Clear-air turbulence

Clear-air turbulence (CAT) is a dangerous weather phenomenon in aviation. Today, we will discuss its essence, threats, and ways to combat it.

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Clear-air turbulence (CAT), unlike other types of atmospheric turbulence, is not accompanied by significant cloud cover and can be observed during clear skies or with a small amount of upper-level clouds. However, there are no obvious signs of turbulence, making it very difficult to detect in advance - both visually and with radar.

What's up today?

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Origins of CAT

There are two major types of CAT and three сommon causes and sources of CAT for it's creation that are currently defined. Let's start step-by-step
Types of CAT:

Mechanical

caused by disruptions of calm horizontal air currents: due to the features of the terrain, other objects in the airspace, and other characteristics of the Earth's atmosphere

Thermal

caused by vertical movements of heated air, temperature anomalies, and seasonal features of the atmosphere in a specific part of the Earth

Common causes and sources of CAT:

Jet Stream

A Jet Stream is a narrow, fast moving current of air, normally close to the Tropopause and generated as a result of the temperature gradient between air masses. Although not all jet streams have CAT associated with them, there can be significant vertical and horizontal Low Level Wind Shear on the edges of the jet stream giving rise to sometimes severe clear air turbulence

Terrain

High ground disturbs the horizontal flow of air over it, causing turbulence. The severity of the turbulence depends on the strength of the air flow, the roughness of the terrain, the rate of change and curvature of contours, and the elevation of the high ground above surrounding terrain. For further information, refer to the article entitled Mountain Waves.

Thunderstorm Complexes

Cumulonimbus (Cb) cells have strong vertical currents. Aircraft passing within 20 nautical miles horizontally, or less than 5,000 feet above the top, of a Cb may encounter CAT.

One of the main principles of flight safety is the ability to prepare for dangerous and complex situations. In the case of CAT, this is very difficult because neither radars nor weather forecasts can accurately predict or show the occurrence of this dangerous phenomenon.

This is where the overall training of pilots, assessment of indirect causes of this phenomenon, and situational awareness come into play.

Effects of CAT

Turbulence in general poses a great danger to a flying aircraft. The different and often opposing air currents that strike the fuselage randomly make controlling the flight trajectory a very challenging task. With CAT, it becomes even more difficult - entering it usually happens unexpectedly and there is very little time to react.

Therefore, it is important to study the negative consequences of this phenomenon in advance in order to be prepared for anything.

Structural Damage

Aircraft can suffer structural damage as a result of encountering severe clear air turbulence. In extreme cases this can lead to the break-up of the aircraft. In even moderate turbulence, damage can occur to fittings within the aircraft, especially as a result of collision with unrestrained items of cargo or passenger luggage. Prolonged exposure to turbulence will shorten the fatigue life of the aircraft.

Physical Injury to Crew/Passengers

If caught unaware, passengers and crew moving around in the aircraft cabin can be injured. In one case, where a B747 encountered CAT over the Pacific ocean, several passengers and crew were severely injured and one passenger subsequently died.

Impaired Flight Crew Performance

Moderate or Severe turbulence can make simple tasks, including reading instruments, near impossible.

As you can see, clear-air turbulence creates serious problems for flight safety. The inability to detect it in advance only adds to the workload of the crew, who are responsible for quickly regaining control of the aircraft and exiting CAT to minimize risks for the plane and passengers.

Possible Defenses

So what should be done if clear-air turbulence cannot be predicted? It is not all hopeless, and there are two main recommendations that help avoid complications when encountering this weather phenomenon.

Awareness

SIGMET charts give forecasts of the location and level of clear air turbulence. Information on local terrain induced CAT may be contained in appropriate Aeronautical Information Publications (AIPs) e.g. Approach plates for Gibraltar contain information on turbulence to be expected for given wind directions.

Restraint Systems

Passengers and crew should fit seat belts and harnesses when seated to protect them in the event of unforeseen turbulence.

Therefore, when you are asked to fasten your seat belts on the next flight at cruising altitude, do not think that it is just a whim of someone from the crew. Most likely, the pilots are predicting possible clear-air turbulence ahead along the route and are trying to proactively ensure your safety <3

Summary

Clear-air turbulence is one of the most treacherous weather phenomena that can be encountered during a flight. It is difficult to predict, gives very little time to react, and can lead to serious consequences such as damage to the aircraft or injuries to passengers/crew members.

This is why it is important to carefully analyze weather information along the route, be aware of both direct and indirect signs of dangerous weather conditions, and take preventive measures to avoid negative consequences.

Sometimes it may seem that meteorology is a secondary discipline for pilots, but situations like this clearly show that a true professional will skillfully use all available resources to ensure flight safety.

Fly safe :)

Name of article: Clear-air turbulence
Release date: 10/26/2023
Acrticle author: Nikita Pavlov

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