Has the second half of November ever been milder than the first half in Chicago?
We just finished a November in which the second half was milder than the first. Has this ever happened before?
— Quentin Calder, Chicago
In 148 years of Chicago temperature records dating from 1870, it has happened 12 times (with 2017 being the 12th). That is an average of once every 12 years. Here are the years in which Nov. 16-30 has been milder than Nov. 1-15: 1875, 1877, 1951, 1970, 1975, 1980, 1982, 1984, 1998, 2001, 2012, 2017.
Note that nine of those 12 occurrences have been between 1970 and the present. A partial explanation is that the temperature observation site was located near Lake Michigan from 1870 to 1941, but farther inland (at Midway Airport or O’Hare) from 1942 onward. Lake cooling is a partial explanation, but it is more likely that our climate is becoming increasingly variable.