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Arcus cloud

Arcus clouds are low-level, wide ranging clouds typically associated with powerful storm clouds and thunderstorms.

Symbol: N/A

Height of base:  Up to 6500 ft.

Shape: Wedge shaped, or a long rolling horizontal column.

Latin:  Arcus = Arch.

Precipitation: None directly, but heavy showers in the vicinity from Cumulonimbus clouds.

What are Arcus clouds?

Arcus clouds are spectacular low-level, long and thin clouds associated with powerful thunderstorms. They are sometimes seen beneath Cumulonimbus clouds. Shelf clouds are attached to the storm cloud, whereas Roll clouds are a horizontal column separated from the storm cloud.

How do Arcus clouds form?

When a cold downdraft from a cumulonimbus cloud reaches the ground, the cold air may spread rapidly along the ground, pushing existing warm moist air upwards. As this air rises, water vapour condenses into the patterns associated with Arcus clouds. The new cloud may roll if it experiences different wind directions above and below.

What weather is associated with Arcus clouds?

As Arcus clouds form with cumulonimbus clouds and downdrafts, they are associated with strong gusty winds, heavy rain or hail showers as well as thunder and lightning.

How do we categorize Arcus clouds?

There are two types of Arcus clouds: Shelf clouds and Roll clouds.

The images below have been sent in by Met Office Twitter followers.

Roll cloud:

Roll cloud over Maldonado, Uruguay by Daniela Mirner Eberl

Arcus cloud:

Shelf cloud over Enschede, Netherlands by John Kerstholt

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