秋天变冷了可以用英文表示为 "It's getting colder in the fall." 或者 "The weather is getting colder in the fall."
"Fall" 是英语中秋天的意思，来源于古英语 "feallan"，意思是 "to fall from a height"。在秋天，叶子从树上落下来，所以秋天也被称作 "fall"。
下面是一些例句：
It's getting colder outside. 户外变冷了。
The temperature is dropping. 气温在下降。
I need to start wearing a jacket now that it's getting colder. 现在我需要穿夹克了，因为天气变冷了。
I can't bepeve how quickly the weather has changed. 我真的不敢相信天气变化这么快。