Turbulence is one of the major causes of aircraft accidents which are brought about by structural failure, forced gains or losses of altitude (up to thousands of feet), or loss of control. The term turbulence in meteorology normally refers to atmospheric motions smaller than the scale which is designated as the mean flow, and therefore encompasses a wide spectrum of motions. Only a relatively narrow band of turbulence is a significant problem for aircraft flight. There are several types of turbulence: convective (near cumulo-nimbus clouds and thunderstorms), low-level clear-air turbulence (caused either by rough terrain or vortices in a large aircraft's wake), violent air motion near mountains (mountain waves), and high-level clear-air turbulence (above 20,000 feet). The latter form of turbulence is particularly hazardous since it is difficult to detect by radar and could prove especially dangerous to supersonic aircraft.
High-level turbulence occurs in patches with horizontal dimensions between 80 and 500 km, and vertical dimensions from 20 up to 600 metres. Between 5 and 10% of high-level turbulence is heavy, and 1 to 3% of that which occurs is violent or extreme.