Why is it that, starting around Dec. 21, the sun stays out longer but temperatures continue to fall into January?
Judy Perkins, Yorkville
The answer lies in the "lag of seasons," the phenomenon in which the coldest portion of the year trails behind the time of minimum heat input (insolation) from the sun. That lag also applies to the peak of temperatures in the summer being delayed until after the time of maximum insolation. Chicago's normal high/low temperatures on Dec. 21 are 33/19 degrees, but fall to 31/16 degrees on Jan. 18, a lag of 28 days. On June 21, the city's normals are 82/60 degrees, but rise to 85/64 on July 12, 21 days later. Even though insolation increases after Dec. 21, it's still insufficient, at least for a few weeks, to replace heat being lost into space. In addition, it takes a while for land, water and air to heat up; the reverse prevails in the summer.