Residents in NSW's Northern Rivers were lucky enough to catch sight of an "extremely rare phenomenon" in the sky on Tuesday morning, struck by its unusual yet familiar-looking shape.
"I'd never seen clouds like that before," Ballina resident Mick Sigle told Yahoo News Australia.
It was around 7.30 am that the "amazed" council worker spotted a wave-shaped cloud ahead and instantly snapped a photograph to send to his partner Kerri. He said he had never seen something so beautiful in the sky before, sharing his delight with a colleague as they both wondered what they were looking at.
Unbeknown to Mick, his partner's friend and fellow resident of the northern NSW town was also amazed by the unusual weather pattern, so much so that she stopped her car to observe the "unusual" clouds in all their glory.
"It just looked so different I stopped the car and took a pic," she shared with Yahoo.
The local residents may not be the only awestruck observers of these clouds, with speculation that these "swirly" clouds were an inspiration for Vincent Van Gogh's famous painting, 'Starry Night'.
Wave-shaped cloud explained: 'Instability in flows'
Martin Singh, from the School of Earth, Atmosphere, and Environment at Monash University, confirmed that what the local residents spotted is called a Kelvin-Helmholtz cloud.
The "rare" cloud is evidence of natural forces that are constantly at play around us, yet aren't usually visible unless two key components complement each other: low level cloud and a change in wind direction.
"Essentially we are seeing the way the air is moving," Professor Singh said, before adding, "Air is moving everywhere but you just can’t see it because there’s no cloud to visualise it with".
"One layer has a cloud, and one layer doesn’t and the instability is happening on that boundary," he continued.
When the density of air is moving with varying height (an instability known as stratification), and wind speed changes in right angles to this air (a phenomenon known as wind shear), it exerts a turning force, creating the shapes.
The "waves" in the sky are like arrows pointing in the direction of the wind movement.
Just like any other cloud, Professor Singh said these wave-shaped clouds can be seen in the sky for mere minutes or for up to a hour, with these lucky residents who caught the phenomenon being at the right place at the right time.
"I think it is safe to say that they are quite rare, and the example in the photo is particularly stunning," Professor Singh concluded.
Do you have a story tip? Email: firstname.lastname@example.org.