As the 2013/14 summer storm season gets under way, I though it would be good to have a look at some of the various types of cloud formations / thunderstorm features that you might see. In this post, we will look at the difference between Roll, Shelf and Wall clouds.
A roll cloud is a tube-shaped cloud and is a rare type of arcus cloud. They are nearly always completely detached from other cloud features and they appear to be “rolling” about a horizontal axis. One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia.
Image used is shared under the Creative Commons Attribution-ShareAlike License.
However, similar features can be created by downdrafts in thunderstorms. This example is from PWL chaser Amery Drage and was taken in Jan 2010. You can clearly see that the cloud is detached from the cumulus cloud above it.
Image credit: ©Amery Drage / Perth Weather Live
One way that roll clouds associated with thunderstorms can form, is when the cool air in the forward flank downdraft at the front of the thunderstorm pushes out ahead of the storm, causing the shelf cloud to become detached from the storm. Known also as the gust front, it marks the leading edge of the rain-cooled outflow from the thunderstorm. If you see a well-defined roll cloud rolling toward you, prepare for strong and possibly damaging winds as it passes.
A shelf cloud gets it’s name from the fact that it often looks like a shelf. Also a type of arcus cloud, a shelf cloud is a low, horizontal wedge-shaped cloud, associated with a thunderstorm gust front. Unlike a roll cloud, a shelf cloud is attached to the base of the parent cloud above it (and can sometimes be seen in non-thunderstorm cold fronts). Gust fronts can be very distructive and in the case below, the wind did a complete 180 degree turn in around 10 seconds, shifting from NE to SW as the gust front passed.
Image credit: ©Jordan Cantelo / Perth Weather Live
Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside can often appear turbulent, boiling, and wind-torn. Shelf clouds form when rain cooled air rushes down to the ground within the thunderstorm and then spreads outwards as it hits the ground. When that cool air hits the ground and spreads out, it forces the warm and moist air upward. Another way to think about it is to picture a wedge of cool air lifting a blanket of warm air, like when you blow air under a sheet of paper.
A strong gust front can cause the lowest part of the leading edge of a shelf cloud to be ragged and lined with rising fractus clouds. In a severe case there will be vortices along the edge, with twisting masses of scud that may reach to the ground or be accompanied by rising dust. A very low shelf cloud accompanied by these signs is the best indicator that a potentially violent wind squall is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as a gustnado.
The following image was sent in by PWL viewer Ben Niven and shows a classic shelf cloud over the Onslow Racecourse on 22nd February 2013.
Image credit: ©Ben Niven / Perth Weather Live
A wall cloud (sometimes called a pedestal cloud) is a large cloud formation that develops beneath the base of a sever thunderstorm or super-cell. It most often forms beneath the rain-free base part of the thunderstorm and indicates the area of the strongest updraft. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm and most strong tornadoes form from these, but not all wall clouds rotate and not all tornadoes form from wall clouds. In this image, you can see the wall cloud ‘hanging’ down beneath the cloud base of a large thunderstorm in the wheatbelt, near Mukinbudin, Western Australia.
Image credit: ©Matt Fricker / Perth Weather live
The following image was captured just recently by PWL chaser Micheal Beazley. It shows a wall cloud very close to the ground. Image credit: ©Micheal Beazley / Perth Weather Live
Sometimes, if there is a lot of moisture in the system, a wall cloud will have a tail, which is a ragged band of cloud (fractus cloud) extending from the wall cloud toward the precipitation. in the following image, you can see a tail cloud forming under the wall cloud and to the right.
Image credit: ©Matt Fricker / Perth Weather live
Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the thunderstorm cloud base, which is commonly called a collar cloud. This classic super-cell image was was captured near Kalgoorlie on 17 March 2013 by PWL viewer Randal Webb, and features in the Perth Weather Live 2014 Calendar. You can clearly see the collar cloud bands in the middle section.
Image credit: ©Randal Webb / Perth Weather live
So next time you are out and you see a thunderstorm approaching, hopefully you will now be able to identify some of these storm features. But remember, thunderstorms can be dangerous and you must always take care to make sure you are not in harms way.
The information in this article is adapted and used with permission under Creative Commons Attribution ShareAlike License.