It’s not something you see every day: a rotating, horizontal cloud that’s taken over the sky.
But that’s just what people got to see online in Russia after several people spotted this “weird-looking cloud” over the the Sea of Azov on June 2, 2017.
Now questions abound: What is it exactly? And what are the chances you’ll get to see one in person?
It’s called a “roll cloud.” It is a type of arcus cloud, which are typically associated with thunderstorms. Depending on the conditions, a roll cloud can last for several hours and extend for several hundred miles.
The rolling motion is the result of winds changing speed and/or direction at the inversion along which the weather disturbance is traveling. The ‘shear’ across the inversion sets up a rolling motion much like that of a rolling pin used in a bakery.
Whether or not you’ll see one depends on a “perfect storm” of conditions, which is hard to predict.
The cloud in the video is comparable to a “Morning Glory,” a roll cloud seen in the months of September and October over northeast Australia. It tends to appear during well-defined inversions with enough accompanying moisture to form a cloud. That’s most likely to happen in the early morning, hence the moniker.
Roll clouds can also occur on the edges of thunderstorms. Especially those that form in areas with strong temperature inversions, as frequently is the case during spring over the Great Plains.
Considering all of the above factors, the odds of spotting a roll cloud over your city are pretty slim. But it never hurts to keep your eye on the sky.