rotor cloud

rotor cloud

[′rōd·ər ‚klau̇d]
(meteorology)
Turbulent, altocumulus-type cloud formation found in the lee of some large mountain barriers, particularly in the Sierra Nevadas near Bishop, California; the air in the cloud rotates around an axis parallel to the range. Also known as roll cloud.

rotor cloud

rotor cloudclick for a larger image
A cloud that forms on the downwind side of a mountain or hill feature. A wave motion is set up on the downwind side. Moist air moving up to the crest of a wave may cool enough to form a cloud. As it sinks to the trough, the cloud evaporates. The clouds thus seem stationary even though the air is moving in waves. The turbulence is often indicated by small rotor clouds at lower levels.
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But the UFOs were lenticular and rotor clouds -created by air being squeezed over mountains.