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.

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

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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

Niagara Falls Rainbow

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After being up at Niagara Falls back in 14 I had to come back for more and wanted to be there when the lights on the Canadian side light up the falls. The night we arrived the light were turned up and got to see some amazing rainbows. The lights would change color and it was a sight to see a rainbow being different colors along its length. In addition to the floodlight bows I also got nice rainbows from natural sunlight and using the super wide angle field of view with my GoPro camera I got nice full circle rainbows. For anyone who is a waterfall or rainbow chaser. Niagara Falls is the place to go and falls are BEST on the Canadian side and this is the perfect bucket list item.

Author: Michael Ellestad, Ohio, USA

“Rainbow”-contrail

Iridescence contrail

Iridescence contrail. Photo: Ron Smith

Contrails are a result of water vapour, produced as a product of combustion, being ejected from the aircraft engines (→ article)

When a contrail forms near the sun, it’s possible to see a rather beautiful ‘rainbow effect’, as in this example. Such iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. The aerodynamic contrail formed by the reduction of pressure in the air as it moves over the wing. When the pressure of a gas falls, then its temperature also falls (the same principle as is used by your refrigerator). The reduced temperature causes small drops of water to condense, which then may freeze. The (frozen) drops get larger as more water condenses on them. The iridescent colours are sunlight diffracted by millions of water droplets condensed by the airflow over the wings. The droplets all have similar life histories and therefore similar sizes, ideal conditions for iridescence.

The photograph was taken by Ron Smith at around 1300 local on 18 July 2015 at Henstridge, Somerset, UK. The aircraft was flying from East to West and, when first seen, was only producing an intermittent contrail. The iridescent contrail appeared as the aircraft approached a cloud layer just below its flight altitude.

One of nature’s works of art!

Authors: Ron Smith, Somerset, UK and Claudia Hinz, Germany

Iridescent pileus cloud

Iridescent pileus cloud · Photo: Gabriele Schröder

Sometimes it occurs that small cloud cap forms above a cumulus or cumulonimbus cloud. These caps, wich are similate to a veil,  are called pileus (cap) and indicate that the air above the cumulus cloud is very humid. The humidity is near the saturation point, so that a cloud can form. If this cloud cap is near the sun and the glare of the sun is in an ideal case reduced by the cumulus cloud covering the sun, iridescent colours appear in the cloud cap.

The intense colour of a pileus cloud indicates that the water droplets in the cloud are very small and of a uniform size.

Such an iridescent  pileus cloud could be observed by Gabriele Schröder on June 6, 2015, at 6.50 P.m. in Schneeberg in the Erz Mountains. The phenomenon appeared in three different parts of the cloud within 10 minutes. Especially interesting is above all the shadow in this picture, which was cast by the lower cumulus cloud and projected upon the clouds. Faint rays can also be seen behind the cloud, indicating that also the surrounding air is very humid.

Red sprites over a convective storm system over the North Sea on July 2nd/3rd, 2015

Sprites · Photo: Laura C. Kranich

Still today we have atmospheric phenomena many people have never heard of, know little about or have at least never seen themselves. For me one phenomenon I had never seen until recently are the so-called red sprites. Red sprites are a high atmosphere light phenomenon (also “transient luminous event” or TLE) related to thunderstorms and extend over altitudes between 40 and 100km above ground. They can have various forms, sometimes like carrots or tendrils, often reticulate, sometimes rather bushy. It has been shown that positive lightning is at least correlated to the occurrence of red sprites, probably triggering them under certain conditions as sprites mostly occur a few to several milliseconds after CG+ (cloud-to-ground positive) lightning. Negative cloud-to-ground lightning (CG-) can rarely cause sprites, approximately 99% of sprites are related to CG+-flashes. Positive lightning is a tropospheric type of lightning where an electrical discharge from the positively charged anvil (top) of a thunderstorm to the ground takes place whereas much more common negative lightning originates in the lower part of cumulonimbus clouds. A discharge from the top of a thunderstorm to the ground requires an enormous amount of charge (hundreds or thousands of a Coulomb) so they only make out a small percentage (about 5-10%) of all lightning in thunderstorms and have been found to be more likely to occur in longer-lived dissipating thunderstorms and winter storms (maybe because the tropopause is a few kilometers lower during winter, hence less charge is required for a discharge from the top of a thunderstorm to the ground). The conditions above a thunderstorm, in the stratosphere, mesosphere and ionosphere are also important for the formation of sprites. Yet the exact processes in and around thunderstorms that lead to the occurrence of sprites are still not fully understood. What is certain is that most thunderstorms do never cause sprites. From satellite observations a global sprite occurrence rate of approximately 1 per minute has been derived whereas the tropospheric flash rate is about 3000 times higher: 44 per second on average. Most sprites appear over Mesoscale Convective Systems (MCS) with a cloud top area of more than 100.000km² while above super-cells or air-mass convective storms rarely any sprites are observed, though super-cells can trigger other TLE like e.g. blue jets.

During the night from July 2nd to 3rd, 2015, I was out in the fields near Felmerholz a few kilometers outside of Kiel, Schleswig-Holstein, actually hoping for some noctilucent clouds. These days they can often be seen here throughout the whole night as the sun never goes below approximately -13° altitude. Fortunately they did not appear which seems rather absurd to say. But instead of focusing on the northern horizon I began to center my attention on the thunderstorms at the convergence line moving from the Netherlands through the North Sea towards Denmark and the extreme northwestern Germany at that time. It is not clear if this system can be characterized as MCS, though its sheer size on satellite images allows of that suggestion reaching from the northerly Netherlands and western Germany to northern Denmark. For me it was visible over the northwestern horizon and steadily producing visible tropospheric lightning about 150-200km away.

