SECT. 5] INTERNAL, WAVES 751 



Internal waves have been observed in closed bodies of water (Mortimer, 

 1952). In the spring the heating of the surface layer, mixing by wind, and 

 differential currents at the thermocline divide the upper and lower parts of 

 the water column with a sharp temperature gradient. Under the influence of 

 wind the upper layer becomes distorted and lighter water accumulates at the 

 leeward end of the lake. This results in thinning of the surface layer, and it 

 occurs at the windward end of the lake (Sonar Data Division, Univ. of Calif,, 

 Division of War Research, 1945). 



The wind causes a slow circulation to be set up that entails a movement of 

 surface water in the same direction as the wind and a current at the thermocline 

 in the opposite direction. The counter flow is initially in the upper layer, but 

 some water just below the thermocline also moves windward. The slope of the 

 distortion depends upon the force and duration of the wind. An equilibrium 

 can be established wherein the wind stress balances the other forces. 



If the wind stops, the currents will reverse, and the thermocline may return 

 to its former level or be tilted in the other direction by oppositely directed wind, 

 aided by the momentum of the return flow. None of the currents have been 

 measured directly, but have been inferred from the change in temperature 

 structure and other properties of the lake. 



A study was made of an internal wave caused by a diurnal wind in Sweet- 

 water Lake.i The prevailing westerly winds start about 1000 h and end 

 around 1700 h. The shifting winds create a standing internal wave some 20 ft 

 high (Fig. 21a, b). Such a major change in the distribution of mass makes 

 studies of the heat budget difficult. The change in heat content of the vertical 

 water column is caused more by the diurnal internal wave than by the daily 

 radiation from sun and sky. 



^ Near San Diego, California. 



