The thermocline (sometimes metalimnion) is a layer within a body of water or air where the temperature changes rapidly with depth. Oceans Since water isn't a perfectly transparent medium, almost all sunlight is absorbed in the surface layer, which heats up. Wind and waves mix the water in the surface layer and distribute heat within it somewhat, such that the temperature may be relatively uniform for up to 100 m (300 ft). Below this mixed layer, however, the temperature drops very rapidly—… (More on Thermocline)