In March, Ingrid Weisse, her husband and two young sons were aboard Alaska Airlines 889 from Portland, Ore., on a flight home to Hawaii when the Boeing 737 began buffeting so fiercely that it felt as if the plane would shake itself apart.

"It got really, really violent," says Weisse. There was lots of screaming in the cabin. A flight attendant was hit by an ice bucket that became a projectile. So many people got sick from the sudden changes in altitude that flight attendants had to hand out more vomit bags, she says. Midway through the approximately 45-minute ordeal, one frightened passenger yelled out, "Please tell us this is normal!"

For Weisse and her family — all frequent flyers — it was like nothing they had ever experienced. Clearly the passengers aboard their flight were rattled, but so were the flight attendants. Before disembarking in Honolulu, one of them confided to Weisse that it was the worst turbulence she'd seen in 23 years on the job.

There are different kinds of turbulence, and it's hard to pinpoint what caused the extreme conditions that rocked Weisse's flight. In an email, Alaska Airlines acknowledged "unexpected turbulence" on the flight. But researchers say there's evidence that a particularly unpredictable type known as "clear-air turbulence" is becoming more frequent, says Paul Williams, a professor of atmospheric science at the University of Reading in England.

As its name implies, clear-air turbulence occurs in the absence of clouds or bad weather. It's caused by wind shear — sudden changes in wind speed and direction — at altitudes above 15,000 feet.

The reason for this increase in clear-air turbulence is climate change, Williams says. As global temperatures increase due to rising levels of greenhouse emissions such as carbon dioxide, the jet stream is experiencing more wind shear.

"Since satellites began observing in 1979, the amount of wind shear has grown by 15%" in the jet stream, he says, referring to a study he co-authored in 2019.

In a follow-up study using climate model simulations, Williams and colleagues predicted that clear-air turbulence in the middle latitudes of the Northern Hemisphere could triple in the next three to six decades, depending on future greenhouse emissions.

A separate 2020 study by a group of China-based scientists points to increased temperatures in the upper atmosphere contributing to "a profound impact on the wind shear and turbulence in mid-latitudes."

It's hard to avoid what you can't see

Broad areas of turbulence can be forecast by meteorologists, but specific, localized areas of clear-air turbulence present challenges for cockpit crews, says Carlo Scalo, an associate professor who studies turbulence at Purdue University.

"The problem is that clear-air turbulence cannot be spotted because it's above the clouds," Scalo says. "You don't really see it. It's just fast air moving into slow air and vice versa."

One way to limit clear-air turbulence would be to avoid the four main jet streams that encircle Earth. Commercial aircraft often take advantage of these narrow bands of strong wind in the upper levels of the atmosphere to increase speed and shorten flight times.

"That's the trade-off," Williams says. "The airline saves fuel and passengers arrive early. But it might be a bumpy flight."

Avoiding the jet stream would likely mean an increase in greenhouse emissions, he notes, because longer flights burn more fuel.

Bumpier skies appear to be getting more common

The increase is already becoming evident to cockpit and cabin crews, passengers and even aircraft manufacturers. In the last few weeks alone, news reports have chronicled a number of stories far worse than the one Weisse tells.

In early March, significant turbulence aboard a Lufthansa flight from Austin, Texas, to Frankfurt, Germany, injured seven passengers, forcing the plane to divert to Dulles International Airport outside of Washington, D.C.

In an email to NPR, Jordan Jewell, a 21-year-old passenger aboard that flight, recounted how two flight attendants "hit the ceiling head first" during one of the drastic altitude changes. Jewell later noticed that one of them left "a massive dent in the ceiling of the plane."

"I had my seatbelt on, despite the fasten seatbelt sign being off, so while I flew up I was kept from hitting the ceiling," Jewell says. "A woman sitting four seats and an aisle to my right did not. So the first time the plane dropped she hit the ceiling, and when we stabilized she landed in the aisle on her back."

American Airlines Capt. Dennis Tajer calls clear-air turbulence "a real nemesis" because it can't be seen or easily detected from the cockpit of a commercial airliner and therefore often comes with no warning. What's more, Tajer, who has more than 30 years of experience as a commercial pilot, believes that the problem is getting worse.

"I can tell you from flying even up at altitude, not just your average thunderstorm, [that] the weather events seem more severe than they have been in the past," Tajer says.

Peter Murray, the founder of turbulenceforecast.com, agrees. He set up the website in 2005 to help travelers see areas of likely turbulence before stepping aboard an airplane. By day, he is a systems administrator at Michigan State University. But thanks to the side project, he's had an armchair view of turbulence in the upper atmosphere for nearly two decades.

"I do feel that things are getting ever-so-slightly rougher," he says.

Just a day after the incident on Alaska Airlines, a United Airlines flight out of San Francisco, bound for Munich, Germany, was forced to divert to Denver when a flight attendant was seriously injured during a short-lived, but intensely turbulent, event.

Dianna Previs was a passenger on that flight. Once it landed, she says, the injured flight attendant was wheeled down the aisle, her leg extended and wrapped in an ice pack with what appeared to Previs to be a compound fracture. "She looked like she was in a lot of pain," she says.

In an email, a United spokesperson praised the pilots on the March 21 flight "for their quick work in addressing this issue and working to ensure the safety of everyone on board."

Flight attendants experience the most injuries

Although it is almost unheard of for turbulence to cause a crash, such events do stress a plane's airframe, says Ryan Pettit, an associate technical fellow and senior controls engineer for Boeing.

Really jarring turbulence "can impart pretty big loads on the airplane," he says. However, "the design standards of modern aircraft are really high."

Nevertheless, the National Transportation Safety Board has said that turbulence causes the most common types of accidents aboard aircraft. From 2009 to 2022, the National Transportation Safety Board tallied 163 "serious injuries" resulting from turbulence. The types of injuries tracked include major fractures, serious burns, internal bleeding or any other injury requiring two or more days of hospitalization. Flight crews incurred 80% of all such injuries, the NTSB notes.

Sara Nelson, the international president of the Association of Flight Attendants-CWA, says the reason cabin crews suffer most of the injuries is pretty straightforward. "We're checking on other people having their seat belts on. We're in the middle of a service. We're preparing for the next service in the galley with a 300-pound cart," she says.

"So, it's often that we're not buckled in when these clear-turbulence events happen," Nelson says.

Two years ago, the NTSB released a study aimed at improving procedures to tackle the problem. Its recommendations range from more stringent enforcement of seat belt requirements to improving pilot reports of turbulence.

Many passenger aircraft use an app called Sky Path to monitor turbulence with a cockpit-mounted tablet as a motion sensor. The app works similar to how Google Maps processes traffic congestion — by automatically collating real-time reports from other users' devices.

Boeing's Pettit says the passenger jetliners already have active flight controls to minimize shaking during turbulence.

Even so, "it's one thing to try to improve the ride quality once you've experienced the turbulence," Pettit says. "But the best-case scenario is you don't experience it in the first place."

One thing being looked at is the use of lidar, or light detection and ranging, a sort of radar that uses light instead of radio waves. It's an existing technology, but there are a number of hurdles to overcome to make it work for detecting clear-air turbulence, he says.

Nelson, president of the flight attendants union, says changing procedures so that cabin crew members remain buckled in during critical parts of a flight would also help. "There's been recommendations from the NTSB to have flight attendants sitting down earlier on descent and staying [seated] longer on ascent, which is oftentimes where we encounter these issues as we're flying through the various altitudes," she says.

Still, there is a simple, already proven safety device. "The seat belt, you know, it's old technology," Pettit says. "There's nothing exciting about it. But the data really points to it working."