Impressive images of an arcus rushing on a beach in Brazil!

This rare and impressive cloud formation surprised the beachgoers of Guaratuba in Brazil a few days ago. What is an arcus and how does it form?

Rory Morrow United Kingdom 2 min

Swimmers on the southeast coast of Brazil witnessed a rare phenomenon this past weekend, with the formation of an incredible arc along the coasts of Paraná and São Paulo Sunday August 28.

Various images and videos from the event were shared on social media, including the video below, which shows the impressive cloud formation stretching for miles along a beach in Guaratuba.

These giant roll clouds, cylindrical in shape, are called because they seem to roll on a horizontal axis. They are a type of arcus and are closely related to shelf clouds, except that they are detached from other cloud features. Associated with powerful thunderstorms, they sometimes appear under cumulonimbus clouds and can extend for hundreds of kilometres.

Arcus form when cold downdrafts produced by thunderstorms reach the ground and spread outward, causing the existing hot air to rise. The water vapor in the warm air then condenses, creating a cloud that can begin to roll if subjected to winds from different directions above and below (wind shear).

The result can be spectacular, the best-known example coming from Queensland, Australia, where, from time to time, conditions align to produce the Morning Glory cloud – a formation comprising several arcus.

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