I question the accuracy of Chicago temperature records set in the 1870s, 80s and 90s. Thermometers back then were not as accurate as thermometers in use today.
Thermometers in use by the Weather Bureau in the 1870s-1890s provided more accurate temperature readings than the readings we get today. However, thermometers then were located in instrument shelters that consisted of wooden louvered boxes set about five feet high. Air in such shelters could warm as much as a few degrees above existing air temperatures on sunny days with light winds.
The National Weather Service currently does not use conventional mercury-in-glass thermometers. Temperature readings today are measured by a platinum wire resistive device whose electrical resistance varies with temperature. The device is accurate but it requires considerable maintenance for accurate readings.