There are two types of arcus clouds, roll and shelf. Both can look almost similar individually, but side by side the differences become more apparent.
A shelf cloud is attached to the base of the parent cloud, which is usually a thunderstorm. It’s curved in the shape of a wedge. Shelf clouds are associated with downdrafts but tornadoes require updrafts. Therefore, shelf clouds will not produce a tornado. They can form by squall lines or downdrafts from thunderstorms.
A roll cloud is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud. They differ from shelf clouds by being completely detached from other cloud features. Roll clouds usually appear to be “rolling” about a horizontal axis. They are a solitary wave called a soliton, which is a wave that has a single crest and moves without changing speed or shape. A roll cloud is long and tubular and appears to “rolling” along a horizontal axis. Although a roll cloud may look like a tornado turned horizontal, they are not associated with tornadoes at all.