Commonly spread Ice Wool

On February 2nd, 2016, we made an excursion to the Anna Tower together with my daughter´s form. The Anna Tower is located in the Deister hills, a small range of hills at about 20 kilometres southwest of Hanover. Its highest elevation ist Mt Bröhn near the Anna Tower with about 405 metres above sea level. We startet from the Nienstädter Pass at 277 metres above sea level. The car park there is covered with gravel and normally rather muddy, but that day it was frozen and hoar frost glittered everywhere. When we startet our excursion, the weather was bright and sunny. But as soon as we left the car park, I noticed a fibrous thing of brilliant white just beside the path which leads along the top of the range. At first sight, it looked like a sheared piece of wool from a sheep. But for me it was clear what it was: ice wool!

The first ice wool I ever found

Up to then, I knew ice wool only from descriptions, and although I had been looking for it for years everytimes there was a light frost, I never found some. And now I found it right beside the path, without having searched for it! After having given others a hint on that phenomenon and explaining it, I took some photographs and then continued my way – slowly and even slower, because there were more and more tree branches which showed ice wool. After having found 10 of these ice wool formations, I roughly counted them, but when I reached 50, I stopped counting. It made no sense, because there was too much. I found a place where at least 20 branches and twigs with ice wool laid around. Not every ice wool formation was well defined, but there were also some very bizarre ones among them!

Large accumulation of branches with ice wool

I arrived at the Anna Tower about one hour delayed. The weather was nice, but it was not really clear. There was a distinct inversion with a pronounced layer of mist, but without any mirages. I think, for this the hill is not high enough. (At really clear conditions you can see Mt Brocken in the east and the Porta Westfalica in the west from the Anna Tower).

Three weeks later, on February 27, I succeeded in finding ice wool at the Nienstädter Pass again. This time I was prepared better and brought a retro adapter to make macro photographs. Thus it was possible to take detailed pictures of the ice wool. Some parts of it had structures which reminded of chains of bacilli. Other parts just looked like shiny and transparent hair. The augmentation effect of the lens with retro adapter was not strong enough to unravel the structures here.

Modified by CombineZP

Modified by CombineZP

Background information:

Ice wool is a physical and biological phenomenon which mainly appears on rotten and decayed wood in deciduous forest with mixed types of trees. It forms hairy ice curls of a brilliant white which remind of candyfloss. Sometimes it looks like paintbrushs with the uppermost parts cut away, others look like wool from a sheep, others remind of minerals or lichen. And sometimes it looks just like a thrown away paper tissue and is often mixed up with this from the distance. But it has always this hairy and cristalline structure which sometimes looks like chains or if it was covered with sugar.

Ice wool is caused by the activity of funguses which decompose rotten wood. During this process, water is set free which gets out of the dead wood through capillaries and freezes at temperatures around or slightly below 0 °C, forming these hairy ice structures. This works as long as the wood itself does not freeze. Contrary to hoar frost crystals, ice wool is formed by liquid water from the wood freezing outside while the atmosphere is not involved. It is not long ago that the process could really be clarified.

Ideal conditions for the formation of ice wool are given when after a period of mild weather with (light) rain the sky clears off at night allowing frost on the ground. So, when you have to scratch the ice off your car windows, there is also a chance of encountering ice wool in the forests. It can be found from October until the beginning of March, except during very cold periods. Best places to find ice wool are under oak and beech trees and maybe also under some larches. Other conifers are not suitable.

But you also need some good luck when looking for ice wool, just as it normally is not wide spread. Similar to the appearing of mushrooms and toadstools, there seem to be good and bad years. Even if the conditions may apparently be perfect, you will not automatically find ice wool. Locations also seem to play an important role as I could see for myself a short time ago. While ice wool was rather abundant up there along the path on the top of the range, I could not find any of it in the Deister forest near my home, although the tree population there is not very different from that at the Nienstädter Pass. Also here lots of branches and twigs in all stages of rotting are lying around, but there is not a single trace of ice wool to be found.

