Why does Lake Michigan act like a heater in the winter but in the summer it acts like an air conditioner?

Dear Tom,
Why does Lake Michigan act like a heater in the winter but in the summer it acts like an air conditioner?
Mrs. Smith’s 3rd Grade Class, Aux Sable Elementary School, Minooka, Ill.

Dear 3rd graders,
It’s an excellent question, and the answer is the difference between air temperatures over land and over Lake Michigan. The temperature of air over the lake tends toward the temperature of the lake itself. That means “lake air” in the winter will often be warmer than colder air over the adjacent land because the lake surface will never be colder than 32 degrees. It’s an opposite situation in the summer. Lake Michigan water temperatures are usually no warmer than about 70 degrees, even on the hottest days, and air over the lake tries to attain that temperature. Thus, “lake air” is usually warmer than “land air” on our cold winter days, but it’s usually chillier than air over the land on our hot summer days.

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