Clear-air turbulence: why bumps come out of a blue sky
Clear-air turbulence happens in cloudless skies, usually near the jet stream. Learn what causes it, why it can't be seen, how pilots forecast and avoid it, and what to do when it surprises you.
The sky is blue, the seatbelt sign is off, and suddenly the plane shakes. That's clear-air turbulence (CAT): bumps with no cloud or storm to warn you. It's the turbulence that surprises people most, but it's made of exactly the same thing as any other kind: moving air.
What causes clear-air turbulence
At cruising altitude, wind speed and direction can change sharply over a few thousand feet, especially near the jet stream, a fast river of wind that can exceed 150 knots. Where fast air slides over slower air, the boundary becomes unstable and breaks into invisible eddies, much like waves curling where a fast current meets still water. Meteorologists call this wind shear.
Because there's no moisture to form clouds, nothing shows the eddies to the eye, or to the weather radar, which detects water droplets.
Where and when it's most common
- Near jet streams, at the altitudes where most airliners cruise.
- Over oceans on long-haul routes that cross the jet stream, such as the North Atlantic and the North Pacific.
- In winter, when temperature differences and jet streams are strongest.
- Downstream of mountains on windy days.
How pilots forecast and avoid it
Even though it's invisible, clear-air turbulence isn't random. Forecasters calculate where the wind shear and instability ingredients come together, and pilots receive turbulence forecasts for their route before departure. In flight, aircraft ahead report bumpy altitudes (these are pilot turbulence reports, or PIREPs), and many airlines now share automatic turbulence data between aircraft. When a report comes in, pilots ask air traffic control for a smoother level.
You can see a passenger version of this for your own route: our free turbulence forecast uses wind shear and stability in the upper atmosphere to show where bumps are likely along your flight.
Is clear-air turbulence dangerous?
For the aircraft, no: it is the same moving air the plane is certified to handle, with a large safety margin. The risk is to people who aren't buckled in when it arrives without warning. That's why crews recommend keeping your seatbelt fastened whenever you're seated, even when the sign is off. Most serious turbulence injuries happen to passengers and crew who are standing or unbuckled.
What to do when it surprises you
- If you're seated, check your belt. If you're standing, sit down (or hold on to a seat) and follow crew instructions.
- Remind yourself: this is moving air, the same as any bump, just without a cloud.
- Breathe in for 4 seconds, out for 6. Drop your shoulders and soften your jaw.
- Watch the crew. If they keep going calmly, so can you.
Frequently asked questions
Can radar detect clear-air turbulence?
Standard weather radar detects water droplets, so it doesn't see clear-air turbulence. Pilots rely on forecasts and reports from other aircraft instead, and research into laser-based detection continues.
Is clear-air turbulence increasing?
Studies suggest moderate and severe clear-air turbulence over some busy routes, like the North Atlantic, has increased in recent decades, linked to stronger wind shear in the jet stream. Forecasting and data sharing between aircraft are improving at the same time.
How long does clear-air turbulence last?
Usually a few seconds to several minutes. Pilots typically climb or descend to escape a persistent patch.
