Sundogs are concentrated patches of light that typically appear on either side of the sun, positioned at the same altitude above the horizon. They are sometimes called “mock suns” or “parhelia,” a term derived from Greek meaning “with the sun.” While they can sometimes appear as subtle, white spots, sundogs often display vibrant colors, resembling fragments of a rainbow. The inner edge, closest to the sun, commonly exhibits a reddish hue, gradually shifting through orange and yellow to a more bluish or whitish appearance on the outer edge. Depending on atmospheric conditions, they can be so intense that they appear as two additional suns, sometimes with a “tail” of light extending horizontally away.
How Sundogs Form
Sundogs form through a precise interaction between sunlight and specific ice crystals high in the atmosphere. The phenomenon requires plate-shaped hexagonal ice crystals, typically found in cirrus or cirrostratus clouds at altitudes ranging from 20,000 to 40,000 feet. In extremely cold conditions, these ice crystals can also form closer to the ground, creating “diamond dust.” For sundogs to become visible, these plate-shaped crystals must drift downwards with their large hexagonal faces oriented nearly horizontally. Sunlight enters one side of these tiny prisms and is refracted as it passes through the crystal before exiting another face.
This bending of light occurs at a minimum deviation angle of approximately 22 degrees, which determines the sundog’s characteristic position relative to the sun. The separation of colors within a sundog happens because different wavelengths of light refract at slightly different angles as they pass through the ice crystals. Red light, for instance, bends less than blue light, resulting in the reddish inner edge. Sundogs are most pronounced when the sun is low in the sky, such as during sunrise or sunset, as this angle allows for optimal light refraction through the horizontally aligned crystals.
Sundogs and Other Atmospheric Optics
Sundogs are part of a broader family of atmospheric optical phenomena caused by sunlight interacting with ice crystals, known as ice halos. A common misconception is to confuse sundogs with a 22-degree halo, a complete circle of light around the sun. While both phenomena involve ice crystals, sundogs appear as distinct bright spots, often on either side of the sun, whereas the 22-degree halo forms a continuous ring. The difference lies in the orientation of the ice crystals; sundogs require horizontally oriented plate crystals, while 22-degree halos can form from randomly oriented hexagonal crystals.
Sundogs also differ from rainbows, which are formed by the refraction and reflection of sunlight through water droplets, not ice crystals. Rainbows appear when an observer looks away from the sun, while sundogs are viewed by looking towards the sun. Another distinct phenomenon is the sun pillar, which appears as a vertical shaft of light extending above or below the sun. Sun pillars are formed by the reflection of sunlight off falling ice crystals, rather than the refraction that creates sundogs.
Cultural Interpretations of Sundogs
Sundogs have inspired various cultural and symbolic interpretations throughout history. In ancient Greek mythology, they were sometimes attributed to Zeus walking his celestial dogs across the sky, leading to the name “parhelia” (meaning “beside the sun”). This association often imbued sundogs with a sense of good fortune or companionship to the sun. In medieval times, the sight of three bright lights (the sun and two sundogs) was occasionally interpreted as a representation of the Holy Trinity, symbolizing divine presence or great fortune.
Many Indigenous cultures also hold significant beliefs about sundogs. For example, some Northern Plains tribes view sundogs as a powerful sign of change or transition, or a reminder of the necessity to live in harmony with nature. Historically, sundogs were often perceived as omens, sometimes signaling changes in weather, such as an incoming cold spell.