Tuesday, 29 December 2020

Halos on the night of 17/18 December in Rovaniemi


When you do spotlight, two things are desirable: a dark location and stable swarm. On the night of 17/18 December in Rovaniemi both were realized, totaling me more than two hours of exposure.

The sky had cleared up in the daytime and for the longest time I saw the sun, only its upper half at noon visible above the horizon from my 6th floor apartment. Some fingers of fog started to rise also here and there, promising a good halo chase for the night.

When I arrived to the slopes at 4 o'clock these fogs had disappeared and there was no diamond dust. Weather readings put the airport at -12 C while railway station was at -11 C. The airport being at a higher elevation (200 m / 650 ft) than the railway station (80 m / 260 ft), this means no inversion. Only when the temperature roles switch can diamond dust grow in clear sky conditions.

This happened after two hours, at 6 o'clock. The swarm spread south-west and engulfed the city center and the industrial area beyond. It was not worth to start photographing, though, given the light polluted locations and poor material streelights had only pillars, no parhelia or tangent arcs.

I was hoping for the swarm to reach the southern end of the industrial district, so I could photograph in the large area there used by some earthmoving company. This location in Anttilanvaara is reached by an unnamed road and has a great dark segment spanning about a half a sky.

Finally the swarm engulfed the Anttilanvaara area and I got to work. By this time the temperature had plateaued in my tripod leg thermometer to around -20 C , and the stuff was mostly plate, with some Parry / column in the beginning. Below are shown stacks from the photos I took during the night.

The night's first set, 57 x 30s. In addition to plates, there is also Parry as evidenced by the helic arc. The arc emerging from subanthelion looks more like diffuse arc than subanthelic arc, telling there is also column, even if no subhelic or Tricker are visible. I saw the diffuse arc visually, but not tangent or Parry arc. The dark line in BR following opposite to the lamp is an artefact. The lamp is about 8 degrees below the horizon. 


As the display of the previous image deteriorated, I looked around and changed to a place where more elevation was possible. Judging by the location of the subsun relative to the 22 halo, the lamp is about 15 degrees below the horizon. 54 x 30s. 

I noticed an additional brightening in the pillar and broke the previous set to turn the camera to photograph it. It was Moilanen arc as expected. 15 x 30s.

Once the Moilanen was gone the plate got again better and I returned to all-sky view. 12 x 30s. After this the swarm disappeared. I packed up to follow, its edge had receded to about 1 km towards the city. I visited a grocery store, buying potatoes and apples.

After shopping I headed back to Anttilanvaara to see it was engulfed again. This gave me this best set of the night before the swarm receded again. The edge moved several kilometers towards the city and I decided wrap it up. That was around 2 am. I think I should have just stayed: when looking from my window at 4 am as I was going to sleep, I could see the diamond dust cloud still resided in the same area over the city and possibly it might have extend its reach to Anttilanvaara yet once more. 57 x 30s. The lamp is here maybe about 30 degrees below the horizon.

Simulations to compare with the image above. The simulation on the left is made with regular hexagons, on the right with full triangles (h/d 0.5 dev 0.1 for both). There may be some of that blue spot at 100 degree azimuth from the subsun visible in the photo, too. It is from subparhelic circle raypath 3257. It seems to remain at the same distance from the sub-120 parhelion as the sub-Lilje blue spot on the other side, so there is some relationship here. Light source elevation is 27 degrees. The bulge this raypath makes in the subparhelic parhelic circle with regular hexagons has been identified in the great spotlight display that Jari Luomanen and Marko Mikkilä photographed in Sotkamo on 22 November 2014. It might be also of interest to see how those photos would look in BR.    

Here is shown only the 3257 raypath. On the left regular hexagons, right triangles. The intensities are comparable. The separate spot with regular hexagons is from 3276543875 raypath, but the part of the sequence that makes full round of the crystal (765438) can be reduced, so it is actually a 3257 raypath. Maybe this spot will be seen some day. In Parry crystals this same ten hit raypath makes subanthelic arc and is likely an important contributor to its intensity in some South Pole displays at least. The raypath is listed in Tape's Atmospheric Halos on page 130 and on page 35 is also a visualization of it.
So that's it folks. This post cleared the backlog and I have nothing more to report. Thus far the winter has been pretty low on the action, only a couple of singular nights of diamond dust. But I am sure in January-February a more hectic gear will be turned on.

