A roll cloud is a type of arcus cloud that looks like an enormous looming roller making its way over the landscape. Here's one pulicized Tuesday by NASA, shot in Wisconsin and credited to Megan Hanrahan (Eazydp on Wikipedia, public domain).
These rare long clouds may form near advancing cold fronts. In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud. When this happens uniformly along an extended front, a roll cloud may form. Roll clouds may actually have air circulating along the long horizontal axis of the cloud. A roll cloud is not thought to be able to morph into a tornado. Unlike a similar shelf cloud, a roll cloud is completely detached from their parent cumulonimbus cloud.
Here's another, by Suzanne Tucker:
