The transition between seasons is subtle and almost unnoticeable. One day is it Spring and it gradually gets warmer until you realize that it is Summer. But the transition from Summer to Fall feels very abrupt. It is signaled by the falling leaves.
It is still 90 degrees out but the leaves are suddenly dying on their branches and floating to the ground. Accumulating. Before long the trees will be bare and the temperature will be cooler. And it will be Fall. We will wonder what happened to Summer.