Monday, 23 December 2019

Winter Solstice elliptical halo

I was outside working on the day of the Winter Solstice and saw AC clouds coming in and I anticipated elliptical halo and got my camera gear ready. Not too long afterward I saw ice crystals precipitating out the AC and using a telephone pole as a sun block got a small elliptical halo. It came and went twice. This I believe is my 2nd elliptical halo for the 2019 year.

Tuesday, 27 August 2019

28° arcs from Yunnan, China

Two rounds of week-long odd-radii outbreaks swept across southern China in July and August. The second outbreak turned out to be the more noteworthy one, involving predominantly Chengdu-like low sun plate displays ( ).

Zhong Zhenyu, a member of the Chinese skywatcher community chat group, went halo hunting with his DSLR on August 21 after being informed of the on-going outbreak in his area. The community's collective effort paid off and Zhong was treated with some great celestial rarities.

© Zhong Zhenyu, shown with permission.
Upon first glance, the scene immediately reminds us of the Chengdu display, with 3 colored arcs piling up to the right side of the sun. The arc in the middle possesses the same color separation as the other two, especially when USM is applied.

© Zhong Zhenyu, shown with permission. Unsharp mask applied.

B-R analysis later carried out by Nicolas Lefaudeux further confirms the arc's authenticity. The left-side component also shows up in the processed image. At this point it's quite clear that we've got the third confirmed sighting of 28° arcs in China.

Processed by Nicolas Lefaudeux.

Over the past few years, this type of weak, low sun odd radius plate displays occur rather frequently during summer monsoon over southern China. Now with 3 confirmed and 1 possible cases of 28° arcs within 3 years, chances of these plate displays involving exotic arcs may not be as slim as we expected.

Jia Hao

Sunday, 18 August 2019

Odd Radius Display, Southern Ohio, USA, 17th August 2019

On the evening of Saturday, 17th August 2019, I observed this complex pyramidal halo display. In total, I saw a faint 9d halo, lower 9d plate arc, bright 18d plate arc, 18d halo, upper and lower 24d plate arcs and a weak upper 23d plate arc. This display is very similar to the one I captured in 2017, the only difference being the one in 2017 the lower 9d plate arc was bright and colorful and in the present display it was weak.

Sunday, 28 July 2019

Possible 28° (plate) arc from Changsha, China

On July 1st, skywatcher Luo Wuping captured a decent odd radius plate display during sunset hours from Changsha, Hunan Province, China.

© Luo Wuping, shown with permission. 3 images stacked.

Upon first glance at his photos, we immediately noticed the display's striking similarities to the previous two Chinese displays involving 28° arcs ( and ). The brightening between the left 35° and 24° plate arcs looks very much like the 28° arcs confirmed in previous cases and its position matches simulations.

© Luo Wuping, shown with permission. 3 images stacked with minor USM applied.

Unlike the Chengdu display, the brightening only appeared on one side of the sun. Uneven distribution of clouds/crystals might be at play here but the absence of the right side component definitely complicates the situation.

Typically B-R analysis on raw files resolves difficult cases like this (which worked well for the Hainan display). Unfortunately, only handphone photos are available at the time of writing, and the jpeg files turned out insufficient for serious image processing.

After discussions with Nicolas Lefaudeux and other halo experts, we reached the conclusion that it's safer to consider the case a possible one for now given the lack of solid evidence. Hopefully DSLR records of the event will surface some day in future.

Jia Hao

Thursday, 11 July 2019

Pyramidal halos 7-10-19

Before leaving for work yesterday, I saw some cirrus clouds moving in and in a short time later 18 and 23d halos along 18d plate arcs, and bright upper 23d arc appeared. The display went on for a while and when the sun was high enough, a bright and well-defined lower 23d plate arc formed. The lower 23d plate arc in this display is my best to date.

Thursday, 2 May 2019

Kern with subhelic arc in the UK

Berkshire (UK), 28 April 2019: Top left corner shows what's left after applying the gradient subtraction and stretching of histograms on a 38-frame average stack. Top right and bottom left, respectively, are with additional blue-minus-red subtraction and colour-channel enhancement. A HaloPoint simulation is included for reference at the bottom right corner.

While relaxing at home in the evening of a mostly cloudy day, a ray of light caught my eye and I checked the halo situation. In fragmented cirrus there was a mediocre circumzenithal arc (CZA) but not much more than that (my view towards the setting Sun is not that great). I took notice of the Sun being low in the sky so odds for Kern arc were on the rise: in the next moment I was setting up my DSLR to get some photos from my backyard.

CZA disappeared soon after I started shooting the first set of images, but it came back a few minutes later so I chose to do a re-run. Fortunately so, as the latter set turned out fruitful indeed in the post-processing.

The processed stack indicates presence of a faint Kern arc as an extension of CZA at the left. Slightly further to the left, there is a white arc that best matches the subhelic arc in the simulation. I think there is also a subtle suggestion of helic arc alongside of the subhelic, but this is less convincing. Furthermore, there are colored patches below the CZA approximately where 46° contact arcs would appear if there were Lowitz-oriented crystals in the mix. The simulation shown above is a synthesis of three distinct populations, including singly-oriented plates, singly-oriented columns, and Lowitz-oriented plate crystals.

