Australia Weather News

Sunsets in northern Australia have looked a bit different lately, with a little-known weather phenomenon slicing through golden hour.

Distinct ribbons of blue sky, dividing typical yellow and orange sunset hues into strips of colour, have been photographed across the Top End on several occasions this month (and over the years).

According to the Bureau of Meteorology (BOM), the stunning views are the result of an optical illusion called crepuscular rays.

"Those are those visible beams of sunlight that we see radiating through the sky, generally at twilight," BOM meteorologist Hanna Whiteside said.

Crepuscular rays can happen any time the sun is visible in the sky — not just at sunset — and anywhere in the world, but chances are you've never heard of them.

Here's what we know about the weather phenomenon.

What are crepuscular rays?

Ms Whiteside said the phenomenon occurs when the sun is partially obscured by low-hanging clouds, giving the illusion of a sunset being split into different colours.

"The beams that we're seeing across the sky, they're actually shadows from the clouds," she said.

The name originates from Latin, referring to the time of day crepuscular rays are most visible from land: twilight.

"They're also commonly known as God's rays or Jacob's ladder," Ms Whiteside said.

Ms Whiteside said crepuscular rays were generally photographed at sunset because, as the sun goes down, light scatters through particles in the atmosphere — which creates the stunning colours that capture our attention.

How do these rays happen?

Ms Whiteside said crepuscular rays can occur "whenever there is some low cloud about", as long as they hold enough "particulate matter" to scatter the light.

"If there's cloud, just above or at the horizon — and the sun is setting behind it — we'll see … the shadows start to radiate in those beams," she said.

"We can also see the phenomenon when the cloud is low in the sky but the sun might not have set yet, so the sun is still above the horizon but we'll get that partial obscuration."

How the rays appear in the sky will "depend on the shape of the cloud and how the sunbeams are passing through", Ms Whiteside said, and more particles in the atmosphere "create more of that scattering" light.

With preventive burns now taking place ahead of the Top End's bushfire season, she said the extra smoke in the air increased the chance of clouds forming in the right spot for crepuscular rays to occur.

You might even be lucky enough to see the inverse effect, according to Ms Whiteside.

"Interestingly, there is a counterpart phenomenon to this, termed anti-crepuscular rays," she said.

"They may appear fainter, on the opposite horizon to crepuscular rays, and they will appear to converge on the horizon opposite the setting sun."

Do crepuscular rays only occur at sunset?

The weather phenomenon of crepuscular rays is possible whenever the sun is visible, but Ms Whiteside said photos captured during the day "generally won't be seeing those same beautiful colours" as those at golden hour.

"At sunset, the sunbeams are travelling through a thicker amount of atmosphere and creating that scattered, beautiful red and orange colour," she said.

"But [crepuscular rays] can happen any time of day."

Why does perspective matter?

Ms Whiteside said crepuscular rays radiate outward from the sun, and to those standing below, the shadows appear to fan out in a circle around that "initial source point".

But the beams themselves tend to run parallel to one another, according to the BOM.

"It's more of a perspective phenomenon, so the beams are actually generally parallel," she said.

She likened this "human perspective" to how we interpret a long road, which apparently narrows as it continues into the distance and grows in size as it gets closer to the individual.

It's the same with crepuscular rays: the sun is further away from us, but the shadows being cast by clouds appear much closer, so they give the appearance of widening as they come towards us.

Darwin no stranger to crepuscular rays

Since this weather phenomenon can occur at any time of year, here are some older photos captured in the Top End by ABC audience members over the years.

For the chance to be featured on the ABC, please email us your original local weather photos.