Bayly`s Beads style sunset

When watching the sun above cold water, you sometimes can observe an unusual phenomenon. The sun does not set as a “ball”, but seems to diverge at the horizon. Sometimes it even appears as a bright horizontal line which adapts a shape reminding of Bayly´s Beads during a total solar eclipse. The last bright beads sometimes disappear only at a few minutes after sunset. This phenomenon was first documented by the British amateur astronomer John Franklin-Adams. He observed the phenomenon several times from board of a ship and attributed it to the swell near the horizon.

Bayly`s Beads style sunset over the Baltic Sea. Photo: Sven Aulenberg, 27.08.2014

Even if it may sound absurd, the conditions above a sea of clouds are similar to those above the ocean. The suface of the moving clouds is undulated, and also the surface of the clouds is cold, just like that of the sea. So the light gets reflected and the light-emitting object (in this case the sun) gets lifted optically. The bright beads then shine throug the gaps of the waves, no matter if they are made of water or of clouds (photo spread).

Bayly`s Beads style sunset over sea of clouds. Photo: Claudia Hinz, 19.02.2015

 

Author: Claudia Hinz

Triple-split rainbow observed and photographed in Japan, August 2012

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Have you ever wondered how many photos of outstanding atmospheric phenomena may exist “out there” without us knowing about them, just because they are not posted on our regular websites, blogs or forums? From time to time, I do Google image search queries on atmospheric optics related subjects to see if something interesting and yet unknown might show up. Some weeks ago, I encountered this way a true rainbow rarity on a Japanese website. The picture had already been publicly accessible for over two years, but went unnoticed by the European or US atmospheric optics community so far. Using the automatic translation function I identified the photographer and contacted him to learn more about his (as of now) unique observation.

Kunihiro Tashima noticed an approaching rain shower on the evening of August 5th, 2012, in the town of Yobuko, Saga prefecture, Kyushu island, Japan (33.54° N, 129.90° E). According to his experience, these showers appear quite regularly after sunny days in the Japanese summer. At 18:24 JST he took the first photographs of a marvellous rainbow display made up from a triple-split primary and an undisturbed secondary (photograph 1, unsharp masked; photograph 2, unsharp masked) from a parking lot. Kunihiro used a Nikon D7000 camera equipped with either a AF-S DX NIKKOR 18-55 mm or a Tokina AT-X 116 PRO DX II 11-16 mm lens at 18 mm and 11 mm focal length, respectively. The sun was located at 9.7° in elevation and 283.8° in azimuth when these pictures were taken.

Within the next minute the shower intensified at his position, so he had to withdraw into his car. Photos taken at 18:25 through the windscreen give the impression that the middle branch had by then already merged with the uppermost one, resulting in a rather broad “traditional” twinned rainbow (photograph 3, unsharp masked). Around 18:32, only an ordinary single primary and a weak secondary were left in front of receding clouds and the blue sky (photograph 4, unsharp masked). At this time, the sun’s position was 8.1° in elevation and 284.9° in azimuth.

Twinned rainbows are nowadays a well-documented phenomenon [1] and several promising steps have been taken to explain their formation [2, 3]. In one of my earliest reports on simulations of rainbows generated by flattened drops with broad size distributions, I pointed out the idea that also split rainbows with three or four branches might occur at very rare occasions [4, p. 117]. However, up to now, no photographs or clear observation records of such highly exotic rainbow displays have been known to the community. Some old reports of multiple rainbows do exist [5], but these are difficult to evaluate due to the lack of further details. Hence Kunihiro’s photos provide to my knowledge the first reliable evidence that multi-split (>2) rainbows exist.

A reflection rainbow generated by mirrored sunlight from a horizontal water surface can be excluded as an explanation here, since the angular deviation from the original bow would have to be larger at this solar elevation. Furthermore, the secondary bow remained unaffected by any anomalies, which is a familiar feature seen in many split rainbow displays.

For further analyses it is necessary to assign scattering coordinates (scattering angle and clock angle) to the individual pixels of the photographs. Unfortunately, no starfield calibration photos or position data for reference objects in the photos are available. Nonetheless I tried to estimate the three orientation angles for one of the images (2nd photo from 18:24) using azimuthal positions of roof-edges etc. as calculated from Google Maps aerial pictures and additional constraints such as the vertical orientation of lampposts and the approximately constant scattering angle of the secondary bow. The lens distortions (deviations from the ideal rectilinear projection) were corrected with predefined, lens-specific data in the RAW converter software UFRaw. Though this estimation procedure is only an error-prone stopgap solution (compared to a true calibration with a starfield image) the results are quite convincing. This can be seen best when the rainbow photos are morphed into an equirectangular projection in scattering coordinates (0° in clock angle = rainbow vertex).

