What was the warmest temperature ever recorded in Chicago on a day that was 100% cloud-covered?

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Dear Tom,
What was the warmest temperature ever recorded in Chicago on a day that was 100% cloud-covered?
—Mike Parenti Palatine
Dear Mike,
Chicago’s daily records of temperatures and sunshine are not easy to cross-check, however, after conferring with some of my colleagues, we have come up with an “educated” guess. A basic input for forecasting the day’s high is the 5,000 foot tempe- rature. This reading warms when the air is lowered to the surface, by 5.5º per thousand feet in a dry environment and by 3.3º/1000 feet in a wet one. A typical 5,000 ft. temp on Chicago’s hottest days is around 77, so in cloudy, moist conditions the maximum high would potentially only reach the low/mid 90s. Chicago climatologist Frank Wachow- ski scanned records from 1950 and found the city’s two warmest “sunless” days were June 24, 1959- 93º and August 24, 1962 94º.

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