Author: Reinhard Nitze, Barsinghausen, Germany

Lunar odd radius display in diamond dust

On four out of last five winters Tapio Koski has photographed lunar diamond dust odd radius halos in the Rovaniemi area. These one-per-winter occurrences are almost solely responsible for lunar diamond dust odd radius displays photographed in Finland during those years. This winter we wanted take part in the tradition. Yet despite numerous odd radius displays we had harvested in the beam, those by the moon – or sun for that matter – were simply not on the offing.

Except on the night of 20/21 January, which was the month’s last diamond dust night in Rovaniemi. During the day, when driving in the city, we paid attention to Fairbanksian amber, a beautiful yellow glow in the sun direction which can be seen in cold weather and with which we became familiar on the succesful halo expedition to Fairbanks in January 1996. This gave us an omen of foreboding that a night of big odd radii diamond dust was finally on the cards for Rovaniemi. Weather forecast was with us too, as the temperature was expected to drop to -33° C – the magic number that Walt Tape has given as being in the center of the temperature range favorable for odd radii.

The display appeared as some thin water cloud that had momentarily overtaken the sky cleared away. The first halo visible was upper 23° plate arc, many others soon followed the suit. In the beam only a crappy plate dominated display was visible – the pyramid stuff was higher up.

Authors: Jarmo Moilanen, Marko Riikonen, Finland

Halos at peak solar elevation, June 28th, 2014, Hörlitz, Germany

Chasing the circumhorizontal arc (CHA) has become a quite popular activity among the German halo observers. Depending on the latitude, there is only a 1-2 h time slot at noon for a few weeks around the summer solstice. Even the highest elevation the sun can reach is still a few degrees lower than the optimal value for CHA formation. This might only be beaten by the moon in a suitable position with respect to the ecliptic.

I was keen on observing the CHA this year as well, and had not had any luck so far. On Saturday, June 28th, there had been a single 22° ring before noon at my home in Hörlitz (51° 32’ N, 13° 57’ E). At 12:45 CEST I got on my bicycle for a visit in the neighbouring village. Already after 500 m I had to stop: The 22° ring intensified, and although there was still nothing else visible with the naked eye, I decided to take a fisheye picture at 12:51 for a later analysis. As seen in the unsharp masked version, the complete circumscribed halo and parhelic circle were already accompanying the 22° halo. With an ordinary wide-angle lens I took a “blindfold” picture deep in the south a minute later, and after unsharp masking both the CHA and the infralateral arc could be distinguished.

Of course this was unknown to me during the observation, but I felt some kind of suspicion that there might be more in the sky than I just saw (even by looking through a grey filter or using a black watch glass mirror). Around 12.53 I noticed the parhelic circle high in the sky, which had a diameter only slightly larger that of the 22° ring (~29°). Within the next few minutes the circumscribed halo became bright enough to appear clearly separated from the 22° ring at the sides. There were no traces of plate halos such as the 120° parhelia which I took as a bad sign for the CHA. There were now also cumulus clouds gathering in the south.

I moved on a bit, but stopped again after a 1 km: The sight of this huge “wedding ring”-like pattern in the sky was just too fascinating. I also scrutinized the south from time to time: Wasn’t there any colourful band appearing in the gaps between the Cu clouds? From time to time I thought that that I could see a part of the CHA, and the photos later proved that it was actually there, but I was not sure if I were just imagining something after staring too long into the sky. Consequently, I do not count this as a successful visual CHA observation. After reaching my destination at about 13.25, the Cu clouds were obstructing larger and larger parts of the sky as the halos were fading away in the gaps. I really had the luck to observe a parhelic circle at almost the highest possible solar elevation at my place (61.7° at 13.07)! Only 0.2° were missing to the ultimate maximum a week before the observation.

