Striking, rare asperitas clouds mesmerize Ottawa

People looking upward after rumbles of thunder in and around Ottawa on Sunday evening saw harsh grey storm clouds cut by pockets of light, emerging from places where the clouds seemed to dip and ripple. 

They're known as asperitas clouds.

"It's something that people may not see again in their lifetimes," said Environment Canada meteorologist Geoff Coulson on Monday.

 "There is a little bit of professional jealousy on my part … I only got to see the pictures after the fact." 

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

They are often seen before 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.

Coulson said meteorologists don't know more about asperitas clouds partially because of how rare they are and partially because they were only recently recognized as a unique cloud formation. 

Like 'the surface of a turbulent, choppy sea'

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

He founded 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 in Europe and here in the U.K.," he said in an interview Monday.

A Latin teacher friend told him to use the Latin verb aspero to name this new cloud, Pretor-Pinney said, because the poet Virgil used it to describe the sea "roughened by the northern winters' gales."

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

Pretor-Pinney proposed the asperitas cloud to the International Cloud Atlas in 2009 and it was officially added in 2017. It was the first new cloud since the 1950s, according to Coulson. 

Ottawa Morning7:33Asperitas clouds seen over Ottawa’s skies on Sunday

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

Climate change and future research

Pretor-Pinney said that some people have asked him whether or not these clouds are caused by climate change.

"I don't think it's that at all," he replied. "It is merely that the technology has emerged for us to be able to see people's views of the sky all around the world." 

Coulson concurred, saying the factors that meteorologists do know contribute to an asperitas cloud would have been possible anytime in history. 

But any time one occurs, it's another opportunity to understand them better.