in fall ，在秋季，表示泛指。

例句The study also found that those born in fall less probably had a mood of depression which may easily drive them to cry, while those in winter were not easier to be angry.这项研究还发现，出生在秋季的人不大可能出现容易让他们哭泣的抑郁情绪，而冬天出生的人则没那么容易生气。in the fall 在秋天;在秋季;坠落走马灯;表示具体的某一个秋天。例句，He has designed to go to Australia for a trip in the fall.他已经打算秋天去澳大利亚旅行。

都表示在秋季，用法都正确，带定冠词the较普遍

in fall释义：

在秋天

例句：

There are dragonflies circling over a pond in fall.

秋天里有蜻蜓在池塘上面打圈飞行。

in the fall释义：

在秋天

例句：

There are dragonflies circling over a pond in fall.

秋天里有蜻蜓在池塘上面打圈飞行。释义：

在秋天

例句：

The leaves fall in the fall.

叶子在秋天都掉了。