The weather is getting worse and worse, which is a warning from nature.

What kind of leading word (relative pronoun or adverb) is used in the attributive clause has little to do with the semantics of the antecedent, mainly depending on what components are missing in the attributive clause behind.

In this question, where is a relative adverb, which acts as an adverbial of a place where the weather is always warm, because the attributive clause of this family table structure lacks only an adverbial;

The second empty that is because the definition clause behind it lacks the subject, and that is used as the subject or object in the attributive clause.