Arcus cloud from a thunderstorm

click to enlarge - image by Scott Gates

Thunderstorm in Summer 2010, Waconia, MN. Backlit by the sun, note the shelf or “arcus” cloud.

From Wikipedia: An arcus cloud is a low, horizontal cloud formation associated with the leading edge of thunderstorm outflow, or outflows of cold air from sea breezes or cold fronts in the absence of thunderstorms.

A shelf cloud is attached to the base of the parent cloud (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn. Cool, sinking air from a storm cloud’s downdraft spreads out across the surface with the leading edge called a gust front. This outflow undercuts warm air being drawn into the storm’s updraft. As the cool air lifts the warm moist air, water condenses creating a cloud which often rolls with the different winds above and below


Posted on May 4, 2011, in Clouds, Thunderstorms, turbulent or shear and tagged , . Bookmark the permalink. 2 Comments.

  1. This is not an arcus cloud. This is the rotating mesocyclone of a supercell in which warm air ascends and rotates as a result of strong deep and mid layer sheer. The dynamics are completely opposite to that of a gust front . Varying humidity levels within the air column combined with shear produce beautifully sculpted and striated updrafts such as this one.

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