arcus cloud
shelf cloud

Although it may have looked ominous, the formation, also known as an arcus cloud, was a sign of nothing more than a few showers.

An arcus cloud is a low, horizontal cloud formation.

Roll clouds and shelf clouds are the two types of arcus clouds.

A roll cloud is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud.

Cumulus arcus clouds have a gust front, and cumulus tuba clouds have funnel clouds or tornadoes.

Cumulonimbus calvus arcus is a sub-type of cumulonimbus calvus, which has arcus cloud ahead of cloud's front.