Nobody aboard an aircraft like cat. Their unpredictable effects range from simply benign, to dangerous, when not deadly. There are even a couple of cases where cat destroyed a plane in flight. You probably understood that I’m not talking about your favorite fur ball, but about a weather phenomenon known as Clear Air Turbulence, designated CAT on weather charts.

There are various sources of turbulence: thermal activity, convective activity, induced by rain, mountain waves, and jet-streams. If technically thermal activity is a variant of CAT, it is not relevant to airlines as it occurs only very close to the ground. Mountain waves and jet-streams can create CAT at almost any altitude from 25′000 to 60′000 feet. It’s easy for pilots to know where convective or rain induced turbulence is located: its position is marked by clouds, and weather radar helps to avoid strong rain areas within the clouds.

The CAT is the source what is known as “air pockets”. Being a gas, the atmoshphere is continuous. There is no such thing like volumes in which there is no air. Otherwise, some of us would die of asphyxia… When you get the feeling that your airliners is falling, it’s not because there is no air, but because the wind is changing direction or speed, inducing turbulence. Because of its nature, detecting CAT is hardly possible. The forecasts are based on position of jet-streams, the super high-speed wind in the higher troposphere. Their speed can reach 150 to 200 knots in their core, which is a thousand feet high. Entering such a high-speed wind definitely induce turbulence. This is why the jet-stream positions are often surrounded by CAT areas.

No system exists for clear air turbulence turbulence. Systems based on sound waves like sodar are limited to heights up to 1′000 feet, and are can not be installed on board. Because forecasted CAT areas are based on the possibility of CAT, they are much larger than the actual size of CAT areas. Even so, not all CAT can be forecasted, and this is why new regulations have been introduced over the last months, requiring passengers to keep their seat-belt fasten at all times.

Each year, several dozen of passengers get hurt in CAT incidents. Invariably, the victims were not wearing their seat-belts. Some times even when the “fasten seat-belt” sign was turned on. Cases where the victims violently hit the cabin ceiling are common. An open, or loosely tightened seat-belt is of no help when an aircraft gets hit by severe CAT. Next time your captain turns the “fasten seat-belt” sign on, make sure to comply, and have it well tightened.

Not less than 30 people got injured – 15 of them seriously – aboard a Qantas Airbus that encountered severe turbulence. The cause is still un-clear, the investigation is running, but whatever the results will be about the cause, the results are there: several serious lacerations and broken bones. Keep your seat-belt fasten and tight at all times.

Read more:

More information about the Qantas incident

CAT page of the Hong-Kong observatory

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