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Clear-Air Turbulence (often abbreviated CAT and sometimes colloquially referred to as "air pockets") is the erratic movement of air masses in the absence of any visual cues (such as clouds). Clear-Air Turbulence is caused when bodies of air moving at widely different speeds meet; at high altitudes (7,000-12,000 metres/23,000-39,000 feet) this is frequently encountered around jet streams or sometimes near mountain ranges. Clear-Air Turbulence is impossible to detect either with the naked eye or with radar, meaning that it is difficult to avoid. However, it can be remotely detected with instruments that can measure turbulence with optical techniques, such as scintillometers.
This kind of turbulence creates a hazard for air navigation. The rapid changes in the speed and direction of the air mass cause the lift created by an aircraft's wings to vary quickly and unpredictably, making for a rough flight. Cabin crew and passengers on airliners have been injured (and in a small number of cases, killed) when thrown around inside an aircraft experiencing turbulence.
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"Clear-Air Turbulence"