NEW YORK (AP) — They literally don't see it coming.

"Clear-air turbulence," which evidently jolted an Air Canada flight Thursday over the Pacific Ocean, strikes almost literally out of the blue, with no visible warning in the sky ahead. An aircraft's radar can't spot it coming either.

But passengers can certainly feel it. Some on the Air Canada flight were slammed against the ceiling, and more than two dozen were taken to hospitals after it made an emergency landing in Honolulu.

Clear-air turbulence happens most often in or near the high-altitude rivers of air called jet streams. The culprit i...