According to the 24 solar terms of the lunar calendar, "autumnal equinox" is the beginning of autumn, and "Start of winter" is the end of autumn, and the "autumn equinox" is just half of the 90 days from Start of Autumn to start of winter. From the day of the autumn equinox, the climate mainly presents three characteristics: the position of direct sunlight begins to enter the southern hemisphere, the day begins to be shorter than the night in the northern hemisphere, until the winter solstice reaches the shortest day and the longest night; The elevation Angle of the sun at noon also began to decrease significantly, so that the daily heat gain began to decrease significantly; Therefore, the temperature decreases, the temperature difference between day and night gradually increases, and the speed gradually accelerates.​ The opposite is true in the southern hemisphere.

During the autumn equinox, the Yangtze River basin and the vast areas to the north of China have entered the autumn division of "pentaverage temperature", and the daily average temperature has dropped below 22℃. The cold air mass in the north began to have a certain force, and the rainy season has just ended in most areas, with cool breezes, blue skies, sunny winds and crisp autumn air. After the autumn equinox, the direct position of the sun moved to the southern hemisphere, and the northern hemisphere got less and less solar radiation, while the ground lost more heat, and the temperature decreased significantly faster.