Dear Tom,
Last week I swam at the Fullerton beach in warm water, but two days later at Warren Dunes in Sawyer, Michigan, the water was positively bone-chilling. Why the drastic difference?
Joanne Lombardo
Dear Joanne,
It is the wind direction that is behind the difference in lake water temperatures that you encountered. The strong, direct rays of the summer sun warm the surface lake water, while the deeper water remains quite cold. When the wind is blowing onshore, the warm surface water is pushed toward the shoreline keeping the beach waters comfortable. When the wind is offshore, the warm surface water is carried out into the open lake and is replaced by the colder bottom water in a process called upwelling. Chicago’s beaches are warmed by onshore east winds, but on the Michigan side of the lake, these same winds are offshore and cool the beach water.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
