Cloud Optical Phenomena Australia: 7 Common Sky Effects

Cloud optical phenomena Australia includes solar halos, sun dogs, circumhorizontal arcs, glories, coronas, cloud wreaths and anthelia. These effects form when sunlight or moonlight interacts with ice crystals, water droplets or fine particles in the air through light refraction, scattering, diffraction and reflection.

Key takeaways

• Thin high cloud can produce clear halo displays when ice crystals line up in the light path.

• Rayleigh scattering helps explain the blue sky, while Mie scattering is linked with haze, smoke and a washed-out sky.

• Circumhorizontal arcs need a high sun angle and aligned ice crystals in cirrus clouds.

• Bushfire haze can make a halo harder to see or can change how bright it looks.

• The Bureau of Meteorology uses cloud and sky observations to help describe the conditions that support these displays.

What are cloud optical phenomena?

Cloud optical phenomena are visual effects caused by sunlight or moonlight passing through the atmosphere. The light can bend, scatter, split or reflect as it meets ice crystals, water droplets or tiny particles in cloud layers. The result can be rings, bright spots, coloured arcs or faint glows.

If you know the cloud type and where the sun sits in the sky, you can often work out which display is likely. The Bureau of Meteorology (BOM) uses cloud classifications to describe the layers that commonly support these effects, especially thin high cloud.

What is the most common cloud optical effect in Australia?

Solar halos are one of the best known cloud optical effects seen in Australia. They appear as a ring around the sun or moon when light passes through ice crystals in high cloud.

A 22° halo around the Sun, observed over Bretton Woods, New Hampshire, USA on February 13, 2021. By Mitchell Underwood - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=99843640

Cloud optical phenomena Australia: how light creates these displays

Cloud optics start with the way light moves through the atmosphere. In high cloud, hexagonal ice crystals can bend light at set angles. In lower cloud or haze, droplets and fine particles can spread light and soften contrast.

Refraction bends light as it enters and leaves a particle. Total internal reflection can bounce light inside a crystal or droplet before it exits. Diffraction spreads light around small droplets, which can create coloured rings or a soft glow. Scattering changes the path of light in the air, which is why haze and bushfire smoke can change how these displays look.

Rayleigh scattering and Mie scattering

Rayleigh scattering is the process that makes the daytime sky look blue. It works most strongly on shorter wavelengths of light. Mie scattering happens when light meets larger particles such as water droplets, dust or smoke. That is why haze can look white, grey or milky.

Rayleigh scattering causes the blue colour of the sky at large angles to the direction of the solar rays and yellow or orange colours for light from the direction of the Sun. By P J Hansen - originally posted to Flickr as Arctic Sunset, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=9496105

This helps explain why a clean halo can look sharper in clear air, while smoke or haze can blur the edges. Researchers at TERN use satellite data to track solar irradiance across the continent, which provides insight into how light interacts with Australia's atmosphere. If you are near a bushfire smoke plume, the sky may still glow, but the shape of the display can be harder to pick out.

Hexagonal ice crystals and refraction

Hexagonal ice crystals are the main reason many halo displays form. These crystals sit in high cloud such as cirrus and cirrostratus. Their six-sided shape acts like a tiny prism and bends light at familiar angles, which is why some displays have a regular size and shape.

How do ice crystal clouds create rainbow-like arcs without rain?

Ice crystal clouds can produce colour because the crystals separate and bend sunlight at different angles. This is not the same as a rain rainbow. The effect comes from light moving through ice rather than raindrops.

Why Australia's Latitude Enhances Circumhorizontal Arc Visibility

Australia’s latitude can help make circumzenithal arcs easier to see because the sun can reach a high enough angle in summer for the display to form. When the sun is too low, the arc will not appear. When the sky is clear except for the right type of high cloud, the rare cloud colors can stand out well.

7 cloud optical phenomena seen across Australia

These are some of the main sky displays you may notice. Their visibility depends on cloud height, particle shape and the angle of the sun or moon.

Diagram explaining the physics of cloud optical phenomena Australia skies produce via refraction and diffraction

1. 22-degree solar halo

The 22-degree solar halo is a ring that forms around the sun or moon. It comes from light passing through ice crystals in high cloud, usually cirrostratus. The ring can look white with a faint red inner edge.

A pale 22-degree solar halo visible through high altitude cirrus clouds

2. Sun dogs

Sun dogs appear as bright spots on either side of the sun. They often sit on the same horizontal line as the sun and can look sharp when the air is clear.

Bright sun dog alignment glowing beside the sun during a low atmospheric winter morning

3. Circumhorizontal arc

This is a colourful arc that sits well below the sun. It forms only when the sun is high enough and the ice crystals are aligned in the right way.

4. Glories

Glories show up around the shadow of your head or aircraft shadow on cloud or fog. They are made by backscatter and diffraction from small droplets.

A distinct glory showing circular rainbow rings cast onto marine stratocumulus cloud from an aircraft window

A solar corona up Beinn Mhòr (South Uist). By Callicious - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=109189731

5. Coronas

Coronas are small rings around the sun or moon. They are usually caused by diffraction from droplets of similar size.

Colourful corona forming as light diffracts around uniform water droplets in an altocumulus cloud layer

6. Cloud wreaths

Cloud wreaths are pale glows or coloured edges around cloud tops. They are often seen where droplets are small and evenly sized.

7. Anthelion

An anthelion appears opposite the sun and can be faint. It is linked with ice crystals and the way light is reflected and scattered back toward the observer.

Distinguishing Between 22-Degree Halos and Australian Bushfire Haze Effects

A 22-degree halo has a regular circular shape around the sun or moon. Bushfire haze usually does not form a neat ring. Instead, it can make the sky look dull, yellow-grey or red, and it can blur the edge of any halo that is present.

If you are trying to tell them apart, look for shape first. A halo keeps its round form. Haze changes the colour and brightness of the whole sky.

How to spot these effects safely

Never look straight at the sun. Use your hand, a building or a tree to block it from view. If you are watching from a coast, hill or open paddock, scan the sky around thin high cloud rather than staring at the bright centre.

For cloud names and layer heights, see the main cloud types in Australia guide. That can help you match the sky shape with the display you see.