CAT: Clear Air Turbulence Explained

In this post I give a description of clear air turbulence and why it’s important that you follow the instructions of the pilot and cabin crew during flights.

Clear Air Turbulence, or CAT for short, is an aviation phenomenon that can suddenly occur in cloudless areas, causing a buffeting effect on aircraft. It can happen in the absence of any visual clues like clouds and is most prevalent during the winter months of the year, particularly when flying through jet streams. CAT is different from other forms of air turbulence experienced when flying in an aircraft because it isn’t associated with cumuliform clouds, including thunderstorms. Also, CAT typically occurs when flying above 15,000 feet.

There are two types of CAT:

• Mechanical CAT – disruption to the smooth flow of horizontal air.
• Thermal CAT – vertical currents of air in unstable atmospheric conditions.

Mechanical CAT is when the air near to the ground traverses over things like buildings, hills, and mountains. Thus, the smooth flow of horizontal air gets disrupted and can be felt several thousand feet above those disruptions.

Thermal CAT is also known as convective CAT and occurs on warm, sunny days when the sun heats the Earth’s surface unevenly. Some surfaces get heated more rapidly than others, resulting in isolated convective currents. In layman’s terms, warm air rises while cooler air descends, resulting in turbulence.

What Causes Clear Air Turbulence?

There are typically three primary sources of clear air turbulence that you’re likely to experience while flying in an aircraft: jet streams, the terrain below the plane, and thunderstorm complexes.

Jet streams are narrow yet fast-moving currents of air that can be found at around 36,000 feet on average. At the poles, jet streams are usually 20,000 feet in the air, while over the Equator, jet streams are as high as 60,000 feet high.

Another leading cause of CAT is due to high ground disturbing the flow of air. The terrain at ground level can cause various intensities of CAT, depending on the terrain roughness, the curvature of contours, and so forth.

Thunderstorm complexes, also known as cumulonimbus cells, produce strong vertical currents of air. The thick and dense cumulonimbus clouds provide the ideal conditions for thunderstorms to occur.

When are you likely to experience CAT?

Flying in an aircraft that crosses a jet stream is one way of increaseing the likelihood of experiencing CAT. The probability increases if you’re flying at an altitude of around 36,000 feet or at lower elevations while flying downwind from mountain ranges, for example. Sometimes even if you’re flying approximately 20 miles from a visible thunderstorm complex, you can experience CAT due to severe thunderstorms.

What happens when you experience CAT during a flight?

When clear air turbulence occurs both the cabin crew and passengers will be asked to fasten their seatbelts as doing so lessens the risk of injuries. In extreme cases of CAT, unsecured objects in the cabin may end up getting strewn across the aircraft. It’s for that reason the cabin crew always requests that items get stowed underneath passenger seats or in the overhead lockers.

The correct storage of personal belongings ensures that injury to people from turbulence during flights is minimal. Cabin crew check where passengers have placed their belongings to proactively prevent injuries in unforeseen air turbulence.

What should you do during CAT?

If you suddenly experience aircraft turbulence, the air crew will immediately request that all passengers fasten their seatbelts. The cabin crew will also ask all standing passengers, including those using the washrooms, to return to their seats and buckle up. Should you have any hand luggage out, be sure to secure it under your seat or in an overhead locker.

It’s also vital that you secure anything loose on your fold-out table, such as laptops, food, drink, etc to prevent injuries to yourself and other passengers and to the crew.

In times of severe CAT, pilots may resort to descending the aircraft to lower altitudes to lessen the impact.

Can pilots detect and avoid CAT?

Clear air turbulence can sometimes be impossible to detect as it’s a phenomenon that isn’t visible to the naked eye. The good news is airlines are investing in technology that can help pilots to detect areas of the sky where there is an extremely high likelihood of experiencing CAT.

Some airlines, such as Hawaiian Airlines, use global turbulence modelling systems to help lessen the risk of pilots flying into CAT hotspots. Other ways that clear air turbulence can get detected are with optical instruments such as scintillometers and Doppler LIDAR.