A colored patch under Moilanen arc

By Marko Riikonen and Jarmo Moilanen

These photos show a colored patch at the bottom of Moilanen arc. We don't quite know what it is. Nicolas Lefaudeux has suggested it could be a segment of circular "Moilanen halo". Such a halo has been photographed by Alexander Haußmann on 27. November, 2015, in Mt. Klínovec of the Czech Republic. Our display occurred in Rovaniemi on 16. December, 2015.

Or then it is the arc which has been observed under Moilanen arc a couple of times in relatively high light source elevation lunar displays. See for example the observation by Jari Luomanen. If so, then the explanation of this arc from rotating Moilanen arc crystals can't be right because the shape is wrong (the lamp is about 7 degrees below horizon). 

There is also 9 and 35° halos - a pretty headache inducing combination were you to try a simulation. A crystal photo is provided. Looks like a lot of pyramids in it, but most are not really textbook pyramids. More crystal photos in the original report in Taivaanvahti.

The Salzburg halo

On the 19th October 2013 Zbyněk Černoch was in Salzburg keeping an eye for halo events, since high cirrostratus clouds were rolling in. As they drifted towards the Sun, he first noticed bright parhelia. A few moments later, rather bright and colourful upper tangent arc with Parry arc and upper Lowitz appeared.





What stunned him later was a vivid arc similar to a parhelion, but located in the 18° region.



The arc lasted only a few minutes as the clouds were drifting away quickly. He haven't seen anything like this before.

After a few discussions the observation fell into oblivion. Until few days later when we thought we would like to figure it out once and for all.

It would seem that the arc itself is a fragment of 22° halo, but the halo itself was quite sharp and narrow in this observation.

After Zbyněk put together 3 photos to show that the 22° halo lies between the mystery arc and 22° parhelion, I decided to run some simulations.

I used pyramidal crystals with std of 15° in the c-axis and joined the simulation picture with the collage from Zbyněk.



My theory is that the mystery arc is actually an 18° plate arc. But there are some inconsistencies in the theory.

1. The color vividness indicates that oriented crystals formed the arc. However 18° plate arc formed by oriented crystals has a typical convex shape which is absent here. 
2. By increasing the std in the c-axis, halos from poorly oriented crystals are becoming more apparent, which is not the case here. 
3. No other odd radius halos were observed. 

If it really is a case of an 18° plate arc, it is a really decent one.

Spotlight display with a strong Moilanen arc (and a crystal sample)

By Marko Mikkilä, Jarmo Moilanen and Marko Riikonen

After having been hunting diamond dust halos since darkfall, at 1am we withdrew along with disappearing crystals back to the apartment to sleep. But this did not mean calling it quits. The forecast was for the conditions getting better, so every one hour each of us woke up in their turn to check the situation.

At 4 am it was time to go – there were streelight pillars over the city. In 20 minutes we were sitting in car and five minutes later arrived to the golf course parking lot next to the ski center. From there we walked to the golf field and away from ski slope lights and stayed there until 7 am.

The display was not dazzling, but it had a Moilanen arc that was strong in comparison to other halos. Consequently, in the crystal sample we expected an abundance of crossed long plates, which are thought to be responsible for Moilanen arc, but they were very few. Perhaps they were small and did no fall in the dish in representative numbers. In general, not much crystals settled on the dish.

Another thing about the crystal sample was that it contained some pyramids, and indeed a short lived 9° halo was seen in the beam when our cameras were not yet ready. Pyramids are often found in samples, but unless they are numerous, they don’t translate to actual odd radius halos in the sky.

There was some variation in the topography at the location, and so for a change, we had the lamp above the camera for all photos. The date is 23 November 2015.

Odd radius display and its crystals

By Jarmo Moilanen and Marko Riikonen

This odd radius display appeared on the night of 17/18 January in Rovaniemi at -29° C. Visible is the usual duo of 9° and 35° halos, and also what seems like 18° halo.

Crystals were collected. It is hard to make sense of most of the crystals. Many seem to have pyramid faces, but obvious pyramids were very few in the sample.

9° and 35° - not your expected combination of odd radius halos

By Marko Riikonen and Nicolas Lefaudeux

This display is far from being impressive. But it has an interesting combination of odd radius halos: 9° and 35°. Not something that would be expected from your textbook pyramid crystals. We had several displays in such a style this winter.

The display was seen on the night of 7/8 January in Rovaniemi in diamond dust that was being displaced by an increasing snowfall. The temperature at the official measuring site 7 km away was -27° C.

