当天气下雨时，用英语可以说 "It's raining." 或 "It's raining outside."
这个表达方式来自于英语的一般现在时动词 "rain"，意思是 "下雨"。
下面是一些例句：
It's raining heavily. (下大雨了。)
The forecast said it would rain today. (天气预报说今天会下雨。)
I need to bring an umbrella because it's raining. (我需要带一把雨伞，因为下雨了。)
I don't pke to go out when it's raining. (我不喜欢在下雨时出门。)