Thermocline

A distinct layer of water where rising warm and sinking cold water meet but do not mix. It is a layer of water where the temperature changes at least one-half a degree per foot of depth. In many of our desert bass lakes, a thermocline often develops during the spring and breaks down in the fall. The colder layer of water is often lacking in oxygen, forcing most baitfish and sport-fish to the upper layer of water. Thermoclines can be so dense that they actually show up on sonar (fish finders and depth finders) as a thick, impenetrable line.

The layer of water where temperature rapidly changes from warmer to colder water.

The middle layer of a lake, separating the upper, warmer portion (epilimnion) from the lower, colder portion (hypolimnion). The middle layer in a thermally stratified lake or reservoir. In this layer there is a rapid decrease in temperature with depth. See metalimnion.

As one descends from the surface of the ocean, the temperature remains nearly the same as it was at the surface, but at a certain depth temperature starts decreasing rapidly with depth. This boundary is called the thermocline. In studying the tropical Pacific Ocean, the depth of 20ºC water ("the 20ºC isotherm") is often used as a proxy for the depth of the thermocline. Along the equator, the 20ºC isotherm is typically located at about 50 m depth in the eastern Pacific, sloping downwards to about 150 m in the western Pacific.