It was a swelteringly hot afternoon in south-west Myanmar on 21 May 2024. Tropical thunderstorms, showers and highs of 36C were forecast for the area around the Irrawaddy Basin. High above, monsoonal heat and humidity were building into a vast convective column of water vapour, a towering cumulonimbus cloud soaring up to 35,000ft (more than 10,500 metres) into the air. From the ground, it would have looked like something from a children's picture book – a soft, fluffy white mass. But for Singapore Airlines flight SQ321 passing just above, it was anything but benign.
In the cabin of the Boeing 777, passengers were being woken up for breakfast with three hours to go on the journey from London to Singapore. For the 210 travellers on board, the past 10 hours had been uneventful. The seatbelt signs were off. Some were milling around, others were using the toilets, and cabin staff were preparing trolleys for service. 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 in an instant.
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.
While SQ321 was an extreme case that made global headlines, turbulence is the leading cause of injuries to passengers and crew in non-fatal accidents, according to the International Air Transport Association (IATA). Figures from the Federal Aviation Administration record about a dozen serious injuries a year from turbulence across the US alone.
It 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 the type caused by mountains and wake turbulence, which is caused by other aircraft, are fairly well understood and avoided. Pilots have always had to contend with the sky's speed bumps as part of the job.
But the threat of turbulence across the world is increasing due to the climate crisis. Not only will pilots need to contend with the frequency and severity of these episodes rising, they're facing a future that includes a large surge in clear-air turbulence – an invisible, largely undetectable turbulence that comes, quite literally, out of the blue.
Flying over the top of a cyclone
Richard Woodward is a former Royal Australian Air Force test pilot and Qantas A380 captain who served the airline for 32 years. He has flown routes all around the world and has seen more than his fair share of difficult conditions, including flying over the top of a cyclone (it was surprisingly smooth and the radar "painted it beautifully").
But Woodward 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. This can be particularly dangerous, Woodward says, given that "there's more energy in a typical thunderstorm than there is in a nuclear weapon".
That massive amount of energy can be seen in lightning – which sometimes even reaches into space, Woodward says – but it also manifests in turbulence. More heat, and greater differences between hot and cold areas, will supercharge already powerful thunderstorms.
It's a problem that pilots and passengers have encountered in Australia. In the same month as the SQ321 episode, a Qantas Boeing 737 encountered severe turbulence while on approach to Brisbane airport after entering a storm cloud at 11,400ft. Three cabin crew, who were not seated at the time, were injured; one was left with a broken ankle. Another crew member with an undiagnosed concussion returned to work on several subsequent flights without medical attention. The Australian Transport Safety Board report was highly critical of the airline's response, saying the crew member "returning to work with an undiagnosed concussion likely compromised [their] ability to perform safety-critical tasks". Qantas has since overhauled its safety protocols for crew injuries.
"In eastern Australia in a typical warm season we get lots of thunderstorms – warm air below, cooler air above, gives you the instability," says Dr Milton Speer, a forecasting meteorologist and a fellow in the school of mathematical and physical sciences at the University of Technology Sydney. He has conducted research that found atmospheric conditions in eastern Australia are increasingly resulting in stronger, more frequent thunderstorm microbursts that can be hard to predict.
Brisbane to Sydney consistently ranks as the most turbulent route in Australia due to these fast-developing and dynamic storm systems.
'The fuel of thunderstorms'
The physical science behind the rise in atmospheric turbulence from global heating is both simple and incredibly complex.
"The science is pretty clear on this: for every degree of warming, the atmosphere can hold 7% more water vapour," says Todd Lane, a professor of atmospheric science at the University of Melbourne. "And water vapour is the fuel of thunderstorms.'"
There's also a strong relationship between the severity of the thunderstorm and the amount of turbulence inside. "So if we have more severe thunderstorms, we expect more turbulence within them," he says. "And also that leads to more turbulence around the storms as well."
Most onboard injuries caused by turbulence are linked to thunderstorms but storms and bad weather are at least to some extent visible via radar and can be forecast.
That's not true for invisible 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%.
Lane, who has worked on problems associated with clear-air turbulence for 25 years, 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, the fast-moving air currents that flow around the Earth.
With global heating, changes in temperature differences between the equator and the poles can intensify the wind shear from jet streams, Lane says.
In 2017 Ignacio Gallego-Marcos was sitting on a plane on the tarmac awaiting takeoff for a flight from Madrid to Stockholm. It was a calm day but the engineer searched for a forecast out of curiosity and realised there was nothing available for the public. It prompted Gallego-Marcos – who has a PhD in fluid dynamics, the study of how liquids (or gases) move and respond to physical forces – to build turbli.com, a publicly accessible turbulence forecasting site.
Anyone can use Turbli to get a read on an upcoming flight. Aviation enthusiasts are among the 300,000 monthly users of the site, alongside nervous flyers.
Gallego-Marcos says turbulence is, to some extent, predictable. And even though global heating may increase its frequency and severity, some types are easier to model than others.
"Clear air turbulence can be very easily predicted when it comes to the jet stream, which are very large flowing masses, so the resolution of the current forecasts can predict them," Gallego-Marcos says. "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."
'The ones you're going to drive into the ceiling'
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 to help avoid turbulence 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. The data, modelled using AI tools, is now providing near-time predictions of turbulence.
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 to hold tight to, along with your armrests: 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. Airlines will have to continue to tweak guidelines and protocol for pilots, cabin crew and passengers when it comes to the ding of that seatbelt light.
"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."