Turbulence explained: causes, clear air turbulence and why aircraft can take it

The coffee sloshes, the wingtip bobs, and somewhere three rows ahead someone audibly draws a breath. If you fly yourself as a spotter – to the next airport trip, to an airshow or simply on vacation – you know these moments. Turbulence is part of flying like crosswinds are part of landing, and yet for many passengers it is the scariest part of the journey. This guide explains where rough air comes from, why the invisible clear air turbulence in particular is so tricky, what loads an airliner must withstand under certification and what the accident statistics say about the actual risk.

Turbulence: the short answer

Turbulence is unevenly moving air. The aircraft flies through areas with updrafts and downdrafts, sudden changes in wind direction and speed or air eddies. Each of these changes briefly shifts the angle of attack at the wing, the lift fluctuates, and the aircraft is lifted, lowered or pushed sideways a little. The so-called “air pocket” does not exist physically: The air keeps carrying the aircraft, it just doesn’t move evenly.

For the safety of the aircraft itself, turbulence is very rarely a problem in airline operations. For people in the cabin it can be – and almost exclusively when they are not buckled up.

The four sources of rough air

Meteorologists and pilots classify turbulence by its cause. That is more than theory: The cause determines whether you can see, forecast or avoid the turbulence.

Convective turbulence: thunderstorms and cumulus clouds

When the sun warms the ground, warm air rises in bubbles, cools and forms cumulus clouds. Inside and around such clouds, updrafts and downdrafts alternate in a small space – in the extreme case in thunderstorm clouds with enormous force. This turbulence is usually visible and can be detected on the onboard weather radar, because thunderstorm cells contain precipitation. Crews fly around such cells at a wide distance. In the analysis by the US accident investigation agency NTSB, convective turbulence was involved in 57.7 percent of turbulence-related accidents in US airline operations between 2009 and 2018 – so it is the most common cause, not the notorious clear air turbulence.

Clear air turbulence: invisible at the jet stream

Clear air turbulence (CAT) forms in cloud-free air, typically at the altitudes at which airliners cruise. The most important trigger is wind shear: Where the jet stream – a narrow band of strong wind at high altitude – borders slower air, layers of very different speed flow past each other. At this boundary, eddies roll up, similar to waves on a sea over which strong wind blows. Other triggers are strong temperature contrasts, pronounced upper-level troughs and mountain waves that reach to great heights.

The tricky part: CAT can be detected neither with the naked eye nor with the onboard weather radar, because the radar “sees” raindrops and hail, not air movement. At the NTSB, clear air turbulence was involved in 28.8 percent of the investigated accidents. Why aircraft fly at these altitudes in the first place is explained in the article Why aircraft fly at around 10,000 meters.

Mountain waves and mechanical turbulence

When strong wind blows over a mountain range, the air on the lee side is set oscillating. These lee waves can continue far downstream and to great heights and produce violent turbulence there. Known rough routes include crossings of the Andes and the Himalayas. Close to the ground, buildings, forests and hills cause mechanical turbulence in strong wind – exactly the gusty, wobbly approaches you see at the fence on stormy days. How crews handle this is described in the guides on crosswind landings and on the go-around.

Wake turbulence from other aircraft

Every aircraft creates two counter-rotating air vortices at its wingtips that sink behind it. The heavier and slower the aircraft, the stronger this wake turbulence. A following aircraft that flies into such a vortex can be thrown into a strong roll. That is why there are minimum distances between departing and landing aircraft, graded by weight class – details in the article Wake turbulence and separation. As a spotter, you can sometimes even see the vortices: On humid days they show up as condensation trails, as described in the guide Photographing vortices and condensation.

Type of turbulence	Typical location	Visible?	How crews handle it
Convective	in and around cumulus and thunderstorm clouds	yes, thunderstorms also on weather radar	fly around cells at a wide distance
Clear air turbulence	cruise altitude, edge of the jet stream	no	forecast charts, reports from other crews, altitude changes
Mountain waves	lee of mountains, partly to great height	only indirectly, e.g. via wave clouds	route and altitude choice, turbulence speed
Wake turbulence	behind other aircraft, esp. on takeoff and landing	only with condensation	separation distances by weight class

How strong is “strong”? The intensity levels

In pilot reports, turbulence is divided into levels: light, moderate, severe and extreme. Light turbulence you feel as brief bumps, and drinks stay in the cup. With moderate turbulence, the movements become more noticeable, walking in the aisle gets difficult and loose objects can slide. Severe turbulence means abrupt, large changes in altitude and attitude – passengers are pressed hard into their belts, unsecured objects fly through the cabin. Extreme turbulence is very rare in airline operations.

Important for putting it in perspective: What feels in the cabin like a fall of dozens of meters is usually an altitude loss of a few meters. The brain clearly overestimates vertical movements, especially when you don’t look out the window.

Why aircraft can take it

Airliners are certified for loads far above what occurs in normal operations. The US design regulation for large airliners (14 CFR Part 25; the European CS-25 is structured very similarly) requires a positive limit load factor for maneuvers of at least 2.5: The structure must carry 2.5 times the aircraft’s weight without permanent deformation. In the negative direction, it is at least minus 1.0 up to cruise speed. On top of that come separate load cases for gusts, in other words precisely for the loading from turbulence.

A safety factor of 1.5 is added on top of these limit loads. The ultimate load calculated from it must also be carried by the structure without failing. Manufacturers like Boeing and Airbus demonstrate this, among other things, in large ground tests in which the wings of a test airframe are bent upward with hydraulic cylinders until the required load is reached. The pictures of the sharply flexed wings of modern carbon-fiber jets like the Boeing 787-9 or the Airbus A350-900 impressively show how much reserve is built in.

