Which nine days are the coldest?

September 9 is the coldest. People say: "It's cold for nine days, but it's cold for three days", and "three days" usually appears on the solstice of winter.

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 also 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 daily heat budget of the ground is still in deficit, so it continues to lose heat and the ground temperature continues to drop.

The deficit of total heat balance on the ground only reaches the maximum in the season of March 9. Because the temperature is mainly affected by the ground temperature, it leads to the lowest temperature in March 9 (of course, this is not the case all over the country, but every year, depending on the region and age). 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.