What Causes a Halo Around the Sun? The Science of Sun Halos and Sundogs

That Ring of Light: Unpacking Halos and Sundogs

Let’s start with a fun word combo: “Sundog.” It sounds like something straight out of a children’s fantasy story—maybe the name of a magical creature that guides solar chariots across the sky. But in reality, a sundog is a real and fascinating atmospheric event, often tied to another stunning sight: the glowing ring sometimes seen around the sun.

You might have looked up on a cold day and seen a perfect, glowing circle encircling the sun—or noticed two brilliant spots of light flanking it like loyal guardians. These aren’t camera lens flares or tricks of the eye—they’re optical phenomena created by ice crystals in the sky. And they’re not just beautiful; they’re also full of scientific significance.

What Is a Halo Around the Sun?

You may have seen it yourself—a circular rainbow-like ring around the sun on a clear day. This breathtaking display is known as a sun halo, or more specifically, a 22-degree halo. Why 22 degrees? That’s the angle at which light gets bent or refracted when passing through certain types of ice crystals in the atmosphere.

Sun halos aren’t just limited to the daytime sun. The moon can wear a halo too, under the right conditions—usually on clear, cold nights when moonlight is strong and cirrostratus clouds are present.

So, what exactly causes this ring of light?

The answer lies high up in the upper troposphere, at altitudes of about 5–10 kilometers (3–6 miles) above the Earth’s surface. This is where cirrostratus clouds form—thin, wispy layers made up not of water droplets but of tiny, six-sided ice crystals. These hexagonal crystals act like miniature prisms.

As sunlight enters these ice crystals, it slows down and bends—a process known as refraction. Most of the light is bent at a 22-degree angle, which is why we perceive the halo at that precise distance from the sun. The result is a large, faintly colored circle, with red hues often appearing on the inside edge and blue on the outer edge due to differences in how light of different wavelengths is refracted.

Why Is It Called a 22-Degree Halo?

This is one of the most common questions, especially from curious skywatchers. The “22 degrees” refers to the angular distance between the center of the sun (or moon) and the halo itself. If you were to stretch your arm out and spread your fingers wide, your hand would span about 20–25 degrees in the sky—meaning you could almost measure the halo’s radius using your own hand!

From Halos to Sundogs: What’s the Difference?

Now, let’s circle back to the sundog. Officially known as a parhelion (plural: parhelia), sundogs are bright spots that appear on either side of the sun, usually at the same 22-degree distance as a halo. But they differ in formation and appearance.

While standard halos occur when ice crystals are randomly oriented in the sky, sundogs form under more selective conditions. When plate-shaped ice crystals drift horizontally in a more uniform orientation, they refract sunlight more directly toward the observer, creating those bright patches that look like mini suns hanging next to the real one.

On especially clear days—particularly in cold climates or during winter months—sundogs can be so vivid that they look like two extra suns in the sky. They can appear alone or as part of a larger halo system and are often accompanied by other optical phenomena like sun pillars or tangent arcs, depending on the shape and alignment of the crystals.

Fun fact: the term “sundog” may have originated from the idea that these glowing spots follow the sun, much like a faithful dog follows its owner. The Greek-derived scientific name, parhelion, literally means “beside the sun.”

Are Halos and Sundogs Rare?

Contrary to popular belief, these phenomena aren’t all that rare—but they do require specific conditions. Halos and sundogs are more likely to appear:

In cold regions or during winter, when ice crystals are more abundant.
When cirrostratus clouds are present, even thinly.
When the sun is low in the sky, typically during the early morning or late afternoon.

Because they are often faint and require just the right viewing angles, many people may not notice them—or they may dismiss them as camera artifacts when taking photos.

What Does a Halo Around the Sun Mean?

Now for the folklore and forecasting! Many cultures throughout history have attached meaning to these celestial signs. While sun halos are natural atmospheric effects, they also serve as weather indicators.

Scientifically, a halo around the sun (or moon) usually means that cirrostratus clouds are present in the upper atmosphere. These clouds themselves don’t bring precipitation, but they often appear as part of a warm front or approaching storm system. In this way, halos can act as early warnings of incoming rain or snow within the next 12–36 hours, depending on the region and prevailing weather patterns.

In weather lore, there’s an old saying:
“Ring around the sun or moon, rain or snow is coming soon.”

While not as precise as modern meteorology, this bit of folk wisdom has some truth to it and was relied upon by farmers, sailors, and sky-watchers long before weather apps existed.

Next Time You Look Up…

If you happen to live in an area with crisp, clear skies—especially during cooler months—take a moment to look up when the sun is shining. You might just catch sight of a spectacular sun halo, or if you’re really lucky, spot a pair of loyal sundogs flanking the sun like celestial companions.

And the next time someone asks what causes that mysterious ring around the sun, you’ll not only have the answer—you’ll have a story. One that stretches from ice crystals in the upper atmosphere to ancient folklore, and from light refraction to weather forecasting.

The sky has many secrets—it just takes a curious eye to uncover them.

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Footnote: Understanding sun halos is a great example of scientific research in action. For students looking for a science fair project, observing and documenting these events can be a form of qualitative research. This topic, often discussed in scholarly sources and international journals on atmospheric science, is a great entry point into STEM fields. It bridges simple observation with complex quantitative research on light refraction, making it a fascinating subject for any budding scholar.