On Tuesday, Erin Bibo was in the North Truro home where she’s vacationing with family when she suddenly heard the crash of thunder outside.
Curious about the storm, Bibo peeked out the window and then walked across the street to the nearby beach to get a better look at the darkening sky.
There, in the distance, she spotted a scene that captivated people up and down the coastline: a dramatic and ominous cloud moving swiftly across the ocean, eating up the skyline.
Before it could disappear from sight, Bibo took out her phone and hit record. Her 13-second clip of the cloud formation — known as a “shelf cloud” — was one of many captured by residents and tourists and then shared on social media this week.
“It was just unbelievable to see those clouds rolling in, and the difference between what the sky looked like,” said Bibo, 41, who is visiting from Washington, D.C. “It was moving in pretty quickly, so I wanted to capture it.”
Glenn Field, a meteorologist with the National Weather Service, said although shelf clouds aren’t considered rare, most formations aren’t “as well organized” in their appearance as the one on Tuesday night.
The striking images of the shelf cloud, which some people likened to an alien spacecraft, even impressed local weather experts.
“Whoa! How about this shelf cloud moving over the Outer Cape!” WBZ meteorologist Jacob Wycoff said Tuesday, in a tweet that included a picture of the clouds spilling across the sky like a tsunami.
A.J. Burnett, a weather producer with WCVB, called the scene “INCREDIBLE,” and tweeted an image of a group of people gazing out at the shelf cloud from a North Truro beach.
Shelf clouds form on the leading edge of strong thunderstorms, usually when storms are more severe, Field said in an e-mail to the Globe. He said the heavy rain from the thunderstorm’s downdraft cools the air, which then “spreads out and literally acts the way a snowplow does in winter.”
“The cooler air is more dense and it forces the lighter, warmer air upward on the leading edge of the storm,” he said. “The upward motion cools and condenses the air,” forming the cloud.
Shelf clouds are typically accompanied by a strong gust of wind as they move overhead, Field said. Then, the rain follows.
Like Bibo and her family, Belmont resident Leslee Masten has also been vacationing in North Truro this week, and was able to get a glimpse of the stunning scene as it barreled through the region.
Masten was on the deck of the Top Mast Resort, overlooking Cape Cod Bay, with her partner, Dennis Hagler, 76, when they saw the cloud “roll in from Provincetown towards us.”
“I don’t think I’ve seen one quite like this!” said Masten, 63. “This one was huge and coming over the bay — there was nothing obstructing our view. It was very dramatic!”
Masten could see lightning strikes in Provincetown below the sizable cloud, as it made its way toward them. The couple was able to watch it for some time before it got too close and they had to head inside and “shut the sliders against the rain, which was coming down sideways.”
Bibo said the “foreboding scene” led to other people also coming out of their cottages to take pictures and videos.
“It seemed like we were all just having this really special collective experience of seeing something that you certainly don’t see every day or often at all,” she said. “I was really glad I had my phone on me to capture it.”