Accuracy of 19th century temperature records

Dear Tom,
I always wonder about the accuracy of Chicago’s temperature records from the late 1800s whenever I hear of a temperature record from say, 1880, being broken. Was the equipment sound back then?
John Tolley
Dear John,
Thermometers in use in the late 1800s provided more accurate readings than the temperature measuring equipment in use today. However, thermometers back then were housed in wooden louvered boxes set about five feet high, and the boxes could warm as much as a few degrees above actual air temperatures in sunny, light-wind conditions. The National Weather Service currently does not use conventional mercury-in-glass thermometers. Its temperature measuring system employs a platinum wire resistive device whose electrical resistance varies with temperature. It’s accurate, but requires maintenance for correct readings.

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