Because water is not completely transparent, almost all sunlight is absorbed by the surface layer, and the surface layer becomes hot. Wind and waves make the surface water circulate and disperse the heat of the surface to some extent. In the first few hundred feet, the temperature may be quite uniform. However, under this mixed layer, the temperature drops very quickly-with the increase of depth 150 meters (500 feet), the temperature may drop by 20 degrees Celsius. This rapidly changing region is the thermocline.

Below the thermocline, the temperature continues to decrease with depth, but at a much slower rate. In the earth's oceans, 90% of the water is below the thermocline. This deep sea is composed of layers with equal density, which are poorly mixed and the temperature may be as low as 0 to 3 degrees Celsius.

On the high seas, thermocline has always been very important in submarine warfare because it can reflect active sonar.