When going through the pictures again, I also found the upper part of the Parry arc in the filtered versions. Remarkably, the part below the parhelic circle is missing, and I do not have an explanation for this at hand at the moment. Nonetheless, the presence of the Parry arc allows to discard plates at all: The CHA may as well be generated by Parry crystals, as seen in this HaloSim simulation. However, when the portion of Parry crystals is increased to the point at which the CHA is rendered at a reasonable intensity, the Parry arc appears too bright.

A representative selection of images from this observation is available here.

Frosting Light Diffraction Patterns

When looking at the sun through the frost patterns on the windshield of my car on May 4, 2014, I saw a well-defined yellow and red corona. The ice crystals on the window pane worked as an obstacle which diffracted the light, which means they deviated it into different directions. So the light waves can reach areas which are blocked out when following the direct way. A diffraction pattern always consists of bright and dark spaces. The dark spaces are those where waves extinct each other, while the bright areas are at the positions where the light waves add each other. Coronae which are really circle-shaped form in grids with periodical gaps as for example in fabric. Irregularly collocated diffracting particles, however, form coronae which change their shape significantly when seen from different angles (1–2).

Author: Claudia Hinz

Colours in a contrail

On this image one can see colour in a contrail at both 22 AND 46 degree positions (the latter just above the electrical power pole on the wide-angle image). It’s analysed carefully recently. The angles were measured using calibration images. I cannot recall seeing other reports of this kind of observation. The aircraft was crossing the Rocky Mountains from west to east in the afternoon.

Author: Alan Clark

Neklid Antisolar arcs: Case closed?

In my last post I outlined several possibilities to explain the great brightness of the antisolar arc (AA) compared to the heliac arc (HA) in the Neklid display from Jan 30^th, 2014. All of them were a bit off the main road of traditional halo science, but traditional arguments did not help to clarify what was observed, hence I had to look for something else.

Both the concepts of plate Parry crystals and trigonal Parry columns should yield weak traces of unrealistic (or better to say non-traditional) halos that might appear in a deeper photo analysis. Claudia Hinz provided me with a set of pictures from the display to unleash any kind of filters that would seem appropriate. Indeed it was possible to pin down traces of the Kern arc in some of the pictures after the initial application of an unsharp mask (1, 2), followed by high-pass filtering (1, 2) or, alternatively, by Blue-Red subtraction (1, 2). Note that the Kern arc was weakly present in the simulations for hexagonal, Parry-oriented plates. This, of course, must not be confused with the recently proven Kern arc explanation relying on trigonal plates in plate orientation. Finally, trigonal columns in Parry orientation are a third non-traditional crystal configuration giving rise to new halos. However, these do not yield a Kern arc.

Obviously, the Kern arc fragments in the photos are very feeble and the whole procedure reminds a bit of the search for higher order rainbows. It is mere guesswork to detect how far the arc stretches around the zenith, but doubtlessly it extends up to 90° and more in azimuth, thus being clearly distinguishable form the circumzenith arc. Nonetheless, one would feel safer with further evidence. Comparing the simulations for Parry columns and Parry plates, three more differences are discernible (apart from the changed AA/HA ratio):

1) For Parry plates, the upper suncave Parry arc does not show an uniform brightness, but appears brighter directly above the sun and loses some intensity towards the points where it joins the upper tangent arc.

2) The upper loop of the Tricker anthelic arc is suppressed for columns, but shows up for plates.

3) Some extensions of the upper Tape arcs appear between the Wegener arc and the subhelic arc.

At least the first two points can be answered in favor of the Parry plates, being visible even without strong filtering. However, I failed to detect any extended Tape arcs as “ultimate proof” so far. This might not surprise since they are, according to the simulation, comparable to the Kern arc in intensity and appear in regions of the sky where the crystal homogeneity was not as well developed as in the vicinity of the zenith.

Piecing the parts together, it seems evident that at Neklid the AA intensity was due to Parry-oriented hexagonal plates. Their traces were detectable, whereas nothing appeared that would hint on trigonal Parry columns. In contrast to this, Parry trigonals were responsible in Rovaniemi 2008. This implies that in nature at least two different mechanisms occur for AA brightening.

