Meaning and Nature
Clear Air Turbulence, commonly known as CAT, is turbulence that occurs in the atmosphere without visible clouds or major storms nearby.
It is especially common at cruising altitudes used by commercial aircraft, often near the jet stream and strong upper-level winds.
CAT is difficult to detect visually because the surrounding air may appear completely clear.
Causes
The main cause is a sudden change in wind speed or direction over a short distance, known as wind shear.
Important conditions associated with CAT include:
- strong jet streams;
- boundaries between fast and slow-moving air masses;
- sharp temperature gradients;
- upper-level troughs;
- mountain waves;
- atmospheric instability near the tropopause.
When adjacent layers of air move at different speeds or in different directions, turbulent eddies may form.
Jet Streams and CAT
Jet streams are narrow bands of very strong winds in the upper atmosphere.
CAT commonly develops:
- along the edges of jet streams;
- below or above the jet core;
- where strong horizontal or vertical wind shear exists.
Aircraft flying through such regions may suddenly experience vertical or lateral movement even when weather radar shows no thunderstorm ahead.
Conventional onboard weather radar primarily detects precipitation and therefore cannot directly identify most clear-air turbulence.
Impact on Aviation
CAT can range from light to severe.
Possible effects include:
- sudden changes in altitude;
- difficulty maintaining a smooth flight path;
- injuries to passengers and crew;
- damage to unsecured objects;
- increased structural stress on aircraft.
Modern commercial aircraft are designed to tolerate substantial turbulence. The greatest practical risk is often injury to people who are not wearing seat belts.
Pilots may respond by:
- changing altitude;
- reducing speed to the recommended turbulence penetration speed;
- adjusting the flight path;
- using reports from other aircraft;
- relying on meteorological forecasts and satellite data.
Climate Change and Future Risk
Research indicates that warming of the atmosphere can increase wind shear in parts of the upper atmosphere.
This may increase the frequency or intensity of clear-air turbulence along some major flight routes.
However, CAT varies greatly by altitude, region and season, so climate change does not mean that every flight will experience more severe turbulence.
Improved forecasting systems increasingly combine:
- satellite observations;
- atmospheric models;
- aircraft-based measurements;
- real-time pilot reports;
- machine-learning techniques.
Conclusion
Clear Air Turbulence is turbulence that occurs in apparently clear skies, usually because of strong wind shear near jet streams and other upper-atmospheric disturbances. Its invisibility makes forecasting particularly important, while the simplest protection for passengers remains keeping seat belts fastened whenever seated.
