On the leading edge of a cold front gliding through the D.C. metro region this morning, a scary-looking tube-shaped cloud stretched across the skies in northern Virginia. It ended up signaling mostly benign weather, in the form of some showers that followed its passage.
This cloud type is known as a roll cloud or arcus cloud.
Resembling a giant dough roller, a roll cloud is completely detached from its parent shower or thunderstorm.
“Sinking cold air causes warm, moist air on the planet’s surface to climb to higher altitudes, where the moisture condenses into cloud form,” explains Live Science. “Winds from the storm “roll” the cloud parallel to the horizon, creating an effect that looks much like a horizontal tornado.”
These clouds are relatively rare, less common than its cousin the shelf cloud, which often can be found along the leading edge of thunderstorms, forming along their gust fronts. Roll clouds seldom produce violent winds, whereas shelf clouds are sometimes a harbinger of severe weather.
Here is a large assortment of the roll cloud pictures, photographed by readers across northern Virginia this morning.
— unbiased (@imunbiased) September 16, 2013
— NWS SFSC (@NWS_SFSC) September 16, 2013
— Jeff Essex (@SXDrums) September 16, 2013
— KSR (@ksrgatorfn) September 16, 2013
— Chase Tralka (@ChasePSU) September 16, 2013
— Matt Brooks (@ma11br00ks) September 16, 2013
— Andy Green (@AndyGreenDive) September 16, 2013