In order to answer this question, you must know what compound, complex, and compound-complex sentences are. A compound sentence is one that is created by joining two independent clauses (complete sentences) with a conjunction such as "and" or "but." A complex sentence contains one independent clause and one or more dependent clauses (phrases). A compound-complex sentence, as you might expect, contains both, at least two independent clauses and at least one dependent clause. Looking at the sentence in question, we see two independent clauses; "Sometimes the air that is rushing in begins to rotate" is a complete sentence, and "a tornado is formed" is also a complete sentence. There are no dependent clauses, but both of these independent clauses are joined by a conjunction (and). Thus, this is an example of a compound sentence.
