A Pronoun In The Weather

"It's snowing."

We say this often, particularly when it's happening. "It is snowing," or maybe, "I hope it snows," if it's not happening where you are.

"It" is a pronoun that stands in the place of another noun. The pronoun is usually used so we writers aren't repetitive. But what is "it" in this context?

We usually think of "it" as the weather outside. "It's sunny outside today." Put it into our original sentence and it doesn't quite fit: "The weather is snowing."

"The sky is snowing," doesn't quite work either. "The clouds are snowing," is probably the best fit, but then there's that switch from singular to plural; that doesn't quite work either.

I don't know the answer, even though I've been thinking about it for the last week while it has snowed. Can anyone give me an answer?