Those strange clouds over Vancouver are called 'asperitas,' and they are very rare

A striking cloud formation, known as asperitas, formed over Vancouver on Friday.

The rare type of cloud, whose name is Latin for "roughness,"was first observed in 2006 by an amateur cloudspotter in Cedar Rapids, Iowa.

CBC science specialistDarius Mahdaviconfirmed the formation saying, "I've only properly seen them once before ... but these ones are even better."

Mahdavi said though the clouds look dark and stormy, they don't produce rain.

Meteorologists don't know exactly what causes an asperitas cloud to form, but there are theories and asense for the kinds of circumstances they form in.

They are often seenbefore or after storms, in an unstable atmosphere with lots of updrafts and downdrafts, and whenever there are significant changes in wind direction higher in the atmosphere.

In 2017, they were added to theWorld MeteorologicalAssociation'sInternational Cloud Atlas, where they were described as an intense, chaotic wave-like formation.

Like 'the surface of a turbulent, choppy sea'

Gavin Edmund Pretor-Pinney argued about 15 years agothat asperitas clouds should be considered a unique cloud formation.

Hefounded the Cloud Appreciation Society in 2005, which shares cloud information and connects cloud spotters. He kept seeing one unusual cloud crop up.

"They would come in every now and then from different places:Australia, from Greenland, from across the U.S., from Europe and here in the U.K.," he said in an interview with CBC News in October after asperitasclouds were seen in Ottawa.

"It's like looking up at the surface of a turbulent, choppy sea from below," he said.

He said they only have one or two confirmed sightings of those clouds in Canada every year, primarily over Ontario.

LISTEN | An interview about asperitas clouds:

Ottawa Morning7:33Asperitas clouds seen over Ottawas skies on Sunday

The founder of the Cloud Appreciation Society helps us understand how the rare wave-like clouds form.