Is March 9th the coldest day of the year?

Old people often say "it's cold for nine days, but it's cold for three or nine days", and the coldest day is coming. The degree of cold in this period can also be seen from the saying "1929, don't shoot, walk on the ice in 3949". "Sanjiu" is the third nine days from winter to the future, which is about the middle and late period of 1. From the tradition of counting nine in China. Counting to nine means counting from the solstice every winter, counting nine every nine days, and counting until the eighty-first day of "99", when the weather was warm.

But obviously, everyone is walking on the ice. Why does everyone think March 9 is the coldest? What about 49?

The picture comes from the weather calendar.

Is "March 9th" coldest?

There is no exact information about when the custom of counting nine originated. To be sure, at least in the Northern and Southern Dynasties, the custom of counting nine became popular. "Liang Chao Shu Jing Chu Chun Qiu" records: "The winter solstice is 998 1 day, which is the end of cold."

Is the coldest statement of "March 9" accurate? Data Emperor selected Beijing, Kaifeng, Nanjing and Xi 'an with a long history and complete meteorological data records, representing North China, Central China, East China and Northwest China respectively, based on the temperature data (195 1) of 60 years since meteorological records were recorded.

According to the data of 60 years from 195 1 to 20 10, the results of average temperature and extreme minimum temperature PK in these four cities during March 9 and April 9 are as follows:

Judging from the average temperature, except Xi 'an, the other three cities all support that "39" is colder than "49". What about the extreme minimum temperature?

Judging from the extreme minimum temperature, North China, Central China, East China and Northwest China, represented by these four cities, are also obviously in line with the statement that it is colder on March 9. The folk saying that "it's cold on March 9" seems to be well confirmed.

Why is March 9 the coldest?

For most parts of China, why are March and September the coldest?

We know that the earth absorbs the heat radiated by the sun during the day and radiates it at night. In summer, it absorbs more heat during the day and emits less heat at night, so it can retain some heat every day. In winter, less heat is absorbed during the day and more heat is released at night. Not only can you not keep it every day, but you should also use some heat left over from summer.

On the solstice of winter, the direct point of the sun is in the southern hemisphere, and the northern hemisphere absorbs less heat during the day, but fortunately there is still some stored heat. After the winter solstice, the stored heat is less and less day by day. By March 9, the heat stored in the northern hemisphere is the least in a year, so it is the coldest.

That is to say, when the solar heat absorbed by the ground is almost equal to the heat emitted by the ground, the temperature will reach the coldest degree.

"March 9 is cold", what about April 9?

In the Yellow River, Huaihe River and Yangtze River basins, "March 9" is still very applicable. Many partners in South China may have to raise objections, but those in the South are not as good as those in the South. Then let's take a look at the PK results of four provincial capital cities in South China.

Judging from the average temperature, April 9 in these four cities seems to correspond to a lower temperature. Although it is not cold, in South China, it is not true that it is cold on March 9. However, judging from the extreme minimum temperature, there was a cold low temperature in all four cities on March 9.

From this point of view, "March 9 cold" is not a universally applicable law. In the south of China, the cold of "March 9th" and "April 9th" have different experiences. Send a map of the coldest time distribution in the country, and friends from all over the country will take their seats, prepare the most cold-resistant equipment in time, and find the right time.

Schematic diagram of the coldest period distribution in China. According to different climatic and geographical conditions, the coldest time in different parts of the country is different. Image source: All Media Product Room of Public Meteorological Service Center.

According to the statistics of perennial climate data in China, the coldest period of the year in most parts of northern China is 5438+ 10 in mid-June (5438+00 -20 in June), and in most parts of southern China it is 1 in late June. This year's "March 9th" is the middle of 10, from June 9th of 65438+ to June 7th of 65438+. For most parts of the north, "March 9" will be colder. However, for some areas in the south, the coldest in late June is 65438+ 10, which is April and September.

However, there are also some areas that are "different": western Sichuan and most parts of Yunnan are coldest in May 438+February at the end of June, probably during the winter solstice; The coldest time in Tibet and parts of Xinjiang occurred in early June of 5438+10.

It is worth noting that this map is based on the data of 198 1-20 10. With the climate change, from April 9 to May 9 (65438+1late October), many areas in the middle and lower reaches of the Yangtze River, such as Nanjing, Nanchang, Hangzhou and Hefei, became cold. Of course, from the comparison of the coldest areas in this picture, even now, "Sanjiu Leng" (65438+1mid-October) still has a "market".

According to Data Emperor, the "March 9th Cold" is a climate generalization, which is still applicable to most parts of China, but it cannot be an absolute reference standard for a single year or two.