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I decided to try to catch some sprites which I have been trying for years when the conditions seemed good. I was pretty sure it would be impossible to catch them as the moon was shining practically at its fullest and midnight twilight was the other reason I did not really believe in this possibility. Though I had a hope. So to maximize the chances of capturing sprites, which I assumed to be a very faint phenomenon, I thought it would be best to reduce the exposure time and increase aperture and ISO setting to compromise between a short light integration time and image quality. So I started continuously capturing images of the distant thunderstorms at 16mm, 3.2s, f/2.8, ISO3200 on a Canon 7D (APSC) for around two and a half hours. After about 30 minutes I recognized the first, my very first sprite on a picture struggling to believe in what I saw. Not only since there was a sprite visible on the image high above the thunderstorm but I was also puzzled about its brightness and size. I continued to shoot for another two hours, the whole observation period was between 22:25 UT and 0:50 UT. As I continued I found another three sprites on my images. When I later analyzed the raw images on my computer I found three more sprites on the images which were rather small and faint compared to the others seen before.

The first (faint) sprite I captured occurred at 22:37 UT, which is just 12 minutes after I started. The next ones were at 23:03 (bright), 23:18 (bright), 23:26 (bright), 23:29 (faint), 23:35 (bright) and 23:39 UT (faint). True midnight, when the sun is lowest, was at 23:23 UT with a sun altitude of a bit above -13°, so it was barely astronomic twilight. Of course there are some gaps between all images (mostly approx. 0.2s, but sometimes several seconds up to a few minutes due to image revision) so that it is absolutely possible that even more sprites actually did occur. I could not see a single one with the naked eye, though I don’t want to say it wouldn’t have been possible. At least the images suggest, it would have been possible to see and my eyes were not too focused on what happened in the sky.

Remarkably all sprites appeared over the northern part of the squall line, which was approximately 200km away from me. There’s one other observation of the very sprite at 23:03 UT from central Mecklenburg-Vorpommern, which suggests that even much greater distances from a thunderstorm of several hundred kilometers may allow suitable conditions for observing sprites but also smaller distances of just around 100km might be suitable. After about 0 UT (2 am CEST), when no more sprites appeared over the northern part, I tried to capture some over the more southern part, but within an hour, no more sprites could be captured by the camera though the tropospheric lightning activity remained high. I did not change the camera settings during the whole image recording, so if they had occurred they would likely be visible in the images. Of course it is still possible I missed some due to the camera reaction time. But from my observations, I want to make the educated guess that there must have been a difference between the northern and the southern part of the squall line, which certainly was not the frequency of the visible tropospheric lightning but probably the fact that the northern part was indeed dissipating with a slowly decreasing frequency of discharges.

During these two and a half hours I took more than 2000 images to get at least seven sprites. If the sky would have been darker I could have used longer exposures and thus had to take less images but I would say it was definitively worth it.

Literature:

Christian, Hugh J. and McCook, Melanie A. “A Lightning Primer – Characteristics of a Storm” @ NASA.  http://thunder.nsstc.nasa.gov/primer/primer2.html

“Charge transfer and in-cloud structure of large-charge-moment positive lightning strokes in a mesoscale convective system”, Blakeslee et al., 2009, doi:10.1029/2009GL038880

Lang, T. J., W. A. Lyons, S. A. Rutledge, J. D. Meyer, D. R. MacGorman, and S. A. Cummer (2010), Transient luminous events above two mesoscale convective systems: Storm structure and evolution, J. Geophys. Res., 115, A00E22, doi:10.1029/2009JA014500.

Victor P. Pasko, Yoav Yair, Cheng-Ling Kuo. (2012) Lightning Related Transient Luminous Events at High Altitude in the Earth’s Atmosphere: Phenomenology, Mechanisms and Effects. Space Science Reviews 168:1-4, 475-516.

Author: Laura C. Kranich, Kiel, Germany

Distrail passing through iridescent cloud

Distrail passing through iridescent cloud · Photo: Claudia Hinz

At 6.35 A.M. on June 25, 2015, I noticed a plane passing through a clear part of the sky without leaving any trace (contrail) behind. Then I observed a beautifully irisating foehn cloud, when suddenly a distrail moved into the cloud dissipating it within two minutes.

Distrail is a short word for dissipation trail. It describes streaky cloud holes caused by airplanes. When a plane flies through or directly above a thin cloud layer, the wake vortices mix the dry air around the cloud into it and the cloud droplets evaporate. This effect is even strengthened by the hot exhausts of the plane, and a clear trail forms behind the plane. Often dust particles in the exhausts act as condensation nuclei making the cloud droplets freeze and form ice crystals. As the saturation vapour pressure above ice is lower than it is above water, the adjacent droplets evaporate. The result is then a white streak of ice clouds between two clear streaks.

Amateur pilots report that the dissipation of clouds also works at small airplanes without jet engines. In this case the propellers stir the air making the cloud dissipate.

Author: Claudia Hinz, Fichtelberg (1215m), Erz mountains, Saxony

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

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