When is the slight cold? When is the great cold? How did these two solar terms come from?

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! It is obviously warmer in winter in our basin. In the dead of winter, in the month 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 the middle of winter in our province is the northern part of the western Sichuan Plateau, with the lowest temperature around -30℃. It is freezing and dripping into ice. The low-altitude valley in southwest Sichuan is the warmest place in the middle of winter in our province. The average temperature of 1 month is about 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 the severe cold season, the average temperature in our basin is mostly 6℃ to 8℃, which is nearly 1℃ 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. For some crops, a proper low temperature is necessary in a certain growth period. Wheat and rape with strong winterness need lower temperature through vernalization stage, otherwise they cannot grow and develop normally. Winter in the basin of our province is warm all the year round, and wheat and rape sown too early often grow too vigorously, jointing and bolting ahead of time, which greatly weakens the cold resistance and is vulnerable to low temperature frost. It can be seen that selecting crop varieties according to local conditions, sowing and planting in time and taking effective measures to promote and control are important links to win high yield. A slight cold is better than a severe cold. Slight cold and severe cold are the two coldest solar terms in a year. At this time, the vast areas in northern China are in the cold weather of "not reaching out in 1919, freezing dogs in 1949". In ancient China, the explanation of the two solar terms, slight cold and severe cold, was: "The cold air is getting colder over time, and the little one is not extreme." "The greatest is the most bitter." It means that the slight cold is very cold, but it is not the coldest time; Great cold is the coldest time. Many people think so now. But in fact, slight cold is colder than severe cold. This view can be confirmed by the meteorological data of China. Slight cold refers to the half month from June 6th (or 5th) to 20th (or19th) in the solar calendar every year; The severe cold refers to the half month from June 65438+1October 2 1 (or 20th) to February 4 (or 3rd) in the solar calendar every year. Looking up the meteorological data in all parts of China, we can see that the average temperature in the lowest ten days of a year is 65438+1mid-October, which is within the slight cold solar terms; However, the average temperature of 65438+ 10 in late October is higher than that in mid-October. For example, it is 0.8℃ high in Beijing, 1.0℃ high in Shenyang and 1.3℃ high in Harbin. So, why is the coldest solar term a slight cold instead of a great cold? This should start with solar radiation. We know that the temperature of a place is related to the direct and oblique sunlight. When the sun shines directly, the ground receives more light and heat and the temperature is high, which is the main reason; Secondly, when shooting obliquely, the distance of light passing through the air layer is much longer than when shooting directly, and more light and heat are consumed along the way, and the ground receives less light and heat, so the temperature is naturally low. In winter, for the northern hemisphere, the sunshine is oblique, so it is cold everywhere. The day when the sun is slanting is the solstice of winter. So, the winter from solstice should be the coldest? In fact, the lowest temperature occurs during the slight cold and severe cold about one month after the winter solstice. This is because the accumulated heat on the ground can supplement a lot of heat loss since summer, although the oblique illumination around the winter solstice is the most severe. During the slight cold, the required heat and the released heat tend to be equal, that is, the surface layer stores the least heat. So the weather is the coldest in the slight cold solar terms. This is similar to the reason why the highest temperature in a day does not appear at noon but around 2 pm. After the slight cold, the temperature rises gradually, so the average temperature of the great cold is slightly higher than that of the slight cold.