ASK TOM: Lowest temp with no snow on the ground in Chicago?

Dear Tom,
What is the lowest temperature Chicago has ever experienced with no snow on the ground?

-Dan Rosenberg, Highland Park

Dear Dan,

Snow, this planet’s most effective reflecting surface, is a prime factor in generating and sustaining winter’s extreme cold. A fresh snow cover can reflect 80 to 90 percent of incoming solar radiation back to space, restricting daytime highs and sending nighttime lows plummeting.

Under ideal conditions of clear skies and light winds, a fresh snow cover can result in minimum temperature readings 20 degrees or more lower than they would be over bare ground. That being said, Chicago’s temperatures will drop below zero without a snow cover, but probably not lower than about minus 10.
Weather historian Frank Wachowski found several subzero occurrences without snow cover, the lowest 8 below on Dec. 14, 1985.