Milwaukee is blessed with terrific beaches, but Lake Michigan will never be confused with the waters of Waikiki. As anyone who’s ever dipped a toe in it knows, it’s cold – like, average surface temperature below 50 degrees cold. But what determines whether it’s #$!@-ing freezing or not so bad, actually, once you get used to it?
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For answers, we turned to Harvey Bootsma, professor and Kohler Endowed Chair of Great Lakes Science at UW-Milwaukee’s School of Freshwater Sciences. According to Bootsma, three main things determine Lake Michigan’s water temperature. One is air temperature, as warmer air heats the water near the lake’s surface. “Another big factor, probably even more important for much of the year, is solar radiation,” says Bootsma. “How much sunshine is there? That’s mainly what causes warming in the spring and early summer. We’re getting more sunlight, and that sunlight heats up the water.”
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The third factor is less obvious but just as important. “Something a lot of people don’t think about is evaporation. When water evaporates, it has a cooling effect,” Bootsma explains. That’s the evolutionary reason you sweat when you’re hot. “The material that water evaporates from, whether that’s skin or the surface of a lake, becomes colder.” Evaporation depends mainly on the difference between air and water temperatures, and on Lake Michigan, much of it occurs in winter. “If you ever drive along the lakeshore on a really cold day and look out over the lake, you’ll see mist coming off it. And that’s because the water, even though it’s cold, is still warmer than the air.”
So with that in mind, what can we expect for lake temperatures this summer?
Last winter saw above-average air temperatures in December and early January and below-average temperatures in late January and early February. To Bootsma, that would imply summer lake surface temperatures in line with long-term averages, about 70 degrees, with a peak of about 73 in late July and early August. The relatively cool May and June – a departure form increasingly warm summers – could lower that ceiling. (For reference, Waikiki’s average summer water temp is about 80 degrees.)
While air temperature, sunshine, and evaporation are largely responsible for Lake Michigan’s long-term temperatures, on a day-to-day basis, wind has a big influence. “If you have a strong wind for a day or two, that will push that surface water eastward toward the Michigan side of the lake,” says Bootsma. When that happens, deeper, colder water gets pulled up near the shore. “So you can have days where surface temperatures will drop down as cold as 46 degrees, even in the middle of summer, if you get that upwelling going on.”
So, sorry, warm-water lovers, but you’re on the wrong side of the lake. “I always tell people, if you like warm water and big sandy beaches, go to the Michigan side of the lake,” says Bootsma. “If you like calmer waters with smaller waves, go to the Wisconsin side.”