That the wings visibly flap in flight is therefore not a warning sign but intended: A flexible wing absorbs gusts instead of passing them on hard to the fuselage. A stiff wing would not be safer but would distribute the loads worse.

Add to this the behavior of the crew: In turbulence, pilots fly a turbulence speed set by the manufacturer. It is chosen so that enough margin to the stall remains while the gust loads don’t become unnecessarily large. The autopilot usually stays engaged and compensates the attitude without chasing every altitude fluctuation hard.

What the statistics really show

The most thorough public analysis comes from the NTSB. In its 2021 safety report, the agency analyzed all turbulence-related accidents in US airline operations from 2009 to 2018 – 111 cases. An “accident” counts as any turbulence encounter in which someone is seriously injured or the aircraft is substantially damaged. The most important results:

• Turbulence is a cabin problem, not a structural problem. In none of the 111 cases was the aircraft substantially damaged. In five cases there was minor damage – to seats, ceiling panels, overhead bins and service units.
• Turbulence is relevant nonetheless. It was involved in 37.6 percent of all accidents in US airline operations in this period. This is also because other types of accidents have become significantly rarer over the years.
• Flight attendants carry the greatest risk. Of 123 seriously injured, 97 were flight attendants (78.9 percent) and 26 were passengers. Cockpit crews were not seriously injured – they are usually buckled in.
• The tail is the most critical area. Almost nine out of ten injuries to flight attendants with a known location happened in the rear part of the cabin, three quarters of them at the rear galley.
• The descent is underestimated. Around half of the accidents occurred in descent or approach, and around 60 percent of these below 20,000 feet – that is, when service and tidying are still going on in the cabin.

Fatal turbulence accidents are very rare. In US airline operations, the last case named by the NTSB at the time of the report was in 1997. Internationally known became the flight Singapore Airlines 321 on May 21, 2024: A Boeing 777-300ER of Singapore Airlines on its way from London to Singapore hit violent turbulence at cruise altitude over Myanmar and made an unscheduled landing in Bangkok. One passenger died and numerous people were injured. According to the preliminary investigation report from the authorities in Singapore, the vertical acceleration switched within fractions of a second from clearly positive to negative values – occupants who were not buckled up were lifted from their seats and then fell back. The aircraft itself remained airworthy. The case shows both at once: how robust the structure is and how important the belt is.

How crews avoid turbulence

Turbulence cannot be ruled out entirely, but a large part of it is flown around before the passengers notice anything. The NTSB found that in a good half of the investigated accidents, the crews knew of a turbulence risk in advance – an indication of how important information and communication with the cabin are.

How air traffic control works in German airspace is explained in the article on DFS, Germany’s air navigation service.

Is turbulence becoming more frequent?

Researchers at the UK’s University of Reading have been studying for years how climate change affects clear air turbulence. Their model calculations show that wind shear in the area of the North Atlantic jet stream strengthens as the atmosphere warms. A 2017 paper concluded that severe clear air turbulence on the winter transatlantic routes could increase by around 150 percent with a doubling of CO₂. A 2023 study already found a measurable increase over the North Atlantic and North America in weather data of recent decades.

That does not mean flying is becoming less safe: The structural reserves stay the same. It does mean that bumpy stretches on some routes could become more frequent and that the importance of forecasting, pilot reports and buckled-up passengers keeps rising. Especially on routes like Frankfurt–New York, where Lufthansa and other airlines operate daily, the jet stream is a constant companion anyway – it already influences flight times and routes across the Atlantic today.

What you can do as a passenger

You can’t do anything about the air itself – but a lot about the consequences. The most effective measure is also the simplest.

• Keep your belt fastened. As long as you are seated, the belt stays loosely fastened, even when the sign is off. This is by far the biggest lever.
• Take the seat belt sign seriously. When it lights up, go straight back to your seat. The trip to the restroom can wait.
• Stow luggage properly. Heavy items belong under the seat in front or well closed in the bin. A loose camera, tablet or telephoto lens on the tray table becomes a projectile in a jolt.
• Mind hot drinks. When rough air is announced, better skip them briefly or have the cup filled only halfway.
• Secure small children. Hold babies firmly on your lap and use a supplemental belt if the airline provides one; a seat of their own with an approved child seat offers the best protection.
• Choose your seat carefully. Over the wings, the movements are usually smaller than in the tail.

If turbulence stresses you a lot, it helps to understand the mechanisms – and to work on the fear in a targeted way. You find tips in the guide Understanding and overcoming fear of flying. Cabin pressure and pressure on your ears is also among the topics that occupy many fellow travelers.

Turbulence from a spotter’s perspective

On the ground, you meet turbulence mainly on approach. On gusty days you see at the fence how aircraft rock shortly before touchdown, correct with ailerons or go around. These are often the most exciting pictures of the day. How to plan such days is covered in the guides on Filming crosswind landings and storms and Plane spotting in bad weather. If you want to know in advance how the wind is at the airport, Reading METAR and TAF and the overview of weather apps for spotters help.

Conclusion

Turbulence is rough air with four main sources: convection, wind shear at the jet stream, mountain waves and wake turbulence. Most of these are visible or predictable and are flown around; the invisible clear air turbulence remains the exception that can’t always be avoided. For the aircraft structure, this is no serious risk – certification requires large reserves, and the statistics show no serious structural damage from turbulence in US airline operations in the years studied. The risk lies in the cabin, and it can be almost completely eliminated with one simple action.