I read on Google books and exactly on this book: Applications of Grammar: Analysis of Effective Communication, Book 3 By Garry J. Moes, Ed Shewan
Adverb clauses page 141
It is clear [that you do not know the facts]. (clause modifies the adjective "clear"
So, as a clause or any item grammar can qualify ( adjective ) = that item ( modifier ) no doubt = adverb
Others would say the clause ( that you do not know the facts) = noun clause appositive of the impersonal pronoun ( it )
( that you do not know the facts ) Is it noun clause or adverb clause ?