Record low for Nov. 21

Dear Tom,
You reported that the record low for Nov. 21 was 1 degree set back in 1880.
How accurate is that measurement, and where in Chicago was it taken?
Tim Guimond, Evanston
Dear Tim,
That 1-degree record low on Nov. 21, 1880, was taken downtown at the city's
official observation site, then located at the Roanoke Building on the
southeast corner of Madison and LaSalle. The reading is quite accurate as
the temperature was measured with high-quality thermometers that recorded
the highest and lowest values for the day. It should be noted that the
record low of 1 degree was taken in the heart of the city, and it was even
colder that morning in outlying areas such as Elgin, which logged 3 below
zero. The high that afternoon was only 11 degrees, and that frigid reading
also established a record low maximum for Nov. 21.