Circles

The Parhelic Circle: Exploring the Enigmatic Halo

When it comes to atmospheric optics, one phenomenon that often goes unnoticed is the parhelic circle. This circular halo is found at the same altitude as the sun and is typically overshadowed by its white color, making it blend in with cloud streaks. However, with practice and keen observation, one can discern faint colors within the circle.

The formation of the parhelic circle involves plate crystals, column crystals, and Parry oriented columns. It is worth noting that this halo has more ray paths contributing to it than any other halo, making it a fascinating subject for study.

The parhelic circle is not a simple halo formed by a single reflection. Its complexity arises from various factors such as crystal regularity, thickness, and tilt. These factors influence the relative contributions of different rays to the formation of the circle. In other words, the parhelic circle is a result of multiple interactions between light and ice crystals, making it a captivating and intricate atmospheric phenomenon.

To better understand the formation of the parhelic circle, let's explore some HaloSim simulations that depict thin plate crystals filtered to allow only a specific number of interactions with crystal faces:

  • Single external reflection: This results in a weak parhelic circle located relatively close to the sun.
  • Passage through 2 faces: No parhelic circle can be formed at this sun elevation, and interestingly, no sundog is formed either.
  • 3 faces: The majority of the parhelic circle is formed by ray paths with a single internal reflection from a vertical face. Rays enter through the crystal's top basal face and exit through the lower one.
  • 4 faces: A faint circle is observed, with the 120-degree parhelion being most evident.
  • 5-15 faces: A weak contribution away from the sun is observed, accompanied by the formation of a sundog. The sundog is a result of an even number of reflections between the top and lower basal faces.

As we can see, the number of crystal faces traversed by rays plays a crucial role in the formation and visibility of the parhelic circle. The intricate interplay between these factors further highlights the complexity of this atmospheric phenomenon.

In addition to the parhelic circle, other halos can be observed near the sun. Sundogs, for example, are far removed from the 22° halo and can be seen at specific solar altitudes. Another notable halo is the bright upper tangent arc/circumscribed halo, which can be observed at the top of the sun.

In conclusion, the parhelic circle is an enigmatic halo that deserves more attention in the realm of atmospheric optics. Its complex formation involving various types of ice crystals and multiple interactions with crystal faces make it a captivating subject for study. By understanding the factors that influence its visibility and appearance, we can gain deeper insights into the intricate workings of our atmosphere. So, next time you find yourself gazing at the sky, take a moment to appreciate the subtle beauty of the parhelic circle and the mysteries it holds.

Complete Parhelic Circle

Imaged at Riga, Latvia on April 9, 2010 by Ivo Dinsbergs who assembled the montage from 14 separate images.

The parhelic circle is everywhere at the same altitude as the sun. It is noticed less often than it might be because it is white and therefore not readily distinguished from cloud streaks. Certain parts do in fact have faint colours but it takes practise to see them.

Plate crystals, column crystals and Parry oriented columns form the parhelic circle.

The circle has more ray paths contributing to it than any other halo.

The simplest path is an external reflection from a near vertical ice face but this is not necessarily the major component.

Below are HaloSim simulations for thin plate crystals filtered to allow only a specific number of interactions with crystal faces.

Single external reflection - Only a weak parhelic circle relatively near to the sun.

Passage through 2 faces - No parhelic circle can be formed. Notice also that at this sun elevation no sundog is formed either!

3 faces - Ray paths with a single internal reflection from a vertical face form most of the parhelic circle. Rays enter through the crystal top basal face and leave through the lower one.

4 faces - A faint circle. The 120 degree parhelion is most evident.

5 - 15 faces - A weak contribution away from the sun. And the sundog is formed! The latter is the result of an even number of reflections between the top and lower basal faces.

Crystal regularity, thickness and tilt all influence the relative contributions of different rays. The point is that the parhelic circle is no simple halo.

Iimage �Ivo Dinsbergs, shown with permission

Halos near the sun. Sundogs at this solar altitude are far removed from the 22° halo. At top is a bright upper tangent arc/circumscribed halo.

Faces traversed by parhelic circle rays

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Reference Atmospheric Optics

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  • "Circles". Atmospheric Optics. Accessed on March 28, 2024. https://atoptics.co.uk/blog/circles/.

  • "Circles". Atmospheric Optics, https://atoptics.co.uk/blog/circles/. Accessed 28 March, 2024

  • Circles. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/circles/.