On a cold, clear morning you might glance toward the sun and notice two bright, glowing patches hovering to its left and right — sometimes tinged with rainbow colors, like a pair of second suns. These are sun dogs (also called mock suns or by their scientific name, parhelia). They're one of the most common and beautiful optical effects in the sky, and once you know what to look for, you'll start spotting them regularly — especially in winter.
Sun dogs aren't a trick of your eyes or your camera. They're created by sunlight bending through millions of tiny ice crystals floating high in the atmosphere.
The magic ingredient is high, thin cloud made of ice crystals — usually cirrus or cirrostratus clouds five miles (8 km) or more above the ground. Specifically, sun dogs need flat, six-sided (hexagonal) plate crystals that drift downward in a nearly horizontal orientation, like tiny falling dinner plates.
| Ingredient | Its role |
|---|---|
| Hexagonal ice plates | Act as tiny prisms, bending sunlight as it passes through them |
| Horizontal orientation | The flat plates float level, so they all bend light the same way |
| 22-degree bend | Light exits deflected by about 22°, placing the bright spot to the side of the sun |
| Prism effect | Different colors bend by slightly different amounts, giving the reddish-inner, bluish-outer tint |
As sunlight enters the side of a hexagonal crystal and exits, it bends by about 22 degrees — the same physics that makes a glass prism split light. Because the plate crystals are all floating flat, they concentrate that bent light into two bright spots exactly 22 degrees to the left and right of the sun, at the same height above the horizon. The red edge always faces the sun and the blue edge faces away, a smaller cousin of the color-splitting you see in a full rainbow.
Sun dogs are part of a whole family of "ice halos." The most common companion is the 22-degree halo — a full ring of light circling the sun at that same 22-degree radius. When both appear together, the sun dogs sit right where the halo crosses the sun's height, like glowing beads on a ring. Other members of the family include the upward-arcing circumzenithal arc (sometimes called an "upside-down rainbow"), sun pillars, and the rare, complex displays seen in polar regions. All are made by sunlight interacting with ice crystals in different shapes and orientations.
There's real folklore here, and a grain of truth to it. Sun dogs form in high cirrus clouds, and a spreading sheet of cirrus is often the leading edge of an approaching warm front and its weather system. So an old saying holds that sun dogs can mean rain or a storm is on the way within a day or two. It's not reliable enough to forecast by on its own — plenty of sun dogs come and go with no change — but as one clue among many, thickening high cloud can genuinely hint that a front is nearing.
Sun dogs are bright, often rainbow-tinged spots that appear about 22 degrees to the left and right of the sun, created when sunlight bends through flat, hexagonal ice crystals floating in high, thin clouds. They're most common and vivid in winter or when the sun sits low in the sky, and they often accompany a full 22-degree halo. Beautiful in their own right, they can also be a subtle sign that high cloud — and possibly a weather change — is moving in. Next clear, cold morning, hide the sun behind a pole and check both sides. Curious about the related winter phenomena? Read about cloud types next.