A Sydney photographer has captured a rarely-observed rainbow cloud phenomenon called “iridescent clouds”.

Maria Milford was driving to the Gold Coast when she spotted the iridescent clouds over the sky in Woolgoolga, NSW.

She quickly whipped out her camera to capture the phenomenon, which she said lasted a couple of hours. 

“The whole time, this effect was in the sky — we watched it all the way until sunset from the car,” Milford told 9News.

“I was really surprised that no one around seemed to notice.”

Here’s how you can spot iridescent clouds.

What causes iridescent clouds and how to spot them

If you want to spot iridescent clouds, you need to know what to look for. 

The phenomenon, which seems magical, will appear as a rainbow shimmering display of colours in the sky thanks to the diffraction of sunlight.

It is most visible when the light source (the sun or moon) is hidden behind a thin layer of clouds.

The phenomenon is caused by the diffraction of sunlight by tiny water droplets in the sky.

“The sunlight is being scattered by the ice crystals in the high cirrus cloud, allowing the viewer to see the different colours similar to a rainbow,” a spokesperson from the Bureau said.

It’s more likely to occur in thin and wispy clouds, but it is possible to spot it on the edges of thicker clouds as well, the weather expert said.

How rare are iridescent clouds?

The Bureau of Meteorology said while the phenomenon isn’t a rare occurrence, it is rare to observe it.

The phenomenon happens randomly and there’s no “best” time of day to spot it – it can even occur at night.

It can be seen when the sun or moon is at a specific relative position to the observing person and the cloud coverage.

“The effect is not uncommon, although the brightness of the sun itself can make it difficult to observe at times,” a BoM spokesperson said.

“Iridescence can also be observed in clouds around the moon, but the effect is typically more muted.”

The Bureau warned people should not look directly at the sun, as it can cause eye damage. 

“People should always take care when observing such phenomena since you are looking directly at the sun, albeit shielded (temporarily) by the cloud.”