To a beginning storm observer, wall clouds and shelf clouds (and more broadly inflow vs. outflow features) — can be confusing. Both of these storm features look to hang down from storms and oftentimes they can be mistook for one another.
However, these two could not be more different.
Shelf clouds are products of storm outflows — or air rushing out and away from a storm. On the other end, wall clouds are an inflow feature — and they typically form at the strongest portions of updrafts where air is being pulled into the storm the most.
Both of these features can have a lot of motion. However, the motion on shelf clouds tends to be more chaotic and less organized in nature. Still, this doesn’t stop many shelf clouds from being reported as rotating wall clouds every year.
A wall cloud can sport rotation along a vertical axis. Still, the presence of a wall cloud does NOT mean a tornado is imminent. For tornado chances to be on the rise with a wall cloud, it must be persistent (lasting more than a few minutes) and it must have increasing rotation with time. A persistent wall cloud that isn’t rotating or a rapidly rotating wall cloud that dissipates quickly tend to not be imminent signs of a tornado. However, ALL rotating wall clouds should be monitored carefully.