Roll Cloud over Canyon Texas 1-20131104
At about 7:30 in the morning on November 4, just as we were departing from our motel in Canyon, Texas for our drive to Albuquerque, this roll cloud suddenly appeared on the northern horizon, The sky was otherwise perfectly blue, and the cloud stretched as far as the eye could see to both the east and west horizon. It moved towards us remarkably fast, and passed directly over us, heading southward. As we drove westward along I-40 about 30 minutes later we could still see it all the way across the sky, eventually joining the southern horizon and disappearing. We noted no unusual wind or temperature change.
A roll cloud is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud. They differ from shelf clouds by being completely detached from other cloud features. Roll clouds usually appear to be "rolling" about a horizontal axis. They are a solitary wave called a soliton, which is a wave that has a single crest and moves without changing speed or shape. One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia, [caused by] mesoscale circulation associated with sea breezes that develop over the Cape York Peninsula and the Gulf of Carpentaria. However, similar features can be created by downdrafts from thunderstorms and are not exclusively associated with coastal regions. Ref: en.wikipedia.org/wiki/Arcus_cloud