Wednesday, 24 February 2021

Surface helic arcs in Finland

By Lasse Nurminen and Marko Riikonen

The cold spell that started in February in Finland brought along some surface helic arc displays. In this post we represent our observations. The past surface helic arcs have been largely symmetrical, but this time asymmetrical displays dominated.
 

Lasse:

Finally some decent winter weather arrived even to the southwestern parts of our country. Lots of snow and the temperatures around -15°C for a long period. In the coldest mornings the temperature dropped to as low as -25°C. It took some getting used to it, as during last winter the temperatures here were stuck mostly around 0°C.

So because of the nice winter I´ve been keeping my eyes open for the possible halos on the ground also. I only observed a few faint 22° rings until this case.

One day the surface of the snow draw my attention as it looked pretty rough. The ice crystals were pointing upwards, usually the surface looks much smoother. It took me a couple of days to wake up my slow brain cells. I remembered seeing some rare halos spotted from a surface looking pretty much the same. A little search from Taivaanvahti website confirmed my thoughts. So the ”hunt” for the surface helic arc was on.

These type of crystals need open space, clear skies and wind to form. I could already tick all of those boxes and I began to search for the possible arcs from a few small landfill hills on 4th of February. There wasn´t too large uniform areas on the slopes to be found, but still I found one spot with the arcs visible to the naked eye. Nothing too fancy though, so I decided to shoot a few photos to stack them in the post processing. The stacked photos helped to identify the arcs even better.

4 February, max stack of 6 photos


Close-up of the crystals

 
The weather then changed a bit, crusting the crystals with a matte coating. The light couldn´t penetrate this layer so the arcs were gone. A week passed and then one day (11.2.) I noticed that there was new growth in the tips of the crystals. The growth was transparent, looking similar to the earlier arc producing ones. So I decided to take a look for the arcs again.
 
This time the view was pretty much different from the first case. I could only see one arc on the surface. If someone would have been in a hurry, looking on the surface, that arc would have easily been mixed with the usual glow of the sun. But I had the time and knew what I was looking for. The glow was clearly way off from the vertical line of the sun. Once again I took a series of photos from the view and decided to stack them as well.
 
I noticed that the phenomenon was right-orientated all over the area. No matter which way the slopes were leaning. The plants sticking through the snow were covered by the new ice crystals much more from the left side of the stem than from the right side. One main factor in generating these kind of arcs might have been the winds blowing from the opposite direction (left) to the visible arc.


 
11 February, max stack of 23 photos (top) and single image (bottom)
 
 
Close-up of the crystals

 
Once more six days later (17.2.) I got to witness this one-sided arc. This time on a more flat area, where I made my first contact with complete arcs. That first time the arcs were barely visible, but this case was easily visible for the naked eye. Stacking did the trick again, making the phenomenon really shine.

During the last case I intended to photograph some crystals a little closer as well. I thought that I had my set of extension tubes in my backpack, but for my disappointment they weren´t there. The next day I had them along, but the situation on the ground had changed. No more arc(s), but the crystals still had the same shape for most parts. This blade like growth in the top part makes the crystal look a little like fish scales with the growth rings to me, too angular though.


 17 February, max stack of 28 photos (top) and single image (bottom)

 

18 February, crystals. No helic arc was seen.

 

Marko:

While the southern part of the country was already enjoying surface helic arcs, otherwise good prospects up north in Rovaniemi were at the same time spoiled by light nightly snowfalls. But then the night between 7 and 8 February was all clear and next day surface helic arc, which other arm was brighter than the other, was visible on river Kemijoki.
 
I took a series of photos and after picking out the bad shots the stack had 244 photos. The max stack is particularly interesting. It has two patterns. One drawn out by the helic arc fork and another by the brighter "random" glints outside and in between the fork.
 
Given the non-overlapping patterns, the two phenomena must be connected in some way. I took a closer look only at the helic arc crystals, in future displays have to check also those brighter glints, it might give some answers. The helic arc crystals were rather small, but as I put my nose to ground it seemed that the halo was formed of a light passing through crystal blades that had terraced structure. I have seen such a formation very clearly in 2014 in a surface helic arc display in Oulu, which had much bigger crystals.

 8 February, from the top: 1. max stack, 2. same with usm, 3. average stack, 4. single frame.

 
Next day in the same place a similar sight was seen but instead I show below a surface helic arc from a different location. This is the Jokkavaara gravel pits where I went to photograph diamond dust halos. The rather weak helic arc is almost completely one sided and appeared only in one small undisturbed spot between areas cleared of snow. I took close-ups with phone. The quality is not good, but the photos manage to show the terraced crystal blades that light up the helic arc arm. There is matte growth on those blades which must have weakened the halo.


