Sundogs are a common and striking atmospheric optical display created when sunlight interacts with suspended moisture. This vivid illusion appears to flank the sun itself.
What Exactly is a Sundog?
A sundog is an atmospheric optical phenomenon known scientifically as a parhelion, meaning “beside the sun.” It manifests as a bright, concentrated patch of light, often displaying subtle rainbow coloring, on one or both sides of the sun. They are frequently referred to as mock suns because they sometimes appear nearly as brilliant as the true sun.
These luminous spots always appear horizontally aligned with the sun, located at a consistent angular distance of about 22 degrees away. Sundogs are distinct from rainbows because their position is fixed relative to the sun. The colors are typically muted, with the red hue closest to the sun and the blue or violet on the outer edge, merging into a white tail of light.
The Physics of Ice Crystals and Light
The formation of a sundog depends entirely on the presence of plate-shaped hexagonal ice crystals suspended in the atmosphere. These crystals are typically found in high-altitude cirrus clouds, but they can also occur at low levels in extremely cold conditions known as “diamond dust.” The crystals must be oriented so their large, flat hexagonal faces are nearly horizontal as they descend.
Sunlight acts upon these crystals, which function like miniature six-sided prisms. A light ray enters one side of the crystal and is refracted (bent) as it passes through the ice, exiting through an alternate side. This bending of light results in a minimum deviation angle of approximately 22 degrees, determined by the 60-degree angle between the crystal faces.
Because the crystals are horizontally aligned, the light is refracted primarily in the horizontal plane, concentrating the light into the two spots 22 degrees to the left and right of the sun. This mechanism dictates the unmoving, horizontal position of the sundogs. The separation of color occurs because the refractive index of ice varies for different wavelengths of light, causing red light to bend less than blue light.
Folklore and Meteorological Significance
Mock suns have captured human imagination for millennia, leading to various folklore interpretations. In ancient Greece, Aristotle noted the phenomenon, and the name parhelion was coined. In medieval times, the three lights (the sun and its two companions) were sometimes interpreted as a sign of the Holy Trinity or an omen.
The term “sundog” may derive from the visual effect of these bright spots following or “dogging” the true sun across the sky. In some Native American traditions, the sundog was viewed as a sign of hope or balance. The historical perception that sundogs predict weather change has a basis in modern meteorology.
The ice crystals that form sundogs are often contained within high-altitude cirrus clouds, which frequently precede an approaching warm front or storm system. While the sundog itself does not cause the weather, its presence confirms specific atmospheric conditions (high-altitude moisture and cold temperatures). These conditions can indicate a change in weather is likely within the next 12 to 24 hours, lending credence to the old weather lore.