What is the climate classification system?

Dear Tom,
Wladimir KÖppen developed a climate classification system in which Chicago’s climate is called “continental warm summer.” Where else, worldwide, are places with similar climates?
Will Warrenburger, Atlanta, Georgia
Dear Will,
The KÖppen climate classification system was first published in 1918 by German climatologist Wladimir KÖppen, but the system has been updated several times, including changes by American climatologist Glenn T. Trewartha. Faced with a lack of adequate observing stations, KÖppen used vegetation to approximate climatic conditions in data-sparse areas of the world. The system defines five major climatic zones. The KÖppen system suggests Chicago, the central Great Plains and New England share climatic similarities with portions of Bulgaria, sections of North and South Korea and northern China.

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