Tuesday, 22 December 2020

Halos on 2. December 2020 in Rovaniemi

At midnight I decided it was time to go. The plume of the heat plant 6 km from my place had developed a kind of bend that could signify an inversion just starting to set in.

Turned out there was still plenty of time. It was windy and dry at the slopes. The gunning plumes died right away. Nothing else to do but wait.

Clouds arrived, covering more and more of the sky, but they were too high. Finally after 3 o'clock the action started, either because the cloud base lowered or it cleared up. It is not always easy to know because in clear sky conditions snow gunning makes often its own cloud some way above the ground, which may be thick enough to block even the full moon.


Lens hood is limiting the view in this seven photos stack when the display was at its best. In BR image inside the Tricker arc is a dark line. The location matches the subanthelic 46° supralateral arc, but it is certainly a borderline case. It is worth noticing how poor the lunar display is in comparison. That's most probably because the swarm was lying low.

Here the lens hood is off. A 14 frame stack. The Wegener arc was really impressive visually from the side of the beam a colored, almost solid arc.


I started with a failed attempt to photograph on the incline of the eastern Totto slope where they had not yet made any snow. The swarm left me just as I got the equipment in place. A little later on the golf track I managed to get a handful of photos before the swarm shifted again. But I botched it by taking only the lens cap off my new 4.5 mm Sigma lens, not the lens hood. When I realized the mistake, the display was already declining. In general, the combined lens hood + lens cap of this lens is not that great. The lens hood thread is endless and you spend a life screwing it off.

There seems to be the other half of circumscribed halo visible here. The photo was taken at 4:48. Moon elevation 37.1 degrees.
Then I ended up photographing the display by the river at end of a short road called Venetie. Lunar this time. The swarm became stable and I think I was in exactly the right spot. A detail of some interest is the circumscribed halo that is seen at lunar elevation of 37 degrees. Well, it is only half a circumscribed halo as the other side is spoiled by trees and light pollution. But in any case I haven't seen before photos with circumscribed halo lower than 39 degrees. Theoretically, the limit light source elevation for circumscribed halo is 29 degrees. Probably it was here full still longer, but it is cut by the trees as moon goes lower.

I called it quits at 6:30 because the display was steadily worsening. As I drove to my place, and out of the diamond dust for first time since the action started, the sky cleared up. So, as to the question posed in the beginning of this post, whether this was a clear sky or punch hole display, the former was true. 

In hindsight, I should have done spotlight instead of lunar. Wouldn't have had to worry about the movement of the light source, and even with all the light pollution from the streetlights and moon, it would have most likely been the more interesting option results wise.


23 frame stack. Moon elevation 37.2-36.7. Moon movement has been accounted for in post processing for this photo and the ones below. All photos in stacks are 30 s exposures.
24 frame stack. Moon elevation 29.8-29.0.
Simulation for the image above, 29.5 light source elevation.
The night's last set 6:30 in the morning by the river at the end of the road Pieskätie. Nine photos stacked.

Wednesday, 16 December 2020

Another night of halos in Rovaniemi – with puzzles

It is more common than not to have something interesting to chew on in diamond dust displays, especially if they are stacked and of the spotlight kind. Such was the case with the displays that I photographed on the night of 24/25 November this year.

The forecast was good and I had come to the field early, poised to leap when the action gets going. It didn't take long. From west came low stratus clouds, and diamond dust appeared inevitably as the tiny ice particles from snow gunning nucleated it.

Yet it wasn't smooth sailing and it took some time before I got to actually photograph halos, first on a small field not far from the hospital. The image above is from this location, a stack of 40 photos, each with 30s exposures.

In the below BR version is seen the first one of the night's two puzzles: there is a weak sub-Kern arc, but no sub-CZA or subparhelia. It would in principle be OK to have sub-Kern without noticeable sub-CZA in this display if the plates were unusually thick, h/d > 0.7, but you still expect at least subparhelia to keep company for sub-Kern. The simulation below demonstrates the issue. 

