Shelf Cloud vs. Roll Cloud

Photographs courtesy Michael Bath and Jimmy Deguara Australian Severe Weather

So early one weekend morning as a cold front pushed through Central Iowa, I was working my usual shift at the TV station. I made a time lapse from one of our Skyview Camera's which we have placed across Des Moines, and later found an intriguing cloud formation.

I thought what I saw was a shelf cloud. However, once while storm chasing I saw a classic shelf cloud, and this appeared different. It turned out to be a Roll Cloud, which is similar yet different to a shelf cloud.

If you're saying to yourself, "Shelf Cloud, Roll Cloud...I'm Confused!!!", then allow myself to explain...myself...or something...let's move on.

Shelf Cloud

A shelf cloud resembles it's name of course, but in more detail it is a low cloud and horizontal (not vertically prominent like many storm clouds).
A shelf cloud is usually curved or semicircular and sticks out like a shelf from a parent cloud (usually a thunderstorm cloud). Personally, I would have called it a Frisbee cloud, because that sounds a lot more flashy and fun than just plain old "shelf".

    Looks Pretty Cool Huh?

Picture Courtesy of NOAA Photo Library                                 

So How Does This Shelf Cloud Form?

I'll tell ya...

Shelf clouds form in association with a gust front. That gust front can be caused by a squall line or downdrafts from a thunderstorm.

What's a gust front? Well, a cold front is the leading edge of cold air meeting warm air. Likewise, a gust front just means the leading edge of gusty or fast moving winds. Makes sense, right?

Anyway, back to the shelf cloud...

When rain falls to earth, it drags cool air with it. The rain cooled air rushes down to earth within the thunderstorm cell and then spreads outwards as it hits the ground. Whomever took this picture was likely standing in a warm and airmass out ahead of this storm, and all that air hitting the ground and spreading out will force the warm and moist air upward. Why? Because cold air is more dense than warm air. Just like when you force water in the bathtub up as you get in and sink to the bottom. Prune hands however, I can't explain.     

  Picture Courtesy of NOAA Photo Library

But I digress, so the warm moist air is forced upwards, and cools forming a really cool looking shelf cloud. So basically the shelf cloud is a result of thunderstorm downdrafts spreading out into and lifting up relatively warm and moist air.

Shelf clouds are associated with downdrafts but tornadoes involve updrafts. Thus, these particular cloud formations will not produce a tornado.

Roll Cloud

Unlike shelf clouds, roll clouds are completely unattached from a parent storm cloud. A roll cloud is long and tubular and appears to "rolling" along a horizontal axis. Although you may think it looks like a tornado turned horizontal, it is not associated with tornadoes at all.

Picture courtesy of Environment Canada

So the cloud I saw was a rare roll cloud. Remember how I said above that a shelf cloud results from downdrafts? Well the roll cloud I saw resulted from a downdraft from a decaying thunderstorm. The outflow from this dissipating thunderstorm had spread outward and lifted what warm and moist air was still at the surface.


Well, I hope you learned a lot and laughed a little.

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