Definition
A turbulent, ragged cumulus-type cloud that forms in the lee (downwind side) of a mountain ridge, marking the location of a horizontal rolling vortex of air called a rotor. The cloud sits roughly parallel to the ridge, often beneath the crest of an associated mountain wave, and indicates severe turbulence within and below the cloud.
Plain English
A churning, broken-looking cloud that forms downwind of a mountain ridge. It marks a spot where the air is rolling like a wheel on its side, and the air there is very rough.
Context Anchor
Seen in mountain flying, weather briefings, and visual weather observations near ridges or ranges when strong winds cross the terrain.
Derivation
‘Rotor’ comes from Latin ‘rotare’, meaning to turn or rotate. The cloud is named for the rotating column of air it sits in — the air rolls horizontally like a spinning rotor, and the cloud forms in the visible, moist part of that rotation.
Why Pilots Care
Signals severe turbulence that can damage aircraft or cause loss of control.
Grounding Statement
Picture strong wind hitting a mountain ridge, spilling over the top, and curling back on itself downwind like a wave breaking — the rotor cloud sits in that curl.
Intuition Check
A rotor cloud is not caused by an aircraft rotor, and it may not look like it is spinning. The key idea is that the air around it is rolling and can be dangerously turbulent.
Example Sentence 1
The pilot saw a line of rotor clouds downwind of the ridge and chose to climb well above the mountain wave before crossing.
Example Sentence 2
Rotor clouds form when stable air crosses a ridge and sets up the rotating flow.