Following on from last it's time for lesson two. I have chosen an Arcus Cloud, specifically a Shelf Cloud.
Shelf cloud A shelf cloud is a low, horizontal wedge-shaped arcus cloud. A shelf cloud is attached to the base of the parent cloud (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn. Occasionally people see a shelf cloud and think they have seen a wall cloud, which is an easy mistake, since an approaching shelf cloud appears to form a wall made of cloud. Generally speaking, a shelf cloud appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm. ~source