What is "clear-air turbulence"

n. The erratic movement of air masses in the absence of any visual cues, especially such movement that causes unexpected turbulence to an aircraft.

n. strong turbulence in an otherwise cloudless region that subjects aircraft to violent updrafts or downdrafts

Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual cues such as clouds, and is caused when bodies of air moving at widely different speeds meet.

The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around as it meets the tropopause. Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus clouds can also indicate high probability of CAT.

CAT can be hazardous to the comfort, and rarely the safety, of air travelers.