What causes turbulence? The five main sources, explained

Key facts

• Turbulence arises from eddies in the atmosphere and is usually grouped by source: convection, wind shear in clear air, mountain waves, surface features and aircraft wakes [1].
• Convective turbulence was present in 57.7% of US airline turbulence accidents from 2009 to 2018, and clear-air turbulence in 28.8% [1].
• Jet streams are narrow bands of strong wind 4 to 8 miles up that can exceed 275 mph [2].
• When wind shear grows strong enough compared with the air's stability (a Richardson number below about 0.25), the air can break into waves and turbulence [3].
• Mountain waves form when wind of more than 15 knots blows across a ridge; their up and down currents can reach 2,000 feet per minute [4].
• Wake vortices behind an aircraft typically last 1 to 3 minutes and sink below its flight path [5].
• At a typical North Atlantic point, severe clear-air turbulence was 55% more frequent in 2020 than in 1979 [3].

What is turbulence?

Turbulence is irregular, swirling motion in the air that bumps an airplane up and down or side to side. Air behaves a lot like water: it flows in currents, forms eddies around obstacles, and makes waves where layers of different speed meet. An airliner flying through that air is like a boat crossing a lake on a breezy day.

The NTSB's turbulence study sorts it into five sources [1]. Here they are, from the most talked about to the most local.

1. Clear-air turbulence and the jet stream

Clear-air turbulence (CAT) is turbulence at medium and high altitudes caused by strong wind shear, with no clouds to mark it [6]. It's what most people mean when they say "it got bumpy out of nowhere."

The main culprit is the jet stream. Jet streams are relatively narrow bands of strong wind in the upper atmosphere, usually near 30,000 feet, varying from 4 to 8 miles up, and they can reach more than 275 mph [2]. They form where warm and cold air meet, and they're strongest in winter, when that temperature contrast is greatest [2]. WMO guidance notes that turbulence may be found close to the edge of the jet stream at the heights airliners cruise [6].

Why does fast wind cause bumps? Because the speed changes sharply with height. Where one layer of air slides quickly over another, the boundary can roll up into waves, the way wind makes waves on water. Meteorologists measure this tipping point with the Richardson number: when shear overwhelms the air's stability (a value below about 0.25), these Kelvin–Helmholtz waves can form and break into turbulence [3].

CAT is hard to spot in advance. It doesn't show up on the weather radar airliners carry, and it's "difficult to observe in advance of an aircraft's track" using remote sensing [3]. Weather models also can't see the small eddies directly, so forecasters compute turbulence indexes from the model's winds instead [3]. That's how turbulence forecasts work.

2. Storms and convection

Convection is the vertical movement of heat and moisture in the atmosphere, especially through updrafts and downdrafts [1]. Thunderstorms are the best-known form, but not the only one [1]. Puffy cumulus clouds on a warm day are smaller signs of the same rising air. WMO guidance puts it simply: turbulence is always associated with convection [6].

Convective turbulence matters most for injuries. In the NTSB's study of US airline turbulence accidents from 2009 to 2018, it was present in 57.7% of them [1]. Storms are also the most avoidable cause: they show on weather radar, and pilots steer around them. That's one reason the seat belt sign often comes on near clouds, during climb and descent.

3. Mountain waves

Mountain waves are up-and-down oscillations of the air on the downwind side of high ground [4]. Picture water flowing over a rock in a stream: ripples form behind it.

They tend to form when [4]:

• the wind blows within 30 degrees of straight across the ridge, without changing direction much with height;
• the wind at the ridge top is stronger than 15 knots and increases with height;
• a layer of stable air (an inversion) sits just above the mountains.

Inside the waves, vertical currents can reach 2,000 feet per minute [4]. If the air is moist, smooth, lens-shaped lenticular clouds can form in the wave crests, a visible sign that waves are present [4]. Forecasters issue mountain-wave forecasts and warnings, and US turbulence guidance maps include them [7]. Long routes over the Rockies, the Alps or the Andes are where you're most likely to feel them; see the most turbulent routes.

4. Surface (mechanical) turbulence

Near the ground, wind rubbing over hills, trees and buildings breaks into eddies, much like water tumbling over a rocky streambed. The NTSB calls this turbulence from surface features: friction between the air and irregular terrain or human-made structures [1].

You feel it mostly on windy days during takeoff and the last minutes before landing. It's short-lived, it's why landings can feel a bit wobbly in wind, and pilots train for it constantly.