Wednesday, 10 April 2019

A Swedish display points to a new halo

Järvsö display at 1137 local time. Photo Louisa Westberg.

Two days before the mighty Borlänge display in Sweden, on 12 December 2018 another display worthy of recognition was seen about 150 km further up north in the small town of Järvsö. We know about it thanks to Louisa Westberg, who immortalized the sighting with commendable photographic coverage. She had published one photo, the last one she took of the display, and it contains a feature that warrants a few lines here: the V shaped arc between the sun and 22 upper tangent arc.

The halo looks like Moilanen arc, the location is just about right. But the shape doesn't quite match the V is too sharp. Rather, the halo looked to my eye more like the so called reflected Parry arc, of which only two sightings are known. A simulation using the highly flattened tabular crystals needed to make the reflected Parry seemed to give good match.

I informed some friends about the display and what Walt Tape saw in the photo was not a reflected Parry, but rather a 22 tangent arc originating from sunlight reflecting from the water. For him there seemed to be enough water surface to make such halo and, moreover, the cusp of the V is filled also with light, which would be the case if the halo was a tangent arc, but not if it were a Parry arc.

Different simulated scenarios for Järvsö display. M = Moilanen arc, RP = reflected Parry. Photo Louisa Westberg. Simulation software HaloPoint by Jukka Ruoskanen.  

Walt's argument sounds like a winner, but what if the light in cusp of the V comes from the strong sun pillar that extends all the way to the 22 upper tangent arc? Then the halo might still have the chance to be reflected Parry, which does not need reflecting external surface for its formation. The simulation I made doesn't test this scenario, though, as the pillar in it is quite non-existent.

In the end, Louisa's other photos of the display seem to clinch the issue in favour of tangent arc explanation. Photos where there is no water in the direction of the sun show no V. Two photos where sun's reflection is at the edge of the far shore have a rudimentary V shape in nearby crystals. This is what would be expected because reflection from far shore edge would provide light only to crystals that are very close to the observer. And finally, when Louisa moves near the other end of the bridge and takes in quick succession three photos from the same spot, there is a long stretch of open water from the sun image backwards. All this water surface would be effective in making the reflected light tangent arc, and true enough, the V in this last triplet of photos Louisa snapped is solid and bright.

Four successive photos of Järvsö display taken before the main photo at the top. The times for photos are 1131, 1133, 1135 and 1136. A rudimentary V is visible in the last two photos. Photos Louisa Westberg.

The location for the photo in the middle (at 1137) is marked by red pins in the map and satellite photo. Yellow line marks the direction of the sun. The location is according to the geographic coordinates in the photo exif data. Sun elevation was 5.3 degrees. 

A diagram by Walt Tape.

Now our attention must naturally turn to the two older observations that have been interpreted as reflected Parry arcs. Could they, too, be tangent arcs from sun reflecting on water surface? For the first display, that I photographed in Rovaniemi on 6 November 2008, the answer looks positive. There was a river between me and the sun at the time of the occurrence of the arc, and a search through all the photos I had taken indeed came up with two that show the river was open. However, if light inside the V is also needed to justify the tangent arc, then here we seem to have surprisingly little of this light.

Top: a photo from the best stage for the V in Rovaniemi display. Bottom: a photo showing sun light reflecting on water (the V is weaker here). 

The V in Rovaniemi display was first observed in the location marked by the red pin on the left. The photos above are from the location marked by the pin on the right. 

The second sighting was by Timo Martola in Turenki of southern Finland on 29 December 2015. The V is filled with light and there is a water body called Ahilammi a lake like widening of a river in the direction of the sun about 800 meters from where the photo was taken (the V shows up also in other photos taken in the same vicinity). But we do not have a photo of Ahilammi at the time of the observation. End of December is already deep winter and Ahilammi would probably be expected to be frozen. However, Finnish Meteorological Institute summary of that winter says December was "uncommonly and at places even exceptionally warm" in Southern Finland. Average temperatures in December in the adjacent sea areas were 4 to 5.5 C above the normal. Add to that the moving water in the river and it starts looking possible that Ahilammi was open and the V in this display could then be interpreted as tangent arc from sun reflection.

Turenki display and two simulated scenarios. Photo Timo Martola. Simulation software HaloPoint by Jukka Ruoskanen.

 Location for the Turenki display photo and sun direction. Photo Timo Martola.

Ahilammi on 3 April 2019. Lakes are still frozen, but Ahilammi is open in the middle because of water movement. Photo Marko Tähtinen

Finally, shape may also be used to distinguish between reflected Parry and tangent arc from sun reflection as the V of the former is slightly wider. The difference is small and not something one might rely too heavily on, but if it was nevertheless used as a criterion for the Järvsö display, it seems the tangent arc scenario might just come out more favourably. For the Rovaniemi and Turenki displays this comparison seems inconclusive.

So where this all leaves us? Perhaps reflected Parry never actually existed and all these three displays instead have tangent arcs from sun reflecting on water. As usual, more observations would be nice and now that we know about the possible role of open water, informed attempts to see the halo can be made.