What is clear-air turbulence? What to know about the "very violent" phenomenon

Video and passenger accounts have painted a picture of chaos aboard Singapore Airlines Flight SQ321 after the passenger aircraft encountered what the airline called "sudden extreme turbulence." 

One person died during the incident. Authorities believe the passenger, identified as a 73-year-old British man, had a heart attack. Dozens more passengers were injured. Six people were treated for severe injuries after the plane made an emergency landing in Bangkok, Thailand, CBS News previously reported. 

The incident has drawn attention to the dangers turbulence can pose. One type of turbulence, known as clear-air turbulence, can be especially difficult for pilots. 

Here's what to know about clear-air turbulence.

What is clear-air turbulence? 

Clear-air turbulence is a "typically very violent" phenomenon that occurs at high altitudes, typically between 23,000 to 39,000 feet above sea level, said Hassan Shahidi, president and CEO of the Flight Safety Foundation. 

What makes clear-air turbulence dangerous is that it can't be seen in advance like turbulence caused by weather, known as convective turbulence. Normally, flights divert or enter a holding pattern to avoid severe turbulence, but if it can't be seen in advance, pilots can't adjust to avoid it, Shahidi said. 

What causes clear-air turbulence? 

Planes often fly through air masses known as jet streams. Within those streams, there are multiple layers of air flowing at varying speeds "almost on top of each other," said Daniel Adjekum, a pilot and aircraft safety consultant who holds a doctorate in aerospace sciences and teaches at the University of North Dakota. The differing temperatures cause friction. That friction, in turn, causes "a lot of disturbance," Adjekum said. 

In convective turbulence, caused by storms or other weather, air is heated and displaced, leading to high moisture content that can be easily spotted on flight instruments. Clear-air turbulence doesn't have that high moisture content level, so radar and other instruments can't detect it until it's too late, Adjekum said. 

"That is what makes it very insidious," Adjekum said.  

Climate change also plays a part. Warmer air caused by carbon dioxide emissions is leading to stronger wind shear at higher elevations, which can result in clear-air turbulence. A 2023 study found that clear air turbulence has increased by 41% over the past 40 years.  

Is clear-air turbulence to blame for the chaos aboard Flight SQ321? 

Experts were hesitant to say if clear-air turbulence was to blame for the scene aboard Singapore Airlines' Flight SQ321. The airline said the death and injuries aboard the plane were caused when the aircraft "encountered sudden extreme turbulence." 

The plane was flying at 37,000 feet, the airline said, putting it in the range of clear-air turbulence, but experts highlighted thunderstorms in the area that could have caused the turbulence. 

Robert Sumwalt, the former chair of the National Transportation Safety Board and a pilot, said on CBS Evening News that it's "too early to know for sure" what caused the incident. 

An investigation is ongoing.

Is there a clear-air turbulence forecast? 

The unpredictability of clear-air turbulence makes it difficult to forecast, though it is more common during winter months. 

Experts said the best way to prepare for the phenomenon is stay buckled in while flying. 

If you're not actively moving about the cabin, your seatbelt should be on, Adjekum said. If the pilot turns on a fasten seatbelts sign, all passengers should return to their seats as quickly as possible. 

Kris Van Cleave and Tracy Wholf contributed to this report. 

In:
• Singapore Airlines
• Climate Change
• NTSB
• Airlines

Kerry Breen

Kerry Breen is a news editor at CBSNews.com. A graduate of New York University's Arthur L. Carter School of Journalism, she previously worked at NBC News' TODAY Digital. She covers current events, breaking news and issues including substance use.

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