Just how dangerous is turbulence?

The accounts from the Hawaiian Airlines flight were dramatic: Passengers hit the ceiling, objects went flying, people were crying.

Sunday’s incident was the result of “severe” turbulence, the airline said, the third of four levels on a scale from light to extreme. Hawaiian chief operating officer Jon Snook said Sunday’s incident, which injured 36 people, was “relatively uncommon.”

But turbulence itself is a frequent occurrence, as any regular traveler can attest - and it typically isn’t cause for alarm, experts say. A day after the incident on the Hawaii-bound flight, a United Airlines flight headed to Houston hit turbulence, and five people were taken to the hospital with minor injuries.

“Turbulence is normal; it’s part of the sky,” said Patrick Smith, a commercial pilot for 30 years who runs the Ask the Pilot blog. “Every flight every day encounters some form of rough air. For crews, by and large, we look at it as a comfort issue, not necessarily a safety issue.”

The Federal Aviation Administration describes turbulence as movement of air that usually can’t be seen and often happens unexpectedly. Roughly 58 people are hurt because of turbulence every year while not wearing their seat belts, the agency says.

“It can be created by many different conditions, including atmospheric pressure, jet streams, air around mountains, cold or warm weather fronts or thunderstorms,” the agency says on its website. “Turbulence can even occur when the sky appears to be clear.”

Bob Thomas, a pilot, former weatherman and assistant professor of aeronautical science at Embry-Riddle Aeronautical University, said there are many different types of turbulence.

“The bad kind is always unexpected,” he said. What’s called “clear-air” turbulence happens without visual cues.

“You can have turbulence from a thunderstorm 20 miles away from the actual worst part of the storm,” Thomas said. “Thunderstorms will create these huge up and down movements of air, and when you get that, you get these big waves that come through and you can just fly through it.”

In the Hawaiian Airlines case, weather officials told media, there were storms in the flight path. But Snook said there had been no warning of the specific patch of air that the Airbus A330 encountered.

“It can be associated with almost any kind of weather,” Smith said. “It doesn’t always matter, and it’s not always predictable.”

He said the tools that pilots have in the flight deck are “amazingly good” at predicting where, when and how bad turbulence might be. They can alter their route or altitude to try to avoid the rough air, or - if that’s not possible - give the flight attendants plenty of warning to prepare the cabin.

“But in a lot of ways, it’s more art than science and sometimes you just don’t know,” Smith said. “It can get bumpy when you just don’t expect.” One place likely to be slightly less bumpy: the middle of the plane over the wings, he said. The bumpiest place to sit is in the tail of the plane, he said, though it doesn’t make a lot of difference.

Smith said the fear that a plane might flip upside down or lose a wing is “at best, science fiction.”

While worst-case scenario fears are extreme, experts say turbulence still poses a risk - especially if people are not buckled in. Snook, the Hawaiian Airlines executive, said Sunday that it wasn’t yet clear how many people on the flight were not wearing seat belts, but that the seat belt sign was on.

“You want to be cautious because the aircraft itself is going to survive,” said Mark Baier, CEO of Aviation Manuals, which provides safety information and systems to smaller flight operators. “You’re going to get thrown around the cabin, or loose objects are going to be thrown around the cabin and cause injury.”