Why is it coldest in March and September?

From the astronomical point of view, the degree of cold and warm in a place is related to the altitude angle of the sun at noon. On the day of winter solstice, the height angle of the sun is the lowest, and Beijing is only 26 degrees. The height angle of the sun is small, and the degree of oblique sunlight is large. It reaches the ground through the thick atmosphere, and a large part of the heat is absorbed by the atmosphere. On this day, the northern hemisphere gets the least sunshine, 50% less than the southern hemisphere, and Beijing only has 9 hours during the day. However, this day is the first day of counting nine, not the coldest time This is because there is atmosphere and water on the surface of the earth, which can store heat, not "save or lose" but "accumulate heat"

From winter to the future, although the solar altitude angle gradually increases, it is a slow recovery process, and the heat lost every day is still greater than the heat received, showing a situation of "making ends meet".

On March 9, that is, from winter to the next 20 days, the accumulated heat is the least, the temperature is the lowest, and the weather is getting colder and colder. If there is the influence of cold air at this time, the weather will be even colder. So "March 9th" weather is the coldest. After this "cold front", there will be a "slight balance" every day, and the weather will gradually get warmer. □