Stay home or strap in: Clear air turbulence is making aviation increasingly dangerous

Climate change is fuelling turbulence on some of our most common flight paths

By climate reporter Jo Lauder

Turbulence is a common part of flying, but severe turbulence that causes injuries, or death, is very rare.

That’s why the world has reacted in shock after a man died of a heart attack and 30 others were injured when a Singapore Airlines flight from London hit severe turbulence.

The plane was cruising at 37,000 feet when the plane suddenly dropped, and passengers were flung from their seats.

As climate change causes global temperatures to rise, scientists say some of the causes of turbulence are already intensifying.

What is turbulence?

Turbulence is caused by a disruption to the air patterns a plane is travelling through.

If you think of the skies like the ocean, turbulence is similar to a wave, according to Professor Todd Lane, an atmospheric scientist at the University of Melbourne.

“Turbulence that aircraft experience is when the wind in the atmosphere changes from being horizontal to going up and down,” Professor Lane explained.

“An aircraft that’s flying along smoothly will start to move upwards and downwards quite radically because the wind is moving up and down.”

The main causes of turbulence are mountains, storms and jet streams, which makes the task of forecasting and avoiding them straightforward in some situations.

Pilots can plan a route to avoid the air that rises over mountains, or around a storm as much as possible.

Jet streams are strong winds in the upper atmosphere where planes cruise, according to Professor Lane.

“Above and below the jet stream there’s what’s called strong wind shear, so the wind is changing speed with height quite dramatically. In those strong wind shear regions, you can get a lot of turbulence.

“So above and below these jet stream regions, there’s quite a lot of what’s typically called clear air turbulence, because there’s no clouds involved.”

It’s not known at this stage what type of turbulence caused the disruption on the Singapore Airlines flight. Tracking service FlightRadar24 told Reuters there were storms — some severe — in the area at the time.

Climate change link to turbulence

As the world continues to burn fossil fuels, global temperatures are rising, and turbulence is just another natural phenomenon that’s affected by that warming.

“It’s affecting the wind patterns and one of those impacts that is being affected are jet streams,” Professor Lane said.

“Those jet streams that are at aircraft flight levels are projected to intensify, which means that those regions will become more turbulent.”

A 2017 study predicted that severe turbulence will become two to three times more common over the North Atlantic by 2050-2080 because of climate change.

A 2017 study predicted that severe turbulence will become two to three times more common over the North Atlantic by 2050-2080 because of climate change.

However, the same study predicted a smaller increase of 50 per cent for severe turbulence over Australia.

“The nature of the jet streams are slightly different in the northern and southern hemisphere because the location of the land masses,” Professor Lane said.

“There is a very strong climate change signal in the northern hemisphere, especially around the Arctic.”

As well as increasingly intense jet streams, climate scientists have warned that storms are worsening as well.

Professor Lane says most of the turbulence in the tropical regions comes from thunderstorms.

“With a warmer atmosphere, the atmosphere can hold more water, which can lead to those most intense thunderstorms being more intense with climate change. As those thunderstorms become more intense, they can also generate more intense turbulence.”

This isn’t just a future problem, it’s already happening.

Turbulence has increased

A study published last year by researchers in the UK found that climate change has already caused an increase in turbulence in the past 40 years.

“We find clear evidence of large increases around the mid-latitudes at aircraft cruising altitudes,” the study from Reading University concluded.

The researchers discovered the biggest increases in clear air turbulence over the United States and the North Atlantic, also some of the world’s busiest flight paths.

For any average point over the Atlantic, the research found that “severe-or-greater [clear air turbulence] increased the most, becoming 55 per cent more frequent in 2020 than 1979.”

However, the research looks at instances of clear air turbulence in the atmosphere; that doesn’t mean there’s been the same increase in aircraft hitting that turbulence.

The modelling predicted smaller increases in turbulence in the Southern Hemisphere and over Australia.

“Following a decade of research showing that climate change will increase clear-air turbulence in the future, we now have evidence suggesting that the increase has already begun,” said Professor Paul Williams, an atmospheric scientist at the University of Reading, when the study was released.

