554 



Internal Waves 



large, the influence of Coriolis force can be expected to increase. Conse- 

 quently, equation (XVI. 43) should be used with caution. 



A particularly beautiful example of temperature seiches in Lake Madii 

 in Pommern, were analysed by Helbfass and Wedderburn (1911) from 

 25 July to 14 August, 1909. The discontinuity layer of the density was located 

 at a depth of 15 m, and the internal oscillations in this layer were very dis- 

 tinct at that time. Figure 230 shows the course of the isotherms in the layer 

 between 10 and 20 m depth. Solid curves represent the isothermes for a station 

 at the northern end, the dashed curves for a station at the southern end of 

 the lake. The complete inverted course of the two groups of curves shows 

 clearly, that one is dealing with an internal seiche with one node and with 



16 20 



4 



10- VIM 



16 20 4 

 1 1- VIII 



4 8 

 12- VIII 



Fig. 230. Internal temperature seiches in Lake Madii (Pommern) from 9th August at 8 h 



to 11th August, 20 h, 1909; , isothermes at northern end of the lake; , isothermes 



at southern end of the lake. 



an amplitude of roughly 6 m. The period was found to be practically ex- 

 actly 25 h. The entire discontinuity layer participates in the vertical movement 

 of the water-masses. The dimensions of the oscillating system, substituting 

 for Lake Madii a rectangular trough, are / = 13-75 km, h' = 23 m, t' = 17°, 

 h = 15 m, t = 8°, in which h' and t' and h and t represent thickness and 

 temperature of the upper and lower layer respectively. Thus one obtains for 

 the period of the internal seiche with 1 node T = 91,000 sec = roughly 25 h; 

 in complete agreement with the observed value. 



It can be assumed that standing internal oscillations occur also in larger 

 or smaller adjacent seas. But because of the great expansion of the water- 

 masses, the period of the standing internal waves will become very long. 

 Conditions will develop similar to those explained on page 550. The effect 



