There are two very common uses of dummy pronouns in English: existential or deictic there
There goes the bell
and weather it
It is raining
which extends to a more general ambient it used in a range of statements about time, place, conditions, and the like
It is pleasant here
It is 4 o’clock
It is the end of the second half of the match.
Leaving there for another occasion, it has been argued that weather it is a dummy subject, and that is where you will find it discussed in Wikipedia, for example, here.
How did it happen? It has been suggested that English, in common with some other Germanic languages, developed weather it (and existential there) as a result of their transition from putting the verb initially, in verb-subject-object (VSO) order, to putting the verb second, in SVO. Some other European languages can drop the subject altogether, therefore have no need for a weather it. Certainly in English you could not get away with
(however Mancunian that may seem); you need to give that verb a subject.
Huddleston and Pullum’s Cambridge Grammar (p 1482) states:
“It is used as a dummy subject with verbs and predicative adjectives denoting weather conditions” as in
“It is raining. It became very humid.”
“It does not represent a syntactic argument and cannot be replaced by any other NP [noun phrase]; it has the purely syntactic function of filling the obligatory subject position.”
However, using substitution, they demonstrate that in other usage for statements about time, place, conditions, it serves as a predicative. They also examine a grey area including
It was a perfect day
which they accept can be substituted to read
The day was perfect
although they claim that “is very unidiomatic.”
I am afraid that I do not buy this distinction, nor do I see such substitutions for weather it as being unidiomatic. In all circumstances the weather it form is at least as idiomatic as a substituted version, but the following seem to work very well for me, and I have heard them used extensively:
It will be hot and sunny tomorrow; the weather will be hot and sunny tomorrow; tomorrow the weather will be hot and sunny; the weather tomorrow will be hot and sunny; and so on.
It was fine today; the weather was fine today; the day was fine; today was fine.
Strangely it would seem that raining and snowing do not substitute as well, with
The weather is raining
sounding stilted and unidiomatic.
Perhaps it would be better to refer to the bad weather it as being a dummy subject, but good weather it being predicative?