Rotor clouds

From Glossary of Meteorology
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rotor cloud

(Sometimes called roll cloud.) A turbulent, altocumulus-type cloud formation found in the lee of some large mountain barriers, particularly in the Sierra Nevada near Bishop, California.

The air in the cloud rotates around an axis parallel to the range. The term was first applied to clouds of this type in Europe, especially in the Riesengebirge and on Crossfell. The rotation may extend to the ground, cause hazards to aircraft, and carry large amounts of dust aloft. Rotor clouds are often associated with lee wave (lenticular) clouds that may be present above.
See Bishop wave.