This Animation explains why you should not worry about Airplane Turbulence
Turbulence is recorded and shared across planes according to a rating system. Light turbulence describes a movement of a foot or two, and rattles the drink tray. Moderate turbulence is the point at which air hostesses strap themselves in. Severe turbulence will launch unsecured objects, including people, although most pilots only encounter severe turbulence a few times in a whole career.