ASK TOM: Average difference between highs and lows

Dear Tom,


What’s the average difference between our daily highs and lows? How does it differ in summer and winter?


-Terry Costlow


Dear Terry,


Chicago’s normal daily high and low temperatures (Midway Airport data) averaged through the year are 59.4 degrees and 43.5 degrees; the difference between those averages is 15.9 degrees.


The high-low difference is greatest (18.6 degrees) mid-May through June and least (12.5 degrees) in December because it is driven mainly by seasonal variations in heat input from the sun.


Heating is greatest on the summer solstice (June 21), when the sun is highest in the sky and day is longest; conversely, heating is least on the winter solstice (Dec. 21). Strong heating elevates daytime temperatures further above early morning lows.