Around October 23 of the solar calendar each year, when the sun reaches the celestial longitude of 210 degrees, it marks Frost's Descent, one of the 24 solar terms. As the last solar term of autumn, it serves as a transition from autumn to winter. In late autumn, the ground loses a great deal of heat, causing the temperature to drop suddenly below 0°C. Water vapor in the air condenses directly on the ground or plants into tiny ice needles, some forming hexagonal frost flowers—white in color and loose in structure.

The Collection of Explanations for the Seventy-Two Pentads of the Lunar Months states regarding Frost's Descent: "In the middle of the ninth lunar month, the air turns cold and condenses, and dew freezes into frost." By this time, white frost has already appeared in the Yellow River Basin of China; across the vast fertile fields, silver ice crystals glisten, while leaves turn yellow and wither, falling one after another. The ancient text Explanations of the 24 Solar Terms notes: "The air turns cold and frost descends; yin energy begins to congeal." It is thus clear that "Frost's Descent" signifies that the weather is gradually becoming colder, with frost starting to fall. In meteorology, the first frost in autumn is generally called "early frost" or "first frost," while the last frost in spring is termed "late frost" or "last frost." The period between the last frost and the first frost is known as the frost-free period. Early frost is also called "chrysanthemum frost" because chrysanthemums are in full bloom at this time. Su Shi, the great literatus of the Northern Song Dynasty, wrote a poem: "Thousands of trees swept into a sea of yellow, only hibiscus blooms alone in fragrance."

"Frost's Descent" indicates that the weather is gradually cooling, with dew condensing into frost. The seventy-two pentads include three pentads related to Frost's Descent: "First pentad: Jackals offer sacrifices of beasts"—meaning animals like jackals start storing food for the winter from Frost's Descent onward; "Second pentad: Plants wither and leaves fall"—at this time, plants turn yellow and leaves cover the ground; "Third pentad: Hibernating insects lie dormant"—animals preparing for hibernation begin to hide in their caves for the winter.

Translator: KAN Bowen

Reviewer: LIU Qingchen