On every flight, for 28 years that I was an International Airline Pilot for Pan Am, I included this message in my passenger announcement:
"I'm turning the seat belt sign off; you're free to move about the cabin as necessary.
However, please keep your seat belt fastened while you're in your seat, just in case we encounter some unexpected clear air turbulence"
This advisory announcement was for passenger safety, and to CMA. Because many people don't listen, I would have preferred to say it like this:
"YO!!! Take the frickin' headphone out of your cotton pickin' ear and listen to what I'm saying here!!! When you're in your seat, keep that seat belt wrapped around your fat ass because if we hit turbulence without any warning, your hard head is going to get clobbered by the ceiling as it comes down to meet you; and I don't want my ceiling damaged."
Clear air turbulence is invisible. It can be forecast at times, but cannot be seen. The usual way of knowing if you're going to encounter CAT is when the aircraft ahead of you runs into it. He'll tell air traffic control, and they spread the word. Sometimes you're on the same frequency and will hear the other pilot send the warning. If you hear him say "Holy Shit", then you know to get your cabin prepared very fast because it is going to be really rough.
CAT can be caused by:
Wake turbulence from another aircraft
You can find descriptions on the internet. Just know that normally the turbulence is not going to damage the aircraft because the wings of the Pan Am aircraft are constructed to take an extraordinary amount of abuse.
When clear air turbulence is encountered, there is a strong downdraft, so the airplane suddenly loses altitude. If you're not buckled into the seat, then you, being lighter than the aircraft, do not drop as fast. You remain suspended in the same position while the airplane drops and the ceiling hits you on the head. Then the downdraft stops and the aircraft goes back up rapidly, and the floor comes up to meet you, or another passenger suddenly finds his head lodged in your dumb ass.
Yesterday, a United Airlines Boeing 777 hit clear air turbulence over Kansas City,
and several passengers were injured because they didn't have seat belts on. One lady was seated, sans seat belt, and she ended up hitting the side of the cabin above the window, cracking the wall. The plane had to divert to Denver to get medical attention for the injured. Unfortunately, one person was reported to be critically injured.
So the next time you fly,
LISTEN TO THE ANNOUNCEMENTS and
KEEP YOUR SEAT BELT FASTENED when seated.
If you don't, and you damage the ceiling with your hard head, then I can say
"I told you so".