Three quarters of a double rainbow, plus an accidental snapshot of a tertiary, Mt. Zschirnstein, Germany, May 15th, 2016

Over the past two decades it has become a tradition among my friends to carry out a bicycle tour to the Elbe Sandstone Mountains (“Saxon Switzerland“) at the Pentecost weekend. We then often pay a visit to a table hill named “Großer Zschirnstein“ (561 m), which features a remarkable cliff of 70 m in height at its south-eastern edge.

Almost 15 years ago, on the evening of June 3rd, 2001, we had the opportunity to observe from there a rainbow extending well below the horizon almost down towards its bottom. Unfortunately, we only had a compact camera without a fisheye lens at hand back then, so the old photos show only some sections of the whole phenomenon.

This year, on May 15th, we were finally granted the proverbial second chance. I already anticipated some rainbow potential in the “Icelandic” weather that day. In the early afternoon, there had already been a rain shower while the sun was shining, but as we had not yet ascended the mountain and the sun was still high in the sky, there was no chance for a rainbow observation.

Some minutes after reaching the plateau in the evening, we had to retreat to the shelter when a rather strong shower of hail and rain set in. To the west a stripe of clear sky widened, and sunshine seemed at hand soon. It took longer than expected, as the clouds were moving rather slow. On the left side, a small rainbow fragment suddenly appeared at the horizon, resulting from sunlit drops a few kilometers off. It was a rather unusual observation to see this rainbow streak vanish and reappear again, as its sight was repeatedly obstructed by scudding (and non-illuminated) mist around the Zschirnstein massif:

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(19:42 CEST, f = 88 mm, Pentax K-5)

Finally the great moment came: Sunshine was reaching the Zschirnstein while the shower, now mostly composed of rain instead of hail, still continued. Within a few minutes we could enjoy this marvelous view:

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(19:56 CEST, f = 10 mm / fisheye)

Unfortunately there was no safe way to access a viewpoint which would have allowed to study the missing quarter, as this would have required some careful climbing around the sandstone rocks for which I already felt too excited at that moment. The fisheye picture can hardly express how huge both rainbows looked like, and how beautiful the raindrop clusters glittered as they drifted around the cliff some 10 m further down. These are certainly the moments that make you understand that famous “double rainbow enthusiasm”, thought not everyone is as outgoing as other people on the internet. Maybe we also stayed a bit calmer because the strong and cold wind added a rather painful component to the taking of photographs and videos.

Later the right part of the primary close to the horizon became especially bright:

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(19:59 CEST, f = 80 mm)

This photo has been processed in a way that no color channel reaches saturation, which is a necessary prerequisite for analyzing possible kinks in the rainbow. In this case, the red rim looks as if would bend inside a bit below the horizon, but this might only be an illusion due to the intensity gradient.

The primary’s right foot above the horizon remained still visible for a rather long time, as the shower withdrew in this direction:

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(20:19 MESZ, f = 50 mm)

But the story does not end here. When going through the pictures later at home, I suddenly realized that I had missed to look for higher order rainbows, or to deliberately take some pictures in the appropriate directions. I was a bit disappointed about my inattentiveness, since this had been my best rainbow display in years and, moreover, I had not been hindered by the limited field of view from a window in a city building. I am often forced to decide between the sunward or antisolar hemisphere when observing rainbows from there.

Luckily I had taken two pictures (an exposure bracket) towards the sun just at the moment when the three-quarter rainbows started to evolve. The reason for this was only the lighting atmosphere – it was the moment when the sun rays had first reached the Zschirnstein plateau. As I deduced later from the movement direction of the shower, there had been rather good conditions for the formation of tertiary and quaternary rainbows when the picture pair was taken. So I decided to apply the strong filtering procedures which are needed to extract higher-order rainbows from photographs. The shorter exposure just gave noise in the interesting region. However, in the longer exposed version something interesting popped up.

Original image:

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(19:54 MESZ, f = 17 mm / fisheye)

Processed image:

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Slightly to the right above the stone pillar, a red-green stripe in the color ordering of the tertiary rainbow can be discerned. For an unambiguous identification it would, however, be necessary to calibrate the picture in order to assign scattering coordinates to the photo’s pixel matrix. Though I had previously calibrated the projection of the lens for the used focal length (the upper end of the zoom range), I would need two reference marks with known elevation and azimuth which are included in this specific photograph to complete the analysis. On the horizon, no distinct remote references could be found. This means that I would have to reconstruct my precise position on the plateau to minimize parallax errors, and then to record a starfield image from there at night, enabling me finally to use the stone pillar or nearby trees as references. Unfortunately, it would take an inconvenient amount of time to access the spot again and the effort for such a trip would be a bit over-the-top for the sole purpose of calibrating a photograph.

