"Nine cold days are counted, and the coldest is thirty-nine." "Sanjiu" usually appears from the winter solstice to the awakening of hibernation. "March 9" refers to the third "nine days" after the winter solstice, that is, the 19th to 27th days after the winter solstice. "The third ninth day after the winter solstice" refers to "三九九", which is also the day when "三九" and "四九" intersect, not "三九", that is, "三九" is nine days rather than one day.
The calculation method is to count from the winter solstice in winter (the beginning of the winter solstice is called "Jiaojiu", which means the beginning of cold). Every nine days is one "nine", and the first nine days is called "one". "Nine", the second nine is called "Two Nine", and so on, until "Nine Nine", that is, to the ninth Nine, until the number of Nine Nine is eighty-one days.
During the winter solstice, the day is the shortest, the angle between the direct sunlight angle and the ground is the smallest, and the intensity of sunlight per unit area of ??the ground is the smallest. Therefore, the ground receives the least solar heat every day, and the daily heat budget deficit on the ground has not yet reached maximum value. After the winter solstice, although the day length increases day by day, the growth rate is very slow. From the winter solstice to the "March 9th" period, the average day length only increases by about one minute. At the same time, the angle between the direct sunlight and the ground also increases very slowly.
The daily heat budget of the ground is still in deficit, so it continues to lose heat and the ground temperature continues to drop. The total ground heat budget deficit reaches its maximum value only during the "March 9th" season. Since the temperature of the air is mainly affected by the ground temperature, this leads to the lowest temperature in the "March 9th" season.
The earth absorbs heat from solar radiation during the day and dissipates it at night. In summer, more heat is absorbed during the day and less heat is emitted at night. In this way, some heat can be accumulated every day. In winter, less heat is absorbed during the day and more heat is emitted at night. Not only cannot it be accumulated every day, but some of the heat accumulated in summer must be used up. On the winter solstice, less heat is absorbed during the day, but there is still some stored heat on the earth. After the winter solstice, the stored heat becomes less and less. By March 9th, the earth's heat storage is the least in a year, so it is also the coldest. Extended information
There are many customs of "counting nine", among which "Nine-Nine Song" is the most widespread and long-standing. These songs skillfully use the phenological phenomena of nature to vividly reflect the weather changes in Jiujiu Middle School. As far as most areas in our country are concerned, the weather is not the coldest after the 29th, but it is just "1929, I can't stretch out my hand." When the fourth nine is over, it is almost the "Great Cold" solar term, so we say "three nine, four nine, cats and dogs will freeze to death."
After May 9th, the earth gradually returns to spring, and the weather gradually warms up from cold, so "In May 9th, 69th, I looked at the willows by the river; in 79th, 89th, I walked in single clothes." By the time of ninety-nine, it is already the "Jiujiu" solar term, so "when ninety-nine hears thunder, the sound will last for a long time."
The custom of "painting nine" appeared in the Ming Dynasty. The so-called painting is actually a picture of calculating the date of spring after the winter solstice. The Ming Dynasty's "Scenery of the Imperial Capital" records: "On the winter solstice, I painted a plain plum branch with eighty-one petals. When the sun dyes one petal, when all the petals are gone and ninety-nine come out, spring is deep, and it is called the ninety-nine to relieve the cold." In addition, in the Qing Dynasty, there was also the "Nine-Nine Poems and Pictures of Eliminating Cold".
There are four sentences for every nine days in the picture, with a total of thirty-six sentences. The content ranges from the "Three Emperors Ruled the World" in ancient times to the "Qing Dynasty sitting on the Golden Palace" in this dynasty. It can be called a novel. An elegant and interesting historical "memorable event".
After "drawing nine", the custom of "writing nine" appeared in the Qing Dynasty. "Writing Nine" also has a strong cultural flavor. It often uses the nine characters (traditional Chinese) of "The weeping willows in front of the pavilion cherish the spring breeze" or "The cypress breeze in the spring front yard brings fragrance to the room." (Similar to "drawing red" in calligraphy practice).
Use a thick brush to write in black, each word has nine strokes, and each stroke takes one day. It takes exactly eighty-one days to complete the nine words. What's interesting is that after filling in one stroke every day, you have to use a fine brush to write down the weather conditions on the strokes in white. Therefore, a line of "write nine" is also a more detailed meteorological data for the ninety-nine days.
Baidu Encyclopedia-Nine Cold Days