Autumn corresponds to Metal in the five phase system, the phase of contraction, cutting, and refinement, following Fire's summer peak as the year's energy turns from outward expansion back toward gathering in. Where Wood in spring was defined by bending and directed growth, Metal's classical association is with cutting and shaping, an image drawn from metalworking that captures both the harvest's literal cutting of crops and a broader sense of paring away what is no longer needed as the year moves toward its more inward second half. In the body, Metal and autumn are paired with the lungs and the large intestine, and with the emotion grief, understood within the tradition as the season's characteristic energetic note, connected to autumn's association with letting go, both of the year's grown produce at harvest and, in the emotional register the system assigns it, of attachment more broadly.
The generating cycle holds that Metal produces Water, commonly explained through the observation that metal, when heated past a certain point or exposed to cold, can be seen to condense or yield moisture, so autumn's contracting energy is read as feeding directly into winter's still deeper turn toward Water's stillness and storage, the phase that follows it to close the four season cycle before the pattern begins again with spring.
Sources Astrology and Cosmology in Early China: Conforming Earth to HeavenDavid W. Pankenier with The Foundations of Chinese Medicine: A Comprehensive TextGiovanni Maciocia