WAVES 



213 



The height of the waves increases until the resistance awak- 

 ened by the orbital motion throughout the mass of water affected, 

 balances the wind work on the surface. As increase in length 

 depends on the increase in the spacing of particles, i. e., the in- 

 crease in rapidity with which adjoining particles are affected, it is 

 seen that the wave length may increase even after the height no 

 longer increases. Thus while the height of hurricane waves or 

 great "seas" is seldom over thirty or forty feet and their length 

 perhaps 500 feet, the latter may increase to 1,500 feet, or three 

 times, while the height does not, or but rarely, reach 50 feet. The 

 velocity of such waves varies from 20 to 60 miles an hour, and their 

 period from 10 to 20 seconds. 



The height of the waves depends not only on the strength of 

 the wind causing them, but also on the size of the water body, 

 especially the diameter of the surface along the path of the wind. 

 Wind velocity is greater over the open sea than along the coast or 

 inland, and this, together with their smaller size, makes the waves 

 of inland lakes less pronounced than those of the sea, and hence 

 lake shores suffer less erosion than the sea-coast. In a small water 

 body the wave height is proportional to the square root of its diam- 

 eter (Stevenson, T\\.-G&:^^8). As soon as the wave height ex- 

 ceeds 0.8 m., it may be determined according to the following for- 

 mula (modified from Stevenson), when H is the wave height in 

 meters, d the diameter of the water body in kilometers (Penck- 

 52 :^(5(5) : 



H = iV'd + o,.8-i^"d 



In the ocean the waves rise quickly to a height of 5 meters, but 

 also decrease rapidly, as soon as the wind producing them ceases. 

 The maximum height of waves in the open ocean, with a given 

 wind velocity, is shown in the following approximate table, ab- 

 stracted from Kriimmel (42:75), 



Table shon.'ing relationships between zvind velocities 

 and wave height. 



