A rare tube-shaped cloud created a spectacle in northern Virginia yesterday (Sept. 16) as it crept across the sky.
The U.S. National Weather Service's Baltimore/Washington office in Sterling, Va., snapped a panoramic photo of the ominous-looking roll cloud as it moved over the area.
"An outflow boundary from the showers associated with the cold front produced a very well defined roll cloud that passed over our forecast office this morning just before 7:30 am," the NWS office said in a Facebook post.
Roll clouds belong to a family of low-forming clouds known as arcus clouds. They are related to the more common, wedge-shaped shelf cloud.
Shelf clouds often form on the leading edges of thunderstorms (or sometimes a cold front) as rain-cooled air ploughs under the warm, moist air that's closer to the ground. This warm, moist air is forced upwards and its water vapor condenses into the scary-looking cloud formation.
Roll clouds are sometimes born out of a thunderstorm's gust front, too, and they are "rolled" into shape by storm winds. But unlike shelf clouds, rolls clouds are not attached to the rest of a storm or any other cloud formation.
Though they may look like horizontal tornados, roll clouds aren't dangerous in themselves. Sometimes, they are even born out of rather innocuous weather. Coastal roll clouds can form as a result of winds coming from the sea in relatively calm conditions, like the so-called Morning Glory roll cloud, which forms regularly in fall months off the shores of Queensland, Australia.
The cloud that stunned the suburbs of the capital was a harbinger of rather mild weather; only showers followed its passage, according to the Washington Post's Capital Weather Gang blog.