Could the record-setting temperatures of the late 1800s be due to the thermometers back then?
Is there reason to think the large number of record-setting temperatures dating from the late 1800s and early 1900s in Chicago was because the thermometers back then may not have been as accurate as today?
— Gary Hall, Crete
Definitely not! In fact, thermometers in use then provided more accurate readings than the temperature-measuring equipment in use today. However, the instrument shelter used then — a wooden louvered box set about 5 feet high — could warm as much as a few degrees above existing air temperatures in sunny, light-wind conditions.
The National Weather Service currently does not use conventional mercury-in-glass thermometers. Instead, its temperature-measuring system employs a platinum wire resistive device whose electrical resistance varies with the temperature. It is accurate, but requires maintenance for correct readings.
Thermometers in use in Chicago in the early 1900s provided more accurate readings than the temperature-measuring equipment in use today.