Why is there a saying that "it's cold in March and hot in dog days"?

"Sanjiu" refers to the third nine days from winter to the future, that is, about 65438+ 10/2 to 20. "Three volts" refers to the front, middle and back three volts, basically counting from long summer to the third Geng Day in the future. On the winter solstice, the northern hemisphere has the shortest day and the longest night, so the surface of the earth gets the least sunshine on that day. However, the solstice in winter is not the lowest temperature day. From winter to the future, although the sunshine time increases, the geothermal expenditure is still greater than the income. Therefore, the ground temperature will decrease later, and the weather will be coldest when the solar radiation energy absorbed by the ground is the same as the heat emitted by the ground. At this time, it is probably in the middle and late part of 1 month (March 9) to say "March 9 is cold". For the same reason, in many areas of China, the summer solstice is the longest day, the highest noon sun height and the strongest solar radiation. Even after the summer solstice, the solar radiation in China gradually decreases, but the heat absorbed by the ground is still greater than the heat released, and the heat continues to rise slowly. By the end of July, the atmospheric heat absorption and release were in a state of equivalence, and the temperature in some parts of China was the highest, so there was a saying that "the heat was at three volts".