the difference in appearance between slicks and surrounding water to the reduc- 

 tion in average angle of reflection caused by the capillary wavelets. 



Thus, slicks are identified by their difference in optical properties from those 

 of their surroundings. As one gets closer to a slick, however, this difference be- 

 comes less obvious. 



Slicks have been observed around the world in the open sea, bays, and lakes.^* 

 According to these investigations, slicks are almost always present when the wind 

 is just strong enough to ripple the water, yet not strong enough to cause whitecaps 

 (Beaufort force 3, i. e., 3.4 meters per second). Frequently, slicks take on the shape 

 of broad, spiderweb-like connecting bands. Occasionally, they occur as isolated 

 patches. In shallow waves over the continental shelf, they often appear as long 

 bands, more or less parallel to the coast. Near shore, a patch or wide band may 

 develop just beyond the breaker zone. Other slicks are found over kelp beds. 



The cause of slicks has been studied by several investigators. Ewing^ ** con- 

 cluded that band slicks are associated with long internal waves in a shallow, 

 lowered thermocline. Since the internal waves are of a progressive nature, and 

 since the slick lies over the trough, it must move in the direction of travel of the 

 internal wave. 



Dill and LaFond' found a lowering of the thermocline in slicks and, in addition, 

 observed that the turbidity of the water near slicks was different from that of 

 adjacent water. They also made related studies of circulation, surface tension, and 

 other features. The present report covers a continuation of this work, particularly 

 the relation of the temperature structure of the sea to slicks. 



MEASUREMENTS 



Temperature 



Early in the work on temperature structure, it became evident that a need 

 existed for some means of recording temperatures at a number of different points 

 in the sea, simultaneously and continuously, over extended periods of time. For 

 this purpose, an instrument was developed that will record temperatures at as 

 many discrete points (up to sixteen) as may be desired. This instrument has 

 proved very successful. 



The recording instrument consists essentially of a string of thermistor beads 

 which are part of a bridge circuit which feeds into a Brown recording potentiom- 

 eter.'* Although sixteen channels and sixteen sensing units can be used, only six 

 beads were used for this study. The beads were arranged so that five of them each 

 actuated three different channels. This reduced the time interval between record- 

 ings on the same bead to about 10 seconds. The sixth bead was placed near the 

 surface and recorded about twice a minute. Other details of the instrument are 

 given in the Appendix. 



Temperature measurements were made from anchored ships off Mission Beach, 

 California, on twelve days between 12 June and 8 August 1958 (fig. 2A). The 

 procedure was to anchor both fore and aft about 1000-1100 and remain there, 

 recording temperature structure continuously until midafternoon, when the wind 

 usually became strong enough to dissipate the slick. The vertical string of sensing 

 elements was buoyed out slightly seaward, about 100 feet from the anchored ship 

 in 50-foot deep water (fig. 2B). The beads were suspended at 2, 9, 16, 23, 30, and 

 37 feet from the surface buoy. As a slick approached the buoy supporting the 



