Image: Brad Smull, NOAA
Wall cloud with lightning in Miami, Texas
Every child knows that converging warm and cool streams of air cause thunderstorms. But next time you notice a zeppelin-like structure forming below one of the cumulus clouds, beware – this might be a tornado in the making! Wall clouds, as pretty and dramatic as they may look, are often just the first warning sign, spelling out “RUN!” as clearly as they can.
Apocalypse now? Dramatic looking wall cloud
Simply put, a wall cloud is the lower portion of a cumulus cloud, caused by ascending warm air that mixes with descending cool and moist air. Thunderstorms are a result of this phenomenon but wall clouds also form the basis for tornadoes.
Image: Colin Grice
Wall cloud over the Danby Beacon, UK during a violent thunderstorm
First, a low cloud base descending underneath the main storm updraft will form the wall cloud, for example in a thunderstorm; then it will start to rotate, thus forming the basis for a tornado. Wall clouds can also form when rising scud – small and ragged cloud fragments – organize and consolidate.