Clear Air Turbulence
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 7,000–12,000 metres (23,000–39,000 ft) 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 cloud can also indicate high probability of CAT.
CAT can be hazardous to the comfort, and even safety, of air travel.
Read more about Clear Air Turbulence: Detection, Factors That Increase CAT Probability, Effects On Aircraft
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—Nadine Gordimer (b. 1923)
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—William Butler Yeats (1865–1939)