The Köppen climate classification system describes Chicago’s climate as “continental warm summer.” What other locations have the same climate classification?
— Walter Brickman, Omaha, Nebraska
The Köppen climate classification system was first published in 1918 by German climatologist Wladimir Köppen. Faced with a lack of adequate observing stations, Köppen used vegetation to approximate climatic conditions in data-sparse areas of the world. German climatologist Rudolf Geiger adjusted some of the climate boundaries and American climatologist Glenn Trewartha made changes by emphasizing average summer temperatures and the length of the growing season. The Köppen system suggests Chicago shares climatic similarities with the central Plains and New England, portions of Bulgaria, Romania, North and South Korea and northern China.