Dear Tom,
What is the difference between official and unofficial weather records?
Janice Victor, LaGrange

Dear Janice,
Official weather observations refer to the weather data that are used to assemble a city’s averages, extremes and climatological normals. It should not be interpreted to imply that non-official weather observations are less accurate or less reliable. O’Hare International Airport became Chicago’s official weather observation site on Jan. 17, 1980. Midway Airport was the official site from July 1, 1942, onward. And before that, from Nov. 1, 1870, to the inception of the Midway data, Chicago’s official observation site was located at several locations in downtown Chicago and at the University of Chicago. The Midway data set began on March 1, 1928; at O’Hare, on Oct. 30, 1958.