Dear Tom,
Chicago’s weather records began over 100 years ago. Are those records as accurate as those taken today?
Janice Koporsky, Berwyn

Dear Janice,
Chicago’s official temperature and precipitation records began on November 1, 1870, but much was lost in the Great Chicago Fire in October, 1871. Regarding the accuracy of weather readings today compared to data recorded a long time ago, say in the 1870s, it is a varied picture. Temperature readings present cases of varied accuracy.

Precision mercury thermometers were in use from 1870 into the 1950s. Those thermometers were calibrated and accurate to 0.1 degree Fahrenheit. Newer electronic thermometers in use since then are accurate only within a degree or two, and they require frequent recalibration. Temperature records from the 1870s (and onward) were more accurate than today’s, but there were problems with thermometer locations. Instrument shelters in the 1870s, for example, were located in Loop buildings ten floors above the ground.

Regarding precipitation and the accuracy of precipitation records, today’s data are measured just as they were in 1870. Rainwater falls into a collecting bucket and its accumulated depth is measured with a ruler.