Clear-air turbulence, explained

The sky is clear, the sun is out, and the aircraft starts to shake. There's no cloud to blame, which is exactly what makes clear-air turbulence feel strange. It isn't mysterious, though.

What causes it

High above the weather, at the heights where airliners cruise, the wind is often organised into jet streams: narrow bands of fast air between roughly 20,000 and 45,000 feet that can blow at more than 240 knots. The speed itself isn't the problem. The FAA puts it plainly: the cause is wind shear, the wind changing speed or direction over a short distance or height. Where layers of air slide past each other at different speeds, the boundary breaks into eddies, much like the swirls at the edge of a fast river.

Clear-air turbulence is most common near jet streams, close to the tropopause (the top of the weather layer) and on the side of the jet nearer the pole.

Why radar can't see it

Weather radar works by bouncing signals off rain and hail. Clear air has nothing to bounce off, and the FAA's manual notes that radar does not detect turbulence itself. That's why pilots lean on two other tools: forecasts built from weather models, and pilot reports from aircraft that crossed the area earlier, which the FAA calls the best source of information on clear-air turbulence.

How big a patch is

According to the FAA, a typical patch of clear-air turbulence is 100 to 300 miles long, 50 to 100 miles wide and about 5,000 feet deep, and it can last from half an hour to a day. Since airliners cover a mile every few seconds, crossing it usually takes minutes, and a change of a few thousand feet in height can be enough to find smooth air.

Clear air versus other turbulence

Type	Where it happens	Can pilots see it?
Clear-air turbulence	High up near the jet stream, in cloudless sky	No; forecasts and pilot reports
Convective	In and around storm clouds	Yes, the storm shows on weather radar; the FAA advises keeping well clear
Mountain waves	Downwind of high ranges in strong wind, sometimes far above them	Not directly; forecasts and reports
Wake turbulence	Behind other aircraft	Managed by air traffic control spacing

In the NTSB's study of US airline turbulence accidents from 2009 to 2018, convective turbulence was present in 57.7% of cases and clear-air turbulence in 28.8%, with mountain waves and wake making up most of the rest.

Where it's common

The UK Met Office names the North Atlantic corridor, East Asia near Japan and mountain regions as hotspots. Over the North Atlantic, the busiest long-haul route in the world, the jet stream is often strongest in winter. Read more in turbulence over the North Atlantic.

Is it becoming more common?

Probably, yes. A 2023 study by Prosser and colleagues in Geophysical Research Letters found that at a typical point over the North Atlantic, the yearly hours of severe clear-air turbulence rose from 17.7 to 27.4 between 1979 and 2020 (about +55%), moderate from 70.0 to 96.1 hours (+37%) and light from 466.5 to 546.8 hours (+17%). The researchers link this to stronger wind shear in the jet stream as the climate warms.

That's a reason to keep your belt fastened while seated, and a reason forecasts like ours matter.

How we forecast it

Our main clear-air turbulence signals are exactly the ingredients above: wind shear and deformation (the Ellrod indices) and air stability (the Richardson number), computed from the ECMWF model at three cruising heights. See how it works, or check where your flight might meet it.