What does [an autumn rain and a cold] mean?

Experts say that in autumn, cold air travels south from Siberia and Mongolia and enters most parts of China. When it meets the warm and humid air in the south, it forms rain. The cold air going south again and again often causes rainfall again and again, lowering the local temperature again and again. In addition, at this time, the direct rays of the sun gradually move southward, and the light and heat irradiated by the northern hemisphere decrease day by day, which is also conducive to the enhancement and southward movement of cold air. After the cold air went south several times, the local temperature became very low This is the truth of "an autumn rain and a cold".