Lake Stratification and Turnover

By Tess Flynn-Belles, Natural Resource Specialist

    39.2°F.  What is the significance of this temperature? We hear “32°F” and automatically think, “Ah, yes, that’s when water freezes”. So, why is 39.2°F so unique? You may be aware that as water cools, it becomes denser and less dense when it warms. A practical example of this would be if you’ve ever tried to see how deep you can swim down in a lake on a hot summer’s day. You can feel the temperature change from warmer to cooler as you dive deeper. In winter, however, once water reaches the temperature of 39.2°F, its molecules actually start to expand and become lighter again, sitting on top of the warmer water!! At this point, the water can continue to cool to 32°F and turn to ice. If you are a pond or lake owner, the following facts will help you begin to understand why, during certain times of the year, your pond goes from crystal clear to cloudy and brown in a matter of days.
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