Halos from oriented pyramids in the spotlight beam

On the night of 20/21 January we photographed oriented pyramid crystal halos in the spotlight beam on two occasions. The first occasion, shown above, got us completely unawares and the odd radii stuff was all gone in the next shot which would have had the exposure right. Colored and solid upper and lower 9° plate arcs were seen, but if they were still present when the camera was placed in the beam, they are nevertheless washed out by the overexposure. In the photo one still sees the 35° halo and lower 20° plate arc. The lower 9° plate arc was of the 23-6 type as the lamp was about 5 degrees below below horizon.

We drove around and switched on the lamp here and there, stubbornly trying to get a rerun, but to no avail. Diamond dust was already on the retreat, the air was drying up even though it was around -33° C. Yet we managed to get something just before it all evaporated, as shown by the image here:

A little simulation study as was necessary to see whether it was column (left) or plate (middle and right) pyramid stuff, but that did not give definitive answer. The plate scenario simulations are identical except that in one is 22° halo and in the other 23°. The 35° and 18/20° stuff above the lamp were initially noticed visually which prompted us take the photo series for this stack.

These last photos were taken at 3:30 am. The two photos below are the first photos of the night at 6 pm. We photographed these lunar and beam displays simultaneously. No odd radii yet at this early hour.

Marko Riikonen / Jarmo Moilanen

originally published in Halo Phenomena

Pyramid crystals and odd radius halos, part II

More halo and crystal photos from the 20/21 January night. While this second crystal sampling and photographing session was under way, the display in the beam was photographed constantly, but again odd radius halos are not visible in the photos, expect perhaps for a suggestion of 35° halo at one stage (not shown). Yet again the sample was littered with classical pyramids as shown by the images in the gallery.

The picture above is an appearance of odd radius halos after the crystal business was over with. It is a “blue minus red” (br) of stack with total of 9m30s exposure and contains 9°, 18° and 35° halos. Three more halo images are in the gallery, first a single frame from the stack above, then two that are versions of another stack, the br displaying full 46° halo.

Jarmo Moilanen / Marko Riikonen

Pyramid crystals and odd radius halos, part I

The night of January 20/21 in Rovaniemi was a night of odd radius halos. Here are shown mostly crystals collected during a half hour period. At the same time photos were taken continuously in the spotlight beam, below is one of those stacks. The sample contained plenty of traditional type pyramids – something we have not yet seen this winter. So it is curious that odd radius halos are not conspicuous in the beam photos and at times even completely absent. Maybe the abundant cavities in the crystals caused too much disturbance.

The stack is from a stage where the odd radius stuff was at its best, taken towards the end of the crystal collecting period. Visible are 9°, 18° and 35° halos. The temperature during the night was around -33° C.

Jarmo Moilanen / Marko Riikonen

Pyramidal crystal halos

A funny coincidence that a halo-observer may want to experience is travelling to Egypt and seeing some pyramidal crystal halos.

This odd-radius display was photographed by Márk Laczkó of Budapest, Hungary somewhere over the southern borders of Romania. He was sitting on an airplane heading towards Egypt on 27th October. When he realised they were flying in cirrus clouds, he deliberately started looking for some halo phenomena. The surprisingly well distinguishable 18°, 20°, 23° and 24° halos were visible for about a minute at 06:21 UTC. Unfortunately, there is no documentation or report of a possible 35° halo. The original photo and two other pictures are also available (#1 and #2).

 

by Ágnes Kiricsi

 

Original post in Halo reports

From archives: odd radius column arcs in 2001

I saw this display in Oulu on 17 September 2001. Because the photos have not been around in the internet, I dare to make posting even though the event is not so recent.

The display has all the odd radius column arcs except the 23° arc. Images are stacked from 3-8 individual images. The one on the lower right is also flipped horizontally to get a bit more definition. For the upper left and lower images, simulations of the pyramid column arcs give crystals as in the figure. Also shown is similarly shaped real crystal, photographed during a weak odd radius column arc display at South Pole in 1999.

From pyramid plate crystals there is the 23° plate arc in all photos, and lower 9° and 24° plate arcs in the upper right photo. The display was discussed in more detail in September 2003 number of journal Weather. It also appears in the book "Atmospheric halos and the search for angle x" by Tape and Moilanen.

By Marko Riikonen

Ice Crystals

"Atmospheric Optics" has a newly updated section about halo forming ice crystals - check under "What's New". Plate, column and pyramidal crystals are illustrated with their orientations and resulting halos.

Elliptical halo in Helsinki

Elliptical halo appeared in the skies today on 26. March 2006. This was one of those rare long lasting displays, with almost an hour duration. The image here shows some of the stacked photos of the display. All together I took more than 600 shots.

The formation of elliptical halos is still somewhat mysterious. While good simulations are produced by obtuse hexagonal pyramid crystals having pyramid apex angles of around 175°, the current knowledge of ice physics does not give much support for such a crystal.

The display was formed - as usual - in Altocumulus virga. More on the cloud associations here.

By Marko Riikonen