Our weather patterns and movement of mega airmasses around the world, combined with high level winds (jet streams) will be mixing all this air surrounding us and causing turbulence. The aim of the game when flying, is to try and find smooth air so cabin service and seat belts can be removed for the comfort of the travelling public.
Turbulence can be encountered not only in clouds but in clear air (blue sky). Clear Air Turbulence (CAT) is common at higher altitudes because it is associated with upper winds. These jet streams are more common at altitudes where all commercial airliners fly. We are talking between 30k and 40k feet altitude.
The good news is if you fly inside the core of the Jet stream, in the same direction, you will gain an increase in groundspeed of over 100MPH or more. Pilots love flying in them because the journey is shorter and there is an increase of fuel savings. The bad news is that the chance of getting a bumpy ride is more predominant in the Jetstream. It is a trade off between time in the air and an uncomfortable ride. It is a fine line if your flight is going in the opposite direction of the Jetstream. The Captain will need to fly well below or in some rare cases, above it, to find smooth air.
Before each flight, the crew will get the flight plan (routes) to follow and a complete weather package which will include maps of where they can expect turbulence. During the flight, communication between ground and airplanes about turbulence encounters will be passed on to other aircrafts so the seatbelt sign can be switched on before hitting the bumps.
I highly recommend keeping your seat belt fastened (loosely) at all times. You never know when the aircraft might hit a turbulent area that wasn’t forecast.
The good news is the aircraft can take quite a beating. It can be tossed around to a point where the pilot might lose control but the aircraft will never break because of turbulence even in a thunderstorm. The manufacture will test the breaking point of each new designed before giving it the airworthiness certificate.