Accuracy of Chicago’s early weather records

Dear Tom,
You have mentioned that Chicago’s weather records began in 1885. I have always wondered how accurate those records are in comparison with today’s.
— Evangelene, Arlington Heights

Dear Evangelene,
Accuracy in the early days of weather observations was not an issue. Chicago’s official records date from Nov. 1, 1870 (not 1885), when the Weather Bureau (now the National Weather Service) began operation. Observations initially consisted of daily high and low temperatures and precipitation, and were extremely accurate. Temperatures were recorded to tenths of a degree from mercury-in-glass thermometers, but now temperature readouts are remoted indoors from outdoor sensors that are reliable only to a degree or two and require occasional recalibration. Precipitation measurements, taken manually then and now, are accurate.