'Very unusual hole in the clouds' captured in Kaikoura

A "very unusual hole in the clouds" has appeared in the sky above Kaikoura which has left one resident describing the moment as a "once in a lifetime event."

Kaikoura's John Vasta photographed the cloud formation around 7.30am yesterday after he looked at the sky when he was walking out the back door of his property. 

"I've never seen anything like it before," Mr Vasta told 1 NEWS NOW. 

"It was one out of the bag."

The keen photographer described the sky looking like an illusion and said he has never witnessed anything like it in the 45 years he's lived in the town. 

According to a MetService spokesperson, the cloud formation is called a fallstreak hole, which is "well documented around the world."

In an explanation post on MetService's Facebook page, a fallstreak hole is a "large circular or elliptical gap that can appear in cloud located in the mid to upper atmosphere."

"Such holes are formed when the water temperature in the cloud is below freezing but the water has not frozen yet due to a lack of ice nuclei (particles that act as initiators and centres for ice crystal growth) - the water is "supercooled," the Facebook description says. 

"When ice crystals are present in the cloud it will set off a domino effect known as the Bergeron process.

"Because the air is saturated with respect to water but super-saturated with respect to ice, when ice crystals are introduced into the cloud the water vapour in the air condenses onto the crystals while evaporating the supercooled water droplets to maintain equilibrium. The result is a large "hole" in the cloud.

"They are called "fallstreak" holes because the ice crystals often kept falling through the cloud, generating brush-like streaks of ice crystals below it."

The fallstreak hole which John Vasta captured yesterday morning. Source: John Vasta