You are correct in identifying that umpires have to call off today’s play as a subordinate clause.
It is not, however, a relative clause. This that is not a relative pronoun; if it were, it could be replaced with which. It is, rather, a marker of subordination (a subordinating conjunction in traditional grammar, or subordinator in the neo-traditionalist approach of The Cambridge Grammar of the English Language). That has at least three different uses: as a relative pronoun (but CGEL denies that that is a relative pronoun), as a subordinator, and as a demonstrative pronoun, and you can only tell which one is play by the context. Here it marks the clause which completes the so ... that ... comparative construction, so it might be called a complementizer, too.
A clause launched by a relative pronoun may act as an adjectival or a nominal:
ADJ: The word which you want is on page 238.
NOM: You have to decide which word you want.
It's an open question whether these relatives are the same words as the identical forms employed as interrogatives:
Which word do you want?
Clauses launched by other relatives (e.g., where, when, whether, why, how*) may act as nominals, adjectivals or adverbials—or, if your grammatical sect permits, interrogatives.