120° Parhelia

The Phenomenon of 120° Parhelia

When it comes to atmospheric optics, one of the fascinating phenomena that can occur is the appearance of 120° parhelia. These unique optical effects brighten the parhelic circle and are caused by multiple reflections within relatively thick oriented plate crystals. While not particularly rare, they can sometimes be difficult to distinguish from bright areas of cloud when they are diffuse.

The parhelic circle, which is typically colorless, exhibits a slight blue hue to the right of the 120° parhelion, often referred to as the "blue spot." This phenomenon is a result of the interaction between sunlight and the oriented plate crystals. As light rays enter the top face of the crystal, they undergo two internal reflections off adjacent side faces before exiting through the lower face. This process leads to a horizontal deflection of the ray at an angle of 120°, giving rise to the distinctive parhelion.

It is worth noting that the angle of incidence of the incoming ray does not affect the horizontal deflection of 120°. This consistency in the angle of deflection distinguishes the parhelion from other rays that contribute to the parhelic circle. Thus, the term "parhelion" is used specifically to describe this particular ray, as opposed to another parhelic circle ray.

To observe 120° parhelia, it is advisable to pay close attention when ordinary 22° sundogs are exceptionally bright. The presence of these vibrant sundogs often indicates favorable conditions for the formation of parhelia. While they may not be as rare as one might think, their diffuse nature can make them challenging to identify amidst bright cloud areas. Therefore, keen observation and careful scrutiny are necessary to spot these intriguing optical phenomena.

Beyond the parhelic circle, even more exotic halos can be generated by singly oriented columns around the anthelic point. These halos, which are located to the right of the 120° parhelion, add further complexity and visual interest to the atmospheric display. The interaction of sunlight with these columns produces a variety of striking optical effects, showcasing the intricate nature of atmospheric optics.

To simulate the occurrence of 120° parhelia and other atmospheric phenomena, HaloSim software can be utilized. By inputting various parameters such as sun altitude and crystal orientation, researchers and enthusiasts can gain a deeper understanding of the intricacies involved in the formation of these optical phenomena.

In conclusion, 120° parhelia are captivating atmospheric optics events that brighten the parhelic circle. They arise from multiple reflections within oriented plate crystals and exhibit a distinct blue spot to the right of the parhelion. While not particularly rare, their diffuse nature can make them challenging to distinguish from bright cloud areas. Nevertheless, keen observation and the presence of exceptionally bright sundogs can aid in their identification. The study of these phenomena not only adds to our understanding of atmospheric optics but also showcases the beauty and complexity of the natural world.

A 120� parhelion brightens the parhelic circle. Far from the sun, these parhelia arise from multiple reflections inside relatively thick oriented plate crystals. To the right of the 120� parhelion, the otherwise colourless parhelic circle is very slightly blue ~ the 'blue spot'. Further right around the anthelic point (the point opposite to the sun but at the same altitude), singly oriented columns generate even more exotic halos. HaloSim simulation centered 150� from a 15� high sun. 120� parhelion ray path through a near horizontal plate crystal. Always look carefully for 120� parhelia whenever the ordinary 22� sundogs are very bright. They are not particularly rare, but when they are diffuse they can sometimes be difficult to distinguish from bright areas of cloud.The parhelia result from at least two internal reflections. Rays enter the plate crystal top face, internally reflect off two adjacent side faces then leave through the lower face. The horizontal deflection of the ray is always 120� regardless of the angle of incidence of the incoming ray ~ hence the parhelion rather than yet another parhelic circle ray.

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  • "120° Parhelia". Atmospheric Optics. Accessed on March 1, 2024. https://atoptics.co.uk/blog/120-parhelia/.

  • "120° Parhelia". Atmospheric Optics, https://atoptics.co.uk/blog/120-parhelia/. Accessed 1 March, 2024

  • 120° Parhelia. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/120-parhelia/.