When it is dew, the summer monsoon is gradually replaced by the winter wind, and the cold air turns from defensive to offensive. In addition, the sun's direct point moves south, so the sunshine time in the northern hemisphere is shortened, the light intensity is weakened, and the ground radiation is fast, so the temperature drop speed is gradually accelerated. The Millennium basically ended the sweltering heat in summer, and the weather gradually turned cold, and the cold gave birth to dew condensation. The ancients used the four seasons to match the five elements. Autumn belongs to gold, golden and white, and described autumn dew with white, hence the name "Bailu".