What Is Clear-Air Turbulence?
Sources: NOAA, FAA, IATA, ICAO · Updated May 2026
Clear-air turbulence (CAT) is turbulence that occurs in cloud-free air, often near jet streams or over mountains. It happens when masses of air moving at very different speeds rub against each other and generate eddies — invisible because there is no cloud, dust, or precipitation to make the airflow visible. Onboard weather radar detects rain droplets and ice particles, so it cannot detect CAT. Pilots rely on forecast charts, pilot reports (PIREPs) from other aircraft, IATA's Turbulence Aware data-sharing platform, and the seatbelt sign to manage the risk. CAT is most common in the upper troposphere (FL300–FL400) along jet stream boundaries, particularly in winter and over mountain ranges. The Singapore Airlines SQ321 incident in May 2024 raised public attention to turbulence safety; the final investigation (TSIB, May 2026) attributed that event to convective turbulence near deep cumulonimbus rather than classical CAT.
How clear-air turbulence forms
CAT occurs in three main settings:
Jet streams are narrow ribbons of fast wind at the tropopause. The contrast between the fast core and the slower surrounding air creates vertical and horizontal wind shear. When the Richardson number drops below a critical value, the airflow becomes unstable and develops Kelvin-Helmholtz waves that break into turbulent eddies.
Strong winds blowing over major mountain ranges (Rockies, Alps, Andes, Himalayas) set up standing waves in the atmosphere downwind. The waves can break into severe turbulence at altitudes up to FL400+, far from the visible mountain.
Strong updrafts and outflow from distant convection can throw off ripples that propagate tens of kilometres into cloud-free air. The aircraft sees clear sky on radar but rough air. The SQ321 investigation classified its event as this category — outflow from deep convective tops 55,000 ft tall.
Why radar cannot see CAT
Airborne weather radar transmits microwave pulses and listens for echoes from precipitation. Liquid water droplets and ice particles reflect well; dry, cloud-free air does not. Even strong turbulence in clear sky returns no echo — so the cockpit display shows nothing. This is the defining feature of CAT: the aircraft has no direct onboard sensor for it.
How pilots manage the risk
- →Pre-flight turbulence forecasts: Significant Weather (SIGWX) charts and high-altitude turbulence forecasts from NOAA/NCEP, Met Office, EUMETNET, and the WAFC (World Area Forecast Centres) show forecast areas of moderate or severe turbulence.
- →PIREPs: Pilot reports of actual turbulence encountered are relayed by ATC to other aircraft on similar routes.
- →IATA Turbulence Aware: A data-sharing platform launched in 2018 that aggregates real-time turbulence reports automatically from thousands of aircraft. Pilots and flight planners use it to choose routes and altitudes.
- →EDR (Eddy Dissipation Rate): An objective, quantitative measure of turbulence severity now standardized by ICAO. Aircraft equipped with EDR sensors automatically report turbulence by intensity.
- →Altitude or routing changes: If forecast or reported turbulence is severe, pilots can request a different altitude or a deviation around it.
- →Seatbelt sign: The most effective single defence against turbulence injuries is keeping the seatbelt fastened whenever seated. Most CAT injuries occur to people not wearing belts.
The SQ321 case (May 2024)
Singapore Airlines flight SQ321 (Boeing 777-300ER London-Singapore) encountered severe turbulence on 21 May 2024 over Myanmar at approximately FL370. One passenger died; 41 were injured. The aircraft diverted to Bangkok. The final Singapore Transport Safety Investigation Bureau (TSIB) report, published 19 May 2026, concluded the turbulence was convectively induced by deep cumulonimbus clouds with tops to 55,000 ft and cloud-top temperatures of approximately -80 °C. The report noted that onboard weather radar may have missed the storms; three radar display malfunctions were recorded on the same aircraft in the weeks before the incident. The case did not match the textbook definition of pure CAT — but it sits in the broader family of "non-cloud turbulence near convection" and reinforced the universal lesson: keep the seatbelt fastened.
Is CAT becoming more common?
Several peer-reviewed studies (notably Williams et al., Reading University) have reported that vertical wind shear at jet stream altitudes increased over the 1979-2020 period, with North Atlantic severe CAT roughly doubling. The mechanism: a warming upper troposphere strengthens the meridional temperature gradient that drives the jet stream. Operational impact so far has been small thanks to better forecasting and reporting, but the trend is a recognized aviation-meteorology concern.
Is turbulence dangerous to the aircraft?
Airliners are designed to far higher load limits than turbulence can impose. The structural risk to the airframe is essentially negligible. The actual risk is to unbelted occupants who can be thrown around the cabin. Modern fly-by-wire aircraft also have automatic load alleviation systems that detect gusts and adjust control surfaces to reduce the load on the airframe.
Sources
- NOAA Aviation Weather Center — Turbulence forecasts and SIGMETs
- FAA Aeronautical Information Manual (AIM), Chapter 7 — Meteorology
- IATA Turbulence Aware Platform — data sharing
- ICAO Annex 3 — Meteorological Service for International Air Navigation
- TSIB (Singapore) — Final Report on SQ321, 19 May 2026
- Williams P. D. (2017), "Increased light, moderate, and severe clear-air turbulence in response to climate change," Advances in Atmospheric Sciences