Wall Cloud
During a severe thunderstorm, air is drawn from Earth's surface up toward the storm cloud in an updraft. As the air rises, it cools and forms a low-hanging, rotating cloud mass called a wall cloud. A wall cloud that remains for a sustained period of time (more than just a few minutes) and is rotating is likely to produce a tornado. The presence of a wall cloud does not guarantee that a tornado will occur, but it is a strong warning sign.