Humid Continental and Humid Subtropical Climates

The modern vocabulary for describing these climates comes from Wladimir Köppen (1846–1940), a German-Russian climatologist and botanist who published the first version of his climate classification in 1900 and 1918, explicitly built around the geographic limits of vegetation types rather than temperature and precipitation alone, and substantially revised it in 1936 in the Handbuch der Klimatologie. After Köppen's death, his collaborator Rudolf Geiger continued refining the system, giving rise to the "Köppen-Geiger" scheme still in standard use, and later formalized in a widely cited updated world map by Murray Peel, Brian Finlayson, and Thomas McMahon (2007) in Hydrology and Earth System Sciences. In this scheme, a "D" designation (humid continental and subarctic climates) requires the coldest month to average below 0°C and at least one month to average above 10°C, while a "C" designation (humid subtropical and related climates) requires the coldest month to stay between 0°C and 18°C; a second letter of "f" indicates no distinct dry season, and a third letter distinguishes hot summers ("a," warmest month above 22°C) from warm summers ("b," below 22°C but with at least four months above 10°C).