Of the thousands of cloud images that had come to his attention, Gavin Pretor-Pinney had never seen anything quite like the menacing, sinewy formation that looked as though it wandered out of Vincent van Gogh's mind during a dark moment.

He posted the photo, taken by Jane Wiggins of Waterloo, Iowa, on his Cloud Appreciation Society site three years ago, and it became an international sensation. Images of similar formations, which looked like so many menacing storm waves about to crash across the sky, started pouring in from all over the world.

The clouds have been sighted as nearby as New York City and State College, Pa., and in all likelihood have appeared over Philadelphia.

Pretor-Pinney believes the world truly is seeing something new under the sun: an unprecedented cloud formation.

Now he is trying to get the World Meteorological Organization to recognize it and give it a fancy-sounding Latin name: altocumulus undulatus asperatus, which translates roughly to one spectacular, mean-looking, don't-mess-with-me cloud.

His efforts have set off a stir in a learned, skeptical - and very tiny - cloud-classification community.

The consensus is that this is nothing new, but the Royal Meteorological Society is taking a hard look at the visual evidence and, if merited, could make a case to the World Meteorological Organization, which has the final say on cloud names.

It would be a momentous change, since the WMO hasn't identified any new cloud varieties since 1956, when it published its first comprehensive, and almost impossible to find, International Cloud Atlas.

The Royal Society says it is taking him seriously.

"He came to us informally," said Paul Hardaker, the society's director, "but we said we would have to treat this properly. We looked at the information and it looked sufficiently interesting, although it wasn't conclusive."

Pretor-Pinney, brandishing several photos, said that a few months back he met with Hardaker and his colleagues at the society's London office. Fittingly, they sat at a table beneath a portrait of Luke Howard, who in a way is responsible for all the hubbub.

Howard, who died in 1864, has been called "the father of meteorology." He and Aristotle can settle the paternity issue, but Howard arguably can be called the biological father in that he invented the cloud-classification system that closely parallels the system beloved by biologists.

Clouds appear in four basic shapes to which Howard applied Latin names: cumulus, the puffy ones; stratus, or layered; cirrus, wispy; and nimbus, what we see when it rains.

However, most of the clouds we see are variations on those themes, such as altostratus, those high, thin clouds that layer the sky before a storm, or stratocumulus, the gray-layered ones that show up as the storm gets closer.

No new cloud formations have been named because just about every one has been identified, says cloud physicist Gregory M. McFarquhar of the University of Illinois.

Margaret LeMone, a cloud specialist at the National Center for Atmospheric Research, in Boulder, Colo., says she has seen Pretor-Pinney's asperatus in Missouri. What's different these days is that people are taking pictures and posting them on the Web.

Hardaker agrees that Pretor-Pinney's cloud might already be classified. "Some people have argued it's just an extreme case of an existing variety," he said. He added that it may be a hybrid form of an undulatus, or wavy cloud, and a mammatus, a pouchier variant of a cumulus cloud.

Jesse Ferrell, a meteorologist and cloud enthusiast with AccuWeather.com, in State College, said he saw a similar formation out his way a few years ago but couldn't be certain it was unique. He thinks it forms when extremely dry air at the surface clashes with more humid air in the upper atmosphere, conditions that could well occur in the Philadelphia region.

Pretor-Pinney says that perhaps his cloud isn't unique, but he believes the WMO should at least consider adding new formations to its list.

"I think it's a fantastic system," he said, "but why should the book on classification be closed?"

Ferrell noted that cloud classification isn't necessarily hard and fast. Almost all clouds are hybrids of different cloud forms. "There's a lot of gray areas," he said.

For asperatus to make the grade, the Royal Society would have to recommend it to the United Kingdom Meteorological Office, which, in turn, would submit it to the WMO. The process could take years, said the office's John Hammond.

In the meantime, Pretor-Pinney, 41, who has a 3-year-old daughter whose middle name is Cirrus, says he'll keep looking up. A philosopher, he says he has been watching the skies since he was 4.

In 2005, he formed the Cloud Appreciation Society because he thought clouds were getting a bad rap. "Everyone complains about them," he said. " 'There's a cloud on the horizon. There's a cloud hanging over them.' I thought someone needed to stand up for clouds."

Two years ago, he published The Cloudspotter's Guide, an offbeat and irreverent treatise on clouds that was rejected by more than 25 publishers before it became a best-seller.

Today, he says, his society has more than 16,000 members in 71 countries.

Wiggins said she had not known about Pretor-Pinney's site until she took the now-famous picture, and a member of her photo club suggested she submit it.

"I named it Armageddon," she said. "It looked like someone was going to come out of the sky. It was so unreal."

But perhaps not quite as unreal as the reaction to her picture.