Finally the question remains how plates may get into a Parry falling mode. But as long as no one understands how symmetric columns do this (though we have the empirical evidence), we should be prepared for surprises. There might also be a connection to recently discussed details of the Lowitz orientation (2013 Light and Color in Nature conference, talk 5.1).

The mystery of bright antisolar arcs

 

 

(photo by Claudia Hinz)

The antisolar (or subanthelic) arc (AA) was one out of the vast range of halo species occurring during the marvelous Neklid display observed by Claudia and Wolfgang Hinz on Jan 30^th, 2014. This kind of halo seems to be exceedingly rare, since it has only been documented during the very best displays, mostly observed in Antarctica. On the other hand, the heliac arc (HA) is a, however not frequent, but well-known guest in Central Europe. Both of them are reflection halos generated by Parry oriented crystals and touch each other at the vertices of their large loops. Fisheye photos towards the zenith from Neklid shows both these halos in perfect symmetry and approximately similar intensity, at least regarding the upper part of the AA.

When trying to simulate the display (solar elevation 17.5°) using HaloPoint2.0, I noticed that the AA was rendered much weaker than the HA, which of course does not match the photographic data. To obtain the Parry effects (Parry arcs, Tape arcs, HA, AA, Hastings arc, partially circumzenith arc, Tricker arc, subhelic arc) I chose a population of “normal” (i.e. symmetrically hexagonal) column crystals with a length/width ratio of c/a = 2 in the appropriate orientation. Since both HA and AA are generated by this very same crystal population, their mutual intensity ratio cannot be influenced by adding plates, singly ordered columns, or randomly oriented crystals. This mysterious issue has also been noted by a Japanese programmer who came across the Neklid pictures.

Inclusions of air or solid particles within the ice crystals are an obvious hypothesis to explain this dissenting AA/HA intensity ratio, since they cannot be accounted for in the standard simulation software. However, a look into literature reveals that there are external and internal ray paths for the HA, but only internal paths for the AA ([1] p. 34-35). That means that inclusions will diminish the AA to a greater extent than the HA. In the extreme case with the interior totally blocked, no AA can arise but a HA is still possible due to external reflection at a sloping crystal face. Hence inclusions cannot explain the bright AA from the Neklid display. Air cavities at the ends of columns which are seen quite often in crystal samples will also inhibit the AA because an internal reflection at a well defined end face is needed for its formation.

Spatial inhomogeneities in the crystal distribution might serve as explanation as long as there is only one single photo or display to deal with, especially when the air flow conditions are as special as they were at Neklid. Maybe there were just “more“ good crystals in the direction of the AA compared to where the HA is formed, either by chance or systematically due to the wind regime. But surprisingly also the observations from the South Pole (Jan 21^st, 1986 (Walter Tape); Jan 11^th, 1999 (Marko Riikonen), also discussed here) show an AA/HA ratio somewhere in the region of unity as far as one can guess from the printed reproductions ([1] p. 30, [2] p. 58). Parts of the AA appeared even brighter than the HA in Finnish spotlight displays. All this implies a deeper reason for the AA brightening. It seems rather unlikely that in all these cases the inhomogeneities should have worked only in favor of the AA.

Hence the crystals themselves must be responsible for AA brightening. Non-standard crystal shapes and orientations are conjectures that can be tested easily with the available simulation programs. For a first try, one can assign a Parry orientation to plates instead of columns. Changing the c/a shape ratio from 2 to 0.5 while keeping all other parameters fixed results in a much brighter AA.

