Social media can be a blessing or a curse; we’ve all seen both sides of that one. However, the ability to immediately share photos with anyone who might be interested is a bonus.

This morning, Rob Thom tweeted a photo of a cloud formation that caught his eye  near Lower Greenwich, on the St John River.  The cloud in the photo is a roll cloud; a cloud that belongs in the arcus family of clouds.

Roll cloudsand shelf clouds are two types of arcus clouds; slight variations in their generation and look being the difference. Arcus clouds are low, horizontal cloud formations associated with the leading edge of thunderstorm out flow, or with a cold front even in the absence of thunderstorms. But there’s another less common type of roll cloud and that’s the coastal roll cloud.

The coastal roll cloud is often referred to as a Morning Glory and is the result of a particular configuration of land and sea.  As moist sea air is lifted to the crest of the atmospheric wave, it cools and condensation forms a cloud. The roll is often accompanied by sudden wind squalls, intense low-level wind shear, and a sharp pressure jump at the surface.The cloud is continuously formed at the leading edge while being eroded at the trailing edge. For this reason they appear to be rolling on a horizontal axis. Sometimes there is just one roll, but there can be as many as 8 to 10 in a series.

Another little bit of meteorological magic!


Chief Meteorologist

Cindy Day