High above the Rocky Mountains, passengers aboard United Airlines flight 1676 had a nightmare at 34,000 feet as the plane bolted, bucked and plunged under severe turbulence. Passenger Kerri Mullens said "It was instantaneous ... Everything that everybody had in their hands was flying through the air. People were screaming." What causes turbulence? Air flows like water, so mountain ranges can create turbulence, as can weather fronts, thunder storms, or small pockets of local instability, experts say.