Wall Cloud

weather · storms · tornadoes · dangerous

What it looks like. A wall cloud is a large, dark lowering that suddenly drops down from the base of a thunderstorm cloud. It creates a striking pouch or bulge hanging beneath the main storm cloud, and it can appear and disappear fairly quickly.

Where tornadoes form. Wall clouds are where the strongest updrafts, or rising air currents, happen inside a thunderstorm. Tornadoes sometimes spin up right from these wall clouds, which is why storm chasers and meteorologists watch them so carefully.

Rotating danger. Many wall clouds spin like tops in a phenomenon called rotation. When meteorologists spot a rotating wall cloud, they know a mesocyclone, or rotating vortex, is forming in the storm, which often leads to the strongest and most dangerous tornadoes.

Storm positioning. Wall clouds form beneath the rain-free base of a thunderstorm, in the area where rain hasn't fallen yet. This location helps meteorologists understand exactly where inside a storm the most powerful forces are at work.

Not all rotate. Interesting fact: some wall clouds stay still without rotating, while others spin violently. Only the rotating ones are reliable signs of a mesocyclone and potential tornado formation.