Why, if water is cooler as you go down in a pond, does the top freeze first?

Question submitted by: Neil Thelen of Ovid-Elsie High

One of water’s unusual properties is the way its density changes with temperature changes.  As water cools, it becomes more dense and heavier, just like other materials.  Once the water reaches 39 degrees Fahrenheit (7 degrees above freezing) it is at its densest.  As it cools below 39 degrees its density begins to decrease: it becomes lighter.

Consider a lake and the change of seasons.  In summer, warm temperatures heat the water.  The warmer the water is, the lighter; the lake will be colder at the bottom, warmer at the top.

During fall, the air and water temperatures drop.  As the water becomes colder, it sinks.  Once the temperature drops below 39 degrees, the water rises again.  This is why ice forms only at the surface of the lake.

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