Friday, October 24, 2008

Sunset sequence

This sequence of images of the setting sun was taken on September 14th, 2008, from Beverly Beach, Oregon, on an evening when sea mist partly obscured the sun. A vague green colour appeared on the upper edge of only one or two of these images.

Author: Alan Clark, Calgary, Canada

Monday, September 08, 2008

Volcanic Twilights over Europe

On August 7, the Kasatochi Volcano, situated on the Aleutian Islands near Alaska, erupted. Clouds of ashes and sulfur dioxide were ejected up to 15 km into the stratosphere.

During the following 3 weeks, the volcanic clouds spread over the whole northern hemisphere, causing widespread intense twilight colours and often also crepuscular rays. These were first reported from Northern America during mid August, but at the end of the month, these "volcanic twilights" were also observed in Europe.

In the evening of August 29, several observers reported a strange and intense yellow light around sunset, followed by a purple light. Some of them were reminded of the unusual twilights between February 17 and 20, which were caused by PSC.

On August 30, skies were clear over Germany, and so many observers could see a kind of silvery cloud stripes a few minutes before sunset. These stripes were orientated north-south and at first glance looked like cirrus or cirrostratus clouds. But during the day these clouds had not been visible at all, and when looking at them carefully, one could see that they were higher than normal high clouds. The contrails of some airplanes were obviously below these clouds, and as the contrails turned reddish in the light of the setting sun, the clouds still remained bright. So they must have been floating higher up in the air, somewhere in the

After sunset, the clouds got a more brownish-yellow hue, but turned pink only about 20 minutes after sunset. Some observers also reported intense crepuscular rays. The purple light faded about half an hour after sunset.

In the morning of August 31, the colours and cloud stripes could also been observed. In the evening, a cold front with thunderstorms approached the western parts of Germany. While even the tops of the cumulonimbus clouds were already dark, the stratospheric clouds still lay in plain sunlight. That evening, instead of the regular stripes of the day before, they looked more like irregular waves.

During the first days of September, the strange twilight colours could still be observed over southern Germany, while for the rest of the country morning and evening skies looked quite normal again.

But as there are still volcanic ashes in the stratosphere, the colours may return. So keep watching the skies before sunrise and after sunset.

Author: Peter Krämer, Bochum, Germany

Friday, August 22, 2008

Glass Bead Rainbow on Kitchen Table

The observation of a rainbow in glass beads on, respectively next to, a fresh route-indicator in a street I made by chance made me think about studying the effect also in the light of a street lamp. But unfortunately the glass beads had been scattered too much by the traffic within a few days. However, also by chance, I learned that glass beads are also used for facing the surfaces of, for example, metal objects, and so I asked our precision tool maker to bring me some...and then I started the experiment.

As a source of light I used a small bulb like those you find in a bicycle lamp, but without a reflector, because the light source should be as similar to a single point as possible. I scattered the glass beads on a black eloxated aluminum sheet (ca. 30 x 40 cm), and the result was overwhelming. Exactly as in the experiment made by Christian Fenn, the bow can be studied under different geometrical conditions when using a laser and a rotating mirror. The easiest way is to realize the reversed geometry just as otherwise the shadow of the head would be rather large as distances are small. Just put the metal sheet on a table, hold the lamp above it and look at it using different positions of your head. I also took some photographs after having attached the lamp to a mounting. This also shields the direct light. The first supernumerary is also visible, and like in the rainbow caused by water, a polarisation of light can also be proved. (1 2 3)

Seen through a microscope, the glass beads look like this:

I estimated the average radius to be at about 50 micrometers with an average variation of about 15 micrometers. But there are different sizes available. Similar to water drops of that size, the colours are rather blurred (this is especially obvious when you look at a glass bow in sunlight). The spectrum of light coming from a bulb is also rather "red" which causes the strange colours of the pictures.

As I was very fascinated by that phenomenon, I also calculated some simulations using the Airy-theory for glass beads (I could reuse some parts of the original text about the twinned bow for this). And in order to show the phenomenon from the observer´s position, I could use the text on halos on snow covers (so after 10 years the circle is closed...). An imaginary depiction seen from above obviously shows the "intersection through the apple", but as far as I know, nobody tried to explain the different width of the colour bands up to now. Tis effect becomes very obvious when "opening the inner bow by merging with the outer one" (This is very difficult to describe; you must have seen it). However, the geometrical data of the simulation are not exactly the same as in my observation because I did not execute any measurements while photographing.

