Twinned rainbow over Dresden, May 11th, 2012

During the late afternoon of May 11th, 2012, a mild thunderstorm moved from west to east over Dresden. I was located at the University campus (51° 02′ N, 13° 44′ E) south of the city and seemingly at the southern edge of the cloud. At 17:30 CEST I noticed the beginning of rainfall, and at 17:32 the cloud already gave way for the sun again in the west, while the peak intensity of the rainfall was just reached at my place. I could trace the primary rainbow in the glittering of nearby drops at this time, similar to the appearance of halos in close ice crystals which are frequently observed at cold placed during winter. Already at this stage, the primary looked unusual, and the idea of twinning occurred to me. So I rushed through the building to fetch my camera and find a suitable location to take photos. I arrived at my preferential observation window at 17:36. At this time, local rainfall had almost completely ceased, and a beautiful double rainbow display could be seen in the east (Fig. 1, 17:37:44, Pentax K-5 + Zenitar 16 mm).

The primary rainbow showed a rather broad single supernumerary around its top, which could be interpreted as a weak form of twinning. Within the next minute, some cloud (maybe a part of the cumulonimbus that was still above my place, though not producing rain anymore) did cast a localized shadow on a part of the veil of raindrops in the east, and a very distinct twinning of the primary bow near its top became visible (Fig. 2, 17.39.06, Pentax K-5 + Pentax DA 18-55 mm at 18 mm, with boosted contrast and unsharp masking)

The most convincing theory for twinned bows so far is the assumption of a mixture of smaller and larger raindrops within the shower, with the larger ones being more flattened or squeezed in the vertical due to the resistance of air during falling [1]. According to this, the primary rainbow produced by the larger drops is shifted inwards (i.e. towards the antisolar point), especially near the top. However, as calculations show, the secondary rainbow remains almost undisturbed. This behavior of the secondary was exactly what I could record during my observation (Fig. 3, 17:39:12, Pentax K-5 + Pentax DA 18-55 mm at 40 mm, with boosted contrast)

The twinning was visible up to 17:41, when the top of the rainbow faded away due to the growing shadow of larger clouds in the west. The left part remained visible for some more minutes (Fig. 4, 17:43:44, Pentax K-5 + Pentax DA 18-55 mm, single frame extracted from video file). At 17:42, a complete and intense double rainbow could be observed also from a location 5 km north of my place, with a well-developed supernumerary inside the primary but without any signs of twinning [2].

For the image taken at 17:39:06, I performed a detailed analysis of the position of the twinned primary and the un-twinned secondary rainbow. The sun was located at this very moment at 26,9° in elevation and 265,9° in azimuth. To calibrate the photo with respect to focal length, elevation and azimuth of the image center as well as rotation around the optical axis, I took a star field image in the late evening of May 14th, 2012 from the very same place. From the position of two stars, the star field photo can easily be calibrated, which allows to calculate the positions of two landmarks at the horizon. Their positions allow in a second step the calibration of the original rainbow image [3]. Lens distortion was not considered, i.e. perfect rectilinear projection was assumed, since all pictures with the zoom lens were taken with activated real-time distortion compensation (camera feature). The results are very convincing, indicating the high accuracy of this calibration method when carried out carefully. This can be illustrated nicely by an overlay of the actual image with the theoretical positions of rainbows made by spheres (Fig. 5)

Only geometric optics was used here, and from it only the Descartes angles for the monochromatic wavelengths of 600 nm (red), 530 nm (green), and 460 nm (blue). Furthermore, I transformed the image into a sun-centered equirectangular projection, in which the rainbows from spherical drops have to appear as straight horizontal lines, indicated by the marks on both the left and right side of the image (Fig. 6)

The inward shift of the lower branch of the primary is obvious, with no part of it extending into Alexander’s dark band, i.e. beyond the Descartes angle. This is in accordance with the theory for flattened drops, as well as the secondary not showing any measurable shift or distortion and furthermore sticking closely to the Descartes angle all along its visible extension. It should be noted that more exotic splittings of the primary have been observed [4] whose explanation requires a different theoretical approach.

Taking a closer look at the shadow edge near the top of the primary, it seems that coming from the left the “ordinary” rainbow, i.e. the upper branch, is dimmed in the shadow region, but can be traced up to +20° in clock angle. On the other hand, one marks a smooth transition from the supernumerary in the bright region on the left into the inner branch of the twinned bow in the shadow region on the right. It is obvious that this localized shadow which is cast by a smaller cloud segment will prevent a certain set of drops in this very direction from contributing to the primary rainbow, and that the remaining sunlit drops are in the right mixture to give rise to a clear twinned bow. Very likely the drops a few degrees to the left will not exhibit a drastically changed size distribution. However, the drops in the foreground are illuminated there and add to the overall primary brightness, thus covering unfortunately the twinned bow. One can speculate that there might be many twinned bows in nature that are hidden from us due to the contribution of “ordinary” drops in front or behind the interesting region along the rainbow cone.

