Chapter 5 — BATHYTHERMOGRAPH 



71.88 



Figure 5-15. — Sound pulse travel through isothermal water conditions. 



Any energy of the pulse that does pass through 

 the negative gradient is reduced greatly, and 

 the sound beam is bent sharply downward. Sub- 

 marines operating 50 feet or more beneath the 

 top of the negative gradient are difficult tai'gets 

 to detect. The shipboard Sonar Technician may 

 get extremely long ranges on a submerged 

 target near the surface (such as the underwater 

 section of the hull of another screening ship), 

 but he may be unable to detect a submarine 

 just a few hundred yards away. Should the 

 submarine enter the positive gradient, however, 

 it probably would be detected. 



Isothermal Layer 



An isothermal layer of water is one of 

 uniform or nearly uniform temperature through- 

 out. This condition is not confined necessarily 

 near the surface of the sea, but often is found 

 between layers or gradients of markedly dif- 

 ferent temperatures. An isothermal condition 

 can be caused by waters of two different tem- 

 peratures mixing. In this respect, it sometimes 

 is referred to as a mixed layer. Isothermal 

 layers are of importance in predicting the path 

 the sound pulse will follow. For this reason 

 Sonar Technicians must be able to read accu- 

 rately the top and bottom depths of such a layer. 

 You are shown an isothermal condition in figure 

 5-15. Now look at figure 5-16 to see how iso- 

 thermal layers sometimes are located in a 



temperature profile of the sea. Notice that 

 every time a straight, vertical BT trace is 

 recorded, the layer of water between its limits 

 is known to be isothermal. 



DAILY HEATING 



Daily heating of the surface water layer has 

 a marked effect on sonar ranges. It produces a 

 condition ASW men call afternoon effect. 



When the surface is heated by the summer 

 sun, and the winds are not strong enough to 

 keep the surface water well mixed, the water 

 close to the surface may be several degrees 

 warmer than the water 20 to 30 feet down. 

 This effect is maximum in the late afternoon. 



TEMPER4TURE- 



SOTHERMAL 



i 



ISOTHERMAL- 



/ NEGATIVE 



ISOTHERMAL 



N^POSITIVE 

 ^.^-^^ NEGATIVE 



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 Figure 5-16. — Temperature profiles. 



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