Left: Seen from above with the observer´s position in the centre of the picture
Right: Gnomonical projection from the observer´s position (“simulated photograph”)

Author: Alexander Haußmann, Hörlitz, Germany

Tuesday, August 19, 2008

Yellow Rainshower

On August 13, I could witness another atmospheric phenomenon in Bochum, Germany.

In the evening, about half an hour before sunset, a rainshower approached from the west. Behind the shower, the sky cleared up very rapidly, so that the low evening sun could shine through the veil of rain. This caused very strange light conditions I never experienced before.

First there was a yellow squall line visible, followed by a bright yellow veil of precipitation. As it came nearer, another shelfcloud-like structure passed from south to east. This cloud was also strangely illuminated and showed orange and brownish colours.

The squall line in the western and northwestern part of the sky turned more and more orange, too, while the rain got an intense and bright yellow. The whole landscape was bathing in a strange and intense yellow light with orange-coloured clouds above and in the east.

When it began to rain, a rainbow appeared in the east, visible as a full semi-circle. The blue part of the bow had already gone, only red, yellow and a faint green were visible.

Then the sun sank beneath the horizon, and all colours faded away within a few minutes. So there was no really red rainbow that evening, but nevertheless it had been very impressive to me.

Author: Peter Krämer, Bochum, Germany

Twinned Rainbow

On August 13, after a rainshower had passed over Bochum, Germany, a twinned rainbow appeared. The bow was rather faint, but the twinning in its upper part was clearly visible. The red and the yellow colour bands of the rainbow were twinned.

On photographs of the rainbow, the twinning effect is hardly visible, but after oversaturating one of the pictures, the twinning turned out well.

During the rainshower, large drops were falling. A second shower about half an hour later with normal-sized drops also produced a rainbow. Even though this bow was very bright, it was not twinned, but showed a secondary bow and supernumeraries.

This is another hint for the theory to be right that the effect of twinned rainbows is really produced by large raindrops getting flattened by the resistance of the air when falling.

Author: Peter Krämer, Bochum, Germany

Friday, August 15, 2008

Light refraction on Fireweed-seeds

While testing my new Tamron AFR70/300 on several plants and flowers I passed a very steep slope overgrown with fireweeds.The sun was relatively low above the slope that I noticed a silky lustre beetween the seeds. As more I was moving the plants beewenn me and the sun, the effect merged into an incredible refraction of light on the silky like seeds. I shot a few pictures and those which where slighty off focus showed very bright colours. This effect seems to be similar with light refraction on spider-webs. In this case though colous where extremely bright.

More pictures: 1 2 3 4 5

Author: Rolf Kohl, Germany

Saturday, August 02, 2008

Summer twilight rays

Bright twilight rays on 16 July, 2008 in Green Bay, Wisconsin. They stretched nearly all the way across the peaceful evening sky. This photo was taken 20 minutes after sunset. I enjoyed the view from our front deck for about 10 more minutes.

Monday, June 09, 2008

African dust over Europe

In the last days of May there was an expansive dust layer above central Europe that caused a lot of interesting atmospheric phenomena. The story begun on 27th of May, when an ochre colored aureole surrounded the sun, and the sunset was pale yellow what repeated the next morning. There was a 7-8 degrees wide, dark greyish band by the horizon (image by Ágnes Kiricsi, in Vecsés, Hungary) this made the dawn to be late and the sunset to be too early. On the next days Bishop’s ring was observed through Central Europe – in the Czech Republic, in certain parts of Germany, and in Hungary. (This Bishop’s ring looks the same as it was produced by volcanic particles.)

In the Alps the dust strongly reduced the transparency of air (to about 3-4 kms).
The photo was taken by Bertram Radelow in Davos, Switzerland. The situaution was the same in the northern and eastern parts of the Alps.

The last two days of May passed away with the attenuation of the dust layer, dust fell out – both dry and wet way. At my hometown the dry version occured and a thin layer of fine yellowish powder subsided on the plants and the parking cars. In Germany there was a reddish-ochre coloured muddy rain falling that caught attention. The same thing happened at some places in Hungary too where rainshowers washed out the dust leaving muddy traces on everything.

