Every winter, the open water of Dickey Lake reminds me that water is at its maximum density at 3.98 degrees Celsius – 39.16 degrees Fahrenheit. Simply enough, when the surface water reaches 3.98 C, it heads for the bottom. Since Dickey lake is somewhere around 74 feet deep, there is a lot of water that needs to be cooled below 3.98 before the surface temperature can reach 0 and ice can start to form. The map (or is it better termed chart for water?) shows why the ice on Dickey Lake forms from the south end first.
Turnover is the simple term describing how lake and pond water reacts to the seasonal changes. It’s a natural phenomenon – I was fortunate enough to have Bruce May of Montana’s Fish, Wildlife and Parks explain the basic physics of temperature, and bring me along to observe temperatures and dissolved oxygen levels in several local lakes back in the seventies. When the water at the top is at the same density as the water at the bottom, wind and wave action provides enough energy to mix the water easily – but when the heaviest (3.98 Celsius) water is at the bottom, and the surface water is significantly warmer, the layers don’t mix so well.