OPOD - A most unusual elliptical halos

OPOD - A Most Unusual Elliptical Halos: A Mystifying Atmospheric Phenomenon

Have you ever heard of elliptical halos? They are a rare and captivating atmospheric phenomenon that occurs when sunlight passes through plate-like pyramidal crystals in the atmosphere. While all elliptical halos are considered unusual, there is one particular sighting that stands out among the rest. This extraordinary display, captured by Jari Luomanen in Finland on March 13th, is not only exceptionally large, measuring 15.3 x 5.6 degrees, but it also exhibits a distinctive feature - a filled rather than a ringed halo. This peculiar observation may potentially be the first credible image of an elliptical halo with an evenly illuminated disk.

Unveiling the Unusual Elliptical Halo

Jari Luomanen, an avid observer of atmospheric phenomena, witnessed this mesmerizing event during a morning when diamond dust, large sectored/stellar plates of ice crystals ranging from 3 to 5 mm in diameter, gently descended from ragged batches of stratus clouds. The air shimmered with an intense sparkle as the crystals glistened in the sunlight. What initially caught Luomanen's attention was the appearance of an evenly lit disk, devoid of the typical arc-like structure found in other halos. Its brightness was so intense that Luomanen had to use extremely fast shutter speeds and narrow apertures to capture the disk near the Sun.

The Metamorphosis of the Elliptical Halo

As time progressed, the evenly lit disk transformed into a distinct elliptical halo, occasionally accompanied by a circumzenithal arc in the sky. Interestingly, no other halos were observed during this period. It is believed that the sectored plates of ice crystals possessed the necessary 90-degree wedge along with other exotic wedges, which gave rise to these elliptical shapes. Luomanen noted that most of the crystals were sectored plates, although some had developed into intricate snowflake-like dendritic stellar plates.

Unraveling the Mystery

The formation of elliptical halos remains an enigma, despite extensive scientific investigations. The prevailing hypothesis suggests that these halos arise from the refraction and diffraction of sunlight as it passes through plate-like pyramidal crystals in the atmosphere. However, the shallow angles required for this phenomenon to occur are deemed unphysical. Moreover, ray tracing simulations often fail to accurately reproduce the multiple arc positions and relative intensities observed in elliptical halos.

To shed light on this perplexing phenomenon, Jari Luomanen's images provide valuable insights. The presence of larger sectored crystals and snowflakes with wedge-shaped structures, tapering in thickness from the center, may play a role in generating elliptical halos. Additionally, the possibility of a range of crystal wedge angles and the existence of even rarer filled ellipses further adds to the complexity of understanding their formation.

In conclusion, the sighting of an unusually large and filled elliptical halo, as documented by Jari Luomanen, offers a captivating glimpse into the intricacies of atmospheric optics. While scientists continue to grapple with the mysteries surrounding these elusive phenomena, the images captured serve as a reminder of the vast and awe-inspiring wonders that await us in the sky above.

Doomsday Ellipse

All elliptical halos are unusual. This one is more unusual than most. It is huge for an elliptical, 15.3 x 5.6 degrees, and the halo is filled rather than having the usual set of nested rings. This could be the first credible image of an elliptical with an evenly illuminated disk.

Jari Luomanen (atmospheric phenomena) took the images in Finland on 13th March. More images.

"This morning I saw a thick swarm of diamond dust precipitating from ragged batches of stratus in the sky. The crystals were really big sectored/stellar plates, in the order of 3 to 5 mm in diameter. There was extreme glitter in the air!

At first the elliptical halo manifested itself as an evenly lit disk that had no distinct "arc" in it, just the blindingly bright disk. It was so bright I was at 1/8000s, f20 at times in order to reveal the disk close to the Sun.

Soon the disk morphed into a distinct elliptical halo! At times there was also a circumzenithal arc in the sky but no other halos whatsoever. Presumably the sectored plates had the necessary 90 deg wedge in them, along with the exotic wedges that produced the ellipses."

All images ©Jari Luomanen, shown with permission

"Most of the crystals were sectored plates. Not stellar, sectored, and probably the 90 degree wedge was at the ends of those sectors. But I recall some had developed into snowflake like dendritic stellar plates.

Something like those below which I imaged some time ago. But these are old pics. I just had a glance at the crystals .in the elliptical display. as they landed on my jacket sleeve. :) "

The filled ellipse evolved into a more normal ringed halo.

Refraction and diffraction, an iridescent cloud drifts across the sun with the halo forming crystals at lower level in the foreground.

Elliptical halos are usually small and they are rare.

They are argued to result from sun rays being refracted slightly by passage through plate-like pyramidal crystals. However, the very shallow angles that are necessary are unphysical. Furthermore, ray tracing simulations hardly ever accurately reproduce the multiple arc positions or relative intensities.

Larger sectored crystals, as imaged by Jari Luomanen, and snowflakes might have a wedge shape tapering in thickness from the centre. Crystal faces might also be curved. These might generate elliptical halos.

And an even rarer filled ellipse? A range of crystal wedge angles?

The most certain thing we can say is that how they form is uncertain.

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  • "OPOD - A most unusual elliptical halos". Atmospheric Optics. Accessed on March 1, 2024. https://atoptics.co.uk/blog/opod-a-most-unusual-elliptical-halos/.

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