Where did the clear autumn come from?

Excerpt from "Twenty-four Solar Terms"

Twenty-four solar terms is a literary work compiled by Wu Cheng, a scholar in Yuan Dynasty. His book is divided into 24 solar terms with "seventy-two periods", and each solar term is divided into three periods, each of which explains why.

The book says that the solar term in September will be cold and condensed. Today is the fifth solar term in autumn, and autumn is crisp.