How does ice form on a pond? | Ponds & Lakes Q&A

How does ice form on a pond?
Jonathan – Denver, CO

You look forward to and enjoy the ice formations that appear on your pond each winter but have you ever wondered how it begins? Why is it that the ice in your pond expands when it freezes instead of shrinking and why does ice float?

Unlike those horror flashbacks of falling asleep in science class we’ll keep this as simple and interesting as possible. As the air cools off outside your water looses heat and becomes heavier. This cool and heavy water sinks to the bottom of the pond as the lighter warmer water rises to the top to cool. This process continues until the pond water reaches about 4 degrees Celsius or 39 degrees Fahrenheit and the water cools enough to freeze. Water is unique in the fact that as is it freezes its molecules form crystals that are spaced farther apart causing ice to expand and take up more space. Once the water in your pond begins to form these crystals and expand it actually becomes lighter than warmer unfrozen water and once again begins to rise to the surface of your pond where it begins to form a sheet of ice. If this all seems a bit too wordy or confusing to you visualize a glass of water with ice cubes in it. The spaced out, crystallized, water molecules of the ice cubes make them lighter than the water in the glass and cause them to float at the surface of the water. The water in the glass that is cooled by the ice cube (but not cold enough to freeze) becomes more dense and sinks to the bottom of the glass and the warmest water rises to the top where it is cooled by the floating ice cubes.

The layer of ice formed by this crystallized frozen water can become fairly strong as it becomes thicker. An inch of ice can be strong enough to hold a small animal without cracking. 3 inches of ice typically is enough to bear the weight of the average person and once a pond freezes to 6 to 8 inches thick it is ready to play host to a hockey game or hold up a snowmobile. Some have even taken their cars out on the ice! While we don’t recommend you give it a try, 8 to 12 inches of ice can support a slowly moving vehicle.

Pond Talk: What do you use your pond for in the winter? Do you skate? Ice fish?