On January 9th, we welcomed the first day of March 9th. In the North China region of China, such a "Nine Nine Song" is widely circulated, which vividly records the folk custom of "counting nine". "Counting nine" refers to counting one "nine" every nine days from the winter solstice until "ninety-nine" and eighty-one days, hence the term "counting nine cold days". In history, people who have been busy for a year have rested and rested in the cold winter, hoping for the arrival of spring warmth and blooming flowers. "Counting nine" to dispel the cold has become a popular folk custom. In addition to the "Nine Songs" that are passed down orally and have slightly different versions in different regions, there are also "Nine Cold Relieving Pictures" that have been passed down through calligraphy, painting, and other forms. According to the average temperature statistics in the past 30 years, nearly 80% of regions in China have seen their lowest temperature periods during the "39" and "49" periods. Among them, most of the northern regions entered the coldest period of the year during March 9th. This is because during the winter solstice, there is still "residual heat" on the earth. During the winter solstice, sunlight shines directly on the Tropic of Cancer, and the solar altitude angle in the northern hemisphere is the smallest, making it the shortest day and longest night in the northern hemisphere. At this time, the surface of the northern hemisphere where China is located receives weak solar radiation energy, resulting in lower temperatures. After the winter solstice, although the solar altitude angle gradually increases and the duration of the day also gradually increases, the heat lost on the surface every day is greater than the received solar radiation. This kind of energy deficit will continue until "39" and "49", when the surface heat balance is close to equilibrium, and the overall "heat accumulation" on the surface is the least. For many parts of China, the lowest temperature of the year usually occurs at this time. However, China has a vast territory, with significant climate differences in different regions, and the coldest periods in different regions are not entirely the same. The eastern part of Jiangnan and most of southern China were colder during the April 9th holiday. Yunnan, southeast Xizang and other places may enter the lowest temperature period of the year after the winter solstice. In the southeastern coast of China and Hainan Island, the lowest temperature period may occur during May 9th. On several cold days, plants in most areas enter a dormant period, and the leaves of deciduous trees fall off. Some animals begin to hibernate. And people are not idle. In terms of agriculture, the northern region happens to use the intermittent period of farmland to carry out the construction of agricultural water conservancy projects, such as clearing ditches and draining water, plowing soil, etc., to enhance the water storage and retention capacity of farmland, and eliminate overwintering pests. In the southern region, people also need to take serious measures to prevent the cold of overwintering crops and ensure their safe overwintering. (Lai Xin She) (Author Li Jiarui is a senior engineer at the Central Meteorological Observatory, interviewed and organized by journalist Li Hongmei)