It was raining heavily.

雨下得很大。

《牛津词典》

It was raining this morning.

今天上午在下雨。

《牛津词典》

It was raining hard outside.

外面雨下得很大。

《牛津词典》

"It's" with an apostrophe is short for "it is," as in "It's raining."

VOA: special.2010.02.11

But I choose to walk, and just walking in the seasons whether it's snowing or raining, or gorgeous sunny fall day.

但是我选择走路，什么季节都是如此,无论是雪天，雨天，亦或是秋高气爽。

普林斯顿公开课 - 人性课程节选

It's really, in a sense, the physical image is of rain; it's raining, so it's leaking all over it.

这在某种意义上确实是，下雨的一种具体的描述;,因为在下雨，所以天空像是在漏水。

耶鲁公开课 - 1945年后的美国小说课程节选

It's when it's dark, it's raining, you think oh I've got to go and walk outside with one baked bean can and that's... and then I'll maybe.. if I do it, then I'll sometimes think oh I won't wash it out.

BBC

If it's raining, I'll stand outside the back door.

BBC: NEWS | UK | England | Kent | Members' club bemoans smoking ban

It's raining, and he's just walking around.

CNN: What happened the night Trayvon Martin died