He called for more work to be done to help predict and prevent aircraft from hitting that turbulence.

“We should be investing in improved turbulence forecasting and detection systems to prevent the rougher air from translating into bumpier flights in the coming decades.”

Evidence for Large Increases in Clear-Air Turbulence Over the Past Four Decades


Clear-air turbulence (CAT) is hazardous to aircraft and is projected to intensify in response to future climate change. However, our understanding of past CAT trends is currently limited, being derived largely from outdated reanalysis data. Here we analyze CAT trends globally during 1979–2020 in a modern reanalysis data set using 21 diagnostics. We find clear evidence of large increases around the midlatitudes at aircraft cruising altitudes. For example, at an average point over the North Atlantic, the total annual duration of light-or-greater CAT increased by 17% from 466.5 hr in 1979 to 546.8 hr in 2020, with even larger relative changes for moderate-or greater CAT (increasing by 37% from 70.0 to 96.1 hr) and severe-or-greater CAT (increasing by 55% from 17.7 to 27.4 hr). Similar increases are also found over the continental USA. Our study represents the best evidence yet that CAT has increased over the past four decades.

Key Points


  • Clear-air turbulence (CAT) diagnosed from reanalysis data has increased over the satellite era
  • The increases are largest over the USA and North Atlantic, which are both busy flight regions
  • Severe-or-greater CAT increased the most, becoming 55% more frequent in 2020 than 1979


Plain Language Summary

Turbulence is unpleasant to fly through in an aircraft. Strong turbulence can even injure air passengers and flight attendants. An invisible form called clear-air turbulence (CAT) is predicted to become more frequent because of climate change. Here we analyze modern atmospheric data based on four decades of observations (1979–2020) to investigate whether CAT has already started to increase. We use 21 different turbulence calculations to ensure our results are as reliable as possible. We find clear evidence of large CAT increases in various places around the world at aircraft cruising altitudes since satellites began observing the atmosphere. For example, at a typical point over the North Atlantic, the upward trend is such that the strongest category of CAT was 55% more frequent in 2020 than 1979. Our study represents the best evidence yet that CAT has increased over the past four decades, consistent with the expected effects of climate change.

1 Introduction

Turbulence is estimated to cost the aviation industry around US$200 million annually in the USA alone (Eichenbaum, 2003). These costs arise partly from additional airframe fatigue, requiring maintenance and subsequent loss of productivity, as well as occasional airframe damage. Additionally, passengers and crew suffer injuries, some requiring costly hospital treatment. Some aircraft turbulence occurs in well-defined locations, such as over mountain ranges or within the vicinity of convective storms, and is largely avoidable. However, clear-air turbulence (CAT) is difficult to observe in advance of an aircraft’s track using remote sensing methods. Furthermore, it is still challenging for aviation meteorologists to forecast CAT, partly because current Numerical Weather Prediction (NWP) models have grid sizes that are many times larger than the turbulent eddies that affect aircraft. Hence, operational forecasters use empirical turbulence diagnostics (e.g., Brown, 1973; Dutton, 1980; Ellrod & Knapp, 1992; Knox, 1997; Knox et al., 2008; McCann et al., 2012) calculated from the temperature and wind fields of NWP output. In recent years, these diagnostics have been combined into multi-diagnostic forecasts (Sharman et al., 2006).

Williams and Joshi (2013) applied 21 CAT diagnostics to the 200 hPa pressure surface (corresponding to a flight level of approximately 39,000 ft) of a climate model using a doubled-CO2 scenario. They found that the future occurrence of moderate-or-greater (MOG) CAT increased substantially during winter in the North Atlantic. MOG CAT is defined by the Federal Aviation Administration (FAA, 2006) as the point at which unsecured objects begin to move, and at which people find it difficult to move around inside the cabin. Williams (2017) expanded the analysis to examine turbulence increases at different severity levels (light, moderate, and severe) and found an increase in the frequency of diagnosed CAT for nearly all threshold–diagnostic pairs. Storer et al. (2017) analyzed a CMIP5 simulation using the RCP8.5 scenario in 2050–2080 and compared it with a pre-industrial control simulation, covering the whole globe and each season at different flight levels. Within the jet stream regions of both hemispheres, the RCP8.5 scenario relative to the pre-industrial control showed around a 300% increase in CAT.

