What's A Roll Cloud?

Roll clouds are completely unattached from a parent storm cloud. This is what makes it distinctly different looking compared to a wall cloud or a shelf cloud. A roll cloud is long and tubular and appears to be "rolling" along a horizontal axis. Although you may think it looks like a tornado turned horizontal, it is not associated with tornadoes at all.

Photograph courtesy Michael Bath and Jimmy Deguara Australian Severe Weather http://www.australiasevereweather.com/

Roll clouds form from outflow, similar to the process discussed in the section on shelf clouds. However, roll clouds are much more rare. They are most often associated with a gust front and do appear to roll or tumble.

Picture courtesy of Environment Canada http://www.msc-smc.ec.gc.ca/education/severe_weather/page20_e.cfm