Frequency of 20 below or lower temps in Chicago

Dear Tom,
I live a few miles west of O'Hare Airport, and I recorded 20 degrees below zero on Jan.
16, when O'Hare went to 18 below. How many times has it been 20 below or colder in
Chicago?
Dan Klein
Dear Dan,
Twenty degrees below zero or lower is a rare occurrence in Chicago. In 138
years of official temperature records, (1871-2008 -- a period comprised of 50,404
days), 20 below or lower has been recorded on only 15 days. That's one day out of
3,360, or about once every nine years. However, their occurrences were not evenly
distributed. Five of the "minus 20 days" occurred from 1872-1899, and then 83 years
passed before the next one, 26 below in 1982. That initiated a "Little Ice Age" here,
because another eight of those frigid days came in rapid succession through 1985. The
last and most recent was 21 below, on Jan. 18, 1994.