A viewer posted on our Facebook page last night asking what the difference between a shelf cloud and a wall cloud is.
The shelf cloud has been a pretty common sight over the last few weeks in eastern Iowa. Our latest round was just yesterday evening when a line of storms rolled across Chickasaw, Winneshiek, and Allamakee Counties. We posted up some cool looking pictures here.
You’ll find a shelf cloud on the leading edge of a line of storms. Sometimes they can look like mother ships and sometimes people report false tornadoes from them. They are one of the coolest yet ominous looking cloud features on the planet (in my opinion).
When rain falls out of a thunderstorm it brings cold air with it. That cold air is more dense than the warm and humid environmental air and therefore when it hits the ground, it pushes out away form the base of the storm. Since it is more dense, it can displace the warm air which has nowhere to go but up and over the cold air. This rising motion allows the warm, moist air to condense into clouds. Because of the conditions that form a shelf cloud, you will notice the strong gusts of wind before you feel the rain hit you.
Sometimes the shelf can be completely detached from the cloud base where it then becomes a roll cloud. They are called this because it basically looks like a Pringle’s tube rolling over on its side. These are relatively rare but are quite the sight!
Sometimes, when the warm air is extremely moist, It will condense before reaching the cloud base. This is what we call “scud” and can often be confused for a tornado. Scud will rise toward the cloud base but won’t be rotating. On Monday, Amy Hagen captured a really eerie shelf cloud north of Waukon and Waterville with what could potentially be considered scud or at least you can see the “fingers” caused by small scale changes in humidity.
These storm clouds are North if Waukon and Waterville but there was mist/clouds coming up out of the valley and joining the clouds in the sky.
Posted by Amy Hagen on Monday, July 1, 2019
Wall clouds can also have scud. A wall cloud is a lowering from the cloud base of a supercell (rotating storm) that is also rotating, sometimes strongly.
A wall cloud can produce a tornado. This formation is much smaller and more compact than a shelf cloud. Some examples are below:
In conclusion, a wall cloud is associated with a rotating storm and the main hazard will be a tornado whereas with a shelf cloud, the main threats will be damaging wind gusts with a line of storms.