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Thermocline

A thermocline is a layer in a body of water, in which temperature changes more rapidly with depth than it does in the layers above or below.

Summer Aeration
Thermoclines is observed in pond and lakes. During the summer, warm water, which is less dense, will sit on top of colder, denser deeper water, with a thermocline separating them. The warm layer is called the epilimnion and the cold layer is called the hypolimnion. Because the warm water is exposed to the sun during the day, a stable system exists, and very little mixing of warm water and cold water occurs, particularly in calm weather.One result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline, as the water below the thermocline never circulates to the surface, and organisms in the water deplete the available oxygen.
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Winter Aeration
In colder climates, this leads to a phenomenon called stratification. As winter approaches, the temperature of the surface water will drop as nighttime cooling dominates heat transfer. A point is reached where the density of the cooling surface water becomes greater than the density of the deep water, and overturning begins as the dense surface water moves down under the influence of gravity. As the temperature continues to drop, the water on the surface may get cold enough to freeze and the pond and lake begins to ice over. A new thermocline develops where the densest water (4 °C) sinks to the bottom, and the less dense water (water that is approaching the freezing point) rises to the top. Once this new stratification establishes itself, it lasts until the water warms enough for the 'spring turnover,' which occurs after the ice melts and the surface water temperature rises to 4 °C. During this transition, a thermal bar may develop.
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