When is it coldest?

It was the coldest during the "March 9th" period. Sanjiu: Some time in the traditional Chinese calendar in China, also called Sanjiu days. There's a saying in China, "Practice dog days in summer, practice Sanjiu in winter". Corresponding to three nine is three volts. "March 9th" refers to the third "nine days" after the winter solstice, that is, the 19th to 27th days after the winter solstice.

March 9 is the coldest time. Sanjiu: sometime in the traditional lunar year in China, also known as Sanjiu days. China has a saying, "Practice dog days in summer, practice Sanjiu in winter". Corresponding to three nine is three volts. 39 "refers to the solstice of the third nine days after beginning of winter", that is, the solstice of the nineteenth day to the twenty-seventh day after beginning 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. On the solstice after winter, although the day is growing day by day, the growth rate is very slow. From winter to March 9, the average daily growth rate is only about one minute. At the same time, the angle between the direct sunlight and the ground is slowly increasing. 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 air temperature is mainly affected by the ground temperature, this leads to the lowest temperature in the "March 9" season (of course, it is not the case all over the country, but it is the case every year, which varies from region to region and from year to year). 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.