570 



Internal Waves 



compressible stratified medium was carried out mainly to explain stationary 

 air waves, which occur in the lee of mountains. When lee waves are present 

 in the air they become visible when water vapour is condensed in the form 

 of one or several banks of clouds which have the shape of waves and which 

 are parallel to the chain of mountains (Moazagotl clouds). Such stationary 

 wave-like displacements of the isopycnals may also occur in the ocean, when 

 a current crosses a submarine ridge. Because of the stratification, which 

 takes place in the ocean, these wave-like displacements may take a somehow 

 different shape than similar displacements of pure discontinuity surfaces. 

 An exact computation of such cases should not be too difficult after the more 

 complicated sample cases were discussed. Figure 239 presents, according to 



; ■" '"^"'""V^' 



2v 



TrnM^m//////////////^//^//////////////^ ~x 



Fig. 239. Stationary internal wave in a stratified medium (streamlines) in the back of rectan- 

 gular shaped obstacle (Lyra). 



Lyra (1940/43) the stream lines of the current, crossing a long extended 

 ridge (see also Praedt, 1940, p. 331). The piling up of the current immediately 

 before the obstacle is clearly discernible; above it the stationary lee-waves 

 extend to great heights. These can still be recognized at a great horizontal 

 distance from the obstacle. There is every reason to assume that the wave- 

 like stationary form of the oceanographic properties, which is frequently 

 observed in the ocean, can also be explained in this way. 



