low water. The expression may also be used in its 

 contracted form - tropic range. (14) 



GRB . Geophysics Research Board of the NAS/NRC 

 (National Academy of Sciences - National Research 

 Council) . 



GREATER FLOOD . See FLOOD CURRENT. 



GRECO . An Italian name for the northeast wind. It 

 was given by Roman sailors to the northeast wind in 

 the Gulf of Lions because it came from the direction 

 of the Greek colony of Marsala (Marseilles). (12) 



GREEN FLASH . A brilliant gr 

 upper edge of the sun as it 

 disappears at sunset when th 

 tinct horizon. It is due to 

 mosphere, which disperses th 

 of light into a spectrum and 

 appear (or disappear) in the 

 The green is bent more than 

 is visible sooner at sunrise 

 (17) 



een coloring of the 

 appears at sunrise or 

 ere is a clear, dis- 



refraction by the at- 

 e first (or last) spot 



causes the colors to 



order of refrangibility. 

 red or yellow and hence 



and later at sunset. 



GREEN MUD . A deep-sea TERRIGENOUS deposit, charac- 

 terized by a considerable proportion of glauconite; 

 calcium carbonate present in variable amounts up 

 to 50 per cent. (27) 



GREEN SKY . A greenish tinge to part of the sky, 

 supposed by seamen to herald wind or rain, or, in 

 some cases, a tropical cyclone. (12) 



GREENHOUSE EFFECT . The heating effect exerted by 

 the atmosphere upon the earth by virtue of the 

 fact that the atmosphere (mainly, its water vapor) 

 absorbs and reemits infrared radiation. In detail: 

 the shorter wavelengths of insolation are trans- 

 mitted rather freely through the atmosphere to be 

 absorbed at the earth's surface. The earth then 

 reemits this as long-wave (infrared) terrestrial 

 radiation, a portion of which is absorbed by the 

 atmosphere and again emitted. Some of this is 

 emitted downward back to the earth's surface 

 (counterradiation) . 



The mean surface temperature for the entire 

 world, 14°C, is almost 40 C higher than the mean 

 temperature required for radiative equilibrium of 

 a black body at the earth's mean distance from the 

 sun. It is essential, in understanding the con- 

 cept of the greenhouse effect, to note that the 

 important additional wanning is due to the counter- 

 radiation from the atmosphere. The glass panes 

 of a greenhouse function in this manner exactly 

 analogously to the atmosphere in maintaining high 

 greenhouse temperatures, hence the name. (24) 



GREENLAND CURRENT . Ocean currents flowing clock- 

 wise around the southern part of Greenland. The 

 EAST GREENLAND CURRENT flows southward and south- 

 westward along the eastern and southeastern coasts, 

 and the WEST GREENLAND CURRENT flows northwestward 

 and northward along the southwest and west coasts. 

 (17) 



GREENWICH CIVIL TIME. See GREENWICH MEAN TIME. 



GROIN (Brit . GROYNE) . A shore protective struc- 

 ture (built usually perpendicular to the shore 

 line) to trap littoral drift or retard erosion of 

 the shore. It is narrow in width (measured paral- 

 lel to the shore line) , and its length may vary 

 from less than one hundred to several hundred feet 

 (extending from a point landward of the shore line 

 out into the water). Groins may be classified as 

 permeable or impermeable; impermeable groins having 

 a solid or nearly solid structure, permeable groins 

 having openings through them of sufficient size to 

 permit passage of appreciable quantities of lit- 

 toral drift. (11) 



GROUND LOG . A device for determining the course 

 and speed made good over the ground in shallow 

 water, consisting of a lead or weight attached to 

 a line. The lead is thrown overboard and allowed 

 to rest on the bottom. The course being made good 

 is indicated by the direction the line tends and 

 the speed by the amount of line paid out in unit 

 time. (17) 



GROUND SWELL . A long high ocean swell; also, this 

 swell as it rises to prominent height in shallow 

 water, however, usually not so high or dangerous 

 as BLIND ROLLERS. (11) 



GROUND WATER . Subsurface water occupying the zone 

 of saturation. In a strict sense the term is ap- 

 plied only to water below the water table. (11) 



GROUP VELOCITY . The velocity of a wave disturbance 

 as a whole, i.e., of an entire group of component 

 simple harmonic waves. The group velocity G is 

 related to the phase speed C of the individual har- 

 monic waves of length L by the frequency equation 



dC 

 G " C - L dL ; 



the phase speed C is thus equal to the group 

 velocity only in the case of nondispersive waves, 

 i.e., when dC/dL " zero. 



The significance of the concept of group 

 velocity lies in the fact that the energy of the 

 disturbed flow is, in a sense, propagated at this 

 speed. The group velocity of several simple at- 

 mospheric models has been studied by T. C. Yeh 

 ("On energy dispersion in the atmosphere.") 



For water-surface waves, the group velocity 

 of deep-water waves is equal to one-half the 

 velocity of individual waves in the group; for 

 shallow-water waves, it is the same as their 

 velocity. (24) 



GROWLER . A small fragment of ice awash, smaller 

 than a BERGY BIT, usually of glacial origin, and 

 generally greenish in color. (25) 



GROWLER ICE . An accumulation of GROWLERS. (25) 



GSMFC. Gulf States Marine Fisheries Commission. 



GTS. Guinean Trawling Survey. 



GREENWICH MEAN TIME . Local mean time at the Green- 

 wich meridian; the arc of the celestial equator, 

 or the angle at the celestial pole, between the 

 lower branch of the Greenwich celestial meridian 

 and the hour circle of the mean sun, measured west- 

 ward from the lower branch of the Greenwich celes- 

 tial meridian through 24 hours. Mean time reckoned 

 from the upper branch of the Greenwich meridian 

 is called Greenwich Astronomical time. Called 

 Greenwich Civil Time in United States terminology 

 from 1925 through 1952. Also called Universal 

 Time. (17) 



GREGALE. A strong northeast wind of the central 

 Mediterranean. (17) 



GUINEA CURRENT . A North Atlantic Ocean current 

 flowing southeastward and eastward along the 

 Atlantic coast of Africa, from the vicinity of the 

 Cape Verde Islands to the Gulf of Guinea, flowing 

 for some distance in close proximity but in the 

 opposite direction to the SOUTH EQUATORIAL CURRENT 

 before curving southward and westward to merge 

 with that current . The Guinea current is the con- 

 tinuation of the eastern branch of the CANARY CUR- 

 RENT, augmented by the EQUATORIAL COUNTERCURRENT . 

 (17) 



GULDER . A double low water occurring on the south 

 coast of England. (17) 



GULF . A relatively large portion of the sea which 



Si* 