It is, however, commonly accepted that due to the air drag only columns can acquire a Parry orientation ([1] p. 42). Furthermore, some halos appear in the plate-Parry simulation which have not been observed in reality, e.g. a weak Kern arc complementing the circumzenith arc. At this stage the question may arise why only due to aerodynamics any symmetric hexagonal crystal (may it even be a column) should be able to place a pair of its side faces horizontally to generate Parry halos such as the HA and AA. Cross-like clusters or tabular crystals ([1] p. 42), from whose shapes one will immediately infer that rotations around the long axis are suppressed, seem much more plausible. Surprisingly, Walter Tape’s analysis of collected crystal samples shows that Parry halos are mainly caused by ordinary, symmetric columns. Parry orientations might be a natural mode of falling for small ice crystals, though up to now the aerodynamic reasons remain unclear. Nonetheless I tested if tabular crystals would give a bright AA. This was neither the case for moderate (height/width = 0.5) nor strong aspect ratio (height/width = 0.3). The AA was in both cases even weaker than in the symmetric standard simulation with which the discussion started.

Trigonal plates have been brought into discussion as possible crystal shapes being responsible for the Kern arc (see also [1] p. 102). Out of curiosity I tested how Parry oriented trigonal columns would affect the AA/HA intensity ratio. In contrast to symmetric hexagonal columns two different cases exist here, depending on whether the top or bottom face is oriented horizontally. As seen from the results, a sufficiently bright AA can be simulated using trigonal Parry columns with horizontal bottom faces, but the upper suncave Parry arc and the lower lateral Tape arcs at the horizon disappear. Obviously they have to, since a trigonal crystal in this orientation does not provide the necessary faces for their formation. On the other hand, the simulation predicts unrealistic arcs like the loop within the circumzenith arc. Choosing a trigonal Parry population with top faces horizontal will diminish the loop of the HA and wipe out the upper part of the AA as well as the upper lateral Tape arcs and add an unrealistic halo that sweeps away from the supralateral arc.

Is it possible to generate a realistic simulation of the Neklid picture with such crystals? Clearly this will require to add a second Parry population of symmetric hexagonal prisms. Doing so, a reasonable compromise can be achieved. In this case the hexagonal crystals produce the Parry arc, whereas the trigonal ones are responsible for the AA. Due to the triangular portion being small, the unrealistic halos become insignificant. However, the fact that a further degree of freedom (mixing ratio trigonal/hexagonal) has to be added to the set of initial simulation parameters is somehow dissatisfying.

The question lies at hand if this result might also be obtained by choosing a single Parry population of intermediate shapes between the symmetric hexagonal and trigonal extremes. This idea is further motivated through pictures of sampled crystals that, though being labeled „trigonal“, show in fact non-symmetric hexagonal shapes. The simulation for these shapes does indeed predict an enhanced AA compared to symmetric hexagons, but the lower lateral Tape arcs and the upper suncave Parry arc still appear too weak. This means that an additional set of symmetric hexagonal crystals is needed again to render these halos at the proper intensity.

Moreover, quite prominent unrealistic halos like the loop crossing the circumzenith arc appear in the simulation. If this assumption for the Parry crystal shape was right, this arc should be visible in an unsharp mask processing of the photos. Its absence hints that these crystals did not play a dominant role in the Neklid display. One could argue that the unrealistic halos may depend strongly on the actual crystal shape and might be washed out in a natural mixture of different “trigonalities“. However, the simulation tests indicate that even in this case the unrealistic halos remain rather strong, as long as one still wishes to maintain an AA at sufficient intensity.

As a conclusion, it can be stated that the intensity ratio between the heliac arc and the antisolar arc in the Neklid display as well as in Antarctic and Finnish observations has raised basic questions about the shapes of the responsible crystals. Simulations with symmetric hexagonal Parry columns, i.e. the standard shapes, render the AA to weak compared to the HA. Inclusions in the crystals and spatial inhomogeneities of the crystal distribution can be ruled out as the cause of this deviation. Plates in Parry orientation or a mixture of Parry oriented trigonal columns with horizontal bottom faces and hexagonal columns both result in a more realistic AA/HA intensity ratio. However, they introduce traces of unrealistic halos and are rather uncommon hypotheses: Plate crystals are not supposed to fall like this, and the existence of “true” trigonal crystals is doubtful. Moreover, the trigonal crystals need an accompanying set of standard Parry crystals to generate other halos like the upper suncave Parry arc.

