Another St. Elmo´s Fire on Mt. Hoher Sonnblick

For the third time, the meteorological observatory on Mt. Hoher Sonnblick (3106m) in the Hohe Tauern mountains registrated St.Elmo´s Fire on its webcams (2-3). But contrary to the two preceding cases (1-2), this time St. Elmo´s Fire occurred during a thunderstorm, the nearest lightnings of which were about 600 metres away. While St. Elmo´s Fire appeared, the intensity of the electric field increased from -3000 up to +8000 V/m (unfortunately no diagram available). At the same time, snowfall was recorded at temperatures of -2°C.

The webcam which recorded St. Elmo´s Fire is located on a ridge between Mt. Goldbergspitze and Mt. Roter Mann at about 2970 metres above sea level looking northeast. On the right Mt. Goldbergspitze can be seen, and in the centre of the picture there is the Kleinfleißkees wit Mt. Hoher Sonnblick. The highest mountain on the left is Mt. Hocharn (3254m). The camera is part of a glaciological research programme around the Sonnblick-observatory. Its purpose is to provide data about the snow cover of the Kleinfleißkees and is operated by the ZAMG (Zentralanstalt für Meteorologie und Geodynamik – Central Institution for Meteorology and Geodynamics).

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

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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.

Crepuscular rays extended to (almost) 180° observed from Mt. Großer Zschirnstein, Elbe sandstone mountains, June 8th, 2014

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Each year during the Pentecost holidays I undertake together with some friends a cycling tour to the Elbe sandstone mountains. This is usually a good opportunity to look for atmospheric phenomena, since we are out in the open the whole day. However this year we just had the sun shining from a plain blue sky most of the time. I feared that nothing interesting would happen, but I was wrong: In the evening of June 8th, thunderstorms were active about 200 km or more to the northwest from our location (Großer Zschirnstein, 50° 51′ 23″ N, 14° 10′ 34″ E, 561m). The top parts of these clouds acted as apertures to cast crepuscular rays through the sky shortly after our local sunset. To the south the view from this mountain is fully unobstructed since the lookout point is located right above a 70 m high rock cliff. Our struggle to thrust the bicycles up there was rewarded by the beautiful sight of a bright, rosy coloured beam extending from the twilight sky in the northwest to the rising earth shadow in the southeast and passing just below the waxing moon.

Even with a (full frame) fisheye lens it was hard to capture due to its extension of about 180°, so I decided to do panorama stitching from an image series (21:26 CEST: local solar elevation -1,5°). One should keep in mind that in reality crepuscular rays are straight lines and the curved shape in the photo is just a result of the cylindrical projection. Likewise it would have been possible to distort the horizon and make the crepuscular ray straight. Having a look at a panning video may be the best way to understand the geometry. Some minutes later (21:31 CEST: local solar elevation -2,3°) a second beam had appeared quite prominently above the first one, and even more might be detectable by image processing. Though all of them being parallel straight lines in 3D space, the mind is always tempted to interpret them as fanning beams like the emissions from a lighthouse.

Until 21.40 the rays disappeared almost completely apart from the foremost part in the northwest, which itself became quite bright at that time. Around 21.48 the cumulonimbus clouds themselves became visible for a while. This change in illumination and visibility must be caused by the increasing solar depression below the horizon which leads to more vertically inclined sunbeams, until the sun finally sets at around 52° N / 12° E (where the clouds might have been) in 10 km of altitude as well.

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

Corona around reflection of Sun

In the evening of June 6, 2014, a reflection of the sun appeared in an inclined window pane of the pyramid-shaped restaurant building on the top of Mt. Zugspitze, while shreds of cumulus clouds coming from the valley passed by. In these shreds not only the shadow of the top of the building was visible from time to time, but there also appeared a distinct corona around the reflection of the sun.

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

Mirages in front of solar disk

On June 7, 2014, there was an especially interesting sunrise on Mt. Zugspitze. There was not the green flash I expected, but the miraged mountains of the Bavarian Forest showed up in front of the rising solar disk (image series). According to calculations of the position of the rising sun and Ulrich Deuschle´s panorama program the mountains visible in the mirage were probably Mt. Hoher Filzberg (1279 metres) and Mt. Sulzriegel (1260 metres), which are 252 kilometres away. The mirages were only visible in front of the solar disk, apart from that visibility was only at 60 kilometres, which is not enough by far for seeing the Bavarian Forest from here.

Author: Claudia Hinz

Fraunhofer lines in rainbow ?

Fraunhofer lines are dark lines in the sun’s spectrum. They are caused by resonant atomic absorption of the sun’s thermal continuum radiation by photospheric gases.

The lines provide clues to the chemical composition of the solar atmosphere, as well as its physical conditions like temperature, pressure, magnetic fields etc.

My rainbow photography dated 11.Oct.2013 showed some greyish bands in the yellow.

Are they traces of the strongest Fraunhofer lines or artifacts of the camera’s sensor being unable to profile intermediate colors?
Is it possible at all to obtain spectral lines in nature without a prism or grating?

Author: Michael Großmann, Kämpfelbach, Germany

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 30th, 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

 

 

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(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 30th, 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 21st, 1986 (Walter Tape); Jan 11th, 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.

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