9 February, max stack of 14 frames
 
Two close-ups of the helic arc

 
Here is a link to all surface helic arc cases this winter in Finland:

https://www.taivaanvahti.fi/observations/browse/pics/3762112/observation_start_time/desc/0/20

The helic arcs by Antti Hämäläinen and Maria Tirkkonen were serendipitous finds. Petri Martikainen spotted these from scenic shots published elsewhere and encouraged the photographers to leave observation in Taivaanvahti.

Monday, 22 February 2021

A fresh 2 o'clock spot display

On 6th February 2021 I noticed a nice 2 o'clock spot outside the 22° halo in the sky. I succeeded to take a stack of 40 photos in 4 minutes, during which the spot vanished and was replaced by a thick arc. Sun elevation was 12.8°.

 
Similar halos were observed in Jutland display 22 April 2020, but the explanation for the spots and arcs remained somewhat unclear then. So I did some simulations to see what could be behind the current display. Here is a video of the first 2 minutes showing how the spot disappears: 
 

The same photos are used in the stacks below. The first 30 seconds (5 frames) are on the top, the first 2 minutes (20 frames) in the middle and the last 2 minutes (20 frames) are undermost.


The possibility of 24° plate arcs was discussed in the Jutland-case, but it was concluded that although the position was correct, the orientation of the spots was not. The spots were understood as intensified parts of subparhelic arcs (Schulthess arcs) - which did not match the simulations either.

The spot was most intense in the very beginning of the current observation. I tried to simulate 24° plate arcs to correspond to the first stack of 5 frames, and included subparhelic arcs to the simulation, too. When the 24° plate arc is weak enough, there is a good match with the actual spot (HaloPoint 2.0, comparison below). So at least in this case the simulation seems to give support to 24° plate arc. A weakness is that the 9° plate arc produced by the simulation was missing from the display. And the subparhelic arcs are not even close.


When the spot started to disappear, the arc became stronger and thicker. It was possible to get rather similar thick arcs by simulating upper and middle Lowitz arcs with more tilt in the crystals (std 30 and 40). So, at least in this case, Lowitz arcs seem more plausible ones than subparhelic arcs.


It is hard to say how well these results correspond to reality or to Jutland display, but hopefully they give some ideas for the interpretation of 2 and 10 o'clock spots and associated arcs.

Saturday, 20 February 2021

A super-February of elliptical halos in Finland

Finland has been treated to an extraordinary outburst of elliptical halos. Between 9 and 18 February, no less than five ellipses were seen, each on their own day. Four were in diamond dust, one in Altocumulus virga. For the diamond dust ones the temperature on the ground as informed by the observer or the nearest weather station was -12...-16 C (10...3 F), which is neatly in the range where elliptical halos are expected to occur.

This surge is all the more dramatic because it comes after a hiatus of over two years, stretching back to a lunar ellipse seen in January 2019. Below are shown four of these fresh sightings. For more photos, and the one case that is not displayed here, look in Taivaanvahti:

https://www.taivaanvahti.fi/observations/browse/onlyPics/3761269/observation_start_time/desc/0/20


18. Feb., Ritva Jyrkkä, Kuhmo


17 Feb., Marko Riikonen, Rovaniemi. Ac virga. A stack of four photos taken in 28 seconds.
 
 
 12 Feb., Markku Siljama, Mäntyharju
 
 
 9 Feb., Timo Martola, Janakkala

Thursday, 11 February 2021

A 46° halo dominated surface display – crystal photos included

This winter's first solar surface display in Rovaniemi came on the first day of February. It was not among the most basic kind of displays, as visually it had only a weak 46° halo. Yet this is either nothing unprecedented I recall of seeing such displays in the past, like do some other folks I have talked to.

 

As we haven't yet seen this kind of a display stacked, I thought I might as well get the job done this time. I was interested to see whether 22° halo would appear in the stack and, taken that I had come to the river ice equipped with microscope, I was curious of the crystals.

After having snapped more than 600 photos of the display I skimmed the surface snow into a petri dish with the dish's edge (no gasoline, it is not needed with surface crystals). What I saw in the microscope appeared to be sectored plates that must have fallen from the sky at some not so distant earlier time, maybe during the previous day. It looked to me like the thinness of the blades combined with a largely poor optical quality was the reason for the lack of 22° halo the light would have hard time to make it through the 60 degree prism unperturbed, especially when the it has to do a basal face ping-pong on those thin blades. But there were also enough clean enough prisms to make me suspect the 22° halo would nevertheless appear when photos are piled into an average stack.