So all in all the mystery of bright antisolar arcs cannot be regarded as solved at this stage. Since this halo species is very rare in free nature, it might be helpful to test perspex crystal models of different shapes in Michael Großmann’s “Halomator“ laboratory setup. Though the refractive index in perspex is higher than in ice, the basic relations between HA and AA stay the same. However the big challenge remains to collect and document crystals during such a display, e.g. with the methods described by Reinhard Nitze.

References

[1]     W. Tape, Atmospheric Halos (American Geophysical Union, 1994)

[2]     W. Tape, J. Moilanen, Atmospheric Halos and the Search for Angle x (American Geophysical Union, 2006)

Addendum

I missed an important piece of information from Finland 2008: The idea of trigonal crystals making Parry halos was already pointed out by Marko Riikonen in an analysis of the Rovaniemi searchlight display. In that case, even one of the halos that I termed “unrealistic“ was observed in reality, thus strongly supporting the trigonal interpretation.

Light refraction in a sunshine recorder

Only at very rare occasions, light refraction can be seen as impressive as in this example. The photo was taken by Hermann Scheer at the Meteorological Observatory on Mt. Hoher Sonnblick (3105m) in the Hohe Tauern mountains in Austria. A layer of ice and rime had formed on the glass sphere of the Campbell Stokes sunshine recorder. This layer split the sunlight up into its spectral colours. That is how impressive physics can be.

Spring halos in Eastern Germany: 46°/supralateral splittings, tangent/Parry arc twins, a great pyramidal show, and biting cold

During the past months the sun was only rarely seen in Eastern Germany, and the number of observed halos was correspondingly low. Moreover, when everybody was hoping for the onset of spring, the winter regained its strength after March 10^th, and people were confronted with masses of snow and untypically cold days and nights for this time of year. But embedded in this belated winter period was a row of days (March 23^rd-28^th) with a remarkable outbreak of halo activity. This report will concentrate mainly on my own observations, though there is also more and complementary material available at the Meteoros message board (in German language).

Saturday, March 23^rd

In Hörlitz, Lower Lusatia (51° 31’ N, 13° 57’ E), the 22° ring and upper tangent arc (or upper part of the circumscribed halo, respectively) were visible from noon on, later to be joined with a suncave Parry arc for some minutes around 15:00 CET (15:01, unsharp masked) as well as a parhelion with a notable blue hue (15:08). From 16.00 to 16.45 the circumzenithal arc was also present. In the evening, the 22° ring, circumscribed halo, both paraselenae and the paraselenic circle appeared at the moon (19:34, USM). The further development is nicely illustrated by a time lapse video I took from 19:54 to 21:54. A weak 9° ring was also present, as visible in the filtered version of the frame from 21:04.

Sunday, March 24^th

Solar halos were again visible from noon on, but quickly changing as the cirrus clouds moved across the sky. I took a second time lapse video (13:23 to 14:40) from the same position as in the night before, showing the 22° ring and the upper part of the circumscribed halo. Note the increase in the wind velocity compared to the night before. This really “fresh” breeze from the East in combination with temperatures below 0 °C even at high noon was challenging for both the observer an the technical equipment. Though the video may suggest that the halo activity decreased during the afternoon, there were occasionally some colourful surprises embedded in the flow of cirrus patches (16:00).

