Thermal layer actually a misnomer
The "Layer" often referred to simply using tempature and in the war that is partially correct. There are three things that effect the Sound Speed Profile Temperature Salinity and Pressure. Typically we deal with temperature but in areas near rivers of monsoon season salinity definnetly has an effect - we can feel it even today in our SSN's. What you are actually looking for is the maximum sound speed in water and to get below that when evading skimmers. IF significant enough it will create a shadow zone which can make detection of a sub damn hard. Most signifcant layers occur above around 400 feet. Too bad the game does not produce a display that either shows just temperature of the Sound Speed Profile.
By the way I have seen them from 270 to 80 feet so far.
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