Dear Tom,
Why do lowest average temperatures of the year occur in the weeks after the winter
solstice? It would seem to make sense that the most extreme temperatures would occur
when the sun is at its lowest angle in the sky.
Tim Arvidson
Dear Tim,
Chicago's normal high on Dec. 21 (the winter solstice) is 33 degrees; on Jan. 15, 29
degrees. On average, the winter's lowest temperatures occur about three weeks after
the winter solstice, and there are three good reasons why. First, the Earth's surface goes
on losing more heat than it gains long after the solstice. Second, the far north (the
source of our coldest air) receives no heat from the sun into mid January, and
temperatures there are still falling. Finally, the average continental snow field continues
to expand into early February. Snow surfaces effectively radiate heat into space and
generate cold air masses.