Monday, March 25^th and Tuesday, 26^th (after midnight)

I continued my observations in the afternoon of March 25^th from the town of Dresden, Saxonia (51° 3’ N, 13° 46’ E). However, as I was later told, I already missed a parhelic circle segment that had been visible around noon. When I had the opportunity to look at the sky, all the halos seemed to reassemble slowly out of nothing (15:58, USM). This pattern of standard halos remained stable throughout the afternoon, and was joined by a photographically detectable supralateral arc at around 17:15. Its left wing became visible to the naked eye at around 17:35. Remarkably, a photo from 17:27 shows both the supralateral arc and the real circular 46° halo in the unsharp masked version, with the former touching the circumzenithal arc and the latter missing it; and both arcs merging at the left side at the spot where I later could see the “supralateral” arc by eye. Very likely this bright region was indeed not a pure supralateral arc, but a mixture with the 46° ring. An alternative way for halo image processing is the subtraction of the blue image channel from the red, which also yielded a convincing result here. Throughout the last months I had the opportunity to record this 46°/supralateral merging (or splitting) effect several times, though it never was clearly visible to the naked eye and could only be revealed by image processing.

At 18:10 (2° solar elevation) all halos had vanished for the naked eye, except for a bright upper tangent arc sitting on a weak 22° ring. Once more, unsharp masking revealed a surprise, namely a weak upper sunvex Parry arc looking like a shifted twin of the tangent arc (USM, R-B). This Parry arc had not been present in photos taken 8 minutes earlier.

Up to this point, the halo activity had already been much higher than what we get in average, but the definite climax was yet to come during the night. A weak 22° ring with a right paraselene (the view to left was obscured) was present around 21:00. At around 21:50 a weak 9° halo could be traced from the photos. At 23:30 the 9° ring was plainly visible, having a brighter spot at its bottom (i.e. the lower 9° plate arc). Due to this encouraging observation, I placed my camera on a cherry pit pillow at the balcony balustrade, and started an automatic time lapse series over almost 4 hours. Occasionally, I entered the balcony from inside to take a glimpse at the sky, but I did not want to disturb the fisheye photo recording by my presence. Hence my visual inspections were not carried out with full adaption to darkness. The 9° ring was very prominent until approx. 03:00, with a bright bottom and from time to time quite bright sides. The 22° halo was rather diffuse, which I took as a sign that further pyramidals might be hidden there. On its top something like a diffuse combination of  an upper tangent arc and a 23° plate or Parry arc was seen. Since the unusual quality and rareness of such an observation was immediately clear to me,  I was very excited what the time lapse video from 23:49 to 03:42 would reveal. The results did even exceed my expectations, especially in the unsharp masked version. In the following pictures (composites of each two neighbouring frames from the time lapse series for the sake of noise reduction) I labelled the halo species I could identify.

00.32.45, lunar elevation 35° 8’:
The distinction between the 23° plate arc and the Parry arc is difficult, but the presence of the other plate arc justifies the interpretation as the former effect. However, there is not enough detail in the bright region at the bottom of 22° ring to decide if more than an ordinary lower tangent arc, e.g. a  20° plate arc, is present. The circular 23° halo is either missing or masked by the outer intensity gradient of the 22° ring. It is however the only smaller halo that requires the prismatic top faces (or bottom faces, as being equivalent for random orientations) of the crystals, and hence it represents a special case. Against this view stands the presence of the 46° halo (at least 1 h later, see below), which requires such crystal faces as well, so the problem remains open.

A version of this photo without the labels is displayed as the title image of this report.

01.29.45, lunar elevation 29° 39’:
At this stage of the display, the bright regions at the sides of the 9° ring appear very prominent, corresponding to the visual impression. They can be associated with column arcs, however, I did not find traces of column arcs of the other halo families in the photos (yet).

01.36.45, lunar elevation 28° 52’:
A very strong unsharp masking reveals the additional presence of the 35° and 46° halos. The clear intersection with the paraselenic circle demonstrates that indeed the circular 46° ring and not an infralateral/supralateral combination is dominant. Note that this situation changes towards the final frames of the video, in which a clear supralateral arc without a 46° ring can be seen.

All radii have been checked by calculating the angular distance of several stars from the moon.

