Dear Tom,
Why is the difference between Chicago’s daily normal highs and lows larger in the summer than in the winter?
— Ron Milis, Bolingbrook
Dear Ron,
On Jan. 1, the normal high/low are 32/17 for a difference of 15 degrees; on July 1, the high/low are 84/63 for a difference of 21 degrees. The amount of heat delivered by sunlight is crucial. Consider that 90 degrees above the horizon is overhead and 0 degrees is on the horizon. The sun rises to 71.6 degrees on June 21 and only 24.7 degrees on Dec. 21. Sunlight is less energetic when the sun is lower in the sky. Add to that the period of daytime heating (days shorten by 6 hours and 6 minutes between summer and winter) and solar radiation dwindles from 473 Langleys in June to 96 Langleys in December. Winter sunlight is just one-fifth as strong, which decreases daytime heating and contributes to the decrease between high/low temperatures during the winter.
