A rare thunderstorm roll cloud formed in the sky of Nizhny Novgorod on December 24, 2015.
Yes, a horizontal cloud rotating in the morning sky over the Volga.
What is exactly a roll cloud?
A “roll cloud” is a type of arcus cloud – formations typically associated with thunderstorms. 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.
And what are the chances you’ll get to see one in person?
Whether or not you’ll see one depends on a “perfect storm” of conditions, which is hard to predict. Roll clouds tend to appear during well-defined inversions with enough accompanying moisture to form a cloud. That’s most likely to happen in the early morning.
Roll clouds can also occur on the edges of thunderstorms. But when there’s too much moisture a roll cloud may be hard to see because it’s hidden among other clouds.
Considering all of the above factors, the odds of spotting a roll cloud in your own backyard are pretty slim. But it never hurts to keep your eye on the sky.