Clear-air turbulence is harder to detect, more dangerous, and according to new research, it's becoming more frequent as the atmosphere warms.

Twenty-five people aboard a Delta Air Lines flight were hospitalized after the flight encountered "significant" turbulence Wednesday, the airline said.

Some passengers were terrified, convinced the plane was going down. One seasoned traveler said it was the roughest flight he’d ever taken. A woman on board felt like she was experiencing an earthquake.