borne instrument that measures the height of waves in the open 

 ocean. This has helped bring some order out of the apparent chaos. 

 Mounted in the weather ship anchored at lat. 6 1 ° N. , long. 1 5 ° 20'W. 

 in the Atlantic, the instrument has shown that fifty-foot waves are 

 not uncommon in winter (the average height for February 1954 

 being twenty-three feet), but in summer the average height fails to 

 about ten feet. The larger the steep waves are, the more work they 

 can do when eventually they crash against the coast. Along the 

 shore the waves are lower but longer. Off Cornwall, in England, 

 even in winter, waves higher than twenty feet are rare. 



As the storm which forms a steep, high sea dies down or moves 

 away, the waves do not suddenly disappear. They travel away from 

 their generating area at various speeds — the longer the wave, the 

 faster it travels. And so the waves, once generated, spread out and 

 begin their long march toward the coast, the longer ones over- 

 taking the shorter ones and becoming symmetrical to form swells. 

 Long, low waves can travel hundreds of miles and more before 

 they lose their energy, but gradually their height diminishes as they 

 move shoreward. This explains why waves at the coast tend to be 

 longer and lower than those in the open ocean; the short waves 

 become attenuated and the long ones become lower. The ability of 

 waves to travel over great distances is impressive, even to the ocea- 

 nographer. We know that waves that have broken on the Cornish 

 coast were formed by a hurricane off the east coast of the United 

 States, about three thousand miles away; similarly, waves measured 

 off California were traced to a storm about seven thousand miles 

 away in the South Pacific. These waves were more than a thousand 

 feet long in the open ocean and up to ten feet high when they 

 reached the coast. 



Waves in the open ocean move independently of the bottom, 

 but as they march toward the shore the shallowing water begins to 

 slow them down. The depth at which a particular wave begins to 

 feel the influence of the bottom depends on the length of the wave. 

 We may say that a wave is in "deep" water if the water's depth is 

 greater than the wave's length. The long, low swells are, therefore, 

 affected soonest. As they near the coast they lose their rounded 

 tops and develop a pointed crest instead, then gradually the crest 

 becomes more and more unstable until the wave begins to curl 

 over with a foaming top, ideal for surf riding. The particles of 

 water within the wave crest are now moving forward as fast as the 

 wave form itself, so they carry the surf rider with them toward the 

 shore. In deeper water the wave form moves much more rapidly 

 than the water making up the wave. The water itself merely rotates 

 in open circular orbits. This is why a boat or other floating object 

 is left very much in the same place as it bobs on the waves, although 

 a floating object does move along very slowly in the direction of 

 wave motion. 



While the shape of a wave changes as the wave advances toward 

 the shore, its length becomes shorter and its speed slower, although 

 it builds in height. The best waves for surf riding are called spilling 

 breakers. Because these waves usually pass over a gently sloping 

 sandy beach, they advance smoothly without losing their foaming 

 crest for some distance. But where the beach is steep the waves 

 usually break up entirely as plunging breakers. 



When waves enter shallow water and begin to feel the effect of 



In the open sea during^a gale, waves build 

 up to twenty feet or more. Giant waves like 

 the one shown above are less common. Rarer 

 still are forty-foot waves like those shown 

 on the wave record below, made aboard the 

 Weather Explorer in the North Atlantic. 

 The highest wave recorded so far measures 

 about seventy feet from crest to trough. 



225 



