Definition
Sudden, often severe turbulence occurring in cloudless regions of the atmosphere, typically at high altitudes near the jet stream where strong wind shear exists between layers of air moving at different speeds or directions. Because there are no visible clouds or precipitation to mark it, CAT cannot be detected by onboard weather radar and gives no visual warning to the crew.
Plain English
Bumpy, choppy air that can strike out of nowhere in a clear sky, with no clouds to warn you it is there.
Context Anchor
Pilots most often encounter this term in weather briefings, pilot reports, and high-altitude flight planning.
Derivation
The name simply describes what makes it dangerous: turbulence in clear air. Most turbulence pilots learn about first is associated with visible weather — thunderstorms, clouds, or terrain. Naming this type 'clear air' highlights the key problem: you can't see it coming.
Why Pilots Care
It can produce sudden altitude changes and airframe stress with no visible warning, so pilots reduce speed and secure the cabin when it is forecast.
Grounding Statement
The key idea is that the air can be moving unevenly even when nothing in the sky gives you an obvious visual warning.
Intuition Check
Do not assume “clear air” means smooth air. In this term, “clear” means the turbulence may not be shown by visible clouds or storms.
Example Sentence 1
The crew turned on the seatbelt sign after a PIREP reported moderate clear air turbulence at FL370 over the Rockies.
Example Sentence 2
Clear air turbulence at cruise altitude caused a brief but sharp bump with no clouds in sight.