Time lapse GIF of an arcus shelf cloud I captured on video on July 28, 2017, in Konskie, Poland.
Soooo, cool things happen when I come to visit my family in Poland. How cool? This cool—I saw an arcus shelf cloud, and was able to get some photos and video of it! I couldn’t get the whole event on video because the rain came pouring, but I got enough to make a little time lapse of it as it blasted through!
Here’s a better look at it in a snap I took before capturing it on video:
Snap of an arcus shelf cloud I captured just before a gigantic rainstorm.
So, what is an arcus cloud? Arcus clouds come in two types: shelf clouds, and roll clouds, with roll clouds being the rarer of the two. Arcus clouds are low, horizontal clouds that are (usually) on the leading edge of a thunderstorm outflow (mine certainly was). These clouds form when the rain-cooled air of a thunderstorm gust front descends and spreads across the Earth’s surface; this pushes warm and moist air upward until it reaches the rain-cooled air, causing the warm and moist air to condense into an arcus cloud. This cloud may or may not be attached to its parent cumulonimbus cloud, a notorious bringer of thunderstorms. Whether they’re attached to their parent cumulonimbus cloud or not is how we distinguish between the two types: shelf clouds are attached to the higher cumulonimbus clouds (like mine), while roll clouds are distinctly detached from them.
When warm, moist air is pushed upward by the downward flow of cool air from a storm cloud, the moist air could condenses right up to the parent cloud, but it this new arcus cloud could get separated from the parent cloud. If the attachment is visible, you have a shelf cloud. If not, you have a roll cloud. Roll clouds could form if the arcus cloud rushes ahead and dissociates from the parent storm cloud, or the roll cloud could be a remnant of a storm cloud that survives after the storm is over. You can read a bit more about these types of clouds, here.
The two also look different. Shelf clouds look like a wedge that heads the incoming stormy cumulonimbus clouds, and the whole feature could be sort of curved. As shelf clouds move forward, the leading parts of the shelf cloud rise upward, while the trailing side is turbulent and wind-torn. Here’s another image of a shelf cloud:
Shelf cloud viewed over Enschede, Netherlands.
Credit: John Kerstholt (original upload by Solitude)
Roll clouds, on the other hand, look like a horizontal, almost cylindrical, cloud form. They are distinct from their parent cloud and appear to roll about a horizontal axis! Here’s what a roll cloud looks like:
A roll cloud above Racine, Wisconsin. Credit: Eazydp
Videos are always helpful, so let’s look at a couple of time lapses of the two types.
Here’s a roll cloud captured in Brownsville, TX, from NWSBrownsville on YouTube:
See how the cloud appears to roll about a horizontal axis? The roll cloud stretches pretty wide across the horizon and keeps its cylindrical form as it surges forward.
Now, let’s look at a shelf cloud time lapse taken in St. Louis, MO, by Tom Stolze:
Here, the attachment of the shelf cloud to the parent cloud is apparent, even though the leading part surging forward may look somewhat like a roll. Pretty similar to the one I captured (I probably missed its formation as I caught it just a couple of minutes before the downpour). I had trouble figuring out which type of cloud I captured until I noticed that the “roll” part in mine appeared to have layers of clouds atop it (more visible in the photo than the GIF). So, not the rarer of the two, but it was really cool to see, and I’m glad I got to capture it. I hope you get a chance to (or have gotten to) see one of these phenomena, too! But if you do see one of these clouds approaching from a distance, take shelter—you may have a terrible storm coming!