Atmospheric optics is a fascinating field that offers a glimpse into the awe-inspiring visual phenomena that occur in our skies. One such phenomenon is the parhelic circle, which becomes particularly intriguing when the sun is low in the sky. In this article, we will delve into the intricacies of the parhelic circle and explore its behavior under different sun angles.
The parhelic circle is a circular halo that sweeps around the horizon, captivating observers with its ethereal beauty. However, it is important to note that this circle is not uniformly bright. Instead, its brightness varies depending on its location relative to the sun and sundogs.
To better understand the behavior of the parhelic circle, let's explore its appearance under different sun angles.
When the sun is only 5° above the horizon, the parhelic circle takes on a subtle glow. However, beyond the 120° parhelia, it brightens once again.
As the sun climbs higher in the sky, reaching a height of 40°, the parhelic circle undergoes a transformation. The rays of light strike the internal vertical faces of ice crystals at more oblique angles, resulting in more efficient reflections. Consequently, the circle becomes more uniform in brightness.
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The parhelic circle is a captivating phenomenon of atmospheric optics that reveals itself when the sun is low in the sky. Its appearance is not only influenced by the sun's angle but also by external and internal reflections within ice crystals. By understanding the behavior of the parhelic circle under different sun angles, we gain a deeper appreciation for the complexities and wonders of our atmosphere. Next time you find yourself gazing at a sunset or sunrise, keep an eye out for the enigmatic parhelic circle and let its ethereal beauty mesmerize you.
Parhelic Circle at the South Pole imaged by Marko Riikonen 11th January 1999 at 0500hrs. The parhelic circle swings through the 22° high sun, brightens outside the sundogs and hugs the flat horizon everywhere at the same height. Image ©Marko Riikonen.
Parhelic Circle for 22° high sun
Sweeping around the horizon, the circle is not uniformly bright. It is fainter through the sun and inside the sundogs where it relies primarily on rays reflected externally.
The halo also becomes faint beyond the 120° parhelia. At these angles, internal reflections become less efficient.
HaloSim ray tracing. Fisheye view centered on the zenith, 40% randomly oriented columns to produce the 22° halo and 60% plate crystals with c/a=0.05.
Very low sun
Here the sun is only 5° above the horizon. Again, the parhelic circle is faint beyond the 120° parhelia - then it brightens again..
The two bright regions 150 to 160° from the sun are Liljequist parhelia first observed in Antarctica by the Swedish meteorologist G.H. Liljequist. Almost perfect hexagonal plate crystals are needed to form them. Their ray paths involve two internal reflections. An example is a ray entering face 3, reflecting off faces 7 then 6 to exit through face 8 (path 3768 see face number key). There is net refraction in this ray path and the Liljequist parhelia are slightly coloured.
40° high sun
Contrast the parhelic circle as the sun climbs higher. The rays strike internal vertical faces more obliquely and are more efficiently reflected. The circle becomes more uniform.
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"Parhelic Circle - Low Sun". Atmospheric Optics. Accessed on December 10, 2023. https://atoptics.co.uk/blog/parhelic-circle-low-sun/.
"Parhelic Circle - Low Sun". Atmospheric Optics, https://atoptics.co.uk/blog/parhelic-circle-low-sun/. Accessed 10 December, 2023
Parhelic Circle - Low Sun. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/parhelic-circle-low-sun/.