Learning about the clear ice on Lake Mendota

It started with a Facebook post: a short video clip shot while skating on crystal clear lake ice, the ribbed sandy bottom seen between skates. Wisconsin Public Television graphic designer Jen Hadley shot and posted the video which got us wondering why the ice on Madison's Lake Mendota is so clear this winter (check out an enhanced version of Jen's video).

Fortunately, our offices are mere blocks away from the University of Wisconsin limnology lab, built right upon the shore of Lake Mendota. There John Magnuson, emeritus professor of limnology and zoology went through the pictures Jen had taken, explained the reason for the clear ice and told us about some of the biological processes in the lake environment that could be observed through the crystalline cover (for more on that check out this slideshow).

"There's more physics to it than biology," Magnuson explained. As anyone who's enjoyed a glass of ice water can tell you, ice floats, and the ice cover of the lake floats on top of the water's surface.

In a typical winter, as temperatures decline ice forms on the lake. This thin layer is as clear as a window pane and maybe as thick, so when snow falls it isn't strong enough to support much weight. Most years, a heavy snow will push down the thin layer of ice floating on the lake surface. Lake water seeps up through cracks in the ice, mixes with the snow, making a slushy mixture.

That opaque slush freezes into what scientists call "gray" ice, which forms what is typically seen on a frozen lake. This year, weather conditions were different, something Magnuson says seems to happen every 12 to 15 years. Cold December temperatures, with no snow, then an early January thaw and a bit of rain left Lake Mendota topped by frozen water instead of the usual slushy mix of water and snow.

Quite a lot of snow has fallen since then, so we're glad to have captured this unusual moment in the life cycle of Lake Mendota.