One of the functions of air conditioning is to remove excess moisture so it stands to reason that normally the humidity would be lower inside than out. That being said there would be a lot of variables, high humidity sites like Houston would have a larger variance than Phoenix. Overall air conditioned domes should be close to the same, however, if you have a downpour in the parking lot and 20,000 soaked bastards are in the stands it could skew the norm.