211 



thousands of highlights implied that little facets on the sea surface were so in- 

 clined as to reflect additional rays from the sun toward his eye. He measured 

 the angular width of the sun path, and from this computed by simple geometry 

 that slopes up to 30° were presented. A refined version of this method has been 

 usedby Hulbert, and later by Shuleikin, with similar results. 



Actually, this method is open to criticism. The choice of the width of the 

 sun path is a somewhat subjective matter, depending as it does on the sensitivity 

 of our eyes. Otherwise, why should the path of moon glitter be narrower than 

 that of the sun? The method can be made quantitative by measuring not the total 

 width of the glitter path, but the change of brightness within the glitter pattern. 

 By the use of such a method. Cox and Munk find that the slopes are nearly nor- 

 mally distributed, with the mean square slope increasing roughly linearly with 

 the wind speed, and that, as a first approximation, the slopes are the same in 

 all directions. Surface slicks reduce the slopes by a considerable amount. 



LABORATORY EXPERIMENTS 



These results have a bearing on laboratory methods for studying the air- 

 sea boundary. Waves generated in wind-water tanks may differ from those gen- 

 erated at sea in several aspects: (1) the laboratory waves are more nearly one- 

 directional, lacking the space for growing at an angle to the wind. We have noted 

 that at sea waves are nearly non- directional. (2) waves in tanks appear to re- 

 semble sine waves more closely than waves at sea. For a sine wave the fre- 

 quency distribution of slopes is the "opposite" of the normal distribution, with the 

 maximum slopes (at the inflection points) the most probable, and zero slopes the 

 least probable, as illustrated by figure 1. (3) the rigid top of a wind-wave tank 

 may modify conditions considerably. What is ordinarily considered a surface 

 wave is actually an internal wave at the boundary between air and water. The 

 air must participate in this wave motion. Even though the density of air is only 

 10"^ that of water, I think the ratio of the thickness of the air column to the wave 

 length must be an important parameter. (4) the effect of slicks, rain, and oth- 

 er naturally occurring phenomena can easily be overlooked in laboratory investi- 

 gations . 



In making these points, I do not 

 wish to imply that laboratory investiga- 

 tions are not useful. In fact, notable 

 progress has recently been made through 

 the laboratory investigations by Francis 

 in England, and by Keulegan in the United 

 States. But until direct confirmation 

 can be obtained by measurements at sea, 

 the laboratory results can only be con- 

 sidered as suggestive. 



slope 



THE REFLECTION OF RADIO AND 

 RADAR WAVES 



Fig. 1. Frequency distribution of 



slopes for (a) a single train of sine 



waves, and (b) a Gaussian sea surface. 



In the earlier discussion the tacit 

 assumption was made that reflection from the sea surface is governed by the 

 laws of geometric optics. This will be the case when the water waves are long 

 compared to wave length of the incoming radiation. For light waves we may 

 safely assume this to be true. For radar waves this is no longer the case. The 

 tiny ripples, which are short compared to radar waves, have an entirely differ- 

 ent effect than the longer ocean waves. It is interesting, in this connection, that 

 radar sea return from an up- wind direction is definitely stronger than from oth- 

 er directions. Since the overall distribution of slope is nearly independent of 



