One morning while out for a jog, I noticed something strange in the sky—two radiant patches of light hovering beside the rising sun, shimmering like a second pair of suns on either side. I rubbed my eyes, thinking they might be playing tricks on me, but the lights didn’t waver.
That was when I realized: this wasn’t an illusion—it was an atmospheric phenomenon called a sundog. These luminous spots are no supernatural occurrence but rather a stunning atmospheric phenomenon.
Seen mostly at sunrise or sunset, sundogs are like nature’s own light show, created through the interaction of sunlight with ice crystals high up in the atmosphere. Their appearance has captured human imagination for centuries, serving as symbols in art, myth, and even weather prediction.
A sundog (or “parhelion,” Greek for “beside the sun”) is a patch of sunlight that appears on either side of the sun, typically at a 22-degree angle. Often, they appear in pairs, forming rainbow-like spots that hug the sun’s sides when it’s low on the horizon.
Dr. Andrew Rice, Associate Professor and Chair of the Department of Physics at PSU, explained the essentials of this phenomenon.
“Sundogs are a sub-occurrence of halos, forming from the refraction of light through small ice crystals high in the atmosphere,” he said. “They typically appear on the 22-degree arc, at horizontal positions, and are best seen when the sun is near the horizon.”
Sundogs aren’t just an occasional, localized wonder, either. “These conditions are quite common throughout the world,” Dr. Rice said.
The reason we don’t see sundogs every day, he explained, is that they require specific conditions in the atmosphere that are more common in winter.
Though they resemble rainbows, sundogs are a different kind of optical phenomenon. Dr. Christopher Butenhoff, a PSU physics professor, explained that while rainbows are formed from sunlight interacting with raindrops, sundogs come from sunlight refracting through ice crystals.
“The main difference is that rainbows result from the reflection and refraction of sunlight in raindrops—liquid water—while sundogs and halos are produced by refraction through ice crystals,” he said.
When light passes through ice crystals, the difference in the refraction angles produces an arc-like effect. “The minimum deflection angle for light passing through these crystals is 22 degrees,” Dr. Rice noted, “which is why sundogs appear specifically at that angle from the sun.” These ice crystals act almost like tiny prisms, bending light in a way that projects these spots of colored light on either side of the sun.
It all starts with cirrus clouds—those high, wispy clouds you often see on clear days. At altitudes of 15,000 to 30,000 feet, temperatures are frigid and these clouds consist mostly of ice crystals. The shape of these crystals is crucial for forming sundogs.
“Sundogs form from plate-like hexagonal ice crystals that resemble the cross-section of a pencil,” Dr. Butenhoff explained. “Light enters the ice crystals and is doubly refracted at both the entrance and exit surfaces.”
But not all crystals produce sundogs. The ice crystals must be aligned with their flat sides parallel to the ground. In windy conditions, the crystals will tumble, leading to halos rather than distinct sundogs. On calmer days, however, the crystals can settle into a more stable orientation, allowing sundogs to appear.
When asked about the exact mechanics of sundog formation, Dr. Rice elaborated on the process. “Ice crystals, when cold and dry, align themselves horizontally as they fall, with their flat, hexagonal faces parallel to the ground. This orientation acts as prisms, refracting light into two spots at 22 degrees on either side of the sun.”
While the right type of cloud is essential for sundogs, the time of day also plays a significant role. Sundogs are brightest and most easily seen when the sun is near the horizon. “The sun tends to be low on the horizon to see sundogs, as the viewer, ice particles and sun are all aligned in the same plane,” Dr. Rice explained.
When the sun is higher in the sky, the angle of sunlight doesn’t interact with ice crystals in the same way, making sundogs less likely to form.
Dr. Butenhoff added, “Sundogs are typically brightest when the sun is near the horizon, due to the relative orientation between sunlight and ice crystals.” The specific alignment of sunlight with the crystals is more likely to occur when the sun is lower, as at dawn or dusk, enhancing the refraction needed to create sundogs.
Sundogs can appear almost anywhere given the right conditions, but they’re most common in regions that frequently have high cirrus clouds and cold weather. This is why sundogs are often associated with winter months and are more frequently seen at mid-to-high latitudes, like northern Europe, Canada and parts of the northern US.
“Colder climates tend to favor halos and sundogs,” Dr. Rice noted. “I’ve seen some spectacular photos from Nordic countries and Alaska, but they are seen all over.”
Dr. Butenhoff mentioned that mid-to-high latitudes during winter provide ideal conditions for sundogs, as these regions experience colder and more moist atmospheres, which produce more ice-crystal clouds. However, because the ice crystals need calm conditions to align in stable orientations, very windy areas might see fewer sundogs.
With climate change on many minds, it’s natural to wonder whether warming temperatures might reduce sundog appearances or shift where they are seen.
While Dr. Rice explained that climate change doesn’t directly impact the conditions that produce sundogs, Dr. Butenhoff noted that shifts in temperature and moisture levels could potentially affect where and how often we see them. “Climate may dry and warm the atmosphere in some locations, but it may also moisten it in others,” he said. “The warming atmosphere may cause sundogs to migrate to higher latitudes.”
Geologist Sheila Alfsen, an adjunct professor at PSU, shared a similar view. “Climate changes will affect sundogs because of changing atmospheric conditions,” she said, though it’s challenging to predict exactly how.
She noted, “Our sun doesn’t send heat to Earth; it sends visible light and some ultraviolet light, which is converted to infrared when it hits the Earth’s surface and is then trapped by greenhouse gasses.”
Sundogs have been a source of fascination for centuries. In ancient Greek culture, they were thought to be omens and indicators of coming storms.
“I have read that the ancient Greeks used them as a forecasting tool, with the term ‘parhelion’ coming from the Greek word,” Dr. Rice noted. Sundogs’ appearance as additional “suns” in the sky has also led them to be seen as supernatural signs.
Although sundogs don’t seem to predict the weather in any magical way, they can be an indicator of changing weather patterns. “High cirrus clouds are sometimes an early sign of an approaching front, which can bring rain or snow,” Dr. Rice explained.
Dr. Butenhoff also noted that early cultures might have viewed sundogs as omens, “Maybe signs of either a good or bad harvest or weather to come. Imagine what you might have thought before science!”
So, the next time you’re outside on a cold day, take a look toward the sun. If you’re lucky, you might see one or even two glowing patches of light—nature’s own dazzling display. Sundogs may look otherworldly, but they’re simply a result of sunlight interacting with ice crystals in just the right way.
Keep an eye out in winter, especially at dawn or dusk, when the sun is low, and the sky is clear. Remember that seeing sundogs requires a bit of luck and just the right alignment of atmospheric conditions. And when you see them, you’re witnessing something that has intrigued people for thousands of years—an ephemeral, yet timeless phenomenon that connects physics, art and mythology, all in one glance.
