Definition
A low, horizontal, tube-shaped cloud that forms on the leeward (downwind) side of a mountain ridge in the rotor zone of a mountain wave. It marks the location of strong, rotating, turbulent air beneath the wave crest and indicates severe turbulence is present.
Plain English
A long, rolling cloud that forms downwind of a mountain ridge. It looks like a spinning tube of cloud and shows where the air below a mountain wave is tumbling violently.
Context Anchor
Pilots may see a roll cloud while visually checking the weather near thunderstorms, especially when learning about turbulence from storm-related wind changes.
Derivation
Called a 'roll cloud' because the air inside it is literally rolling — rotating around a horizontal axis like a log turning end-over-end. The name describes the motion, not just the shape.
Why Pilots Care
Signals strong, shifting surface winds and possible wind shear that can affect takeoff, landing, and low-altitude flight.
Grounding Statement
Picture a long, horizontal cloud sitting downwind of a mountain ridge, slowly turning over on itself like a rolling pin — the air inside is doing exactly that, and any aircraft caught in it gets thrown around violently.
Intuition Check
Do not assume a roll cloud is just any cloud that looks slightly rounded. In aviation weather, it means a distinct, low, tube-shaped cloud separated from the main storm cloud and often tied to strong wind changes.
Example Sentence 1
The pilot spotted a roll cloud forming downwind of the ridge and diverted well clear of the mountain wave system.
Example Sentence 2
Roll clouds often mark the leading edge of a gust front and indicate a rapid change in wind direction and speed.