In English we often use the idiom, "it is raining." We don't think much about the use of a pronoun without antecedent here. Nobody ever asks, "what is raining?"
I raised this question recently on a message board and got three answers, dashed off together in an irritated fashion, as if to shoo me away: the sky, or the world, or nature is raining.
Those are three very different answers, and the first of them refers to a mere visual phenomenon, the illusion of a blue dome above an observer's head that we call the "sky".
Try out the phrase "the sky is raining." Does that trip off the tongue?
There are profundities in the neighborhood of this simple seeming idiom.
"It is raining, but what is 'it'?" could serve as a zen koan.
After writing the above, I discovered that I explored the idiom in question in a post on this blog in June 2012. Ah well, repeating oneself at three-year intervals doesn't sound too bad.