Dear Tom,

Are Chicago’s weather records from 100 years ago less accurate than today’s modern records?

—Fran Williamson, Westchester

Dear Fran, Records taken 100 years ago were very accurate and in some respects more reliable than those taken now. Weather Bureau thermometers in the early 1900s were accurate to one-tenth degree. Dew point temperatures were hand calculated using ventilated wet-bulb readings. Today’s electronic temperature sensors are considered accurate with a tolerance of plus/minus two degrees; dew point temperatures, plus/minus four degrees. Precipitation was measured using accurate eight-inch rain gauges, but today’s automated systems employ a less reliable tipping bucket that can give faulty readings in windy conditions or in times of heavy frozen precipitation.