Aerial Rare Halos, China - OPOD

Last updated on September 16, 2023

Aerial Rare Halos, China - OPOD

When it comes to atmospheric optics, there are a multitude of fascinating phenomena that can occur. One such remarkable display took place in China, where a magnificent halo show was observed in the high and cold cirrus haze over NE Yunnan Province on May 4th, 2014. The sun was positioned at an angle of approximately 42°, creating a stunning visual spectacle for observers.

Upon closer examination of the image captured during this event, several rare halos can be identified. By enhancing the image through color subtraction, the colored halos become more prominent. Notably, an upper Wegener arc can be observed, which is formed by horizontal column crystals. This arc is quite rare and its continuation below the sun is even more uncommon.

In addition to the Wegener arc, three Parry arcs are also visible in the image. Parry arcs are already considered rare, but the presence of high sun Parry arcs makes this display particularly noteworthy. One Parry arc can be seen above the sun, reaching towards the parhelic circle, while two arcs are positioned below it.

The generation of helic arcs from external reflections on the side faces of Parry-oriented crystals is a rare occurrence. Typically, such crystals are artificially generated on ski slopes. However, in this natural high cirrus display, there appears to be a faint trace of one such helic arc.

To better understand and visualize the halo display, a HaloSim ray tracing computation was conducted. The simulation, based on a single wavelength of green light, accurately reproduces the observed halos. However, due to the narrow appearance of normally colored halos like the infralateral arc, it is necessary to use color subtraction to enhance their visibility.

The ray tracing simulation incorporated a mixture of randomly oriented crystals (producing 22° and 46° halos), singly oriented columns (resulting in the circumscribed halo, infralateral arc, and Wegener arc), as well as a few Parry-oriented columns. This comprehensive simulation allows for a more detailed analysis and understanding of the observed halo phenomena.

Specifically focusing on the Wegener arc, it is worth noting that it is tangent to the top of the 22° halo and curves over the sky to intersect the parhelic circle directly opposite the sun, known as the anthelic point. Continuing below the parhelic circle, the arc becomes tangent to the 22° halo once again, this time below the sun. The Wegener arc's unique path and formation make it a captivating feature of this halo display.

Among the rare halos observed in this display, the lower sunvex Parry arc stands out as a true rarity. Its ray path is defined by passing through the two lower inclined side faces of Parry-oriented column crystals. The presence of this arc adds another layer of intrigue to an already extraordinary atmospheric phenomenon.

While Parry-oriented column crystals are relatively unlikely to occur naturally, they do occasionally form. These crystals have a horizontal long axis, with their upper and lower prism side faces also positioned horizontally. By convention, the topmost face is labeled as number 3, with other side faces numbered from 4 to 8, while the end faces are designated as 1 and 2.

Examining another image captured during this halo display, we can observe the lower sunvex Parry arc just above the aircraft's winglet. Enhancing the halos through levels shift and unsharp masking provides a more aesthetically pleasing image, although color subtraction remains a more effective method. By utilizing color subtraction, other Parry arcs and the upper and lower Wegener arcs become more clearly visible.

Lastly, it is important to acknowledge that atmospheric conditions can change rapidly, even at high speeds. The absence of the parhelion in the top image can be attributed to these rapid changes in crystal populations, highlighting the dynamic nature of atmospheric optics.

In conclusion, the aerial rare halos observed in China during this particular event provide a captivating glimpse into the intricate and awe-inspiring world of atmospheric optics. The combination of rare phenomena such as Parry arcs, Wegener arcs, and helic arcs, along with the unique properties of Parry-oriented column crystals, make this display truly exceptional. By employing advanced ray tracing simulations and image enhancements, scientists and enthusiasts alike can delve deeper into the complexities of atmospheric optics and gain a greater appreciation for the wonders of our natural world.

Remarkable Halos, China

A magnificent halo display in high and cold cirrus haze. Imaged over NE Yunnan Province by JI Yun (it) on 4th May 2014. The sun was about 42° high.

There are many rare halos. Mouse over the image to see a labelled and enhanced version. Enhancement was by colour subtraction. It highlights coloured halos.

Note the upper Wegener arc from horizontal column crystals and its more rare continuation below the sun.

Parry arcs are rare. High sun Parry arcs more so. There are three in this image - One above the sun and reaching towards the parhelic circle and two below it.

Parry oriented crystals generate helic arcs from external reflections from their side faces. They are exceeding rare unless from the somewhat artificially generated crystals of ski slopes. Here there is a possible faint trace of one in wholly natural high cirrus.

Images ⓇJI Yun (itz), shown with permission

A HaloSim ray tracing computation of the display. The projection matches the camera angle. The simulation is for a single wavelength of green light and normally coloured halos like the infralateral arc therefore appear narrow.

The simulation used a mixture of randomly oriented crystals (22 and 46° halos), singly oriented columns (circumscribed halo, infralateral arc and Wegener arc) plus a few Parry oriented columns.

Below: An all sky zenith centred ray tracing of the display to better show the Wegener arc.

The Wegener is tangent to the top of the 22° halo and curves over the sky to cross the parhelic circle directly opposite the sun (the anthelic point). It continues below the parhelic circle to become tangent to the 22° halo again - this time below the sun.

Note the lower sunvex Parry arc - a rarity indeed. Its ray path is 5,7. Rays pass through the two lower inclined side faces.

A Parry oriented column crystal. The long axis is horizontal and the upper and lower prism side faces are horizontal. Unlikely but it happens!

The topmost face is, by convention, numbered 3. Other side faces are 4-8. End faces are 1 and 2.

A singly oriented column. Unlike Parry crystals, this much more common orientation has the crystal taking all rotational positions about the long axis. Individual crystals do not necessarily 'spin'.

Another image showing the lower sunvex Parry arc just above the aircraft's winglet.

Mouse over for a view where the halos are enhanced by levels shift and unsharp masking.

This is more aesthetic but less effective than colour subtraction. On this image the latter shows the other Parry arcs and the upper and lower Wegener arcs.

Note the parhelion. It was absent in the top image. Crystal populations change quickly at 500 mph!

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Reference Atmospheric Optics

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