations. In certain special cases 

 of water waves, e.g., the Gerstner wave, or the 

 solitary wave, a theory of finite-amplitude dis- 

 turbances exists. (24) 



GREASE ICE . A kind of SLUSH formed from the con- 

 gelation of ice crystals in the early stages of 

 freezing. It gives the sea surface a greasy ap- 

 pearance. (25) 



GREAT DIURNAL RANGE (Gt) . The difference in height 

 between mean higher high water and mean lower low 

 water. The expression may also be used in its con- 

 tracted form - diurnal range. (14) 



GRAVITY POTENTIAL . The work required or gained in 

 moving a unit mass from sea level to a point above 



GREAT TROPIC RANGE (Gc) . The difference in height 

 between tropic higher high water and tropic lower 



53 