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I calculated such projections for the 1st and 2nd photo from 18:24, as well as for the last photo from 18:32. The orientation angles I only estimated once (for the 2nd picture from 18:24), whereas I pursued a “dead reckoning” approach using some reference objects to transfer the initial orientation calibration (including its errors) to the other two photos. This allows for a consistency check of the method by evaluating the last picture which shows an ordinary rainbow display. The non-split primary appears, according to the expectation, as an almost straight line with only a slight curvature towards the antisolar point around its vertex.

With the orientation being now somewhat trustable, I took a closer look at the finer details in the triple-split bow. The uppermost branch of the primary is shifted by approximately 1° for clock angles > –60° into Alexander’s dark band, i.e. towards the secondary, when compared to its left foot at around –70° in clock angle. Such a behaviour cannot be explained by the current theory for rainbows generated by flattened drops, since it predicts an inward shift of the primary at its vertex, i.e. away from the secondary, for this elevation of the sun. Elongated rather than flattened drops will yield a shift towards the secondary, but such shapes far from the equilibrium are not stable and will occur only temporarily during drop oscillations. Since these oscillations have periodicities in the range of milliseconds for common raindrop sizes, it is doubtful that a well-defined rainbow, required to be stable over the typical exposure time of a camera (or the human eye), can be generated by oscillating drops with considerable amplitudes. Obviously, such oscillation blurring will be reduced for smaller amplitudes as the oscillations damp out over time, but simultaneously the drop shapes will converge towards their flattened equilibrium states.

Summing all up this means that Kunihiro’s pictures do not only represent the first photographic proof for multi-split bows, but will also give the rainbow theorists something to think about. It might be that we have to take into account additional influences such as electrostatic fields, refractive index variations, or anomalous wind drag.

Red Sky Glow by reflected Dawning

In the morning of January 24, 2015, I noticed that the sky was covered with low clouds, except for a small gap right above the southeastern horizon, where the sun would rise about 20 minutes later. So I expected a wonderful dawning, but nothing happened. But at 8.10 a. m., I noticed a strange red light coming from the west. When looking out of the southward window, I saw the western and southwestern sky glowing in a dark red colour.

During the following minutes, the red glow slowly extended eastward and became more and more intense. At last it was so intense that even the ground took this colour, giving that morning a quite eerie mood. In the picture taken in northeasterly direction (2) you can see this reflex on the ground, especially on the gravel path and the pedestrian crossing at the lower left. And the picture also shows that towards the east the low clouds still had their normal dark grey colour. In the southeast, no trace of a normal dawning was visible, but higher in the sky there was also this strange red glow from above. (3) This is vice versa to the normal course of a dawning, where the red colour spreads from east to west.

During the next 5 minutes, the light from above became even brighter and turned into a more orange colour (4). At 8.25 a. m., just before sunrise, a bit of sunlight reached the lower surface of the low cloud layer, but it was by far not as intense as the glow coming from above (5).

Author: Peter Krämer, Bochum, Germany

“Shadow-Corona” observed on top of Fichtelberg and Zugspitze

It is not unusual that one can see some shadow rays in the sky due to clouds in front of the sun. One can also observe coronas in consequence of diffraction of the sunlight or moonlight by small waterdrops of thin clouds. But it’s a rareness to notice both phenomena at the same time.

It would be even more interesting to be at the top of a mountain with the clouds very close. So, thin wisp of clouds racking only a few meters over your head. Sometimes these wisps cause also beautiful coronas. If a building or a mast obliterate the sun, its superstructures can cast long shadows into the clouds.

Kevin Förster observed both phenomena on top of the Fichtelberg Mountain (Erzgebirge) on January 24th, 2015. This time the sun was behind the tower of the weather station and the different appliances at the top of it afforded the shadows. The origin of the clouds was found in the “Böhmische Becken” situated at the southern slopes of the mountain range. Therefrom they drifted into the direction of the Fichtelberg Mountain. First it consisted of ice crystals and caused ice halos. Over the Fichtelberg there were widespread clouds of waterdrops, which caused a nice corona additional to the shadow rays.

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“Shadow-corona” at the top of the Fichtelberg. (c) Kevin Förster.

A similar event was observed on Mount Zugspitze in the Bavarian Alps by Claudia Hinz on May 5th, 2013. The sunlight was blocked by a mast and its shadow fell on very thin clouds. Simultaneously there was a bright corona. (12345)

In both cases the sun was lower than the top of the tower so that the shadow of the tower was projected on the cloud layer above. This is a very uncommon phenomenon.

