late 14c., "horizontal zone of the earth's surface measured by lines parallel to the equator," from Old French climat "region, part of the earth," from Latin clima (genitive climatis) "region; slope of the earth," from Greek klima "region, zone," literally "an inclination, slope," thus "slope of the earth from equator to pole," from a suffixed form of PIE root *klei- "to lean."
Ancient geographers divided the earth into zones based on the angle of sun on the slope of the earth's surface and the length of daylight. Some reckoned 24 or 30 climates between Meroe on the upper Nile in Sudan and the mythical Riphaean Mountains which were supposed to bound the Arctic; a change of climate took place, going north, at a place where the day was a half hour longer or shorter, according to season, than the starting point. Others counted 7 (each dominated by a particular planet) or 12 (dominated by zodiac signs).
Change of temperature gradually came to be considered more important, and by late 14c. the word was being used in the sense "a distinct region of the earth's surface considered with respect to weather." The sense shift to "combined results of weather associated with a region, characteristic condition of a country or region with reference to the variation of heat, cold, rainfall, wind, etc.," is attested by c. 1600. Figuratively, of mental or moral atmosphere, from 1660s.