The old man often said 'count nine cold days, cold in September', the coldest day is coming. The coldness of this period can also be seen from the saying of "no action in 1929, walk on the ice in 3949". "39" is the third nine days after the winter solstice, about the end of January. Thirty nine days is the coldest time of the year. Do you know why? When does the March 9th day begin?
January 9-17
"39" is the coldest time of the year
There is no exact information about the origin of the custom. To be sure, at least in the southern and Northern Dynasties, the custom of counting nine had become popular. Liang Daizong's Jingchu chronicle of the age of Jin and Chu said: 'it is customary to use the number of winter solstice days and 9981 days for the cold. '
The data emperor selected four cities with a long history and complete meteorological data records, namely Beijing, Kaifeng, Nanjing and Xi'an, to represent North China, central China, East China and Northwest China, and used the temperature data of 60 years since they had meteorological records (1951) as the basis.
According to the data from 1951 to 2010, the PK results of average temperature and extreme minimum temperature in these four cities during the period of September 9th and September 4th are as follows:
In terms of the average temperature, except for Xi'an, the other three cities all support a cooling ratio of 39 to 49. What about the extreme minimum temperature?
From the point of view of extreme minimum temperature, the four cities represent North China, central China, East China, northwest and other regions, which are obviously in line with the saying of "three nineties are colder". It seems that the folk saying "cold in September" has been well confirmed.
Why is it the coldest day
We know that the earth absorbs the heat of solar radiation during the day and emits it at night. In summer, it absorbs more heat in the daytime and emits less heat in the evening. It can retain some heat every day. In winter, it absorbs less heat in the daytime and emits more heat in the evening. Not only can it not be retained every day, but also the heat retained in summer can be used up.
On the day of the winter solstice, the direct sunlight is in the southern hemisphere, while the northern hemisphere absorbs less heat during the day. Fortunately, there is still some stored heat. After the winter solstice, the heat stored in the northern hemisphere is less than that in a day, and the heat stored in the northern hemisphere is the lowest in a year, so it is the coldest.
That is to say, when the sun's heat absorbed by the ground is almost equal to the heat emitted from the ground, the temperature reaches the coldest level.
'it's cold in September. What about September
In the Yellow River, the Huaihe River and the Yangtze River, the "cold in September" is still very applicable. A lot of small partners in South China may have to raise objections. There is no comparison between those in the South and those in the south. Let's take a look at the PK results of the four capital cities in South China.
From the perspective of average temperature, it seems that the fourth and ninth seasons of these four cities correspond to lower temperatures. Although this is not cold, the saying that the third year is not true in South China. However, in terms of the extreme minimum temperature, the four cities all experienced colder temperatures in September, 2013.
In this way, "cold in September" is not a universal rule. In South China, "39" and "49" have different experiences. Send a map of the distribution of the coldest period in our country. Small partners from all over the country will take their seats and prepare the most cold resistant equipment in time.
According to China's annual climate data statistics, the coldest time in most of the year in the north of China is the middle of January (10-20 days), while in most of the south, it is the coldest in the last ten days of January. This year's March 9th is January 9th to 17th, which is exactly the middle of January. For most parts of the north, '39' will be colder. However, for some parts of the south, it was the coldest in late January, that is, in April and September.
However, there are also some areas that are 'different': most of the year in Western Sichuan and Yunnan is the coldest in late December, probably in the winter solstice solar term; some areas in Tibet and Xinjiang are the coldest in early January.
It is worth noting that this map is based on data from 1981-2010. With climate change, many areas in the middle and lower reaches of the Yangtze River, such as Nanjing, Nanchang, Hangzhou, Hefei and so on, have become colder during the period from September 4 to May 9 (late January). Of course, from the comparison of the coldest area in this picture, even at present, there is still a "market" for "cold in September" (mid January).