Distorted Solar Disk by Smokestack Exhausts

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The Hungarian observers Attila Kovács and György Répás witnessed an unusual kind of refraction. The temperatures in the exhausts of smokestacks differ widely within a small space, which causes very bizarre distortions of the sun.

Attila reports that such strangely distorted solar disks can be seen almost every day in the exhausts of smokestacks. So it is worth while trying it by yourself to get a live impressions of physics.

Photographs:
   Attila Kovács at the hospital of Eger, November 11, 2014:
   György Répás, smokestack in Budapest, March 29, 2014:

Cloud Iridescence along the Alps and in Hungary

On January 10, 2015, unusually bright and colourful iridescent clouds were observed along the Alps between Switzerland and Hungary. To display the huge area in which the observations were made, Kevin Förster plotted all known observations into the satellite image taken at 12 noon that day.

The cloud iridescence was observed in 7 countries (Switzerland, Liechtenstein, Austria, Germany, Italy (Southern Tyrol), Slowakia and Hungary). The westernmost observation point is Fribourg in Switzerland, the easternmost one is Tápiószolos in Hungary. This means that the iridescent clouds were observed along a distance of 965 kilometres and in an area measuring about 122,500 square kilometres, which ist about a third of the area of Germany. There is no case of a similarly distinctive iridence known so far.

Many observers reported iridescence stretching up to large angles from the sun and a great similarity to nacreous clouds. These form above northern latitudes at very low stratospheric temperatures beneath -80°C in the ozone layer. The iridescent clouds were visible until 20 minutes after sunset, followed after an intense afterglow on clouds which still received sunlight up to 45 minutes after sunset. At some places eye-catching crepuscular rays were also observed. The 30 hPa-Chart, however, shows that it was much too warm for polar stratospheric clouds to form.

Nevertheless the cloud layer must have formed at higher altitudes than normal. One observer reportet that all airplanes flew beneath the clouds, and also many pictures show contrails below the cloud layer. So the clouds probably formed at more than 12,000 metres above ground.

Discussions about the weather situation in our forum and measurements by the Austrian weather service (Central Institution for Meteorology and Geodynamics ZAMG) showed several peculiarities of the situation: Strong foehn winds caused gravity waves which peaking at about 14,000 metres above ground. This was the level of the tropopause, which was unusually high for these latitudes that day. And it also was unusually cold, as a radiosonde launched in Vienna measured a temperature of -75.7°C. The highest of the multilayered foehn clouds formed along the tropopause. Due to their high altitude, their droplets were of the optimal size to cause iridescence. Unfortunately, it can not be clarified if there also formed small ice crystals like in nacreous clouds because strong vertical movements may impede the freezing of the droplets.

Video from Thomas Klein, Miesbach, Southern Germany

Thanks to all who put their pictures at our disposal and helped us with data, special knowledge and hints to clarify the reason for this phenomenon. The discussion can be found, together with a lot of photographs and some time lapse videos in the forum of the Arbeitskreis Meteore e.V.

Authors: Claudia Hinz and Kevin Förster

Glories and cloudbows observed during short-distance flights

On Sept 25th and 27th, 2014, I was traveling by plane from Dresden to Brussels and back, with stops at Frankfurt and Munich, respectively. As usual, I booked window seats to study sky phenomena. The sunward side was not very interesting, since these short-distance flights are carried out at heights below the cirrus clouds and therefore no sub-horizon halos can be observed (at least in autumn). On Sept 25th only a single 22° halo appeared in the cirrus clouds above the plane, whereas on Sept 27th ice crystal clouds seemed to be fully absent.

Accordingly, the viewing direction towards the antisolar point proved to be much more interesting. As most of the Atmospheric Optics enthusiasts I had seen glories and cloudbows before (especially when traveling to the Light&Color meetings in the US) but this time the conditions seemed to be especially favorable. I could observe an an almost textbook-like development of both phenomena right after piercing through an Altocumulus layer after the take off from Dresden (Sept 25th, 11:13 CEST):

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From Debye series simulations (intensity sum of the p = 0 to p = 11 terms in order to prevent artifacts from the small-scale inter-p-interferences as present in the Mie results) a mean drop radius of about 8 µm with 0.5 µm standard deviation can be estimated (assuming a Gaussian drop size distribution):

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This simulation was calculated for the original lens projection with added ad-hoc gray background. It is also available as a fisheye view centered on the antisolar point without background [1], together with the corresponding simulation for monodisperse drops (no spread in size) of 8 µm in radius [2].

