Why is there a saying that "the first frost is three times, and the vegetation is yellow"?

In the first frost, the spring breeze blows, the summer rain falls, the autumn falls, and the winter snows.

Grass, safflower, swimming fish, birds, pond grass, castle peak, safflower, fish out of the water and birds into the forest.

The first frost is one of the 24 solar terms of the lunar calendar. Entering this solar term, the weather is getting colder and frosty. The first frost usually occurs on the 23rd day of the Gregorian calendar 10 year 10. At this time, the first frost appeared in the Yellow River basin in China, and most areas were busy planting crops such as three wheat.

In ancient China, the first frost was divided into three stages: "One stage was that the jackal sacrificed animals; Second, the vegetation is yellow; Three waiting insects are salty. " In this solar term, jackals will show their prey before eating them.

Every year, around Gregorian calendar 10 year 10 23rd, when the sun reaches longitude 2 10 degrees, it is the first frost. The first frost said that the weather was colder and the dew condensed into frost. "Twenty-four solar terms": "In mid-September, the essence is cold and condensed, and dew cream". At this time, frost has appeared in the Yellow River Basin of China, and a silver ice crystal shines in the fields thousands of miles away. At this time, the leaves turned yellow and fell. The ancient book "Twenty-four solar terms" says: "When the frost comes, the yin begins to coagulate." It can be seen that "first frost" means that the weather is getting colder and it begins to frost. Meteorologically, the first frost in autumn is generally called "first frost" or "first frost", and the last frost in spring is called "late frost" or "final frost". The interval from the last frost to the first frost is a frost-free period. Some people also call the first frost "Chrysanthemum Frost", because chrysanthemums are in full bloom at this time, and Su Shi, a great writer in the Northern Song Dynasty, said in a poem: "All trees are sweeping away pornography, and only Furong is alone."

Frost is formed by condensation of water vapor. How can water vapor condense into frost? Lv Benzhong, a poet in the Southern Song Dynasty, wrote in Luzi IV in the Southern Song Dynasty: "The moon invades the post, and the bridge spends the night frost." Lu You wrote in Frost Moon that "the withered grass is frosty and gray, and the cold window is full of new shadows." It shows that frost appears on a clear moonlit night in autumn. There are no clouds in the autumn night, and the ground is like a quilt, which radiates a lot. When the temperature suddenly drops below 0℃, the water vapor on the ground will condense on streams, bridges, leaves and soil, forming tiny ice needles, and some will become hexagonal frost flowers. Frost can only form on sunny days. People say that "the thick frost is fierce" is the reason. There is a poem in "The First Frost", which says, "Every autumn evening, it turns into frost and somewhat condenses into yang. Lotus is defeated in a thousand pools, and cotton is harvested in a thousand hectares. "

References:

Baidu encyclopedia: the first frost (one of the 24 solar terms)