Why is it sunny and cool in autumn?

In autumn, the subtropical high began to retreat southward, and the mid-latitude westerly airflow once again became the main weather system affecting Chinese mainland. At this time, the summer monsoon is weakened, and the low-level southwest jet is weakened, so there is less water vapor transported to China and less precipitation in the whole country. The westerly airflow often brings cold air at high latitudes, causing Chinese mainland to cool down.

As the direct point of the sun began to move southward from summer to the future, China received less and less solar radiation, which caused the national temperature to drop.