Cloud of the Day – Roll Cloud
Roll cloud is a form of arcus cloud, defined as a low, horizontal cloud formation. Roll cloud is the milder of the two forms of arcus cloud. The other, shelf cloud, is often associated with violent weather. Roll clouds get their name not only from their appearance, but also from the fact that they seem to roll around their horizontal axis.
They are formed when a downdraft from an approaching cold front pushes warm, moist air up in front of it. If it goes over top and gets pushed down again, it can set up a circulation. If the warm, moist air is cooled to the condensation point, a roll cloud forms.
Queensland, Australia is blessed with predictable, relatively frequent appearances of normally rare roll cloud, which they call Morning Glory Cloud. They can be seen every October, the result of seasonal patterns of ocean and atmosphere. The link will take you to a video compiled from footage shot by Rob Thompson over a seven year period. This link will take you to a YouTube video shot in Texas.
Roll cloud is harmless, but it might presage weather with the advancing cold front.