Mainly because of the autumn tiger. Due to the weakening of subtropical high, the temperature dropped month by month, and the temperature was still high in early September. In some years, the highest daily temperature in the first ten days of September is above 30℃, and there is high temperature weather, commonly known as "autumn tiger", but it generally lasts for a short time. Since mid-September, the main body of the Pacific subtropical high has rapidly withdrawn southward, the cold air has become increasingly active, and the temperature has dropped significantly.