What are “official” weather records?

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Dear Tom,
You often refer to official weather records and unofficial weather records. What is the difference?
Kelley Nemathis, Bolingbrook
Dear Kelley,
“Official” weather observations refer to the weather data that are used to assemble a city’s climatological normals, averages and extremes. “Official” should not be interpreted to imply that other (non-official) weather observations are less accurate or less reliable.
O’Hare International Airport became the official weather observation site for the city of Chicago on Jan. 17, 1980. Prior to that date, Midway Airport was the official site from July 1, 1942, onward. And before that, from Nov. 1, 1870, to the inception of Midway data, Chicago’s official observation site occupied a variety of locations in downtown Chicago and at the University of Chicago. The unofficial Midway data set extends back to March 1, 1928; at O’Hare, back to Oct. 30, 1958


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