SECT. 5] 



727 



Contrary to the behavior of gravity waves, ripples of finite amphtude have 

 sharper troughs than crests. In the extreme form, corresponding to breaking of 

 a gravity wave, an air bubble is formed at the trough. Schooley (1958) has 

 found that ripple profiles resembling the calculated form can be produced by 

 winds of 6 m sec~i in a small tank. The anemometer level in Schooley's experi- 

 ments was about 2 cm above water level ; therefore very much higher wind 

 speeds, measured at normal anemometer levels, may be required to produce 

 trough sharpening in the open sea. 



Fig. 5. Photograph of the sea surface showing relation of ripples to crests of gravity waves. 

 Scale in meters. 



Roll 



4. Growth of Ripples 



[1951) has observed and photographed the formation of ripples and 



short gravity waves in ponds on tidal flats. He finds that near the up-wind edge 

 of a pond there are two stages of development. In the first, short ripples, with 

 crests almost parallel to the wind, and long ripples (length about 1.7 cm) 

 forming a rhombic pattern, are visible. In the second stage the latter grow 

 down wind in amplitude and length and become associated with short ripples, 

 as discussed in section 3 of this chapter. 



