Irregular motion of an aircraft in flight, especially when characterized by rapid up-and-down motion, caused by a rapid variation of atmospheric wind velocities. This can occur in cloudy areas (particularly towering cumulus and lenticular clouds) and in clear air. Turbulence is the leading cause of nonfatal passenger and flight attendant injuries. The U. S. Federal Aviation Administration (FAA) classifies aircraft turbulence as follows:

• Light: Causes slight, erratic changes in altitude and/or attitude, and rhythmic bumpiness as occupants feel a slight strain against seat belts. * Moderate: Similar to light, but of greater intensity, with rapid bumps or jolts, and occupants feel a slight strain against seat belts. * Severe: Turbulence that causes large, abrupt changes in altitude and attitude, and large variations in airspeed, with the aircraft temporarily out of control. Occupants are forced violently against their seat belts and objects are tossed about, with food service and walking impossible. * Extreme: The aircraft is tossed about so violently that it is practically impossible to control, and structural damage may occur.