Autumn is upon us. Trees burst with colors of red, yellow and orange before their leaves plunge to the earth. The winds gradually change from pleasantly warm to uncomfortably cold, and a veil of clouds enshrouds the once-radiant glow of the sun.

But is all that a good thing?

According to weather.com, nine out of the next 10 days in Ann Arbor will be partly to mostly cloudy.

A 2006 study of Seasonal Affective Disorder, which tracked whether more people become depressed when there is less sunlight present, showed it is cloudier during the winter in middle latitude states like Michigan.

The reason?

Canada, Wisconsin and the Great Lakes.

Dr. Robert Aron, a meteorology professor at Central Michigan University, said air that blows from those locations has a lot to do with the cloudiness.

“It has to do with the great lakes,” Aron said. “When cold air from Canada or Wisconsin comes over the relatively warm water of the Great Lakes there is both a lot of evaporation from the lakes and the air is heated from below causing it to rise.”

The Great Lakes yield one positive, though. The warm water from Lake Michigan moderates the temperatures in the state throughout the winter.

Derek Posselt, an assistant research scientist at the University of Michigan, said the lake keeps bordering land warmer in the winter.

“In addition to moderating our temperatures, the lakes act as a source of water vapor,” Posselt said in an e-mail message. “The benefit to us is that the air is not only cloudy, but quite a bit warmer than it would have been if it had not been modified by the lake.”

So the Great Lakes are both a gift and a curse, depending on how you look at them.

One can only imagine how frigid the temperatures would be in the winter without the Great Lakes.

E.J. Horstman

Have a campus mystery you want the Daily to solve? E-mail hchris@umich.edu.

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