A roll cloud is a low, horizontal, tube-shaped cloud associated with a thunderstorm gust front or a cold front, and it is completely detached from the parent storm above. Along its long axis it can appear to slowly rotate about a horizontal line, giving it a rolling, tumbling look.
How it forms
Roll clouds develop for the same basic reason shelf clouds do. A thunderstorm's cold outflow, or a cold front behaving like a density current, undercuts warmer, more humid air ahead of it and lifts that air along a sloping interface. Where the lifted air cools to saturation, cloud forms.
The rolling appearance comes from the wind shear across the boundary. Warmer air rises up over the front on one side while cooler air sinks or moves in the opposite direction below. This produces a horizontal circulation that shapes the cloud into a tube. The tube separates from the parent storm when the outflow is far enough ahead of the main cloud deck that no continuous connection to the storm remains.
How spotters interpret it
A roll cloud in front of a thunderstorm is a signal that a well-defined outflow surge is arriving. Wind shift, a temperature drop, and gusty winds usually follow within minutes. The key visual test in the field is whether the tube is attached to the storm above. If cloud material bridges the gap, it is a shelf cloud. If the tube stands alone against clearer sky above, it is a roll cloud.
The rotation to report from a roll cloud is horizontal, spinning around an axis parallel to the ground. That is fundamentally different from the vertical rotation that produces tornadoes.
What it is not
Roll clouds are not tornadoes and cannot spawn them from their rolling motion. They are outflow structures. Actual tornado threat comes from a rotating storm updraft, not from a horizontal cloud tube along a gust front.
Roll clouds are also relatively rare compared with shelf clouds. Most low, horizontal cloud features along a gust front are shelves, not rolls, so confirm the detachment before calling it in.