Shelf Cloud Season in Full Force: What are They and How They Form
Social media’s most popular cloud made some spectacular appearances this week. Welcome to the stage, the super scary shelf cloud!
Last week’s #WackyWednesday “Hulk invasion” clouds were a tough act to follow, but the “cloud on a shelf” still reigns supreme.
Residents of the Twin Cities posted dozens of snapshots of their ominous guest Tuesday evening. Here are just a few…
And on the same day, it looked like a scene out of the movie Independence Day in our nation’s Capital. Some Twitter users even called the storm a “mothership” (not without scientific controversy though).
What are Shelf Clouds and How They Form
A shelf cloud certainly looks scary. And it’s usually a good idea to head indoors when you see one. But there are also some misperceptions about them we can clear up.
The World Meteorological Organization (WMO) refers to a shelf cloud as an “accessory” to a parent cumulonimbus cloud. It’s official name is Arcus. Shelf clouds are often falsely reported as wall clouds or funnel clouds, but the National Weather Service (NWS) in Louisville clarifies.
“A shelf cloud will usually be associated with a solid line of storms. It may appear to rotate on a horizontal axis,” the NWS said.
“[But] wall clouds will rotate on a vertical axis, sometimes strongly. The wall cloud is much smaller and more compact than a shelf cloud and is usually under a rain free cloud base,” they add.
Both cloud types are created by adjoining air masses moving in opposite directions. However, the shelf cloud forms from a mature cluster or line of storms. And here’s how.
A cumulonimbus cloud is created by violent upward motion in the atmosphere. As the storm matures, what goes up must come down. This is why it rains so hard when the storm is overhead. The intense downward motion (called a downdraft) spreads out over land, often creating destructive wind gusts up to 60 or 70 mph. This is what creates the dark colors on the underneath side of the “shelf”.
Now, think about where the relatively calm air in front of the storm must go. It can’t go underground. So it is naturally pulled back up into the storm on the front side of the shelf. This condensation process creates brighter clouds on the edge (or top) of the shelf. These are usually illuminated by sunlight or translucent rain-free clouds prior to the storm arriving.
The adjoining upward and downward motions that make up a shelf cloud can sometimes give the appearance of carpet being rolled out across the sky. As the storm weakens, the shelf cloud can even become detached from the parent cloud. This is referred to as a “roll cloud”.
So the next time you see a shelf cloud, you’ll certainly want to grab your camera. But do it quickly! Dangerous winds, frequent lightning, and very heavy rain almost always immediately follow the passage of the “shelf”.