Wall cloud with lightning in Miami, Texas
Image: Brad Smull, NOAA
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: 
Image: NOAA
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.
Wall cloud over the Danby Beacon, UK during a violent thunderstorm:
Image: Colin Grice
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.
A rather spooky looking wall cloud over Spearman, Texas:
Image: NOAA
The end of a supercell – a thunderstorm with a deep, continuously rotating updraft – often forms wall clouds that can range in size from a few hundred feet (0.25 km) to over 8 km (5 miles). Especially in moist environments, many wall clouds have tails, an extension formed by a ragged band of cloud and cloud tags.
Wall cloud with a gigantic tall cloud at the Oklahoma-Texas border:
Image: NOAA
Severe storm researcher Ted Fujita, a.k.a. “Mr. Tornado”, was the first to identify wall clouds and their role in tornado formation or tornadogenesis in the 1950s. Thunderstorms containing a rotating wall cloud are most likely to produce tornadoes and are therefore a phenomenon that tornado spotters watch out for. But typically, a wall cloud precedes tornadogenesis by only ten to twenty minutes, so quick action is needed.
Wall cloud of the 1981 Alfalfa Tornado in Oklahoma:
Image: NOAA
If a thunderstorm with accompanying wall cloud is especially persistent with rapid ascension and rotation, it can take as little as one minute for a tornado to form. If this ascension and rotation is slower, this process can take more than an hour – not much time for a warning in any case. Here’s an example of a typical supercell with wall cloud. The different images for this panorama were taken between Rouvres and Oulins in Eure-et-Loir, France.
Tornadic wall cloud panorama:
Image: Matthieu Pron
Below is the incredible sequence of the tornado formation near Dimmit, Texas on June 2, 1995, often called one of the best observed violent tornadoes in history.
Image: NOAA
The wall cloud below nicely shows the rear flank downdraft (RFD), an area of descending air said to be essential for tornado formation.
Like tentacles waiting to reach down:
Image: NOAA
This rather friendly looking wall cloud should not mislead anyone as this is the stuff tornadoes are made off.
Pretty but don’t be fooled:
Image: NOAA
One hell of a wall cloud over a small town in Poland:
Image: Ziijon
A picture-book wall cloud over Columbus, Ohio:
Image: marada
Getting darker and darker – wall cloud over Texas:
Image: Wesley Fryer
We leave you with this video of two rotating wall clouds over Strasbourg, Saskatchewan.
