Your lake has layers

Your lake has layers

A year ago at the Wisconsin Lakes convention, a featured speaker asked for a show of hands: “How many know what a thermocline is?” Only about half the hands went up, and this in a room filled with lake lovers – lake residents, lake association people, anglers, DNR staffers, watershed managers.

So that tells me if I say your lake has layers, not all of you will say, “I knew that!” Yes, your lake has layers – and it’s for the simple physical reason that a less-dense liquid floats on a more-dense one, as oil floats on water. OK, that’s not a pretty image, but it makes the point. Warm water is less dense than cold water and will float on top of it.

What happens as spring moves into summer, and warm air and sunlight pump heat into the lake, is that the deep water stays cold, while the water nearer the surface gets warm. So in summer, your lake has a layer (a thick layer, mind you) of warmer water floating on cold. Between them there’s a zone where the temperature changes rapidly.

The technical term for this condition is thermal stratification. And again in technical terms, the warm upper layer is the epilimnion, the cool lower layer is the hypolimnion, and the zone in between is the thermocline.

On most lakes in Wisconsin, where I live, it’s easy to experience the thermocline – and you may well have done so without knowing it. Next time you go swimming on a nice summer day, get out where the water is fairly deep, say 15 feet or so. The surface water you’re in will likely be comfortably (or at least reasonably) warm. Now, do a feet-first surface dive. With an upstroke of your arms, propel yourself down. Soon your feet will feel a sudden change to much cooler. That means they have hit and passed through the thermocline. And with this simple test you can determine roughly how thick that epilimnion is.

This thermal stratification is a seasonal thing – it actually less reverses itself in winter, when the colder water is on top and the (only slightly) warmer water is on the bottom. This happens because water has the unique property of actually becoming less dense as its temperature falls below about 39 degrees F and heads toward the freezing point (32).

Stratification makes all sorts of interesting things happen in your lake – about which, more in later posts. For now, just be glad that if you’re asked amid a roomful of people what a thermocline is, you’ll be able to raise your hand. And that you now have something new and interesting to tell your grandkids next time they visit your lake.