But there was still a piece of hope: From the shorter exposed version (-2 EV),  I could estimate the position of the sun quite accurately, as there is only a small overexposed area around it. This allowed me at least to draw lines of constant angular distance from the sun into the photograph in order to decide if the colored stripe appeared at the correct position or not. Using the previously measured spectral sensor response of my camera, and estimating the temperature of the water drops to be around 5°C, I derived the following values for the Descartes angles of the tertiary and quaternary rainbows: 41.7° / 43.7° (red, 620 nm), 40.6° / 45.1° (green, 530 nm), and 39.3° / 46.8° (blue, 460 nm). In the following animation, these angular distances from the estimated position of the sun have been marked by their respective colors:

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The colored stripe seems to fit reasonably well to the Descartes angles of the tertiary rainbow, especially when taking into account that the positions of maximal intensity are shifted a bit inward from the Descartes angles for the tertiary (and outward for the quaternary) due to wave-optical effects. This shift was also noted in the analysis of the very first photograph of a tertiary rainbow. Further contributions form distorted drop shapes are of minor importance here, as the sun elevation is small and we are looking at the rainbow’s sides. Therefore the effective cross section of the drops should remain nearly circular, even if they are squeezed in the vertical. I leave it to the readers to decide if also traces of the quaternary might be visible among the color noise slightly to the left above the stone pillar.

Addendum: A short video clip from the observation can be found here.

Iridescence near the ground

Iridescence is caused by light diffraction of water droplets of clouds. The wave nature of light forms new waves at the small drops. In certain directions they interfere and can amplify each other. It is important that the droplets are very small, not considerably larger than the wavelength of light (micrometre range). Such drops occur mainly in medium-high and high clouds. The edges of lenticular clouds iridesce most frequently.

But in very rare cases iridescence emerges below the sun in near-surface layers of fog. Two cases have been seen in the last time.

On 05th December 2015 Claudia Hinz observed cold fog from the Bohemian valley of river Eger accumulating at the crest of the Ore Mountains (German: Erzgebirge; Czech: Krušné hory). As in the afternoon the sun was above the cloud wall, the clouds edge iridescend first and later the complete wall of clouds appeared in slight pastel colours. Iridescence on the frequent Bohemian fog couldn’t be observed previously.

Iridescent Fog

On 09th March 2016 Richard Löwenherz observed slight iridescent shallow fog. It was a windless and sunny late afternoon in the Swedish Jämtland. An anticyclone had establish and caused a gradually clearing sky. In a deep depression originated shallow fog already before sunset, as well as above the frozen Hällsjön at Kaxås in the north of Storsjön. But this scene was unusual. As the ceiling of the flat layer of fog was slight iridescent between 17:10 to 17:15 CET (directly below the sun) it was a real surprise. At this time the air temperature was a little bit below the freezing point. Perhaps in the valleys, where the fog was formed, the temperature decreased below -5°C.

It is worth mentioning, that there was striking iridescence in stratus and stratocumulus fractus since the morning.

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Authors: Claudia Hinz, Richard Löwenherz

Moonbow with Alexander’s dark band

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On July 31, 2015 was a “blue moon” (second full moon in a month). The weather forecast for that night in Spain was storms and heavy rains. I was travelling from Madrid airport to the north of Spain. The first atmospheric phenomenon was a 22 degree halo that I photographed in the rural areas of Castilla. Then the storms began and the blue moon disappeared. At 5am I was already at my home in Villaverde, Leon. It was raining all night but then only for a few minutes the moon appeared again on the wester horizon and produced this double moonbow with Alexander’s dark band.

I created also a short timelapse video of a second  moonbow , late that night, just before dawn. Pictures taken with Nikon D5300, Nikkor fisheye 10,5 mm, f:2,8, ISO 400, 20 sec exposure.

Author: Roberto Porto, Spain

Halo Blog is back

Haloblog

We have reactivated the separate Halo Blog so that it can serve as an international forum for observations of halo phenomena and for discussions about halo theory.

http://www.haloblog.net/

We hope for an interesting exchange!

Lunar odd radius display in diamond dust

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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

Oblique supernumeraries to the primary rainbow

August 1, 2015: Rainbow with oblique supernumeries. Photo: Claudia Hinz

August 1, 2015: Rainbow with oblique supernumeries. Photo: Claudia Hinz

Last fall, two AKM members observed a rainbow with supernumeraries, which were clearly oblique to the primary rainbow.

On August 1, 2015, they were observed by Claudia Hinz on a red rainbow just before sunset in the Fichtelgebirge / Erzgebirge mountains. A rain front had just passed and the last precipitation from the departing clouds evaporated in the air, so that the raindrops did not reach the ground anymore. Virga were clearly visible and at the same time an intensive Zero order glow could be seen at the Sun side.

