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; The leaves on the earth turned yellow and fell; The stinging insects are motionless in the cave and hang their heads into hibernation.
The first frost occurs when the sun reaches the longitude of 2 10 degrees every year around Gregorian calendar 10 year10 23rd. 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 of the Southern Song Dynasty, wrote in Luzi IV of the Southern Song Dynasty: "The moon invades the post, and the bridge is frosted." 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.
The solar term of the first frost means that the weather is getting colder and first frost begins. To the south of latitude, the average temperature is around 16℃, and there are three solar terms before the first frost day. In the southern valley of South China, frost can only be seen in the dead of winter. Of course, even at the same latitude, the temperature and humidity of the air close to the stratum are different due to the difference of altitude and topography, and the first frost period and frost day are also different.
Frost is the condensation of water vapor when the ground encounters cold weather, so it is not the frost in the first frost, but the phenomenon that the ground will produce the first frost when the weather is cold.
From a scientific point of view, the saying that "dew becomes frost" is not accurate. The frozen dew formed by dew freezing is hard small ice beads. Frost refers to the phenomenon of crop freezing injury caused by the sudden drop of temperature, and its harmful temperature varies with crops, varieties and growth periods. To form frost, the temperature of the ground or ground objects must drop below 0℃, and the water vapor content in the air on the ground should reach a certain level. Therefore, there may not be frost in first frost, and there may not be frost in first frost. However, due to the low temperature during frost, if the temperature continues to drop, it will easily lead to frost.
"The first frost kills a hundred herbs", plants that have been hit by severe frost have no vitality at all. This is because the liquid in plants forms ice crystals due to frost, protein precipitates, and the water in cells extravasates, which makes the protoplasm seriously dehydrated and deteriorated. "Wind knives and frost force swords", which shows the ruthlessness and cruelty of frost. In fact, frost and frost are inseparable, but it is "freezing" that harms crops, not "frost". Someone once tried to put two leaves of a plant in a box with the same low temperature, one of which was frosted and the other was not frosted. Results The leaves were seriously damaged in frost-free period, but there were only slight frost marks in frost period. This shows that frost can not only harm crops, on the contrary, when water vapor condenses, it can also release a lot of heat. 0 g of steam at 1℃ condenses into water, and the heat of vaporization released is 667 calories, which will turn heavy frost into light frost and light frost into dew to avoid frostbite.
It is better to say "frost kills a hundred herbs" than "the first frost kills a hundred herbs". Frost is the expression of cold weather, and freezing is the enemy that kills crops. Because there is frost when it is frozen (sometimes no frost is called black frost), autumn frost and spring frost are collectively called frost.