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: 1-2-3

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  (1-2).

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 (1-2-3).

 

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 (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

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