What is sundog, the unique illusion noticed on the winter sky?

Sundogs are also referred to as mock suns or parhelia (meaning 'with the sun') . It can be seen in the sky flanking the real sun on cold winter days as bright patches of light to the left, right, or in some cases on both sides of the sun. Although these very vivid scenes can be cool and different, they are simply a natural wavelength illusion created by the Earth's atmosphere.

What causes Sundogs on the winter sky?

The sun’s rays pass through atmospheric ice crystals to create sun dogs. Ice crystals are generally present in high-altitude clouds known as cirrus or cirrostratus clouds, which can be located between 20,000 feet and 40,000 feet above ground level. In extremely cold climates, the same phenomenon can occur much closer to the surface. This near-surface formation of ice crystals is known as diamond dust and occurs when the temperature is below 22 degrees Fahrenheit (30 degrees Celsius).

Sundogs are caused by flat, hexagonal ice crystals that are in the atmosphere and that fall from the sky. As they do so, the flat face of the ice crystal is oriented parallel to the ground when viewed from above, making it easier for sunlight to enter into one side of the ice crystal and exit through another side. Because the path through the ice crystal allows for light to bend twice as the ray passes into and then out of the ice crystal, the light is bent by at least 22 degrees.

Both sundogs exhibit colorization and can show slight rainbow-like coloration with red closest to the sun and progressively paler colours extending outward. Sundogs also exhibit wide variability of brightness of far greater range than either their respective colorization will indicate. For example, they can be weakly coloured patches on either side of the sun or can be so highly intensified that they appear to be two additional suns.

Sundogs sometimes have lines of light trailing away from them as well. The “tails” form when light is reflected from the vertical surfaces of flat, hexagon-shaped ice crystals. The term sundog has been reported to originate from Greek mythology. One explanation suggests that the sundogs were associated with Zeus, the god of the skies who was said to have dogs walking beside him in the sky.

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