In this tremendous capture by Gry Elise Nyland, we see an amazing roll cloud over Calgary, Alberta, Canada on the morning of June 18, 2013. According to the NOAA’s National Weather Service:

“A roll cloud is a low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front (or sometimes with a cold front). Roll clouds are relatively rare; they are completely detached from the thunderstorm base or other cloud features, thus differentiating them from the more familiar shelf clouds. Roll clouds usually appear to be “rolling” about a horizontal axis, but should not be confused with funnel clouds.”

 

Featured as the Earth Science Picture of the Day on July 9, 2013, NASA’s Jim Foster says the clouds, although ominous looking, don’t usually generate dangerous winds.