Roll Clouds

November 4, 2012

When I lived in New Mexico many years ago with my young children we used to stand on the porch of our home in Coralles and watch the storm clouds fighting on top of the mountains. I wrote of this often in my diary. They appeared to shoot forward, then roll back on themselves, forward, then backward, creating rolls over the Sandia Mountains that sometimes stretched along the range and could be seen hovering over Albuquerque. Sometimes it was a single roll, sometimes more. Occasionally the clouds would break free and shoot across the valley dropping rain upon the cities of Bernalillo, Coralles, Albuquerque and Rio Rancho. Other times they would simply roll along until the winds seemed to shred them like threads pulling from a carpet.

It was 25 years later when I finally learned that these clouds created by this great battle actually have a name, and a logical one at that–roll clouds.

A roll cloud is a type of arcus cloud. There are only two types of arcus clouds. The other is the shelf cloud. Roll clouds form when outflows of cold air form without thunderstorms. Watching their formation is fascinating. The clouds visually appear to roll horizontally, which you can see in the picture below. They are actually a single wave of cloud that has a single crest, moving without changing speed or shape.

There are areas of the world that have regular roll clouds, such as the Morning Glory formation in Queensland, Australia and roll clouds seen over the English Channel, the Shetland Islands, and in Lithuania. Roll clouds can be seen over the Sea of Cortez, Uruguay, Nova Scotia, Ontario and Brazil. The picture below, however, was taken over the Sandia Mountains as seen from the Rio Grande Valley in New Mexico. The roll clouds extended from approximately Sandia Peak all the way across Albuquerque.




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