"This" is clearly ambiguous. I think the grammarians call it an "unclear antecedent".
Also, when someone says "It is raining", to what does the "it" refer to?
Beats me.
Apparently it's called a syntactic expletive. Basically it's there because English requires there to always be a subject in every sentence. "Rains" is not a complete sentence and is very confusing, so "it" is essentially an artificial subject, so it is "it" that rains instead of nothing.
"This" is clearly ambiguous. I think the grammarians call it an "unclear antecedent".
Also, when someone says "It is raining", to what does the "it" refer to?
Beats me.