What's the difference between autumn and beginning of autumn?

Beginning of autumn is the solar term that begins in autumn, which indicates that hot summer is coming and autumn is coming. But it's not the same as autumn.

"Autumn" is defined according to the standard of dividing four seasons in climatology. Only when the average daily temperature is stable below 22℃ for five consecutive days can it be considered as autumn. Moreover, the climatic conditions are different every year, and there is a great difference in the morning and evening when you enter autumn, and the time difference can reach two or three weeks.