SECT. 5] 



INTERNAL WAVES 



745 



h. Relation of internal waves to slicks 



Sea-surface slicks, which often represent visible evidence of internal waves 

 below, are seen as streaks or patches of relatively calm surface water sur- 

 rounded by rippled water. The absence of wavelets in a slick gives it a glassy 

 appearance in contrast to the adjacent rippled water (Fig. 16). 



From most angles, a slick appears brighter than its peripheral area in day- 

 time because a smooth surface reflects the sky more than a rougher one. At 

 night, when ambient light may exist, slicks contrast with adjacent, rippled 

 water because their unruffled surface is less susceptible to surrounding reflec- 

 tion. Surface slicks appear darker in full sunshine when the visual angle is such 

 that light is directly reflected toward the viewer. This is because slicks do not 

 produce the glitter that radiates from mutual reinforcement of reflected rays 

 on a contiguous rippled surface. 



Fig. 16. Sea-surface slicks off Mission Beach, California. 



Slicks have been studied in oceans, bays and lakes (Dietz and LaFond, 

 1950 ; Woodcock and Wyman, 1946 ; Forbes, 1945). According to such investiga- 

 tions, slicks are generally present when the wind has enough force to ripple the 

 water, but not enough to cause whitecaps (Beaufort force 3, i.e., 3.4 m/sec). 

 Slicks frequently assume the shape of broad, web-like connecting bands, and 

 they occasionally appear as isolated patches. In shallow ocean over a con- 

 tinental shelf, slicks are often contoured as long bands, more or less parallel with 

 the coast. Near shore, a wider band may develop just beyond the breaker zone. 

 Some slicks have been discovered on surfaces above kelp beds. 



During a 1958 study of slicks and internal waves, it was found that slicks 

 were present about 10% of the time. During the periods of observation, 105 

 slicks were recorded. The duration of a single slick, as it passed any point, was 

 from 0.35 to 5 min, the average being 1.3 min. 



