Definition
Turbulence encountered in air where no clouds are present. It is most often associated with the jet stream and wind shear at high altitudes, particularly near the tropopause, and cannot be detected visually or by onboard weather radar.
Plain English
Bumpy air that hits you out of nowhere on a clear, cloudless day. There is nothing to see and nothing on the radar to warn you about it.
Context Anchor
Pilots encounter CAT in weather briefings, forecasts, pilot reports, and in flight at cruise altitude when the air looks visually clear but may still be rough.
Derivation
The phrase is literal: turbulence in clear air, meaning air that contains no clouds or visible moisture. The point of the name is the contrast — most turbulence is associated with visible weather like thunderstorms or clouds, but this kind is not.
Why Pilots Care
CAT produces sudden vertical accelerations that can injure passengers and crew, damage the airframe, and occur with no visual warning.
Grounding Statement
An aircraft can be flying in blue sky at cruise altitude and suddenly enter rough air because two nearby layers of air are moving at different speeds.
Intuition Check
Clear-air does not mean smooth air or perfectly safe weather. It means the turbulence is not shown by visible clouds.
Example Sentence 1
ATC issued a ride report warning of moderate clear-air turbulence near the jet stream at FL370.
Example Sentence 2
Passengers were asked to keep seat belts fastened because the forecast showed possible CAT over the Rockies.