You are midway in your flight. Everything is smooth. You looked out the window and the sun shines bright in clear skies. Suddenly, the aircraft shakes and all of a sudden drops abruptly then goes up again! Those who are walking in the aisle that time are thrown up the ceiling. Bags fall off the overhead compartments. The galley is a mess. The cabin is a mess. What you actually went through is called Clear Air Turbulence or CAT.
CAT is actually undetected, not even an aircraft radar could detect it in order for pilots to avoid it. Plus, it happens in clear skies, something you would least expect.
So what is CAT or Clear Air Turbulence. By its name alone, it is defined as sudden severe turbulence that occurs in cloudless regions or areas usually at very high altitude which causes sudden violent movements on the aircraft. This is commonly attributed to higher altitude turbulence associated with wind shear. This always happens outside of convective clouds.
Standard aircraft radars cannot detect a CAT as this is not associated with clouds that show unpredictable movement in the air. Airlines and pilots are made aware of the factors that causes or indicates CAT to reduce the probability of meeting it.
There are basically two type of CAT, mechanical and thermal.
There is no history of an aircraft crashing due to CAT but this has caused already a lot of injuries and a few deaths to people who are unbuckled due to the severe abrupt movement of the aircraft. During such moment, passengers experience weightlessness for a few seconds that they are thrown up the ceiling.
Pilots nowadays though are fully trained to handle a situation where they fly in through CAT. Airlines and aircraft manufacturers have provided rules and instructions for pilots on what to do should they fly through one.
To better understand CAT, we have a few diagrams below. We hope all these will convince you to always keep your seatbelts fastened even if the fasten seatbelt signs are off.
(All diagrams below CTTO)