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.