Please fasten your seat belts.

We’ve all heard the announcement on commercial airline flights long-range and short. As illustrated by Thursday’s midair incident involving sudden turbulence, there’s a reason behind the plea.

You can get hurt.

Thirty-seven passengers and crew members were injured in Thursday’s violent encounter aboard an Air Canada flight traveling at 36,000 feet and 600 miles southwest of Hawaii. The plane, en route to Sydney from Vancouver, British Columbia, was diverted to Honolulu.

The culprit was a common flight hazard known as clear air turbulence, or the sudden and erratic movement of air that cannot be seen.

According to the Federal Aviation Administration, clear air turbulence is caused when hot and cold air mix or when speedy wind currents at different altitudes and in opposing directions clash. It can happen near a jet stream or near mountains and often in the winter in northern latitudes.

Such turbulence is a problem that affects all aircraft operations, the FAA says, and is especially problematic because it happens without warning or often without visual clues to warn pilots.

Seventeen passengers and crew members were injured by turbulence in 2017, according to the FAA, and more than 300 were hurt during the previous decade, with a large percentage of the victims vulnerable crew members working without seat belts.

Former airline pilot and Hawaii aviation expert Peter Forman said clear air turbulence is fairly common between Hawaii and the mainland, but severe or extreme turbulence is uncommon because the airlines route traffic around those areas.

The airlines often know where turbulence is on popular routes, Forman said, because pilots report their experience, allowing others to adjust their route or altitude.

But reports of injuries from turbulence seem to be rising, he said, especially with more airliners flying long-range routes that are not frequently traveled.

“Jets traveling a long way on routes, such as from Vancouver to Sydney route, may not know about the turbulence until they fly into it. If it’s the only plane in that area in a long time, they are more susceptible,” he said.

In Hawaii the southern approach to Kahului Airport through Maui’s Central Valley is among the most turbulent flight paths, especially when the tradewinds are blowing.

“I recommend as long as you’re in your seat, keep your seat belt on,” Forman said.