Tuesday, March 26^th and Wednesday, March 27^th (after midnight)

During the afternoon the halo activity rose again, until at around 14:00 both a complete 22° ring and 9° ring were visible again in rather structured cirrostratus clouds. Over the next hour, the clouds became more uniform, but also more dense (15:12). Unsharp masking and subsequent Red-Blue subtraction revealed also a weak 35° halo and 46° halo, both not being visible to the naked eye (USM, R-B). In the R-B picture, an additional ring-like feature is visible at about 12° distance from the sun, likely an artefact of this processing mode in connection with the camera and lens. It could be traced in later photographs (15:22, USM, R-B, composite of two images), maybe together with faint traces of the pyramidals near the 22° halo. As in the night before, the pyramidals faded over time, until a pattern of prismatic halos remained (16:35, USM).

Moon halos seemed at first unlikely due to the increasingly dense clouds, but after midnight once more the 22°/9° ring combination stood in the sky, however rather diffuse and less colourful than before (00:38, USM, composite of two images). A supralateral arc (or 46° halo) additionally appeared around 02:00 (02:12, USM, composite of two images).

Wednesday, March 27^th and Thursday, March 28^th (after midnight)

Around noon, a complete parhelic circle together with the 22° ring, circumscribed halo and both parhelia could be seen in the region of Dresden, though I personally missed this observation. When I began to look at the sky in the early afternoon (I was somehow a little afraid that this flood of halos would never end), the parhelic circle had lost most of its brightness, but was still detectable at the sunward side of the sky. No pyramidal halos showed up anymore, so maybe the most exotic halo species at this point was a small Lowitz arc reaching from the right parhelion to the 22° halo. However, the detection is difficult due to the presence of contrails and lower clouds, that produce artefacts in the image processing (13:34, USM). R-B subtraction also revealed a weak 46° halo.

Again, the clouds did thicken towards the evening, but this day before midnight a light snowfall set in. The series of halos seemed to have come to definite end. Nonetheless, during the night the upper part of the 22° halo appeared on the moon, just as to wave goodbye after an astonishing week full of surprises and challenges (02:11) and certainly one of the most remarkable periods in my 18 years of skywatching.

All images and videos from this report can be found here in chronological order. Any details concerning camera and lens type, focal length, precise time stamps etc. will be provided on request.

Author: Alexander Haußmann, Dresden, Germany

Sodalite interference colours

Sodalite – sodium aluminium silicate chloride – is a mineral of volcanic origin (chemical formula: Na8Al6Si6O24Cl2) and it comes from hydrothermal fluids in a volcanic rock’s cavity. The sodalite containing rock itself is not homogenous but consists of many different, small minerals beside the blue sodalite.

The mineral itself is very nice deep royal blue in general, the piece illustrating this article was mined at Mt. Vesuvius and bought in a mineral shop in Italy. The sodalite pieces are full of other crystals, usually well visible whitish veins which mostly consist of calcite.

When looking at the mineral with the help of some magnifying device we can see small parts of it having thin and colourful layers! These coloured parts are concentrated at the edges of the calcite veins or patches and only visible in a magnified form. Here, the translucent calcite was built on the blue sodalite mineral in a later process different from the forming of the blue crystals from the original hydrothermal solution. These places must also contain a very thin layer of air which is responsible for the colours with its interference.

What is unknown: the forming of the air layers. Are they originally there or are they created when the stone is cut from the rocks? I think the later is more possible as the sodalite rocks can more easily break where the white veins run, so the chopping of the rock might create the gaps, resulting interference patterns. The process might be the same as the ice pieces with fissures showing interference colouration too.

The pictures (1 – 2 – 3) were taken with a cheap digital microscope, the magnificiation which shows the interference colours is 200X. Smaller magnification also shows it but only in tiny coloured spots.

Author: Mónika Landy-Gyebnár, Hungary

More posts to this topic:

• Colours inside a cracked piece of ice by Noli
• Colours on fissures in ice by Reinhard Nitze