Flight explained
Why aircraft move in turbulence
What happens to the air, wings and passengers when an aircraft encounters turbulence.
Turbulence is the inhomogeneous movement of air. Ascending and descending streams, air mass boundaries, mountains and storm clouds change the speed and direction of the flow around the aircraft, so passengers feel swaying and short overloads.
What causes turbulence and how it affects the aircraft
The wing of a passenger aircraft should not be absolutely rigid: it bends elastically and takes part of the load. The design is tested with a margin regarding the modes of normal operation, and the crew receives forecasts, on-board radar data and messages from other aircraft.
The main practical risk is associated with unfastened people and unattached objects. Therefore, it is better to keep the belt fastened all the time, even when the scoreboard is turned off.