SCIENCE AND THE SEA 



SLOW DOWN SHIPPING 



8 10 12 14 16 18 20 22 

 Wave height (ft.) 



. CUT RADAR EFFICIENCY 



6 8 10 



Range (nautical miles) 



. RAISE SONAR NOISE LEVELS 



10 

 Frequency (cps) 



Fig,.l. Lowered ship speeds reflect not only added drag rmposed by 

 higher waves but also voluntary reductions made to ease strain on 

 vessel, cargo, or passengers. Radar efficiency, expressed by blip-scan 

 ratio, and deep-water sonar noise background both vary with sea state 

 (defined in the margin). 



The family tree of ocean waves 



Waves on water differ fundamentally from waves in water. 

 Internal waves of various kinds develop below the surface at the stable 

 interface between water layers of differing densities. They are 

 difficult to study because they can only be detected by the way 

 temperature or salinity of the water changes across the interface 

 (Fig. 2) and by the slicks of smooth appearing water that they create 

 at the sea surface, as they travel along at depth. Although their 

 periods are long in relation to their length— internal waves, in other 

 words, move very slowly compared to other waves— their height can 

 easily reach 50 or 60 ft. They are a quite interesting and important 

 class of waves; one line of informed speculation concerning the loss 

 of the US nuclear submarine Thresher invoked their influence, for 

 example. But waves on water are our prime concern here. 



In order of decreasing wavelength, waves on water include the 

 tides, seiches and storm surges, tsunamis, wind waves and swell, and 

 the diminutive capillary waves. Each of these could justifiably claim 

 our attention. 



The tides, for instance, with their typical heights of 2-10 ft (up 

 to 50 ft in extreme cases, as at the Bay of Fundy) and their clocklike 

 regularity at most places, may someday be as important in power 

 generation as they long have been in determining ship-arrival and 

 departure times. 



Travel direction 



2000 

 Time (hr) 



Fig. 2. Internal waves with varying 

 heights and periods, generally 

 moving at speeds less than 1 knot, 

 form between subsurface water 

 layers whose densities differ be- 

 cause of differences in temperature 

 and/or salinity- They are usually 

 detected by oscillations in the 

 depth of isotherms, as shown, as 

 one water mass replaces another 

 along an undulating boundary; but 

 their presence at depth frequently 

 can be inferred by "slicks" of 

 smooth water visible at the sur- 

 face, located above the trailing 

 slope of larger internal waves. 



(c) INTERNATIONAL SCIENCE AND TECHNOLOGY, CONOVER-MAST PUBLICATIONS, INC., 1964 



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