Air turbulence is responsible for 58 injuries each year in the United States. These injuries occur not as the result of a crash but rather due to violent movement of the airplane while it is airborne. Most injuries can be avoided by simply wearing a seatbelt, even when the seatbelt sign is turned off.
Turbulence is the term used to describe air disturbances that cannot be seen. Turbulence can be caused by any number of conditions including air rushing over mountain ranges, weather fronts, and even other aircraft. Pilots generally report when they encounter turbulence so that following planes can divert around the turbulence or advise passengers to buckle up. Sometimes however there is no warning, with turbulence hitting an aircraft suddenly. While the planes themselves are designed to withstand all but the most severe turbulence, unbuckled passengers may not be so lucky.