Sunday, 20 September 2020

Mysterious sunvex 28° arcs in Yunnan, China

On August 26 2020, Zhang Yibing from Pu'er, Yunnan captured a sunset odd-radius display, in which a bright 28° arc dominates the show. 

Stacked and sharpened. © ZHANG Yibing, shown with permission.

Evolvement of the display. No sharpening. © ZHANG Yibing, shown with permission.

The arc’s brightness is almost on par with the 20° plate arc, while other plate arcs such as the 24° and 35° ones are exceptionally weak and barely show up in stacked photos. What’s more interesting about the 28° arc is its shape. What we are seeing in this display is a gently sun-vex arc which doesn’t quite follow the 28° sun-centric circle.

Stacked and background subtracted photos reveal a left 28° sun-vex arc as well. Note how both arcs curve away from, instead of follow, the 28° circle.

Such appearance contradicts current theories. In the 30-32 pyramid and cubic ice models ( ), the predicted 28° arcs do not appear sun-vex like the 24° plate arcs do. Quite the opposite, when the crystals are wobbly, the arcs should look more sun-cave and follow the 28° sun-centric ring.

The puzzling shape can still be achieved by tweaking the available theories though. JI Yun and I did some experiments in ZHANG Jiajie’s simulation program and came up with a bizarre solution.

First is to tilt a triangular 30-32 upper pyramid by 109.5° until one of its pyramidal faces goes horizontal. Then, apply a loose ( 15° wobble or more) sun-ward azimuthal lock on both tilted 30-32 crystal and cubic ice.

The revised models produce identical sun-vex 28° arcs which closely match the ones seen in the display.

The idea of az-locking crystals in the air is certainly outlandish and should hardly be taken seriously unless concrete evidence is found in real world. That being said, there’s only so much we can do with available theories. New ideas/experiments are much needed.

This display marks the 11th record of 28° arcs in China since 2016, and more importantly, the 1st record revealing the arcs' real shape. Is it possible that what we're seeing in this display are actually new halos, different from the previous 10 Chinese records and the legendary Lascar arcs? Or is it that all these Chinese 28° arcs are a same new breed different from the Lascar ones? We'll need many more similar displays to draw a conclusion. Looking forward to the next summer.


Nicolas Lefaudeux proposed another possibility for the arc's peculiar appearance:

"On my side, the sunvex shape is still not so obvious to me. I see more a vertical elongation of the 28° arc + some patch of 28° halo. 

Effects like diffraction by "vertically narrow" crystal faces would create such vertical elongation (like a plate crystal with exotic faces instead of regular prism faces). Unfortunately, such effects cannot be simulated with our current softwares. The appearance of the halos (smooth and rather undefined) make me think of diffraction-affected halos (small crystal faces), like most of the odd radius halos. We would need cases with sharper arcs in order to be more conclusive."

Friday, 18 September 2020

Long subparhelic arcs in Rovaniemi spotlight display


Diamond dust season is about to begin, the hissing of guns at the Ruka resort may be heard already at the end of September. To get in the spirit, let's reach into the drawer and grab this display which appeared on the night of 7/8 February 2017 in Rovaniemi. At first it was bog-standard plate with CNA, sub-120, and sub-Lilje / subparhelic circle patch opposite to the lamp, but then it changed, giving these long subparhelic arcs. The lamp sits at the usual ~5 degrees below the horizon. The location is Sieriaapa bog some 6 km east from the ski center. This is the direction where the diamond dust most often heads to, guided by the prevailing wind and topography.


Friday, 4 September 2020

August halos from Moravia, Czech Republic

Jiří Kaňovský from Černotín, Czech Republic observed three separate halo complexes this August.

August 2nd, 2020 – bright odd radius halos

I (Jiří Kaňovský) was visiting my aunt when I’ve noticed a halo making cirrostratus cloud shield. It was obvious that what I saw were not classical halos, but odd radii ones. I was able to observe 9°, 18°, 20°, 22°, 23°, 24° and 35° halos. Near the end of the observation, a bright 23° parhelion joined the party.



August 10th, 2020 – classic halos with a twist

These halos were observed due to an extensive cirrus/cirrostratus cumulonimbogenitus shield from a previous thunderstorm activity. At the evening hours, some of the cirrus clouds began to sublimate. Among the classic halos like 22° halo, UTA, CZA, SLA and 22° parhelion, later image stacking revealed a bright spot near supposed 9° column arcs and a 24° halo or a 24° column arc as well.

August 24th, 2020 – almost invisible uppercave Parry

There was a surprise 14 days later in the form of a weak but nice halo complex. A 22° halo, parhelions, weak UTA and CZA were observed. Later image processing revealed a weak uppercave Parry and a hint of a supralateral arc.

All these observations are a part of an endeavour to bring an old project back to life – the Czech HOP (Halo Observation Project). After 10 years of inactivity, multiple people are joining again to work on the project so it could be a place for amateurs and “professionals” alike where they can share their photos of observed complexes.