it is表示天气吗?

对，可以用来表示天气
比如:it is sunny today.今天晴天。

可以的,it的用法很多:
1.表时间
eg:It's Monday.

2.表天气,大气状况
eg:It's cold.
如果是名词表示天气,则不能用"it",要用"there be".

3.表距离
eg:It's a short way now.

4.表不明身份的人或物.
eg:Who's that at the door?
It's me.

5.作形式主语,代替不定式或主语从句.
eg:It's difficult to learn swimming.

6.作形式宾语,代替不定式或宾语从句.
eg:I think it a pity that he can't come tomorrow.

不一定，可以表示很多。表示天气的话应该是：The weather is....

可以，it当作形式主语。
It is cloudy .