Expert: Delta plane likely hit 'clear air' turbulence, which can be unpredictable

Passengers on a Delta flight from Utah to Amsterdam found themselves diverted to Minneapolis-St. Paul International Airport Wednesday night after severe turbulence prompted an emergency landing.

Their plane took off from Salt Lake City Wednesday afternoon. About an hour into the flight, they ran into turbulence that caused the plane’s altitude to drop about 1,000 feet within a minute — according to the flight-tracking website, FlightAware.

Delta said 25 people on board were injured. They were taken to hospitals after landing.

We wanted to know more about what causes severe turbulence. Kwasi Adjekum is professor of aviation and aerospace sciences at the University of North Dakota.

The following conversation with Adjekum and MPR News guest host Chris Farrell has been edited for length and clarity. Use the audio player above to listen to the full conversation.

What might have caused the severe turbulence on this flight?

The FAA is investigating, but it looks like probably they ran into clear air turbulence. A lot of times, clear air turbulence happens because you have turbulent movement of air masses within the region of what we call the jet stream. So you're talking about between 23,000 to 39,000 feet in the higher troposphere and it's quite difficult to detect. That's why we call it clear air turbulence, because basically, it's not associated with any visual body, like a cloud mass.

Can pilots predict they’re entering into clear air turbulence?

It's very difficult to detect, because most times, pilots have what they call weather radar on board. But this normally detects the water, moisture that will be found in a cloud. So if you don't even have clouds, or you don't have moisture within the air, it makes it very difficult to detect clear air turbulence.

Is severe turbulence becoming more frequent?

The body of evidence suggests that with the increasing temperature associated with climate change and the global warming there's been a higher level of frequency of clear air turbulences. And so now it looks like there's a relationship between a sort of increasing atmospheric temperatures and clear air turbulence.

What should passengers do to stay safe?

I think one of the most important things is basically following the instructions that are issued by the cabin crew as much as possible. Anytime you are seated, just make sure that you are well strapped in. Because, remember, if you are to hit a clear air turbulence — and from the experiences of Delta 56 I mean — it can be chaotic. The rule of thumb is to make sure that at all times, unless you have to get out to visit the washroom, make sure your seat belts are well fastened.