Ok - last post on this subject and only so that people who are not familiar with the concept of a thermocline can understand what it is and how it works. Please note that there will
NEVER be water that is almost 20 degrees above freezing at the bottom of your pond in winter. The difference in temperatures could only be that great if the water depth was in the hundreds of feet.
Important points:
- water is densest when it hits 39 degrees - in the summer, warmer water will sit on top of water that is 39 degrees - when the fall rolls around and all the water gets to that temp or below, the 39 degree water falls to the bottom and the water above gets colder and freezes into ice at 32 degrees.
- this
does not happen in a garden pond as it is not deep enough. The water at the bottom will be warmer (if the pond doesn't freeze solid), but not necessarily due to a thermocline and not necessarily as warm as 39 degrees and generally due to the soil's frost depth in your area.
- in the quote below, they are talking about lakes that are 30 meters (90 feet) deep
http://en.wikipedia.org/wiki/ThermoclineThermoclines can also be observed in relatively shallow lakes. In colder climates, this leads to a phenomenon called turnover. During the summer, warm water, which is less dense, will sit on top of colder, denser water that sinks to the bottom, with a thermocline separating them. Because the warm water is also exposed to the sun during the day, a stable system exists and very little mixing of warm water and cold water occurs. 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. As winter approaches, the temperature of the surface water will also drop until it approaches 4 °C (39 °F), which is the temperature at which water is densest (remember, water expands as it freezes into ice - that expansion actually begins before the freezing point). 4 °C is, generally speaking, the temperature of the water below a thermocline. When the entire body of water is at or close to 4 °C, 'fall turnover' begins - the thermocline disappears, (or, to say a different way, it reaches the surface) and the water from the bottom of the lake can mix freely with the water from the top. This process is aided by wind or any other process that agitates the water.
As the temperature continues to drop, in those locations where it does, the water on the surface begins to get cold enough to freeze and the 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.
Waves can occur on the thermocline, causing the depth of the thermocline as measured at a single point to oscillate (usually as a form of seiche). Alternately the waves may be induced by flow over a raised bottom, producing a thermocline wave which does not change with time, but varies in depth as one moves into or against the flow.
And Finally . . . . . another quote:
Depending on the geographical location, the thermocline depth ranges from about 50m to 1000m.
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