Minnesota lakes are undergoing a rite of autumn in which their temperature becomes more uniform top to bottom, and oxygen is stored for winter inhabitants under the ice. The process is known as the fall overturn.
In summer, our lakes have warm water on and near the surface, with cold water below. Only the warm top layer circulates, and the oxygen supply can become depleted in the deep water. It's not unusual for the surface temperatures of Minnesota lakes to be in the upper 70s and even 80s during July and August, but with the onset of cooler weather, the temperature in the upper layer drops until it is the same as the lower layer's. The fall overturn occurs during this time. A temperature of 50 degrees throughout the lake is the average for Lake Minnetonka, Lake Harriet and other area lakes now.
The uniform temperature allows mixing, induced by wind and currents created by differences in a lake's water density. From time to time, the currents bring all the lake's water to the surface, where it absorbs oxygen from the air. As the water in the lake circulates, oxygen is returned to the depths during the fall overturn, and nutrients are brought to the surface.
This is how lakes store oxygen for the fish, frogs, turtles, worms, and other living things that spend the winter under the ice. Those people who spend time on the lakes, or walking around them, at this time agree that the lakes smell different than they did last month.
The temperature of the lake, which will continue to drop until the lake freezes over, is a result of the mixing water. The cool surface water goes to the bottom of the lake because cold water is heavier than warm water. As it sinks, warmer water from below is forced to the surface, where it in turn is cooled. The whole lake will cool to 39 degrees, the temperature at which water is the heaviest. As the surface water continues to cool below 39 degrees, it expands, becomes lighter, and remains on the surface to freeze.
Ice forms at 32 degrees. On the first calm, freezing day or night after a particular pond or lake reaches 39 degrees in all parts, an ice cover will form. The temperature of the water in contact with the ice sheet is 32 degrees, but a few feet below the ice the temperature remains above freezing, reaching 39 degrees on the bottom.
Jim Gilbert's observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.