By Dr. David Wartinbee, for the Redoubt Reporter
As we shivered through this past week’s zero degree (and less) temperatures, there was really a silver lining for some of us. That is, if you are interested in ice fishing or skating on some of our local lakes.
Several weeks back I flew over a number of local lakes and some were completely covered with ice, while others were completely open. This time of year reminds me of the changes taking place within the lakes, as well as the physical properties of water that put ice where it is.
In order for a lake to form ice, the entire lake must first reach 4 degrees Celsius, or 39 degrees Fahrenheit. The way this happens is quite interesting. As water cools and gets closer to the magical 4 degrees C, it becomes more dense and sinks to the bottom of the lake.
This way, warmer water is forced to the top of the lake and it gets exposed to the colder air. When that surface water cools, it then sinks and warmer water from a little bit lower takes its place. Eventually, the entire lake water is the same temperature. Since shallow waters around the edge of a lake, or shallow coves of a lake, can more quickly reach the critical temperature of 4 degrees C, ice usually forms there first.
Lakes like Tustumena and Skilak contain huge volumes of water at great depths, so it takes a long time for the entire lake to reach the 4-degree C level. Only then can ice formation begin.
When the surface water starts to get even colder than 4 degrees C, the density reverses itself. Water colder than 4 degrees C becomes a little bit less dense and stays on top. Now it can cool to zero degrees C. Once some of the surface water has reached freezing temperatures, the “heat of fusion” needs to be satisfied. This simply means that once the water has reached zero degrees C, it must now give up 80 calories of heat per gram of water in order for it to change into ice. After all that heat loss, the newly formed ice is still only zero degrees.
Ice crystals are a hexagonal latticework of molecules bound together by hydrogen bonding. This helps explain why snowflakes are six-sided. Previously, the water molecules were packed together but once in crystalline form, the molecules are pushed apart and their density is much less than liquid water. The result is something we all know — ice cubes float on the top of our glass of ice tea.
Ice is a better insulator than liquid water, since the molecules are separated from one another. That means it takes longer to transfer heat from the underlying liquid water to the air. So once ice has formed on a lake surface, it takes a little longer to cool the same thickness of ice underneath. There is a lesson for all of us there: If the temperatures remain constant, ice formation takes longer and longer as the ice thickens.
Snow deposits on top of lake ice also act as an insulator. Because snow traps air between the flakes, it makes it even more difficult for heat to pass from the underlying water and ice to the air. Lakes can become very dangerous when they freeze with a thin ice cover and then receive a covering of snow. It may take a long while to form new ice underneath that is safe to walk on. In the springtime, this snow cover can insulate the underlying ice from warming air temperatures and slow thawing.
While many of our lakes are now covered with a fair amount of ice, the deeper lakes remain unfrozen or with deceptively thin ice coverings. Before heading out on your favorite lake this winter, check the ice thickness in a number of places. Your life depends on it.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.