Roll Cloud:
A rare roll cloud appeared over Portugal’s coast recently, surprising beachgoers as strong winds hit during an ongoing extreme heatwave.
- A roll cloud is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud.
- Roll clouds usually appear to be “rolling” about a horizontal axis.
- They usually appear at an altitude of 2,000 to 7,000 meters, and continue to curl visibly.
- They are more often seen in areas with complex terrain and changeable weather—both in the mountains and on the coasts of the oceans. For example, in the Alps, around Japan, Scotland, and northern Australia.
- During severe weather, roll clouds can be confused with funnel clouds, as the roll cloud will show clear signs of rotation along the horizontal and vertical axes, like a tornado.
- Roll clouds are helpful to meteorologists because their appearance sometimes heralds bad weather—strong winds, thunderstorms, storms, or even tornadoes.
- A sharp, rapid roll of clouds can indicate a storm is about to intensify, while a smooth movement can mean calmer weather.
- Along the edge of a thunderstorm, they can serve as a precursor to more active and serious weather in the immediate future.
- An arcus cloud is a low, horizontal cloud formation that typically forms along the leading edge of a thunderstorm outflow or cold front.
