Internal Waves 



531 



with a shift of the phase in x of a quarter of the wave length and a quarter 

 of the period in /. we get internal horizontal progressive waves. Their 

 velocity is 



g 



V 



1 + 



1 67l" Q 



(XVI. 29) 



The velocity of such internal waves therefore depends upon the wave length 

 as well as upon the vertical density distribution. It can be seen that with 

 the same phase of oscillation for all values of z (A = 00) this velocity of 

 internal waves is proportional to the wave length, and not to the root of the 

 wave length as for ordinary surface waves. 



Their physical nature becomes understandable only if it is considered 

 that in unstable stratified water masses according to more recent ideas the 

 convection mostly does not take place between individual layers in form of 

 irregular up- and downwards movements of water bodies of arbitrary size, 

 but rather stationary circulations of quite definite form ("cells" of definite 

 extent and shape) occur, in the outer portions of which the medium sinks 

 and rises in their central parts. (Benard-cells, see Vol. 1/1, Chap. 5, Fig. 92.) 



Also in case of stable stratified water masses oscillatory processes develop 

 during forced displacements in vertical direction of water bodies (i.e. during 

 dynamic convection), which are of cellular nature. The movement initiated 

 by a single impulse after surpassing the equilibrium position by a certain 

 rate will reverse its direction and the disturbed mass distribution tends 



2 mm 



FlG. 220. Stability oscillations (cellular waves) in the Baltic with a period of 45 sec. Taken 

 from temperature registrations in a depth of 7-5 m on 31 July 1944. <f> = 54°34-5'N. 



X = 12°18-8'E. 



towards its previously occupied equilibrium position. In this way a cellular 

 oscillation of definite form develops, which will depend upon the stability 

 of stratification. Such cellular gravitational waves are thus denoted as sta- 

 bility oscillations. 



Ever since the first continuous recorded temperature observations in strati- 

 fied water masses in lakes and in the ocean, short temperature oscillations 



34* 



