Airplane turbulence is getting worse. Scientists explain why.

The skies can be clear, blue, and tranquil. "And all of a sudden, boom, you hit it," Dan Bubb, a former airline pilot and now an aviation historian at the University of Nevada, Las Vegas, told Mashable.

This boom is "clear-air turbulence," a well-known hazard to aircraft and the passengers aboard. It's created by unstable air that commercial planes sometimes cruise through at higher altitudes. It's not visible from the cockpit. It's doesn't show up on the flight deck's weather radar. "It's...