5. Wake turbulence

Every wing that makes lift leaves two spinning vortices behind its wingtips [5]. The heavier the aircraft, the stronger the vortices, and large wide-body jets make the strongest ones [5]. Vortices tend to sink after they form and drift with the wind, and typically last between one and three minutes [5].

Air traffic controllers keep aircraft spaced apart, with more distance behind bigger aircraft, so the wakes have time to sink and fade [5]. On the rare occasions an airliner crosses an old wake at cruise, passengers usually feel a short, sharp jolt or roll.

Is climate change making turbulence worse?

For clear-air turbulence, the evidence says yes. A 2023 study in Geophysical Research Letters analyzed 1979 to 2020 with 21 turbulence calculations [3]. At a typical point over the North Atlantic, the yearly time with severe-or-greater CAT rose 55%, from 17.7 to 27.4 hours; moderate-or-greater rose 37%, and light-or-greater 17% [3]. The increases were largest over the USA and the North Atlantic [3].

The same study points out that better forecasting should be reducing the chance of actually meeting turbulence, even as there's more of it [3]. For how much it matters to your safety, read is turbulence dangerous?

Why some flights are bumpier than others

Putting the causes together explains most bumpy flights:

• Route: flights along a strong jet stream or across big mountain ranges meet more CAT and mountain waves.
• Season: jet streams are strongest in winter [2], while storms are a bigger factor in warm, humid weather.
• Phase of flight: climbs and descents pass through cloud layers and storm areas; about half of US airline turbulence accidents happen during descent or approach [1].
• Time of day: see the best time to fly for less turbulence.

How Sorapuff helps

Sorapuff's route turbulence forecast looks at the causes you can't see. It downloads NOAA's GFS winds along your route, computes a well-known clear-air turbulence index on your phone, and adds storm and mountain-wave warnings (SIGMETs) and recent pilot reports from aviationweather.gov. It has been checked against thousands of real pilot reports, and it's a likelihood, never a promise. One forecast is free; Pro adds unlimited forecasts (pricing).

FAQ

What is clear-air turbulence?

Clear-air turbulence is bumpiness at medium and high altitudes caused by strong wind shear, usually near the jet stream, in air with no clouds to mark it. Because it's invisible and doesn't show on weather radar, it's the type most likely to feel like it came out of nowhere. Forecasters predict it from weather-model winds, and pilots share reports of it.

Why can't pilots see turbulence on radar?

Airliner weather radar detects water droplets and ice, so it shows storms and heavy rain clearly. Clear-air turbulence happens in dry, cloudless air, so there's nothing for the radar to bounce off. That's why pilots rely on forecasts and on reports from other aircraft for it, and turn on the seat belt sign when bumps are expected.

Does the jet stream cause turbulence?

Often, yes. The jet stream itself is fast but can be smooth in its core. Turbulence tends to form at its edges, where wind speed changes sharply with height or across distance. Airlines like riding a jet stream eastbound for the tailwind, and crews change altitude to find smoother air when it gets bumpy.

Why is it bumpier over mountains?

Wind blowing across a mountain range can set the air oscillating up and down downwind of it, like ripples behind a rock in a stream. These mountain waves form best when strong wind blows straight across a ridge and stable air sits above it. Forecasters warn about them, and pilots choose altitudes and routes to keep the ride as smooth as possible.

Can another plane cause turbulence?

Yes. Every wing leaves two spinning vortices behind it, strongest behind heavy aircraft. They sink and fade within a few minutes. Air traffic controllers space aircraft to give wakes time to clear, so passengers rarely feel them, and when they do it's usually a brief jolt.

Sources

1. Preventing Turbulence-Related Injuries in Air Carrier Operations Conducted Under Title 14 CFR Part 121, Safety Research Report SS-21/01 (2021), NTSB
2. The Jet Stream, NOAA JetStream: An Online School for Weather
3. Evidence for Large Increases in Clear-Air Turbulence Over the Past Four Decades, Geophysical Research Letters (2023)
4. Mountain Waves, SKYbrary Aviation Safety
5. Wake Vortex Turbulence, SKYbrary Aviation Safety
6. Aviation Hazards: Turbulence and Wind Shear, World Meteorological Organization
7. Graphical Forecasts for Aviation help: Turbulence, Aviation Weather Center (NOAA)