Clear Air Turbulence (CAT): Why Planes Shake in Clear Skies

Short answer: clear air turbulence happens when layers of air moving at different speeds or in different directions slide past each other, most often near the jet stream at cruise altitude. There is no cloud or moisture to see it by, so it doesn't show on weather radar, but forecasters can predict where it is likely from the wind and temperature patterns, and pilots share reports of it in real time.

Clear air turbulence is the bump that seems to come from nowhere on a sunny day. Here is what causes it, where it happens, and how it is forecast today.

What is Clear Air Turbulence?

CAT occurs where wind speed or direction changes sharply over a short distance (wind shear). The faster layer drags on the slower one and the boundary breaks into eddies, a little like the swirls where a fast river meets still water. Unlike convective turbulence, which happens in and around storm clouds, CAT happens in clear skies.

Where CAT Occurs

Jet Streams

The most common source:

• Polar jet stream: around 30,000 to 40,000 feet, typically between 30° and 60° latitude
• Subtropical jet stream: usually around 25° to 35° latitude
• Both are strongest in winter

The Edges of Jet Streams

The bumpiest air is usually not in the core of the jet but above, below or beside it, where wind speed changes fastest. The tropopause, the boundary between the lower atmosphere and the stratosphere, is another common spot.

Mountain Waves

Strong wind flowing over a mountain range sets up waves in the air downwind. They can:

• Reach cruise altitude (30,000 feet and above)
• Extend a long way downwind of the mountains
• Break into turbulence in otherwise clear air

Why CAT is Hard to Predict

Invisible to Radar

Weather radar detects water and ice in the air, not air movement. With no cloud, there is nothing for radar to see. (More on this in can pilots see turbulence on radar?)

It Changes Quickly

CAT can form and fade within minutes, vary in strength, and drift with the jet stream.

It is Small

Many turbulent eddies are smaller than a weather model's grid, so models forecast the conditions that produce CAT rather than each individual bump.

How CAT is Detected and Forecast

Pilot and Aircraft Reports

Pilot reports (PIREPs) remain a key source, and many airliners now also report turbulence automatically as an EDR (eddy dissipation rate) measurement, shared with other airlines through programmes such as IATA's Turbulence Aware.

Turbulence Forecast Models

• NOAA GTG 4.0 (US airspace): hourly, on a 3 km grid, calibrated against pilot reports; the forecast US dispatchers use. On 30 July 2026 NOAA's Aviation Weather Center debuted the GTG Nowcast, which refreshes every 15 minutes from live aircraft measurements.
• WAFS (worldwide): the ICAO-mandated World Area Forecast System, which provides global turbulence guidance for flight planning.

Physics-Based Indices

Where a dedicated model isn't available, forecasters use indicators calculated from wind and temperature data:

• Richardson number: how stable the air is against overturning
• Ellrod index: wind shear combined with deformation of the flow
• Multi-level wind shear: comparing winds at neighbouring flight levels

This is how Turbcast forecasts flights outside the US; for US flights it uses GTG 4.0 directly. See how turbulence forecasting works.

Is CAT Getting Worse?

Research suggests it is, modestly. A University of Reading study published in 2023 found that the strongest category of clear air turbulence over the North Atlantic increased by about 55% between 1979 and 2020, as sharper temperature contrasts strengthen wind shear at cruise altitude. More in climate change and turbulence.

How Pilots Handle CAT

1. Flight planning: reviewing turbulence forecasts and choosing altitudes before departure
2. Reports in flight: listening for pilot reports and automated EDR data from aircraft ahead
3. Altitude changes: CAT often sits in a thin layer, so a climb or descent of a few thousand feet usually finds smoother air
4. Speed adjustment: slowing to turbulence penetration speed
5. Routing: steering around areas where CAT is forecast or reported

CAT and Flight Safety

• Airliners are certified to withstand at least 1.5 times the largest loads they are expected to meet in service, far more than CAT produces
• The one airliner ever lost to turbulence alone, BOAC Flight 911 in 1966, met an extreme mountain wave near Mount Fuji; none has been lost to turbulence since (the full history)
• Most turbulence injuries happen to people who are standing or unbelted, so keeping your seatbelt fastened while seated is what actually helps

Conclusion

Clear air turbulence can't be seen, but it can be forecast: today's models predict where it is likely, and aircraft share what they meet in real time. Keep your seatbelt fastened whenever you're seated, and a surprise bump stays just that: a bump.