1.3.1.2.1. Arcus clouds

  • 10 months ago
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  • Author: SW101

ROLL CLOUD

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.