It’s a very unusual air mass able to produce relative humidities low enough to flirt with the all time record. But, that was the situation in Chicago Thursday. Not only were skies 100% cloud free for a 2nd consecutive day, humidities here were lower much of the day than in the desert Southwest. At 6:05 p.m. Thursday evening, Chicago’s relative humidity dropped to just 14%. On only three occasions since weather records began here in 1871—on May 10, 1934, April 11, 1956 and April 8, 1971 —has the air been any drier —and, on those occasions, by only 1%! Air that dry cools quickly because there is little moisture to retain heat. But, the same air heats rapidly once the sun rises. That’s one reason temps soared to 66° Thursday afternoon in the Fox Valley. It also explains the wide 30° spread between Thursday’s 63° and 33° extremes at O’Hare. The normal range is 57° and 37°.
WGN-TV Chief Meteorologist Tom Skilling and the WGN Weather Center staff provide daily coverage of weather in the Chicago area.
