A roll cloud is a long, low, tube-shaped cloud separated from the cloud deck above. It can form along atmospheric boundaries and sometimes appears to roll around its horizontal axis.
Why it resembles a tube
A sharp contrast between moving air masses creates rising and sinking motions that can condense moisture into a long cylindrical formation.
How it differs from a shelf cloud
A shelf cloud is attached to its parent cloud, while a roll cloud is detached. Both can be associated with outflow boundaries, but the shape and connection are key clues.
Is it a tornado?
No. Tornadoes are rotating columns of air extending from a thunderstorm to the ground. A roll cloud rotates about a mostly horizontal axis and does not itself indicate a ground-reaching vortex.
What weather to expect
Roll clouds may accompany gusty winds along a boundary. Observe from safe shelter and do not treat the absence of lightning in a photo as evidence that the wider weather situation is harmless.
Frequently asked question
Do roll clouds always happen with thunderstorms?
No. Some form along other atmospheric boundaries without an active thunderstorm overhead.
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