Roll clouds stretch like a long horizontal tube, spinning as they process across the sky. This class of arcus cloud is relatively rare but occasionally forms in areas where cool air is sinking, along the downdraft of an oncoming storm or in coastal regions as a result of sea breezes. The cooler, sinking air displaces warmer, moist air to higher altitudes where the moisture condenses into a cloud. Winds then roll the cloud parallel to the horizon. Roll clouds are a form of soliton, a solitary wave with a single crest that moves without changing its shape or velocity; this is why the cloud appears so regular as it moves across the sky. These clouds are sometimes also called Morning Glory clouds and form regularly off the coast of Queensland, Australia around October. (Video credit: T. and B. Mask)
A reminder, for those attending the APS DFD conference this weekend: my FYFD talk will be Sunday evening at 5:37pm in Rm 306/307. I will be discussing, among other things, the results of July’s reader survey and science communication.