Thermocline
An omni sonar term, in plain English.
A boundary where water temperature changes quickly with depth. It bends sound and can shorten detection range.
See it in action
The free lessons show where this shows up on a real screen: start with the learning path, try it yourself on the omni trainer, or look it up on real CSH-10 captures in the field guide. Owners and captains talk settings every day on the forum.