There is also a category of extreme turbulence, although without actually trawling around for a definition, I couldn't say with certainty that it is an 'official' category.
This is the fabled 'Vindaloo' level. Some hairy chested experienced pilots (like 411A and me) have experienced it. There is one more level 'Phal'- you don't want that! Nobody has survived to tell the tale!
I guess I have 'half a brain', but the truth must be told.
A lot is being made of this 'pilots will avoid areas of severe turbulence'. I'm afraid not. When one looks at the weather charts, these are frequently marked. Whilst notice is taken, one will
never reroute on that basis. Otherwise nobody would be able to get to SIN in thunderstorm season- often you have to just batten down the hatches and penetrate thunderstorm areas as best you can. It is remembered that these charts are produced within the bounds of modern meteorological knowledge. They are
very unrelaible. One would warn the cabin crew when it was likely to be encountered (to assist with planning cabin service), but that is as far as it goes. I have never ever altered a planned flight for this reason. Once airborne, one can attempt to get other pireps (pilot reports of
actual turbulence encountered) and study the weather radar, but no other means is available or reliable enough. One will usually not even bother leaving seat belt signs on if you are not certain it will be encountered (unless you are American with a plane load of potential litigants). Maybe that will change as this new era of humanity tends to sue if they go to sea in a cruise ship and get hit by a storm!
That is the true current policy. I see nothing wrong with it. There will always be the odd unexpected encounter that will leave head marks in cabin furnishings, but it has been found to be of acceptable safety in aviation today.