The Arcus cloud formation presents itself as a very long and low level formation. It is usually connected with a cold front or an incoming thunderstorm.
The two variations of Arcus cloud roll clouds and shelf clouds. These formations are stunning and memorable features if you ever get the opportunity to witness them.
A shelf cloud is a lower, horizontal arcus cloud which will usually present itself as a wedge shape. Whereas the roll cloud is detached from other cloud structures, the shelf cloud is connected to the base of the approaching thunderstorm clouds.
At the leading edge of the cloud the air can be seen as rising in the outer aspect of the shelf formation, whilst the bottom of the cloud rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often seem squall-like and turbulent.
Source : Nick Nerbonne