[1] http://www.atoptics.co.uk/rainbows/twin1.htm
[2] http://www.meteoros.de/php/viewtopic.php?t=9492
[3] http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-50-28-F134
[4] http://atmospherical.blogspot.de/2009/11/twinned-rainbow.html

 

Author: Alexander Haußmann, Dresden, Germany

Reflected Crepuscular Rays

In the evening of April 18, 2012, I observed some light rays from 2962 metres high Mt Zugspitze [1]. These rays did not emanate from the sun which at the moment I took the photograph was above the rays under the cloud layer. And there were bright reflections of the sun on the ground indicating that there must have been a large lake [2].

Taking a look at the map I realized that Lake Constance was in that direction, but there were no reports that it is visible from Mt Zugspitze. Just the bright reflections of sunlight made it possible to recognize it. The surface of 536 square kilometres large Lake Constance, the third largest lake in Europe, reflected the sunlight scattering it towards me. The shadows of the mountains between me and Lake Constance were projected upon the cloud layer from below creating this wonderful natural spectacle.

A similar observation can be found here: Reflected sunray

Author: Claudia Hinz, Germany

Colours in Citric Acid

Lately I experiment a lot with crystal growing and I knew that Citric Acid can refract the light, but when I saw this I was more than surprised. These flat crystals of about 1-2 cm in diameter (the larger ones) were grown between two glass plates, then put in front of a polarized light source and photographed with a polarization filter. It took a few tries to find a proper way to grow them flat enough for this purpose though. More Photos see here and here.

Author: Rolf Kohl, Germany

Dark ring around the sun

“On the afternoon of 16th November I noticed a dark ring around the sun outside of an aureole. However, the effect could be catched better with the naked eye than the photographs assume.” (2)

“I took some picture in close succession, but none of them shows the ring. Conspicuously, the ring was distinguished by high regularity and evenness. Subsequent enhancing of the contrast let the dark ring become visible clearly on these pictures.”

Quite often, you can spot clouds moving in front of the sun. With a little luck and a cloud layer, that isn’t too thick, beautiful halation and aureoles can be seen at these conditions.

Since a cloud act as an obstacle, it causes a shadow. Apriori, this shadow is invisible to the observer. But if the shadow is projected onto a lower layer of haze, the shadow gets visible in the haze.

Hence, the example above generates a dark ring around the sun, induced by altocumulus cloud shadows on the hazes below.

Time : 16 November 2011
DSLR Camera : Nikon D 3100
Exposure : 1/500 sec, f/22#18mm, F/11, ISO 100

Author: Alec Jones, Bolton/Lancashire, United Kingdom; Michael Großmann, Kämpfelbach, Germany

Spiderweb-Halos

Trying to photograph a spiderweb against the sun to get a completely forward-scattered image of it led me to the discovery of the “spiderweb-halos” (Foto 1). I got a rather dim image of the web against the darker background, but I was struck by a much brighter circle-like structure. This was the moment of the discovery of “spiderweb-halos”.

All you need to get them is a spiderweb between yourself and the sun and a device to block the sun. I used my self-made filter usually used when photographing halos, made of a small piece of black slide film inserted on one end of a ball pen’s tube. It has the right transparency to show also the contour of the sun disc on the photographs (Foto 2). One of the best pictures I get is this (Foto 3).

The spiderweb-halos belong to the general phenomenon called “Circles of light in treetops” (M. Minnaert, Light and Colors in the Outdoors (1993), §29: Foto). As seen on the pictures, the bright forward scattering part of each thread is that one, which is perpendicular to the sun. These segments form an (interrupted) circle between the spiderweb centre and the sun.

Taking several pictures I discovered the dynamics of the spiderweb-halos (Foto 4). The diameter of the circle increases with increasing distance of the sun from the centre of the web (Foto 5), until the halo becomes a dim and open circle segment when the sun is far outside the web (Foto 6). With the sun in the centre of the web, the whole web will be lit up as a “full halo” (not perfectly realized in Foto 2).

An outer circle segment may also be seen, depending on the construction of the web (Foto 4). Another feature of the spiderweb-halos known to halo observers is the “pillar” between the sun and the center of the web. The pillar may vary in appearance from a truly pillar-like structure (Foto 7) to a triangular segment (Foto 8). The pillar may also be seen outside of the sun (if the sun is located near the centre of the web; Foto 9) and extend to the other side of the center of the web (“counter-pillar”, Foto 7).

The full-scale of  “spiderweb-halos”  is shown in this very schematic diagram – it visualizes also why I termed them “halo”: all appearances are known also to the atmospheric halo observer.

Author: Christoph Gerber, Heidelberg, Germany

Reflected sunray

It was the 21st of June in 2010, when I came back from work in the evening and prepared my photo equipment for some time-lapse experiments of the very intensive sunrays currently shining. I was late and just wanted to get one last visual impression from my balcony before walking down to the river and shooting the pictures. What I saw was pretty amazing, so that it took some seconds to get the camera working.

