22-Degree Solar Halo: How Ice Crystals Paint Rings Around the Sun

A 22-degree solar halo is an optical phenomenon caused by refraction of sunlight through hexagonal ice crystals in high-altitude cirrus clouds. Light bends as it passes through the crystals, then forms a pale ring around the sun.

Key takeaways

• A 22-degree solar halo forms when sunlight passes through ice crystals high in the troposphere.

• The halo appears as a ring around the sun with a 22-degree arc radius.

• High-altitude cirrus clouds and cirrostratus clouds are the cloud types most often linked with halos.

• Halos can appear before a change in weather, but they do not guarantee rain.

• Never look directly at the sun. Use a solid object to block the solar disk if you want to view the halo safely.

The atmospheric physics of a 22-degree solar halo explained

What is a 22-degree solar halo?

A 22-degree solar halo is a ring of light around the sun formed by refraction through ice crystals in upper-level cloud. The crystal shape matters because sunlight enters one face and leaves through another, creating the halo's bright circle. This is one of the most 7 common sky effects seen in Australian skies.

Graphic illustrating light refracting through a hexagonal ice crystal to form a 22-degree solar halo arc.

The science of minimum deviation in Australian skies

The ring forms because light bends as it enters and leaves a hexagonal ice crystal. At one angle, the ray takes the least-deviating path through the crystal. That minimum deviation is what gives the halo its regular size.

The exact geometry depends on the crystal shape and the refractive index of ice. Snell's Law describes how light changes direction when it moves between air and ice. Millions of crystals in the same cloud layer then scatter light into a circle that surrounds the sun.

Snell's Law and the refractive index of ice

Snell's Law explains why the ray bends at the air-ice boundary. In a halo, the refractive index of ice is the key property that controls the change in direction. That is why the 22-degree solar halo has a stable shape rather than a random blur. The process is similar to how cirrostratus clouds act as a canvas for solar displays.

The halo often looks white, with a red fringe on the inside and, in rare cases, a violet fringe on the outside. That colour split comes from the way different wavelengths bend by slightly different amounts. Rayleigh scattering can also affect how the sky looks around the halo, but the ring itself is mainly a refraction effect.

A stunning moon halo at night, demonstrating how ice crystals create rings around celestial bodies, similar to solar halos. By Ross Corkrey - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=90011079

Cloud formations that produce halos

High-altitude cirrus clouds

You need high cloud made of ice, not rain cloud, to form a halo. Cirrus clouds sit high in the troposphere and are made up of ice crystals rather than liquid drops. The Bureau of Meteorology describes halo displays as a sign of ice crystals in upper cloud.

Interpreting halo brightness: cirrus vs cirrostratus density

The brighter and more complete the ring, the more uniform the cloud layer is overhead. Thin cirrus may show only a section of the halo. A broader cirrostratus layer is more likely to produce a full ring. If the cloud thickens too much, the sun can disappear behind it and the halo fades.

Hexagonal ice crystals: columnar and plate crystals

Halos form from hexagonal ice crystals, especially columnar and plate crystals. Their flat faces and six-sided structure let sunlight enter and leave at the angles needed for the ring. The crystals do not all need to point the same way, but they do need to be shaped correctly for the optical phenomenon to appear.

Weather conditions and halos in Australia

Winter cold fronts in southern Australia

Halos are often easiest to see ahead of a change in weather, especially when higher cloud spreads in from a front. In southern Australia, that often means winter and the passage of cloud bands that bring cirrus and cirrostratus. If you see a halo, it is worth watching the sky for a later change, but not treating it as a rain forecast on its own.

Split visual comparison showing a bright rainbow near the ground and a pale solar halo high in cold upper cloud.

Are sun halos rare in Australia?

No, they are not rare. Many people miss them because they are looking away from the sun, or because the ring is faint. If the cloud is thin enough and the sun is not hidden, the halo can be easy to spot.

Visibility over the Great Dividing Range and cold air masses

Clear air helps the ring stand out against the sky. That means halos can be easier to notice over higher ground and in places with less haze. You may also spot a stronger ring when cold upper-level air supports ice crystal formation in cloud.

Comparing atmospheric optics

Difference between halos, arcs, and sun dogs

Halos are circular rings around the sun, while sun dogs are bright patches that appear to the left and right of the sun. All of these are atmospheric optics linked to hexagonal ice crystals, but they form in slightly different ways depending on crystal shape and orientation.

For example, columnar and plate crystals can create different features when they are aligned in the cloud layer. That is why one sky can show a halo, a sun dog, or both at once. You can see these varied formations in atmospheric optics time-lapses.

Why halos are not rainbows

Rainbows form when sunlight reflects and refracts inside water droplets, usually with rain in the opposite part of the sky. A 22-degree solar halo forms much higher up in ice cloud, so it can appear even when the ground below is dry. The two features can look similar at a glance, but their causes are different.

22° halo at Großkrotzenburg, Germany. By Patrick Looß - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=93889081

How to view a 22-degree solar halo safely

Never stare at the sun. Instead, shade the sun with a building, tree, pole, or your hand while keeping the surrounding sky in view. If the halo is bright enough, you will see the ring without needing to look at the solar disk itself.

If you want to photograph it, use the same rule. Keep the sun blocked, lower the exposure if needed, and avoid pointing your eyes straight at the glare.