A wall cloud is a shelf-shaped
cloud hanging down from a
severe thunderstorm. They have a very distinctive wedge-shaped form and are usually associated with very powerful
thunderstorms containing internal rotation (
mesocyclones). A wall cloud may show signs of
rotation itself - if it does, it is not unlikely that a
funnel cloud or
tornado will form. Wall clouds are also associated with extremely strong winds, frequent
lightning, heavy rain, and
hail.
Wall clouds are most common in the Midwestern United States, but may be seen out of any powerful thunderstorm. They are harder to see in moister areas such as the Southeastern US, where the air is less clear. I've even seen one or two out of unusually powerful thunderstorms in California