Many people have been mystified by these clouds as they are beautiful to see, but, if you look behind them they carry a dark and ominous storm. These clouds are arcus clouds. Arcus clouds have two different types: shelf clouds and roll clouds. Both of these types of clouds are seen on the leading edge of a cumulonimbus cloud as they are associated with the outflow from the storm. As cold air descends from the cumulonimbus cloud in a downdraft, it hits the ground a spreads out. Once it spreads out, warm moist air leads up into an updraft into the cumulonimbus cloud. This outflow is where we can see both the shelf and roll cloud.
Shelf clouds are a low lying horizontal cloud that is connected to a cumulonimbus cloud. This type of cloud can be seen at the leading edge of a storm, and can be associated with powerful winds. This is because they can come from gust front, squall lines, or even cold fronts. Even though shelf clouds hang low like a tornado would, there is usually no vertical rotation to produce one. The same applies to roll clouds. Although, roll clouds may appear different than shelf clouds as they are not attached to the cumulonimbus cloud, but are still associated with said cloud. They appear to roll on a horizontal axis because of the different wind direction at the different levels of the atmosphere.
Many have seen clouds that appear like shelf clouds but some may be mistaking it for wall clouds. Wall clouds appear low lying in the atmosphere like shelf clouds, but they will not appear in the leading edge outflow portion around a cumulonimbus cloud. They can emerge toward the trailing part of the cloud where inflow would be prominent. Wall clouds are also known to have a rain free base because of the inflow.