Frost is one of the twenty-four solar terms. The weather is getting colder and frost begins. Frostfall usually occurs on October 23 of each year in the Gregorian calendar. When the sun reaches 210 degrees of ecliptic longitude, it is the frostfall in the 24 solar terms. Frost's Descent is the last solar term in autumn and the transitional solar term from autumn to winter. In autumn evening, the ground dissipates a lot of heat, and the temperature suddenly drops below 0 degrees. The water vapor in the air condenses directly on the ground or plants to form tiny ice needles, and some become hexagonal frost flowers, which are white in color and loose in structure.
"The Collection of Seventy-Two Hours of the Moon Order" says about frost: In the middle of September, the air is solemn and condensed, and the dew turns into frost. "Frostfall" means that the weather gradually becomes colder and dew condenses into frost. In ancient my country, frost was divided into three periods: in the first period, jackals were sacrificial animals; in the second period, vegetation yellowed and fell; in the third period, stinging insects dropped their bodies. The wolves began to capture their prey and sacrificed the beasts, using the beasts as a sacrifice to the heavenly retribution, and to offering sacrifices to the autumn gold. The leaves on the ground withered and yellowed and fell; the stinging insects all stayed in their holes and did not eat, hanging their heads and entering a state of hibernation.
The ancient book "Explanation of the Twenty-Four Solar Terms" says: "When the air is calm and frost falls, the clouds begin to condense." It can be seen that "frost falls" means that the weather gradually becomes colder and frost begins to fall. In meteorology, the first frost in autumn is generally called "early frost" or "first frost", while the last frost in spring is called "late frost" or "final frost". The period from the last frost to the first frost is the frost-free period. Some people also call early frost "chrysanthemum frost" because chrysanthemums are in full bloom at this time. Su Shi, a great writer in the Northern Song Dynasty, wrote in a poem: "Thousands of trees have turned yellow, and only the hibiscus is fragrant."
Frost is made of water vapor. How does water vapor condense into frost? Lu Benzhong, a poet of the Southern Song Dynasty, wrote in "Nan Gezi·Lv Si": "The slanting moon invades the post, and the night bridge is protected by the late frost." Lu You wrote in "Frost Moon" that "the withered grass is white with frost, and the moon is new in the cold window." This shows that frost appears on a clear moonlit night in autumn. There are no clouds in the autumn evening. The ground seems to have been uncovered, dissipating a lot of heat. The temperature suddenly drops below 0℃. The little water vapor on the ground will condense on the streams, between bridges, on leaves and soil, forming tiny ice needles, and some become hexagonal frost flowers. Frost can only form on a sunny day. This is why people say "thick frost fiercely hits the sun".
