The turbulence that struck United Airlines Flight 967 was nothing new to pilots. Sixty-five thousand reports of turbulence over the U.S. are filed by pilots every year, according to the National Center for Atmospheric Research, and 5,500 of them are "severe or greater."
At least 21 people were taken to Denver-area hospitals Tuesday evening after the Boeing 777-200, which was flying over Kansas, hit a downdraft, throwing people around the cabin if they were not wearing seat belts. The plane, which was headed to Los Angeles from Washington's Dulles International Airport, diverted to Denver so the injured could get medical help.
"This is depressingly familiar," said ABC News aviation consultant John Nance, a veteran pilot. "About 60 people a year get hurt by this. We've all heard the prohibitions about sitting down in your seat without putting your seatbelt on, but unfortunately there's always a number of people in the cabin who don't have their seatbelts on when something like this hits without warning."
This afternoon the National Transportation Safety Board said it was beginning an investigation. This is the third time this year that United passengers have been injured in planes that encountered turbulence.
The turbulence typically is caused by convection -- the natural tendency of hot air to rise and cold air to descend toward the ground. On a hot summer afternoon over the American Midwest, this pattern can be more violent than anywhere else on earth, according to the Federal Aviation Administration.
"Pilots don't fly through storms, not because of the bad weather itself, but because of the updrafts and downdrafts," said Laura Brown of the FAA.
The air currents can be powerful, moving at more than 100 mph.
On Tuesday, there were thunderstorms in the area around Flight 967, and planes were being diverted around it, the FAA said. The plane carried 255 passengers and 10 crewmembers, according to United. Passengers said the seat belt light was on, but not everyone paid attention.
"Sometimes with very strong thunderstorms, strong turbulent waves will form near the top of storm," said Clinton Wallace, deputy director of the Aviation Weather Center of the National Weather Service, in an e-mail to ABC News. "This is often at the same altitude that planes fly. These turbulent waves spread out from the top of the storm similar to the way waves move in a pond when a rock is thrown into it. In rare cases, these strongest waves can move great distance from the storm and vertically displace the aircraft very quickly."
Wallace said it is possible that Tuesday's turbulence "was far removed from the thunderstorm" itself -- that the pilots may have thought they are going around the edge of the storm system, but got caught in strong drafts anyhow.
"Current weather radars are very good at detecting turbulence in rain," said John Hansman, a professor of aeronautics at the Massachusetts Institute of Technology. But "clear air turbulence" outside of a storm is another matter. NASA and other research agencies have been working on how to detect it, but there have been no major breakthroughs.
"For those couple of seconds, I just saw people fly up like in front of me," said Brian Liu, a passenger interviewed at the Denver airport after United 967 landed. "Actually, the lady sitting beside me, she actually flew up to the roof and she hit the ceiling and she landed on my lap."