A sign of the time: Phenomenal Roll Cloud over Curitiba, Brasil - Dec 17, 2012

A sign of the time. A phenomenal roll cloud formation filmed over Curitiba, Brasil on December 17, 2012.

An arcus cloud is a low, horizontal cloud formation. Roll clouds and shelf clouds are the two types of arcus clouds. A shelf cloud is usually associated with the leading edge of thunderstorm outflow; roll clouds are usually formed by outflows of cold air from sea breezes or cold fronts in the absence of thunderstorms.



Source: freehenrique