The subject in 'It is raining.' 'it' is indexical, and in any sentence about the weather, 'it is raining' or 'it is sunny', 'it' refers to the weather. — Herg
You're mixing up your forms like
Terrapin did.
The "it" in "it is raining" cannot syntactically refer to the weather in the trivial way the "it" does in "it is sunny" because the syntax differs. This is made obvious when you consider that "a sunny day" is a correct form but "a raining day" isn't. The day can be "sunny" but the day cannot be "raining". Rather, it can be
rainy.
So, the proper syntactical parallels are:
It (regular pronoun indexing the weather) is sunny (adjective). ✓
It (regular pronoun indexing the weather) is rainy (adjective). ✓
and
It (dummy pronoun with no clear referent) is raining (present participle). ✓
It (dummy pronoun with no clear referent) is sunning (present participle). X*
*But we can say "She is sunning herself" meaning sunbathing. The reflexive version seems to have stolen the opportunity for a non-reflexive parallel to "It is raining". So, why the difference? Maybe because of the different characteristics of each phenomena (we lie in the sun not in the rain and the rain falling seems more active and verb-like than the emission of less tangible light rays) or maybe because of some etymological accident. Fact remains, the former is valid and the latter isn't, and the most straightforward and commonly accepted logical analysis of the former is the non-indexical dummy pronoun angle, the objections given in this discussion so far having being based on misunderstandings.