Why is it so cloudy in the winter in Chicago?


Dear Tom,
Why is it so cloudy in the winter in Chicago?

Freshmen students in Ms. Bernhard’s division
Northside Learning Center, Chicago

Dear Freshmen at Northside Learning Center,

Winter cloudiness tends to be layered, or stratiform because cold air at and near the surface is denser than warmer air aloft. This is called a temperature inversion. Inversions inhibit vertical mixing in the atmosphere lending to clouds that are great in horizontal extent, often covering the entire sky. Incoming sunlight, and long hours of wintertime darkness support inversions which can last for days, keeping clouds “locked in”.  As warmer weather approaches and sunlight becomes stronger, surface heating breaks up inversions allowing vertical mixing of air. This results in growth of cumuliform clouds which are vertically developed and seldom cover the entire sky.

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