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- Updated: August 29, 2024
We look at one research paper that investigates the long term trends in reported clear air turbulence events
In recent years there has been a noticeable uptick in publicized incidents of turbulence affecting commercial aircraft. These events, sometimes unexpected and severe, have led to passenger injuries, emergency landings, and a recent rare fatality.
Turbulence can be caused by a variety of factors, from the mixing of air masses at weather fronts, to disturbances caused by terrain and mountains. One person died and dozens were injured after Singapore Airlines flight SQ321 experienced severe turbulence over Myanmar in May 2024. This was originally believed to have been a clear air turbulence event, but has since been shown to have been caused by fast developing storms.
Clear Air Turbulence (CAT) is an atmospheric phenomenon that typically occurs at high altitudes where clear skies and more serene conditions prevail. Unlike the turbulence associated with thunderstorms and other visible weather systems, CAT is not usually visually identifiable.
Amongst heightened awareness of CAT as a phenomenon, there has been a growing conversation around whether CAT events are becoming more common, and if climate change is a driver of these events. The apparent increase in incidents aligns with scientific observations and projections that climate change is intensifying atmospheric conditions conducive to CAT, making it a pressing issue for aviation safety. Let’s take a look.
What causes Clear Air Turbulence?
The primary cause of CAT is wind shear, which is a difference in wind speed or direction over a short distance in the atmosphere. When fast-moving air currents, such as those found in jet streams, come into contact with slower moving air, the resulting friction can create turbulence. This is particularly common around the tropopause, the boundary layer between the troposphere and the stratosphere, at altitudes between 7,000 and 12,000 meters (23,000 to 39,000 feet).
Several factors contribute to the formation of CAT.
- Jet Streams: Jet streams are narrow bands of strong wind in the upper levels of the atmosphere. These winds can reach speeds of over 160 kilometers per hour (100 miles per hour). The edges of these streams often experience significant wind shear, which can lead to turbulence.
- Temperature Gradients: Sharp changes in temperature can cause changes in air density, which in turn can lead to turbulence. For instance, the temperature difference between the warm air in the jet stream and the colder surrounding air can contribute to instability.
- Mountain Waves: When strong winds blow over mountain ranges, they can create waves in the atmosphere that propagate upwards. These waves can become turbulent, especially if the air is stable and the wind is strong and steady.
- Atmospheric Circulation: Large-scale atmospheric circulations, such as the Rossby waves and the Coriolis effect, also play a role in the formation of CAT by causing the jet stream to meander, creating regions of intense wind shear.
Are Clear Air Turbulence events happening more often?
In a word, yes. Recent studies have shown a significant increase in the frequency and intensity of CAT conditions over the past few decades. This increase is linked to several factors, most notably climate change.
In the study ‘Evidence for Large Increases in Clear-Air Turbulence Over The Past Four Decades’ by Prosser et al. (2023), CAT trends were analyzed globally from 1979 to 2020 using reanalysis data. The research found substantial increases in the probability of a flight experiencing CAT at aircraft cruising altitudes, especially around the midlatitudes. For instance, over the North Atlantic, the total annual duration of light-or-greater CAT increased by 17%, moderate-or-greater CAT by 37%, and severe-or-greater CAT by 55%.
It’s likely no coincidence that the most intensive locations CAT was observed correlate with global Jet streams. Jet streams are fast flowing, narrow air currents found in the atmosphere near the altitude of the tropopause and play a critical role in determining the weather and climate patterns.
The Prosser report outlines one of the primary reasons for the increase in CAT events as the intensification of the jet streams, driven by the warming of the planet. As global temperatures rise, the temperature gradients between the equator and the poles become more pronounced, strengthening the jet streams and increasing the likelihood of turbulence.
Additionally, Williams and Joshi (2013) projected that climate change could lead to a substantial increase in the occurrence of moderate-or-greater CAT in the future. Their model indicated a potential increase of up to 83.9% for moderate-or-greater CAT in a scenario where atmospheric CO2 levels double from pre-industrial levels.
Implications
The increase in CAT poses several challenges for the aviation industry. CAT is estimated to cost the aviation industry around $200 million annually in the USA alone due to additional airframe fatigue, maintenance, and the potential for passenger and crew injuries (Prosser et al). Since CAT is difficult to predict and often occurs without warning, it remains a significant safety concern.
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Chris Lomas
Chris is a Commercial Pilot and lover of all things aviation. Based in London, Chris researches and writes articles on what’s happening in the world of aviation for Flightradar24.
Chris Lomas
Chris is a Commercial Pilot and lover of all things aviation. Based in London, Chris researches and writes articles on what’s happening in the world of aviation for Flightradar24.
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