When is 202 1 February 1 1?
Jiujiu, also known as Jiujiu in winter, is a folk method to calculate the date of cold spring and warm flowers in China. It is usually the coldest in March and April, which is the coldest time of the year. When you count to nine "nine days" (998 1 day), spring will be deep and warm, and everything will be full of vitality. It's time for spring ploughing.
Extended data
During the winter solstice, the daytime is the shortest, the angle of direct sunlight on the ground is the smallest, and the sunshine intensity per unit area on the ground is the smallest, so the ground gets the least solar heat every day, and the daily heat budget deficit on the ground has not reached the maximum. The solstice after winter, although the days are getting longer day by day, the growth rate is very slow. From the winter solstice to the "March 9" period, the average daily growth is only about one minute.
At the same time, the angle between the direct angle of the sun and the ground increases slowly, and the daily heat budget of the ground is still in a deficit state, so it continues to lose heat and the ground temperature continues to drop. The total heat balance deficit on the ground only reached its maximum value during April 9.
Because the temperature is mainly affected by the ground temperature, this leads to the lowest temperature during "March 9 and April 9". In addition, around March 9, the cold air mass in Siberia also developed to a very strong stage, and the frequent invasion of strong cold air is also one of the reasons for the low temperature.