Not only were temperatures fall-like Wednesday, but humidity values, the likes of which we rarely see in July, greeted us courtesy of summer brand of arctic air that moved into the area. Dewpoints yesterday dropped to an incredible 42F in Chicago and spent 6 hours at or below 44F, a feat that occurs less than 1% of the time in July!! More information on why dewpoint is a better measure of how humid it is than relative humidity, check out this short YouTube video from your NWS Chicago's YouTube channel: http://ow.ly/zg32f
Using computer forecast models, NOAA's Air Resources Laboratory can calculate the approximate trajectory of our incredibly dry low level air mass from yesterday afternoon. Notice that the origins of yesterday's air mass are from just north of the arctic circle, which helps explain the remarkably low humidity levels for this time of year!