What is the origin of the great cold solar terms? What are the weather characteristics of severe cold?

Slight cold is cold, and slight cold indicates the degree of cold. As the saying goes, "March 9th is cold". "Sanjiu" is mostly from 65438+1October 9 to 17, which is also within the slight cold solar terms. But this is just a general rule. In a few years, a bad cold may be colder than a mild one. People still remember the winter of 1975, and the lowest temperature solar term was actually heavy snow! The winter warmth in Sichuan Basin is remarkable. In the winter of 65438+ 10, frost and snow invaded the east of the Three Gorges, often freezing, and the lowest temperature was about-10℃. However, the lowest temperature in the northern part of the basin is rarely below MINUS 5℃, and the low temperature below 0℃ in the southern part of the basin is even rarer. The coldest area in Sichuan in the middle of winter is the northern Sichuan Plateau. The lowest temperature can reach about -30℃. It's freezing, dripping into ice. The low-altitude valley in southwest Sichuan is the warmest place in Sichuan in the middle of winter. The average temperature in 65438+ 10 is around 12℃, and only a few teenagers may have a low temperature below 0℃. In addition, the inversion effect is very significant, so tropical fruits such as bananas and mangoes can grow well. The lowest temperature in the basin in winter is not low, which is beneficial to non-spring production and diversified management. Citrus, which is forbidden to move, grows in the south. Generally, the minimum temperature is not lower than minus 5℃, and the annual temperature is higher than 15℃, which can be met in most areas of the river basin, and almost all subtropical plants exist. Sichuan is called "Land of Abundance", and its unique climatic conditions should be a very important factor. Great cold, like slight cold, is also a solar term representing the degree of cold weather. Although many records of modern meteorological observation show that in most parts of China, the severe cold is not as good as the slight cold, in some years and a few coastal areas, the annual minimum temperature will still appear in the severe cold solar terms. In severe cold season, the average temperature in Sichuan Basin is mostly 6℃ to 8℃, which is nearly 65438 0℃ higher than slight cold. The proverb "slight cold and severe cold, cold into a ball" shows that the severe cold solar term is also a cold period of the year. Therefore, it is necessary to continue to do a good job in the cold protection of crops, especially to protect livestock from wintering.