Let's say someone random comes to you and says:
In this case, the usage of "it" is (almost) completely subjective. You both didn't create a dialogue that produced a context in order to make the existence of "it" justified.
So, I was discussing with some folks about the need of "it" in there and I said that it is referring—and always will be, in that context—to the weather. I mean, if someone comes to you and say that's raining, this person is necessarily referring the "raining" to the subject weather.
The point is that one of these folks asked me if the "it" is precisely referring to weather when it becomes "It's raining money." and I said yes. As far as I know, this case is dealing with a metaphor which is still talking about rain.
My argument is that the comprehension about the usage of "it" is that it always should have a context, even if it is abstract or hidden. It's there, but not explicitly defined.
Is this argument right? Is "It's raining (something)" referring necessarily to the weather and will it always be?