Indeed, the 600 photos did their job and the stack revealed a 22° halo, albeit an extremely weak one. This calls me back to the recent lunar display where parhelic circle was seen without parhelia. In the comments I suggest that thin plates of poor optical quality might be the culprit. After this surface display the explanation has gained somewhat on its appeal on me.

Some technical info. The halo photos were taken with Samyang 8 mm on Canon 80D. Here are shown b-r and bgr (the Lefaudeux background removal method) versions, the two lowermost ones further flip-stacked. After winnowing the photo set, exactly 600 photos were left in the stack. Let it be known also that it took 24 minutes to take those photos, which I hazard were perhaps around 620 of all of them.


A typical 22° halo raypath in thin crystals with several basal face reflections. The image is from a HaloPoint simulation. 

 









A fine column display in Rovaniemi, 17/18 January 2021

 

Here is yet another column dominated display in Rovaniemi. It was pretty stable, giving me the longest stack this winter, 31.5 minutes (63 x 30s).

When I arrived at the slopes, the diamond dust was slightly elevated and the pillars that were visible on this crystal stream didn't reach to the ground. But it seemed like some kilometers to the east, on the north side of the river Kemijoki, the swarm was coming down to the ground.

As I drove towards that approximate location, a foggy scene ahead became visible and upon diving in streetlights birthed great parhelia and tangent arcs. I got out of the car and asked a passer by who was coming my way on the sidewalk if there is an access to the riverfront. Luckily, such a place was very close: the next road off back towards the city took me to a tiny public swimming spot where I could start photographing.

The long duration gave plenty of time to enjoy the display also visually. While my camera is often pretty close to the lamp, say, at 15 to 20 meters, that's no good for the visual. That close it is harder to distinguish the halos because the glitter, although bright, is too sparse. 

When you go further the volume of the lit area increases and the display solidifies until a point is reached where it is almost like you were watching a classical display. The problem, though, is that the more you take distance the more the beam shoots overhead when the lamp is below horizon, and you will see only a segment of the display.

Even so, it is quite all right. This time, as I took distance to lamp, I really paid attention what was seen. The subhelic and Tricker arcs were solid and awesome. Then I started looking for something that I had already planned to look for, and which I had not yet seen visually: the "column 351, 361", which is a halo related to the 46° supralateral arc, and which in the image looks like the 46° supralateral arc a 46° supralateral arc for the subsun, if you will. After a little brain haze about its location, I spotted it. An unmistakable solid arc faint but distinct. Seeing it with my eyes after having first photographed it in 2008 this was the highlight of the night.

I am starting to think that I should place also the camera further away in the coming up displays to get the benefit of more solid display even if the glints are dimmer further away. I used to do that back in 2007-2008 when I started spotlighting. Just a few photos in the stack gave a nice display. 

With a lamp of given brightness, there must be some sweet spot distance where the competing benefits of the brighter glints nearer to the lamp and increased lit volume further from the lamp come to an optimal balance. This is not something that you can apply mechanically every time, though. When the swarm has a plenty of crappy crystals, you can't go very far. That's because with increasing distance those crappy crystals start making a mask that blocks the display more and more, until at some point the display is completely gone. I have seen this change to a total masking happen on some occasions very steeply and not at all that far from the lamp.

Well, I wandered off a bit here. Back to the display. But there is really nothing special to it. Two less often photographed halos are visible, the "column 135, 136" which is related to the 46° infralateral arc, and "column 3152, 3162", which is related to the (as yet unobserved) anthelic 46° supralateral arc. We might need to set up an international committee and come up with real names for these and many others some day. Walt Tape has already written of the naming of "column 351, 361" even if he doesn't offer an answer.

I thank Olli Sälevä, a colleague here in Rovaniemi. This display was photographed with his camera. He lent his Canon 80D to me when my camera broke.

 

Four versions of the image. Upper left is b-r, lower right bgr. The black and white image to its left is just to fill empty slot, I played with Photoshop's Black & White sliders. This shows the blue circle somewhat better than other versions.


A HaloPoint simulation and its b-r version. The two most rarest features in the display are marked with an arrow. Left arrow marks the "column 135, 136", the right arrow marks the "column 3152, 3162". I have not included Parry crystals in the simulation, which are evident in the display by the faint helic arc.

 

 Parameters for the simulation.

Sunday, 7 February 2021

Low sun odd radius display in Rovaniemi 14 Jan. 2021 – and some words on Canon 80D

It was fortunate that I discovered a new webcam for Rovaniemi on this day. After having spent the whole night out at below -30 C temps waiting for diamond dust to rise, and which never happened, I called it quits in the morning and was at bed 8 am. But I set the alarm for 11 am to check the webcams if things had changed. The sun was already reaching 2.5 degrees above the horizon at midday, which was starting to make solar display a realistic prospect.

