The average coldest day of the year falls about three weeks after the shortest day on Dec. 21 and the average hottest day is three weeks after the year’s longest day. Why the delay?
Chicago’s average daily temperature falls to a minimum (at 31 and 16 degrees) in the days around Jan. 18 and rise to a maximum (at 85 and 64 degrees) in the days around July 12 (at O’Hare International Airport). Both dates are about three weeks after the shortest and longest days.
On average, after Dec. 21 temperatures continue to fall into mid January because heat lost into space is still greater than incoming heat from sunlight, even though incoming heat begins to increase slightly after Dec. 21. By mid January, incoming heat finally overtakes heat loss, and temperature start to rise. Similarly, though opposite, with heat loss.