What was that strange cloud seen over the Piedmont on Tuesday?

Photo taken near East Bend around 7:30 p.m. (Credit: Cory Austin Gillenwater)

Photo taken near East Bend around 7:30 p.m. (Credit: Cory Austin Gillenwater)

Photo taken near East Bend around 7:30 p.m. (Credit: Cory Austin Gillenwater)

Photo taken near East Bend around 7:30 p.m. (Credit: Cory Austin Gillenwater)

Photo taken near Clemmons (Credit: Ashlee Hemmings)

Photo taken near Clemmons (Credit: Ashlee Hemmings)

Several people sent us photos of a strange-looking cloud that rolled over the Piedmont on Tuesday night.

The cloud moved across the Piedmont between 7-8 p.m. on Tuesday. Meteorologist Emily Byrd said the cloud appears to be a roll cloud, a rare cloud that typically forms near advancing cold fronts and coastal areas.

What is a roll cloud? A roll cloud is a low, horizontal, tube-shaped type of arcus cloud. Roll clouds usually appear to be “rolling” about a horizontal axis. They are a solitary wave called a soliton, which is a wave that has a single crest and moves without changing speed or shape.

Photo taken in Clemmons (Credit: Heather Alva)

Photo taken in Clemmons (Credit: Heather Alva)

What causes a roll cloud? A downdraft from an advancing storm front can cause moist, warm air to rise, cool below its dew point, and form the cloud. When this happens along an extended front, a roll cloud can form.

It looked like a tornado! While a roll cloud may resemble a tornado, it is rarely associated with severe weather. Unlike a shelf cloud, it is completely detached from other cloud features.

Are roll clouds common? Roll clouds are most commonly observed near the coast. One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia.

Coastal roll clouds have been seen over California, the English Channel, Shetland Islands, Lithuania, Eastern Russia, and many other coastal areas.