Sundogs: An Interesting Atmospheric Phenomenon

Sundogs and halos exist only if people are there to see it. It is just refracted sunlight ready for reception at our end. For every set of eyes, there exists a different halo. Every individual crystal is at a different position, receives light from a different angle, and refracts it to a different direction.

Hence, a person standing far from another person will see a different halo. If there is someone to receive this light, a halo is there to receive him!

Nature often amuses us with the beauty of its colors, with sky being a huge medium. We have grown up watching clouds, rainbows, sunrises, and sunsets, which are a part of our daily lives. But there are many more atmospheric phenomena that are unusual, beautiful, and make us forget everything for a few moments. One of them is sundogs.

They occur everywhere at any time of the year, but mostly during winters and are present for a short duration. So if you get to see them often, consider yourself lucky. Nature gives a grandstand every now and then. We just need to see it!

This happens only in the presence of ice crystals in the air. These ice crystals, which are hexagonal plate-like in shape, are needed to be horizontal at their flat surface.

When light from the sun enters these crystals, it refracts at an angle approximately equal to 22º (index of refraction of ice being 1.31, and considering hexagon to be a section of the 60º equilateral prism). As light deviates from its normal path through the ice crystals, it results in the formation of sundogs.

Sun altitude
The appearance of sundogs is altitude-dependent. At 0º - 10º, the rays of the Sun passing through the crystals are parallel to their upper and lower basal faces, resulting in a perfect 22º deviation. This makes the sundogs appear within a halo.

As the Sun rises, the path of light through the crystals is inclined to the basal faces, giving a different angle. Due to this, sundogs appear lighter and farther from the halo. The ideal time for sundogs is when the Sun is at the horizon, i.e., sunrise and sunset.

It is a similar phenomenon in which the orientation of the ice crystals decides whether a halo or a sundog will emerge. If the hexagonal ice crystals do not have their flat surfaces in a horizontal position and are randomly oriented, they form a 22° halo.

The molecular structure of ice crystals is always hexagonal, but they vary in shape from a plate to a column. The dispersion of light in a prism due to different wavelengths causes the separation of white light into red, green, and blue depending on the index of refraction.

Since an ice crystal is a part of the prism, it shows similar manifestation. The inner side, closest to the Sun, is reddish, while it gradually changes to blue and fades off consequently.

It is formed in cirrus clouds, which are a genus of atmospheric clouds and look like thin strands of hair. These are massive clouds, and can stretch through continents.

Cirrus clouds are formed when water vapor undergoes deposition (a process in which gas transforms into solid), and sometimes look like huge trails in the sky, which happens when ice crystals start falling down, but evaporate before reaching the Earth's surface.

A light ray passing through the two side faces of the ice crystal, deviates at an angle ranging from 22º to 50º. 22º being the minimum deviation, the rays cluster at this angle, making a brighter halo than that formed at 50º, where the rays get scattered.

Since no rays deviate at an angle less than 22º, the color of the sky inside the halo is darker. Sometimes two halos are formed where the 22º halo extends to 50º halo; the outer one being considerably lighter, looking like a disc with a hole.