"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 raining outside.

外面下着雨呢。

Is it okay 实战 - SpeakingMax英语口语达人

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

They got this miserable modern one with the sort of Vietnam like conditions that they have out there; it's raining all through the God-damned battle of Agincourt.

为了在现代世界重现当时的惨状,摄制组远赴越南,在阿金库尔战役中,该死的雨下个没完没了

耶鲁公开课 - 古希腊历史简介课程节选

And now let's suppose that your parents are going away for the weekend and they leave you with the following instructions: If it's raining tomorrow morning, when you wake up, you are to close the windows, open the door and let the cat in, phone the grocer and have him send over the following list of groceries. All right.

现在假设你们的父母这周末,离开了，给你们留下了,以下的一些指示：,如果明天早上下雨了，醒来的时候，要把窗户关好，打开门,让猫进来，打电话给杂货店并且让他,送来以下的生活用品，好的。

哈佛公开课 - 计算机科学课程节选

What could be more prosaic than "It is raining?" All of a sudden you notice that string of "i's." You notice all kinds of other things about it.

最基本的意思就是“在下雨“，但是你注意到,“i’s“这个词，你注意到它的相关意义。

耶鲁公开课 - 文学理论导论课程节选

What kind of a definition of "it" is "It's raining?"

里头的“it“是什么意思呢？

耶鲁公开课 - 文学理论导论课程节选