A roll cloud is a rarer type of arcus cloud. It is a horizontal, roughly tube-shaped cloud. In contrast with a shelf cloud, a roll cloud is completely detached from the base of the thunderstorm.
For a roll cloud to form, a relatively stable airmass needs to be present close to the ground. This may be a nocturnal inversion layer or outflow from a preceeding thunderstorm. The gust front forces the weakly buoyant air to rise, but due to limited buoyancy and entrainment of cooler outflow airmass, it ceases to rise and begins sinking.
A roll cloud will appear to “roll” its horizontal axis.
Note: this section only talks about roll clouds associated with thunderstorms. Other instances of roll clouds, such as Morning Glory, are not discussed here.