How does a sundog form?
A sundog, also called a parhelion, is a bright and sometimes colored patch to the left or right of the Sun. It usually forms when sunlight refracts through plate-shaped ice crystals floating in preferred orientations. The patches are not additional suns; they result from the atmosphere directing more light toward particular viewing directions.
Mechanism and everyday examples
1. Why does crystal orientation matter? How a crystal sits in the air matters as much as its shape. Thin hexagonal plates may tend to orient approximately horizontally. Light traveling through suitable faces becomes concentrated around particular deviation angles. Rather than producing an equally bright ring all around the Sun, this arrangement can create conspicuous patches beside it. The presence of ice alone is insufficient: crystal shape, orientation, and viewing direction must be appropriate together. 2. Why do the patches appear at similar heights? In the common sundog arrangement, light passes between vertical side faces of horizontally oriented crystals. The appearance depends on angular geometry around the Sun, and patches often lie at approximately its elevation. With a low Sun they may appear near the 22-degree halo; their position and visibility can change as the Sun rises. A fixed screen distance or pixel spacing should therefore not be expected under every condition. 3. Where does the color sequence come from? Ice has a refractive index that varies slightly with wavelength. Different colors deviate by different amounts, sometimes giving a reddish edge nearest the Sun and paler bluish tones farther outward. Crystal variation, cloud thickness, and brightness may wash out the colors. Seeing only a white bright patch does not invalidate the explanation; the visible detail of the phenomenon depends on the conditions along the line of sight.
Observation, applications and misconceptions
4. Why can sundogs occur with a halo? The same high cloud layer may contain crystals with several shapes and orientations. Randomly oriented crystals can contribute to a ring, while preferred orientations strengthen the side patches. A photograph may consequently show a broad ring and two bright points together. Sometimes only one sundog is visible because clouds are unevenly distributed or one side is obscured. Asymmetry does not imply that the event requires a different light source. 5. How can it be distinguished from lens flare? Internal reflections in a camera lens can also produce colored spots near a bright source. Rotating or repositioning the camera may make these ghost patches behave differently within the frame. Atmospheric sundogs retain their physical positions relative to the Sun and can also be seen directly when observed safely. A cropped fragment of a single photograph is often insufficient to distinguish the possibilities confidently without additional viewing information. 6. When and how can they be observed? Thin high clouds or suspended ice crystals in very cold air may provide suitable conditions. Looking directly at the Sun or using optical magnification is unsafe. A structure can block the solar disk while the surrounding area is examined. Record the date, time, approximate solar elevation, and positions of the patches. These observations identify the event more reliably. A sundog alone is not a warning of an approaching disaster or a guarantee that precipitation will follow.