The reflected sunray remained for 50s since my first view. I took 4 pictures of it and made a small animation. The occurrence of that common sunray on the same cloud baseline seems to be at random, due to the fact that the reflected ray moves with reduced speed.

Some discussions revealed that the river itself could not cause the mirror effect, because the surface of running water is too unsettled and not plane enough to produce such a shapely reflection. A calm and wind-protected surface is the harbour basin in a distance of about 3km in the direction of the sun. Further waters in that direction are more distant (>10km).

I kept an eye on comparable situations to get these reflections again, but without success so far. You should be watchful on the following conditions:
- intensive sunrays of course
- low altitude of the sun (to get long distanced rays)
- dark clouds in the short distance (to get the contrasts)

Place : Dresden, Germany
Time : 21 June 2010
DSLR Camera : Canon EOS 1000 D
Exposure : 1/80 sec, f/55mm, F/7.1, ISO 200

Author: Eik Beier, Dresden, Germany

Sodalite interference colours

Sodalite – sodium aluminium silicate chloride – is a mineral of volcanic origin (chemical formula: Na8Al6Si6O24Cl2) and it comes from hydrothermal fluids in a volcanic rock’s cavity. The sodalite containing rock itself is not homogenous but consists of many different, small minerals beside the blue sodalite.

The mineral itself is very nice deep royal blue in general, the piece illustrating this article was mined at Mt. Vesuvius and bought in a mineral shop in Italy. The sodalite pieces are full of other crystals, usually well visible whitish veins which mostly consist of calcite.

When looking at the mineral with the help of some magnifying device we can see small parts of it having thin and colourful layers! These coloured parts are concentrated at the edges of the calcite veins or patches and only visible in a magnified form. Here, the translucent calcite was built on the blue sodalite mineral in a later process different from the forming of the blue crystals from the original hydrothermal solution. These places must also contain a very thin layer of air which is responsible for the colours with its interference.

What is unknown: the forming of the air layers. Are they originally there or are they created when the stone is cut from the rocks? I think the later is more possible as the sodalite rocks can more easily break where the white veins run, so the chopping of the rock might create the gaps, resulting interference patterns. The process might be the same as the ice pieces with fissures showing interference colouration too.

The pictures (1 – 2 – 3) were taken with a cheap digital microscope, the magnificiation which shows the interference colours is 200X. Smaller magnification also shows it but only in tiny coloured spots.

Author: Mónika Landy-Gyebnár, Hungary

More posts to this topic:

• Colours inside a cracked piece of ice by Noli
• Colours on fissures in ice by Reinhard Nitze

Cloud bow

On December 23, I observed on Mt. Wendelstein (1838m, Bavarian Alps) my first real cloud bow which formed on the rim of a cumulus – or might be even a cumulonimbus – cloud. Although a snow shower had formed in the centre of the cloud, the bow clearly did not appear in the shower itself but on the outermost rim of the cloud. There, the cloud droplets must have been even big enough to make the bow show faint colours.

When the phenomenon started to appear, there was the left end of a rainbow visible at the rim of the shower, so it might have rained there. But the picture clearly shows how the droplets become rapidly smaller as the bow extends into the cloud. Even its diameter seems to be smaller in its upper part (1 – 2 – 3).

Spider silk glitter path

While taking a walk, I noticed a field that was covered with fine spider silks. The sun was rather low (about 10°) and made a kind of lower light pillar appear in the silks. When I took a closer look, I could also see concentric circles of light with the sun in their centre.(2)

These circles are caused by the perspective and the angle in which the light strikes the surface of the spider silks. The light gets reflected best to the observer when the reflecting surface is positioned at an angle of 90° to the source of light. A similar effect can be observed when a street lamp shines throug wet branches of a tree.(3)(4)

The “lower light pillar” can be seen better because the sun as the source of light shines vertically down onto the field and all spider silks in this direction reflect the light towards the observer.

Place : Kämpfelbach, Germany
Time : 01 November 2011
DSLR Camera : Canon EOS 450d
Exposure : 1/60 sec, f/22mm, F/10, ISO 100

Author: Michael Großmann, Kämpfelbach, Germany

Sirius scintillation

Caused by the numerous cities and industrial areas south of my observation site (North-Rhine Westfalia, Ruhr Area, Germany), there are rather strong air turbulences (bad seeing) near the horizon. But what is bad for astronomical photographs, however, can be very nice to demonstrate atmospheric aberration and dispersion in the star trails on photographs without tracking. The lower a star is in the sky, the more pronounced is this effect, especially at very bright stars.

In this case, Sirius had an elevation of 11° on October 23, 2011, at 3.10 hours. The sky was clear, wind was at 1-2 Bft, temperature 3°C and humidity at about 80%.

I took this photograph using a Canon EOS 350D, which was focally adapted to a Maksutov (6”, 1800mm, f 12,0). After having been adjusted and properly focused, the telescope was driven at maximum speed over the right ascension axis. This makes the star transit rapidly through the field of view causing a star track on the camera chip which records the chronological sequence of the flickering of the star.

Author: Ronald Blendeck, Germany