The fog rolling in off Lake Michigan was striking this morning. This trip out to shoot the scenery prompted a quick look into the basic meteorology behind the phenomenon—a question the excellent Chicago Weather Center blog has usually already answered.

Here’s the deal with Chicago fog in general

The nation’s largest fog machine? That would be Lake Michigan, at least for tonight. An expansive area of dense fog that’s been sitting over the southern end of the lake all day is starting to work its way onshore.

The fog is a by-product of moist air (dew point temperatures in the lower 60s) interacting with lake cooled air. As moist air drifts over the lake, it is cooled to the saturation point, which results in low level cloud or fog formation that hugs the surface. Another key ingredient for lake fog in this instance is light winds.

This time, it was the intense heat and humidity from early this week rapidly cooling down overnight. Or, to put it in weatherman terms: “Nighttime cooling brought temperatures down to the dew points in sections of the Chicago area overnight producing areas of thick fog … Light winds aided the process.”

Whatever the science behind it, it was beautiful—and a great reason to take an impromptu trip to the beach.

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