On October 5, 2015, Sirko Molau observed in Günzburg/Bavaria a similar phenomenon. Also here the rain shower had already passed and a strip of blue skies was visible near the horizon. Over one hour after the rain Sirko was surprised to see a bright rudiment of the rainbow. On the first glimpse it looked like a split rainbow. However, a closer look revealed that two interference bows disemminated obliquely from the root of the rainbow.

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October 5, 2015: Rainbow foot with oblique supernumeries. Photo: Sirko Molau

The oblique interference arcs can be explained best with different raindrop sizes. In both cases, the rainbow appeared after the rain had disapperead and just when the Sun showed up. We can assume that dry air had already moved in, causing the last drops to evaporate on their way to the ground. So the raindrops quickly reduced in size after they left the cloud. The simulation of Les Cowley shows that with reduced drop size not only the number, but also the distance of the interference bows decreases.

Authors: Claudia Hinz, Sirko Molau, Germany

Reflected Sunlight Dewbow

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Reflected Sunlight Dewbow. Photos: Jérémie Gaillard

On August 19, 2010, Jérémie Gaillard made an interesting discovery when looking at the surface of the lake Etang de l´Alleu which is located in the French community of Saint-Arnoult-en-Yvelines. The water was covered with pollen, on which droplets of dew had formed. In these droplets two colourful rainbows were visible. Dewbows can be understood as the lower part of a rainbow projected onto a horizontal plane. When a dewbow is fully developed, a semi-circle which opens towards the sides should be visible, the apex of which is situated at the lower end of the observer´s shadow. Equivalent to normal rainbows, primary and secondary dewbow should run parallely, but in Jérémie Gaillard´s observation they did not.

Instead, the second colourful bow fragment is a reflected sunlight dewbow. The surface of the water acts as a large mirror reflecting the sun. The reflected image of the sun now acts as a second source of light, which is situated as far below the horizon as the sun is above it. (angle of incidence = emergent angle). So the antisolar point for the reflection of the sun is above the horizon. This reflected antisolar point, which is located the double of the real sun´s elevation above the antisolar point, is the centre of the two rainbow circles for the reflected sunlight. So the additional rainbows are displaced upwards by the double sun elevation compared to the primary and secondary rainbow, making a rather unfamiliar appearance in the open nature.

Author: Claudia Hinz

Two Days of Diamond Dust Halos in Boži Dar (CZ) Nov. 27th/28th, 2015

The attempt of a group of pictures. Photo: Andreas Zeiske

About half of the participants under a diamond dust halo. Photo: Andreas Zeiske

The 13th meeting of German halo observers was a quite bountiful one as it presented halos on two of the three days. The location at the top of the western Ore Mountains, which straddle the German-Czech border with peaks surpassing 1200 m is well known for complex displays as documented by the organizers Claudia and Wolfgang Hinz. [e.g. 1234]

Although this area was being readied for the skiing season, ice crystals from snow guns were not the dominant source for the observed halo phenomena. Rather, the local winter weather can make fog from the often persistent huge mass of cold air in the Czech/Bohemian basin ascending toward the mountain ridge, where it can precipitate as diamond dust. Claudia Hinz has also observed that it does not need to be below -10oC for good halo displays.

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The Mt. Keilberg in a diamond dust cloud. Taken from Mt. Fichtelberg by Andre März.

As a case in point, during the meeting, the temperatures were between -2oC und -5oC only. However, clouds and fog were not coming from the South this time, but rather slowly from the Northwest. The photo taken on the evening of Nov. 27th, 2015, shows a hood composed of this ice fog covering Mt. Klínovec (Mt. Keilberg, 1244 m) as seen from Mt. Fichtelberg (1215 m) a few kilometers to the west. These two mountains are the highest peaks of the range, and both were accessed in less than 10 minute by car from our meeting place in the cosy winter resort Boži Dar (Gottesgab).

On that very night, the ice fog at Klínovec unfolded a wide array of lamp and lunar halos, namely the plainly visible 22°-ring, upper and lower tangent arcs, Parry arc, 46°-ring (or supralateral arc)  parts of the parhelic circle, and just bordering at visual detection Tape arcs, helic arc (from the Moon, however) and Moilanen arcs, as well as Minnaert’s cigar.

But there were daylight halos too, and it all started unexpectedly on the same day’s morning, when murky morning weather all of a sudden made room for ice fog precipitating into clear Southeastern skies. This also happened at the road toward and on the summit of Klínovec. Although being seasoned observers, most participants of the meeting had never stood in diamond dust and its halos. So, this day proved to be a spectacular experience for all senses, of course starting with the three-dimensional flickering of otherwise well-known halos up to the sounds and sights of ice crystals falling on their jackets like fine semolina.

