Abstract: An elongated hollow tube extending between the forward and after ends of a hopper trailer serves as a ridge pole for a protective tarpaulin covering a commodity, such as rice or wheat, forming the trailer's cargo.Conduits connected to the ends of the tube extend downwardly below the margin of the tarpaulin so as to conduct atmospheric air into the tube. From the tube the air discharges downwardly through an opening in the tube vertically above the hopper door and thereby equalizes the air pressures on both sides of the tarpaulin as the commodity is withdrawn from the hopper and tends to form a "vacuum" beneath the tarpaulin.