What phenomenon does "clear air turbulence" describe?

"Clear air turbulence" specifically refers to turbulence that occurs in clear air, typically associated with the presence of jet streams. Unlike turbulence caused by other factors, such as storms or thermal activity, clear air turbulence is often found at high altitudes in the absence of visible weather patterns like clouds. This type of turbulence is primarily a result of wind shear, which is a rapid change in wind speed or direction over a short distance, commonly experienced when aircraft fly in and out of the jet streams.

The correct answer captures the essence of the phenomenon, emphasizing its occurrence in clear air and its connection to the high-speed winds of jet streams, which can create a sudden and unexpected roughness that can be challenging to detect and predict. Understanding this is crucial for pilots, as recognizing the conditions that lead to clear air turbulence can help in flight planning and enhance safety.