Unsharp masking and saturation increase processing of the photograph reveals that the sequence of supernumeraries can be traced until they merge with the glory rings:

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Over the next minute I mounted the fisheye lens to my camera in order to record a broader view. Unfortunately, some of the outer glory rings and inner supernumeraries had already vanished, indicating an increase in the drop size spread:

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Note the smaller angular size of the plane’s shadow as the distance to the Ac layer had further increased. A well fitting simulation to this photo can be calculated by assuming again a mean drop radius of 8 µm and setting the standard deviation now to 1 µm:

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For comparison, the fisheye simulation centered on the antisolar point was calculated for the 1 µm drop size spread as well [3]. Furthermore, I recorded a video sequence showing the movement of both glory and cloudbow across the uniform Ac layer (11:15, [4]). When later the edge of the Ac field was reached, the glory showed an appreciable degree of distortion (11:18 CEST [5], processed version [6]).

On Sept 27th, not a uniform but a fractured Ac layer was present after the take off from Brussels. Nonetheless the glory appeared circular (12:34 CEST [7], processed version [8], video at 12:37 CEST [9]), with the exception of occasional larger disturbances in the layer (12:34 CEST [10]). The cloudbow was not as prominent as two days earlier. During the later part of the flight only occasional Cumulus clouds were present, which did not allow for further glory observations until the plane started descending when approaching Munich. At this point the angular size of the clouds became large enough again to act as suitable canvas for the glory (13:14 CEST [11] [12]). During the final passage through a Cu cloud I recorded a further video (13:15 CEST [13]). Remarkably, the angular size of the plane’s shadow varies rapidly (indicating the distance to the drops) whereas the the angular size of the glory remains rather stable (indicating the drop radius).

Photos and videos were taken with a Pentax K-5 camera equipped with either a Pentax 10-17 mm fisheye or Pentax-DA 18-55 mm standard zoom lens. A gallery view of my photos can be seen here [14].

Alexander Haußmann

Full Circle Rainbows

Most people know a rainbow only as a bow in semicircular shape which becomes smaller with increasing sun elevations and disappears beneath the horizon at a sun elevation of 42°. But this is only half of the truth, just as a complete rainbow is a full circle. But it is not easy to observe and photograph it in its full beauty. This is because the lower part of a rainbow can only appear when there are enough drops of water below the horizon to make it bright enough. During the last two months, both variations could be captured.

One case in which a rainbow can be seen as a full circle is when the sun shines through the spray of a waterfall. With the sun standing low behind the observer, the rainbow continues downward in front of the background, and with a little luck the full circle becomes visible. The photograph above was taken by Wolfgang Hinz on July 29, 2014 at the Seljalandsfoss-Waterfall in Iceland. More pictures: 123

But it is also possible to see the lower part of a rainbow from an airplane. During a flight on August 12, 2014, Peter Krämer could even look upon a part of a rainbow from above, whith the houses of the city of Essen behind it. But unfortunately the windows in a plane are rather small, so that a full circle rainbow can only be seen from the cockpit. But nevertheless, Peter Krämer could catch the right part of the rainbow when the pilot made a light left turn  (12).

The lower part of a rainbow can also be seen from a mountain. Here it is necessary that opposite the sun rain falls into a deep valley. In the morning of July 8, 2014, Claudia Hinz saw a rainbow which appeared on an approaching rain front from Mt. Zugspitze (2963m), the right part of which excended downwards until the village of Ehrwald. As the sun was just rising, the spectral colours were filtered out of the sunlight by the atmosphere due to the oblique angle in which the sunlight fell in. So the rainbow showed only a long waved red colour. The other colours appeared only a few minutes later (123).

 

Authors: Claudia Hinz, Peter Krämer, Germany

Reflected pillar & rays in windows

On April 9th, 2014, Uwe Bachmann observed a pillar of light, produced by sunlight falling onto the building of the European Central Bank (EZB) in Frankfurt. He was observing from the German Weather Service’s (DWD) headquarters in Offenbach, i.e. from a distance of 3 kilometers.

For his first photo taken at 6.17 UTC, the sun was at an elevation of 13.9° and at an azimuth of 94.8°. The upward beam of light is thought to be produced by reflection from the building’s front, which is at an angle of 9° to the vertical, with scattering from aerosol producing the luminous pillar.

With the rising of the sun and the changing of its azimuth, this pillar is “cut down” subsequently. The second photo shows the situation at 06.35 UTC for a solar elevation 16.8° and an azimuth of 98.4°. The skewness of the reflected beam of light at an angle of 45° is evident.

The last photo taken at 06.52 UTC for a solar elevation 19.5° and an azimuth of 101.8° shows the beam being reflected almost at a right angle. Here, the azimuth of the sun is almost coincident with the observer’s.

More pictures: 1234

Author: Michael Großmann,Kämpfelbach & Uwe Bachmann DWD, Offenbach, Germany

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 (23). But contrary to the two preceding cases (12), 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).

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