The sub-Kern appears to be not all alone here, though. It seems there is – just like in the simulation – a faint subparhelic through the subanthelic point. And even faint sub-Liljequist parhelia, at least the other one, against the trees in the 8.30 o'clock position in the curve of the sagging wire. These features are made by plate oriented crystals just like the sub-Kern. Could it then be that the subparhelia are there, too, but are dissolved in the background? To my eye the background doesn't seem that restless or lit up to make subparhelia disappear. Neither does it seem that they would be outside the image field either. So it is a bit of mystery here.


In this HaloPoint simulation the attempt was to get a very weak sub-Kern (arrows), just like in the photo, to see whether subparhelia would not be made. This didn't pan out, the subparhelia come inevitably. I happened not to save the parameter file. Light source elevation was 7 or 8 degrees from the horizon. Plates were thick h/d 1.0 to not make at all sub-CZA. The blue spot on Tricker arc seems to be visible also in the image. I don't recall seeing it in earlier displays.


The relative brightness of sub-Kern / sub-CZA allows knowing how thick the plates are. In past observations the plate thickness inferred this way has capped at around h/d 0.5-0-6. Simulations with HaloPoint. Regular hexagons, 0.2 variation of prism face lengths, light source elevation 7 degrees from the horizon.

The session was finished abruptly by the swarm turning into snowfall. To my surprise the crystals melted on the lens, I would have thought the lens cooled enough by then, the temperature at the railway station official measuring point was -8 C. 

As I left the small field to see where the diamond dust had gone, I was wondering why in the part of the windscreen that was cleared of snow by wipers a quite nice 22 halo was visible under streetlights. I briefly considered taking a photo, but kept on driving. 

It didn't take long to locate the swarm. Streetlights had awesome parhelia, but the action was always in places where I couldn't do neither spotlight nor lunar. (And although it would be awesome to do videos of these 3D halos, I don't have video in my old Canon 40D.) After a good while I finally got a lunar display on a bog called Matkajänkä. The image below is a 10 frame stack of 30s exposures, the photos are rotated to account for the moon's azimuthal movement in the sky, which is why the trees and streetlight pillars around are blurred. The display was better before I was ready to photograph. It was a beautiful thing to look at.


Once the lunar display was gone I again struggled to find a place to photograph. After more than two hours I finally ended up on a little light polluted clearing near the Jätkänkynttilä bridge, where a plate display was visible. Only three 30s frames are stacked here, but the display is nevertheless quite smooth in the image because the wind had picked up, sweeping great masses of clumpy diamond dust fast across the beam.

And this gives us the second puzzle of the night: the 120 subparhelia look tilted. No matter how much I try, I can't get myself to say they point towards zenith, which is what would be expected. I don't know what's going on


 This half-image is a flip stacked version.


The night's final display I got in the Saarenkylä crop field. Just like in the previous location, I was late, and got only a couple of photos before it started fading. Nothing special is seen, the BR reveals blue spots on both Lilje and sub-Lilje. 

Note to self is in place here. After I had taken two photos with zenith centered view, I decided to change to side view. That's no good. Never change horses in midstream. You want to have as many photos in the stack as possible to smooth out the display. Always stick to the view you have chosen for as long as the display lasts. This allows you to do also a time lapse movie which may turn out useful.



Let's do a little digression for the end. The simulation I made for the display has this extra arc crossing the subanthelic point, fleshed out due to fine dot in the simulation. This is a "rotated Wegener arc" from raypaths 32567 and 32457. And as the filtered simulation I made below shows, it has also its own "rotated 22° tangent arc" which arises from similar raypaths, but without the basal face reflection. These theoretical halos are 120 degrees rotated versions of their ordinary cousins: their common point of contact is 120 degrees from the 22° tangent arc and Wegener as measured along the great circle passing through sun and zenith.