S. H. Lee et al. (2019) examined three reanalysis datasets over 1979–2017 and found evidence of a 15% increase in vertical wind shear strength at 250 hPa over the North Atlantic (30–70°N, 10–80°W). As is well known, when the ratio of thermal stability to vertical wind shear (Richardson number Ri) is less than some critical value, typically 0.25, Kelvin–Helmholtz instabilities can form and ultimately result in CAT. Therefore, stronger vertical wind shear is expected to increase the amount of turbulence. However, studies examining whether the amount of CAT measured by aircraft has increased due to recent warming trends are limited by the availability of suitable data. The record of automated, quantitative eddy dissipation rate (EDR) turbulence measurements is too short. Pilot reports (PIREPs) have a longer record, but are not quantitative, and the geographical distribution of CAT based on PIREPs is limited in spatial and temporal extent (Wolff & Sharman, 2008). Furthermore, long-term improvements in operational CAT forecast skill should be acting to reduce the probability of encountering turbulence, even if the amount of turbulence in the atmosphere is increasing.

Outside aviation, CAT is also of interest as a mechanism allowing the mixing of air between the troposphere and stratosphere. Jaeger and Sprenger (2007; hereafter JS07) used ERA40 reanalysis data (Uppala et al., 2005) to compute a winter and summer northern hemisphere CAT climatology (1958–2001) near the dynamic tropopause, using four CAT diagnostics: Richardson number, negative squared Brunt–Väisäla frequency, negative potential vorticity (Gidel & Shapiro, 1979), and Ellrod’s turbulence index (Ellrod & Knapp, 1992). An increase in all four of these indices over the north Atlantic, European, and US regions was found. However, for aviation purposes, aircraft fly along constant flight levels as opposed to the dynamic tropopause. Furthermore, the first 22 years of JS07’s climatology uses reanalysis data from before the start of meteorological satellite era in 1979, leading to data quality concerns. Marlton et al. (2021) and Simmons et al. (2020) showed that the commissioning and decommissioning of meteorological satellites can introduce biases. J. H. Lee et al. (2023) have recently updated JS07’s work (see Section 4 for a discussion).

Renalysis packages now contain four decades of data entirely in the satellite era, during which the world has continued to warm. Therefore, the present study aims to analyze CAT trends during 1979–2020 in the ERA5 reanalysis data set, which has more advanced model physics and higher vertical and horizontal resolution than ERA40. The 21 diagnostics used in Williams and Joshi (2013) and Williams (2017) will be computed, as opposed to the four in JS07, to yield an improved quantification of inter-diagnostic uncertainties. These 21 diagnostics have previously been validated using aircraft measurements of CAT and have generally been found to be skillful (e.g., Sharman et al., 2006). To make the results more applicable to global aviation, the diagnostics will be calculated on the 197 hPa pressure level, corresponding approximately to a flight level of 39,000 ft (FL390), rather than the tropopause, and for the entire globe as opposed to just the northern hemisphere considered by JS07 and J. H. Lee et al. (2023).


Citing Literature

Number of times cited according to CrossRef: 3

  • Why (Still) Studying Turbulence in Fluids and Plasmas?, Perspectives of Earth and Space Scientists, 10.1029/2023CN000215, 4, 1, (2023).
  • Effects of subgrid‐scale turbulence parametrization on the representation of clear‐air turbulence using kilometre‐ to hectometre‐scale numerical simulations, Quarterly Journal of the Royal Meteorological Society, 10.1002/qj.4557, 149, 757, (3301-3322), (2023).
  • Urban boundary‐layer flows in complex terrain: Dynamic interactions during a hot and dry summer season in Phoenix, Arizona, Quarterly Journal of the Royal Meteorological Society, 10.1002/qj.4752.
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