How is clear air turbulence defined?

Clear air turbulence (CAT) is specifically characterized as turbulence that occurs in clear skies, meaning it happens without any visible clouds. This type of turbulence is primarily caused by wind shear, which is the difference in wind speed and direction over a small distance. Wind shear can occur at various altitudes, but it is particularly significant at high altitudes where jet streams are present.

The nature of CAT makes it difficult for pilots to detect visually or through instruments until the aircraft experiences the turbulence. Understanding CAT is crucial for pilots, as it can occur suddenly and without warning, affecting flight stability. The definition focuses on the critical aspect of being free from cloud formations, which sets it apart from other types of turbulence that may accompany weather phenomena or visual cues, such as thunderstorms or cumulus clouds.