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.

Delta Air Lines says a flight from Salt Lake City to Amsterdam was hit by serious turbulence, sending 25 people to hospitals and forcing the flight to divert to Minneapolis-St.