Have You Ever Seen A Sundog? Here’s Why They Form

I am constantly scanning the sky. It’s just what I do. On my commute home from the University of Georgia yesterday, I saw something that always excites me. I quickly grabbed my phone, rolled down the window, and snapped the picture below. It is a Sundog. Sundogs have special meaning to me because I think they are very high on the “scientifically cool” index. During my time as a scientist at NASA Goddard Space Flight Center, I also played on a softball team with a group of mostly atmospheric scientists called the Sundogs. What is a Sundog and why do they form?

Sundogs are more scientifically known as an optical phenomenon called a parhelion, which means “with the sun.” Theoretically, a similar phenomenon is possible with moonlight, but I have never seen it so will stick with an explanation involving the sun. There are two conditions usually present when a Sundog forms: (1) the sun is low in the sky as typically observed at sunset or sunrise, and (2) there are lots of clouds with ice crystals in the sky. With those basic conditions noted, I should spend a few minutes defining the words “reflection” and “refraction.” According to a NOAA website, reflection of the visible part of the electromagnetic spectrum happens “when the waves encounter a surface or other boundary that does not absorb the energy of the radiation and bounces the waves away from the surface.” Refraction, on the other hand, happens when waves change direction after passing from one medium to another at an angle not equal to 0° or 90°. If you have ever witnessed the “pencil appearing to bend in a glass of water” demonstration, that is an example of refraction of light. When light is refracted within a water droplet or ice crystal in a cloud, it is actually broken out into colors. Most people are familiar with this effect when they see a rainbow. Interestingly, the formation of a rainbow involves reflection and refraction.

Sundogs form when light is refracted because of ice crystals in the sky. Though my picture above only shows one of them, there are often two of them (picture below). They are typically located 22 degrees to the right and/or left of the sun depending on how the ice crystals are distributed. The colors transition from red (closest to the sun) to blue (on the edge way from the sun). Sundogs are often called “mock suns.” Other similar phenomena include halos and sun pillars. 

Halos are rings of light that form around the moon or sun as light is refracted in ice crystals associated with a veil of cirrus-type clouds. Though typically white, halos can have color. A Sun Pillar is a vertical shaft of light directly above the sun. Like Sundogs, they are often seen at sunset or sunrise. They form due to sunlight being reflected off of falling ice crystals. A reasonable question might be, “What is it about atmospheric conditions that produce a Sundog rather than something else?” According to Britannica.com,

Sun dogs occur when the Sun or Moon shines through a thin cirrus cloud composed of hexagonal ice crystals falling with their principal axes vertical, as opposed to the halo phenomenon that occurs when the principal axes are randomly arranged in a plane perpendicular to the Sun’s or Moon’s rays. The red end of the spectrum, being bent the least, appears on the inside, with the blue, when visible, appearing on the outside. Sun dogs most commonly appear during the winter in the middle latitudes.

Keep watching the sky. It is full of fascinating views.