Clear-air turbulence is increasing as the planet warms, and it remains the leading cause of injuries to passengers and crew in non-fatal accidents, according to the International Air Transport Association (IATA). The invisible phenomenon, which cannot be detected in advance using radar, is a growing concern for airlines and pilots.
A violent encounter over Myanmar
On 21 May 2024, a hot afternoon in south-west Myanmar saw monsoonal heat and humidity build into a towering cumulonimbus cloud soaring up to 35,000ft (more than 10,500 metres). For Singapore Airlines flight SQ321 passing just above, the cloud proved anything but benign.
In the cabin of the Boeing 777, passengers were being woken for breakfast with three hours to go on the journey from London to Singapore. The seatbelt signs were off. At 2.19pm and 23 seconds local time, moderate turbulence shook the plane for 17 seconds and the seatbelt sign was switched on. Then, as one passenger described it, it was “like a grenade had gone off”. In just 4.6 seconds, any passenger not strapped in hit the roof with 1.5 times the force of gravity and was thrown back on the floor.
Passengers reported hearing people screaming in agony, blood streaming down faces, with overhead panels broken by the force of bodies hitting them. One person was left dead and 79 were seriously injured. The turbulence had lasted just over 21 seconds.
Thunderstorms and the climate crisis
Turbulence can be caused by a multitude of factors, including geographic features such as mountains – the top 10 most turbulent routes are all over either the Andes or the Himalayas. Some forms, such as mountain and wake turbulence, are fairly well understood and avoided. But the threat of turbulence across the world is increasing due to the climate crisis, with a large surge in clear-air turbulence expected.
Richard Woodward, a former Royal Australian Air Force test pilot and Qantas A380 captain who served the airline for 32 years, says airlines and the aviation industry are increasingly aware of the threat posed by global heating. “There’ll be an increase in the severity of thunderstorms and storms in general because, as the planet warms, the amount of energy inputting into these events will become massively greater,” he says.
Thunderstorm or convective turbulence – the cause of the SQ321 incident – creates strong updrafts, then strong downdrafts as the air and water vapour cool. It may be encountered several thousand feet above clouds and as far as 20 miles (more than 30km) on either side. Woodward notes that “there’s more energy in a typical thunderstorm than there is in a nuclear weapon”.
Rising clear-air turbulence
Clear-air turbulence is usually found in the upper atmosphere, at cruising altitude, often at the edges of jet streams. While it can be modelled or reported by other aircraft, it can’t be detected in advance using radar. An analysis by scientists at the University of Reading of four decades of atmospheric data collected between 1979 and 2020 found that severe clear-air turbulence increased by 55% and moderate and light turbulence had increased 37% and 17% over the same period.
Clear-air turbulence will increase even under the most optimistic scenarios of global heating – a 1.5C increase above preindustrial levels will lead to a 27% increase; with a 2.75C increase in global temperatures that figure jumps to 56%. Todd Lane, a professor of atmospheric science at the University of Melbourne, calls it a “particular hazard for commercial aircraft” and says it occurs when there’s an increase in wind shear – a change in wind speed or direction over a relatively short distance, often at the edge of jet streams. With global heating, changes in temperature differences between the equator and the poles can intensify the wind shear from jet streams, Lane says.
Predicting and preparing for turbulence
Ignacio Gallego-Marcos, who has a PhD in fluid dynamics, built turbli.com, a publicly accessible turbulence forecasting site. Aviation enthusiasts are among the 300,000 monthly users of the site, alongside nervous flyers. Gallego-Marcos says clear air turbulence can be very easily predicted when it comes to the jet stream, but “clear turbulence is also found in the vicinity of thunderstorm clouds and there it becomes difficult to predict because a thunderstorm is very small – it has a lot of features that you cannot predict in weather models”.
Stuart Fox, IATA’s director of flight and technical operations, says the traditional approach of pilots relying on reports from other pilots and weather advisories has some limitations given “information may not be available at the right time and can vary in quality”. In 2018 IATA began collecting turbulence observations from about 30 airlines running more than 3,700 aircraft and sharing them with participating airlines, which include United Airlines, Qatar, Etihad, ANA and Lufthansa. The project uses onboard sensors to record turbulence information and share that instantaneously, with the data modelled using AI tools.
Some manufacturers are looking at aircraft design and sophisticated sensors including Lidar, which uses lasers to detect atmospheric density ahead of an aircraft. And some airlines are becoming more risk-averse. After the SQ321 injuries, Singapore Airlines enforces much stricter rules in the cabin when turbulence is expected, with meal services stopped and crew members seated. On some flights along the same route, it’s routine to have the seatbelt sign on for most of the flight.
But this approach has one particular flaw. “No matter how many times you tell people to put their seatbelts on, there’s a chunk of them in the back who find it uncomfortable to have a loosely fitted seatbelt, for some stupid reason,” Woodward says. “People still need to go to the toilet, they get bored, and once the seatbelt sign’s been on for a small period of time … they’ll get up and go to the toilet anyway. And they’re the ones you’re going to drive into the ceiling if you get a sudden event.”
For nervous flyers, there’s a reassuring fact: there hasn’t been a crash of a passenger airliner attributed solely to turbulence since flight HN431, a Dutch plane that lost control after entering a tornado in 1981. “The reality is, structurally, the aircraft are built to withstand far more than they would ever be expected to encounter in the normal line of operations,” says Steve Lieb, a senior lecturer in aviation at Central Queensland University. The real problem lies not in the plane itself but what’s inside. “The single biggest limitation I will say in modern airplanes is not the reliability of the engines or anything in it,” Woodward says. “It’s the humans on board.”
