We just learned about Cloud Étages
Some clouds only come around during big storms. One of these types of clouds is called a shelf cloud.
Usually when you see a shelf cloud, it is a low sideways wedge cloud right at the edge of a thunderstorm.
The upper part looks like a rising cloud, and the bottom part looks like a wild windy blowing storm.
(from: wikipedia - arcus cloud)
Kid Facts - Blast from the past: Slate