Actually, one of the group, Reinhard Nitze, often refrained of looking at the halos, but rather collected and photographed the fallen crystals. These proved to be almost exclusively needles. This is compatible with the observed halos, comprising once again strong tangent arcs, a weaker 22o ring, and later then an occasional display of the suncave Parry arc and parts of the 46°-ring (or supralateral arc) and traces of the helic arc, outlined by individual glistening crystals in the zenith only .

The next display came to happen after a late lunch on Fichtelberg. In it, halos of the 22o family were nicely contrasted against a blue sky, with the upper tangent arc assuming a huge “longhorn steer like” appearance at times. Due to rapid variations in the thin drifting ice fog, many halos “pulsated” in 3D between a couple of meters distance and maybe 50 to 100 m. Not farther away than a few arm lengths, one could see both a weak 22o ring as well as a snow surface halo from the just fallen crystals. The whole display was ending with a blindingly bright pillar of light around the sun.

The second day (Nov. 28th, 2015) eventually saw a westerly snow and rain front drawing in, but in the early morning  there were halos, once again on Fichtelberg (22° ring, UTA, and 46o ring und mehr?).

For the subsequent interesting talks and demonstrations by Elmar Schmidt, Alexander Haußmann, Claudia Hinz, Michael Theusner, Georg Dittié, Kevin Förster, Reinhard Nitze, Michael Großmann, Thomas Klein, and Richard Löwenherz, which formed the other major part and reason of the meeting, the reader should refer to the German language publications [1].

Before leaving for their respective homes, on Sunday, Nov. 29th, 2015 the group was given a tour of the Fichtelberg active mountain top weather station which by now was surrounded by a thick ground cover of snow.

 

Author: Dr. Elmar Schmidt, Bad Schönborn, Germany

Munich Süddeutscher Verlag Tower Sunrays in Fog and “Ghost Windows”

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A lot of discussions went on before the SV-Tower was built. The headquarters of the publisher “Süddeutscher Verlag” was originally designed to be a 145 metres high 39-story building, but had to be limited to 100 metres (28 stories) after a popular petition in Munich.

It was built between 2006 and 2008, when in September the SV employees rather unwillingly moved to their unloved new workplace.

There were several discussions on the visual appearance of the new high-rise building. Some considered it as a boring square log, while others admired the special feature of its storefront.

This storefront consists of a straight inward and a prismatic outward glazing. As the individual panes of the outer storefront are inclined to each other, they reflect landscape and sky alternately. So, depending from the incidence of light and the observer´s position, the appearance of the tower changes permanently. The inner glazing is normally not important here.

Already in 2010, a friend showed me an unsharp photograph taken from his mobile phone, which showed sunrays in dispersing fog around the SV Tower. Although I pass the building almost every morning, it took five years until I could experience this phantastic light show myself.

Hoping to be lucky this time, I took my camera with me on my way to work on November 3, 2015. The tour didn´t start very promising, as there was no fog around Munich. But when I reached the fairground east of the city, some fogbanks appeared, which already had started to disperse. Above them, the sun was shining, and so I got more and more excited. Should I really be lucky today?

Short before the end of the A94 motorway, the SV Tower  provided a stunning show with its reflected sunrays in the fog. Just a few hundred metres further I took an exit and went back on a road parallel to the motorway. From a parking lot I could watch and photograph the permanently changing sunrays.

Intensity, direction and appearance of the rays constantly changed when I changed my position and the wafts of mist moved. And even the inner glazing played a role now, because the light caroming the straight inner glazing becomes reflected parallely. This caused an effect of “ghost windows” in the fog.

When I started my observation at about 9.50 a.m., the fog was still relatively thick. At about 10.25 a.m., the fog had completely dispersed and the show was over.

Author: Rainer Timm, Munich, Germany

Green flash over the North Atlantic Ocean

Knsipel

I am currently working as an astronomy lecturer for a German tour group sailing the Norwegian cost on the Hurtigruten vessel MS Nordnorge. On October 8, around sunset we crossed the Vestfjord a stretch of open sea between the Norwegian mainland and the Lofoten islands. Since I expected to see a green flash, I prepared everything to capture the phenomenon.

I was not disappointed. Through my 600 mm telephoto lens I could clearly see the green an blue flash. Closer inspection of the images afterwards also revealed that I also manage to capture a purple flash in the last fractions of a second before the upper limb of the Sun entirely disappeared.

I am attaching a panel which collects crops from the last 30 images in my picture series which show the development of the phenomen over the last 12.66 seconds (according the time stamps created using GPS time). I have also created a very nice gif animation of the event, which you can find (along with additional pictures) on my homepage.

Author: Benjamin Knispel, Hannover, Germany

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