Sun Dogs: How Parhelia and Ice Crystal Optics Work

Sun dogs, or parhelia, are bright spots that can appear beside the sun when light passes through ice crystals in high cloud.

Key takeaways

• Sun dogs are also called parhelia.

• They are an atmospheric optical phenomenon, not a storm or weather system.

• They form when sunlight is refracted by ice crystals in upper cloud.

• The effect is linked with hexagonal ice crystals, which form as long columns or thin plates, but sun dogs specifically result from the refraction of sunlight through these hexagonal crystals acting as prisms, not from plate-shaped crystals alone.

• A 22-degree halo can appear with the same kind of ice crystal optics.

Sun dogs: what parhelia are

Thin cirrostratus clouds creating atmospheric optics near sunset.

Sun dogs, or mock suns, are bright patches of light that sit near the sun. They are part of atmospheric optics and are caused by refraction in ice crystals high above the ground. According to Britannica, these occur specifically when hexagonal ice crystals fall with their principal axes vertical.

Very bright sun dogs in Fargo, North Dakota. Also visible are parts of the 22° halo (the arcs passing through each sun dog), a sun pillar (the vertical line), and the parhelic circle (the horizontal line). By Gopherboy6956 - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5985196

The Bureau of Meteorology describes cloud types and sky features that can support these displays. In simple terms, you need ice crystals in the right part of the sky, with the right shape and orientation, for the light to bend into a visible patch.

The science behind sun dogs and parhelia

Comparison between a circular 22-degree halo and isolated mock suns.

Parhelia form when sunlight enters ice crystals and bends as it leaves. That bending is called refraction. The crystals act a bit like tiny prisms, sending some light toward an observer on the ground. Similar effects can even be seen at night; for instance, a moon dog is the lunar equivalent of this phenomenon.

Hexagonal ice crystals and plate-shaped crystals

Sun dogs are linked with hexagonal ice crystals. In many cases, plate-shaped crystals are the key form because their flat faces can line up in a way that sends light to the side of the sun.

When many crystals are aligned in a similar way, the bright spots stand out more clearly. If the crystal shapes or angles vary too much, the effect can look faint or disappear altogether. Experts have even explored how celestial effects could appear in alien skies under different atmospheric conditions.

How a 22-degree halo differs

A 22-degree halo is a ring of light around the sun, while sun dogs are bright spots to the left and right. Both come from ice crystal refraction, but the pattern depends on how the crystals are shaped and lined up.

Cirrus clouds and cloud height

Sun dogs usually appear with cirrus clouds or related high cloud. These clouds sit high in the atmosphere, where temperatures are low enough for ice crystals to form.

If you see thin, wispy cloud near the sun, that is often the kind of setting that can produce parhelia. The sky may look pale or milky around the sun, with the bright spots showing off to each side. These are commonly associated with cirrostratus clouds which can cover the sky in a thin veil.

Diagram explaining ice crystal refraction and the 22-degree angle required for sun dogs Australia.

Why they are called mock suns

Mock suns is another common name for parhelia. The term fits because the bright spots can look like smaller suns sitting beside the real one. An article from ABC News clarifies that these large spots are a well-documented optical occurrence.

They are not reflections and they are not caused by dust or smoke. The main ingredient is light passing through ice crystals in the upper atmosphere.

Southern Hemisphere solar cycles and when to look

In the Southern Hemisphere, sun dogs can be easier to spot when high cloud is present and the sun sits lower in the sky. That is when the angle of the light makes the effect more noticeable.

For Australian readers, the best clue is the sky itself. Watch for thin high cloud, a bright sun, and pale patches or a ring near it.

Two sun dogs, and a partial 22° halo in Sun City West, Arizona - By Markkawika - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=154478522

What the Bureau of Meteorology says to watch for

The Bureau of Meteorology uses cloud observations to describe the sky conditions that can support optical effects like parhelia. If you are looking for sun dogs, focus on high cloud, not on surface temperature alone. You can find more definitions in a weather glossary.

That means you may see them on a cool day or a warm day, as long as the upper air has the right ice crystals.