Thermocline

People who know about thermoclines include submariners, serious fishermen* and readers of contemporary naval combat novels.  I fall into the latter category.

I'm fascinated by novels and movies about submarines and the deadly, 3-dimensional game of hide n seek they play.  Thermoclines play a large role in those games because they mess up the sonar used to locate and track other vessels.

The wavy dotted line is the thermocline.  

Radiated noise bounces when it hits the thermocline, and can only pass through

 it if at or near a 90 degree angle.  So, the destroyer can't get a fix on the sub unless the two are directly above/below each other.

Thermocline definition: a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere) in which temperature changes more rapidly with depth than it does in the layers above or below.  At sea, the thermocline is typically about 300 feet below the surface, warmer water above, cooler water below.  It's much higher in freshwater lakes that are 30+ feet deep.

Serious fishermen know about them because fish like to hang out in waters of a certain temperature, and those temps are typically above the thermocline.  They - the fishermen, not the fish - use depth finders to locate the transition layer.

*For several years I was a not-so-serious fisherman.  Perfect weather, chores all done, hook up the boat, tow it down to the marina, launch, motor out to where the fish are rumored to be, drop anchor, hook some bizarre-looking piece of hardware onto my line, open a beer, fire up a cigar, sit back and enjoy life.  Then, 2-3 hours later, head back to the barn, stop at Safeway and buy fish - if I really wanted fish for dinner.

Had I known about thermoclines, I may have actually caught a few fish.  Or not.  In retrospect, I was more of a boater than a fisher, just enjoyed being out on the water.