Nothing had changed. There was no diamond dust visible in the three ski center cameras, nor elsewhere in the city. It was totally dry. But as I was about to close the browser and continue sleeping I happened to chance upon a Rovaniemi webcam that I had not known about. And it had a diamond dust display going on. Moreover, I thought of seeing an upper 24° plate arc in it.

I was out quick. When I come to my apartment from a halo hunt session I just throw my clothes in one pile on the floor. If I have to suddenly leave for another round, like now, they are readily arranged to get dressed fast. Five minutes and I am out.

Of course I then wasted too much valuable time looking for a place to photograph, making stupid navigating decisions. Yet I managed to get some photos:

First set, 19 photos in 1min 10s. Sun elevation 2.5 degrees. The display moved on and I went after.


Second and the best set. This stack has 104 photos taken during 5 minutes. Sun elevation 2.3 degrees.
 

The strong 20° upper plate arc got me looking for the upper 35° plate arcs, and I think I also saw them on the camera display. But I couldn't be sure whether or not I was seeing them with my eyes. Sometimes I though of seeing, the next moment not. To my chagrin I had forgotten (yet again) the convex mirror at the apartment.

But in the end I did happen to get an unquestionable visual of the 35° plate arcs. Not on the upper arcs, but on the lower ones made of glittering crystals against the ground. At first I was wondering why there is a 22° lower tangent arc, but soon realized it is of course the lower 20° plate arc. And at that point I caught also the 35° plate arcs flanking it. 

This happened when the display had already markedly waned, with no odd radius halos in the sky visible to the eye. So these against-the-ground versions must probably be considered something of an easy catch in odd radius displays. I regret of not looking for them earlier, especially when I was taking the first photos on that bridge. 

This is by no means the first account of 35° plate arcs observed this way. Walt Tape writes in his 2006 book with Moilanen the following on page 152: "In low level displays in Fairbanks we do sometimes see the lower 35° plate arcs below the horizon, showing up as concentrations of sparkles between ourselves and the ground, sometimes in the space of only a few meters."

While there is nothing special about this Rovaniemi display as such (we all know the Chinese routinely catch similar displays in high clouds spiced up with 28° arcs), the conditions that birthed it got me puzzled. My initial webcam survey that snow guns were not giving anything was actually right. As I arrived to the scene, I could see there was no cloud whatsoever growing out from guns. This display was made by the city main power plant, which grew an extensive cloud. Given that snow guns should by all experience be superior diamond dust makers, spewing out nuclei that themselves are ice, it beats me how this is possible. How can the guns fail but power plants work? I have no answer, but this is not the first time to have encountered such an anomaly. In 2017 there was a display that was formed by another, smaller power plant in the Saarenkylä area, and the guns were giving nothing at the same time.

I finish with some words on the performance of camera used, Canon EOS 80D. Olli Sälevä, a colleague here in Rovaniemi, lent it to me after my 40D broke. Because I had already placed an order for the same camera, I was horrified to see color banding artefacts reminisicent of those from the Nikon 3000 series cameras. It appeared the issue was not nearly that serious, though. Jari Luomanen, who took a look at one of the raws, pointed out that the artefacts change location according to the color temp. So they are not a property of image files, but rather something made by the editing software. You can play with color temp to move color bands to a position where they don't disturb so much or, better, stack the images with different color temps to largely wash them out. Of course, the latter increases significantly the after processing work burden, but at least it is relief to know they are not a fixed property of the files. The images below demonstrate these issues. There is also a comparison that shows how two softwares place the color bands differently in the image.

 

These stacks show how the colored artefact band moves upwards with increasing color temp. The upper left image is an average of color temps in the indicated range at 100K steps. This works quite well to wash out the artefacts. Double usm. Photoshop 2021.

 
Comparison with Canon Digital Photo Professional 4 and Photoshop 2021. The former has several artefact bands while latter has one broad band. A one time heavy usm was given for both images in the Photoshop.
 

In general, it would be great to see people give little reviews of their cameras in this blog. As of now, buying a halo camera is basically a jump in the dark. Having experiences shared here could avoid some frustrations. I already know from my own trials that Nikon 3000 series is a stay-out, and the same seems to be with 5000 series. The best camera for halo photography that I know is the one that Nicolas Lefaudeux had for his automatic halo camera in Paris. When I look at the images on his page, I don't see any banding artefacts. It soothes the soul to look at those images.