A closer look reveals there is also another rotated tangent arc and rotated Wegener pair. It comes about by triangular crystals, raypaths 3573 and 32573 respectively. The sharp red edge of these arcs is pointing away from the sun because they are rotated by 240 degrees. All these rotated arcs are shown in the side view simulation below. I have called them as 142°/262° tangent/Wegener arcs according to how much they are rotated from the sun. However, because the 262° rotated arcs are in reality found 98 degrees from the sun on the great circle, that number probably makes for a better name. Greenler, who considered the 3573 raypath to look for possible explanations for Hevel's halo and 90° parhelia in his book, talked of a 98 halo concerning the version of this effect in randomly oriented crystals. Indeed, to call it a 262° halo would sound rather quixotic. 
Whatever they will be called in the future, the rotated Wegener arcs seem distinct enough to become reality once a quality swarm in a dark location gets deep stacked. Parry orientation version of these rotated arcs, of which there are much more (including sub versions), may be discovered in monster Parry displays.  
As for those dark places, they have become more scarce, unfortunately. Four years has passed since I last chased in Rovaniemi, and now the light pollution is on a whole another level thanks to leds having come to replace the faithful old sodium vapour and metal halide lamps. Also some new roads – with streelights of course – have been built in some places that used to be good.
I also see now why it is good to preserve degree symbols in halo names. I have recently tended to drop them, but with these three digit names it started feeling like I am looking at raypath sequences.

Wednesday, 9 December 2020

Divergent light tangent arc on sparsely falling diamond dust

In Cluj-Napoca (Hungarian: Kolozsvár), Romania it is a common winter weather situation to have a dense layer of fog sitting on the city, with temperatures a few degrees below 0⁰C throughout the day with only the surrounding higher hills and mountains rising above this fog. High levels of humidity combined with low temperatures and aerosols floating in the air often results in industrial snowfall or sometimes (especially near the edge of the fog) diamond dust. On the evening of 2nd December we had the above described conditions but the weather was about to change, with gentle southern wind slowly sweeping out the fog.

Around midnight as I prepared to go to bed, I looked through my window and instantly noticed the unmistakable tiny sparkles of diamond dust under the streetlights. It was too sparse to form consistent halos (although the most intense moments produced visible lower pillars), but the sparkles hinted to the presence of the lower tangent arc. After a quick set up, I began taking videos of the area below the closest lamp (at 8-10 m from me, and approx. 8 deg. below eye level). The videos were around 4 min each, but many of them were rendered useless by the sudden drops in the amount of the diamond dust. Between two videos I also snapped a quick picture of a distant lamp that shows the pillar roughly as it was visible to the naked eye (see below).

A clearly visible pillar formed on diamond dust in the light of a distant lamp

On my computer I extracted every frame from the videos and saved them as jpg files (separately for each video), and stacked them together using a StarStax (a free startrail-making software) in ’lighten’ mode, to get the brightest value for each pixel in the resulting image (this is important as averaging the frames would wash out the individual sparkles that make up the halo). With this method I got an integrated image from each video that contained all the sparkles that appeared throughout that video. The videos were taken at various focal lengths ranging from 18 mm to 55 mm. When the first stacking results came out, I was simply delighted to see the lower tangent arc, and did not think much about it. However, on the next day I noticed something fishy: the tangent arc and the lamp comfortably fitted into the same field of view at 55 mm, which was not usual: this arc had to be way closer to the light source than a normal tangent arc. With the help of the solar disk (picture taken with the same equipment and settings, and resized to match the dimensions of the video), I measured and got approx. 7 degrees for the distance of the tangent arc from the lamp (see below).

Measuring the distance between the lamp and tangent arc. The solar disk on the top was used as a reference for the 0.5 deg. unit of the scale

This was completely new to me as I’ve never encountered any observations that describe tangent arcs so close to the light source, but I suspected the divergent light as the cause, since I knew that it can considerably alter optical phenomena compared to their familiar counterparts caused by paraller light rays. With the help of Walter Tape’s detailed article on streetlight halos (1), I confirmed that it is indeed the divergent light that caused the unusual appearance of the tangent arc, its distance from the lamp being normal for the lamp’s elevation relative to the horizon (since my observation of the below-horizon lamp with a lower tangent arc is equivalent to an above-horizon light source with an UTA at same angular distance from the horizon).

Based on Tape’s article, I also concluded that the strip connecting the lamp with the tangent arc is not a simple pillar, but part of the tangent arc. The slight widening at its center corresponds with the figures in the article.

On the following link you can watch a short piece taken from the video used for the picture on which the measurement was made, and the (sped-up) image stacking process using the frames from the original video: (2).

Beáta Ujvárosi