thermocline

n. layer of water in some oceans and lakes which is characterized by a greater temperature gradient than the layers above and below it (Geophysics)

The thermocline (sometimes metalimnion) is a layer within a body of water or air where the temperature changes rapidly with depth.
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n. thermocline, layer of water in some oceans and lakes which is characterized by a greater temperature gradient than the layers above and below it (Geophysics)

The middle layer of a thermally stratified lake or reservoir. In this layer, there is a rapid decrease in temperatures in a lake or reservoir.