In the meanwhile the dust „cloud” could even reach southern Scandinavia, where it also produced „colorless” sunset. The photo was taken by Andras Uhrin in Stavanger, Norway.

The origin of the dust is in the deserts of Africa. Late spring and early summer there are proper conditions in the Sahara and Sahel region for the fine dust to lift up high in the atmosphere where hot, dry winds transport it towards Central Europe, causing the same phenomena almost each year. There were „blood rains” in Europe caused by this Saharan dust over the centuries, like the ’Blutregen’ in 1901 in Germany, when the dust mass that fell out was 1-4 grams / square meter! The micron sized particles are mixed of fine sand and fine mica pellets.

Tuesday, May 06, 2008

Rainbows and supernumeraries

During last weekend (Sunday 4th May -08) at city of Espoo, I observed seasons first rainbow. Local thunderstorm came along with rainshowers produring primary and secondary bows. I visually saw 3 primary supernumeraries.

In the heavily enhanced image (USM) of primary bow there is small hint of 4th supernumerary visible (1). Outside of seconday bow (2) there is larger arc (dark pink color), assuming it indicates supernumerary also... The droplet sizes may been quite even, due to easily visible interference bows.

Thursday, May 01, 2008

Heiligenschein and dewbow in moon light

Well, I wanted to observe moon halos that evening, but the sky was totally clear without any cirrus clouds.
I noticed that some blades of the plants on a field were rather wet. I walked some steps into the field and immediately noticed a clear brightening around the shadow of my head. I did a few steps to the left and to the right and could see a very faint dew bow. So I took some photographs at different exposure times and ISOs. The best photographs (1 2) I achieved using stop 4, an 18-mm-lens and at a time of exposure of 45 seconds at ISO 400.

Author: Michael Großmann, Kämpfelbach, Germany

Thursday, April 03, 2008

Polarization Colours in Airplane Window

Patrice Touhar took this photograph through the window of an airplane using a polarization filter and so made these polarization colours appear.

The colours are generated by the combination of three circumstances:
  1. There is polarized light behind the window
  2. The window is made of a double refracting matter
  3. A polarization filter is used in front of the window

Under these circumstances, the light gets split up into two coherences of polarized components which travel through the window at different speeds. This causes a phase difference the amount of which depends from the wavelength. This means that the polarization gets transformed once into a linear polarization of a different direction than the origin for a certain colour, and into a circular polarization for another colour (Lissagiu interference of two waves). As the polarization is now dependent from the colour, the colours appear when a polarization filter is used, no matter if this filter is placed in front of the eye or in front of a camera.

These colours can be seen and photographed under the following circumstances:

  1. There is polarized on one side of the window coming from the blue sky (Rayleigh scattering) and reflection from water and the scattering caused by clouds
  2. The window is made of plastic. In this material there are mechanical tensions which cause a double refraction.
  3. There is a polarization filter in front of the camera.

The last condition is not necessary if you look at the window from an acute angle. Then the light becomes already polarized by the refraction. In this case there is no filter necessary to see the colours. However, the colours you can see under this circumstance are fainter than the colours seen through the polarization filter because refraction polarizes only a part of the light.

You can also find another observation of this kind in this former thread.

Spectral Colours on Metal Surface

During my work I noticed that a grinded metal surface divides the sunlight into its spectral colours (1 2 3). But this happans only when there are small scratches with a maximum depth of 0,003 to 0,005 millimeters. The scratches shown here are about 30 – 40 mm long, but unfortunately I cannot tell you what shape they are. The worked article was positioned horizontally on a workbench and the sun elevation was at about 50-55° (28.06.2007 at 11.30 hours CEST in Pforzheim). The visual angle onto the worked part was about 80°. The question I think about is: How can the white light be split up into its spectral colours that way, and, much more important: What shape of the hollow is necessary to reflect the spectral light this way?

Author: Michael Großmann, Kämpfelbach, Germany

Tuesday, March 11, 2008

Multiple rainbows

Jan. 12, 2008
Riding home on the bus this afternoon, I noticed the rainbow off to the East. The sun was low in the west and was not far from setting, so the clouds were beginning to take on a golden hue. The rainbow looked like it was repeated numerous times and fading off in the distance. I got off at the next stop and pulled out my new pocket-sized digicam and snapped a couple of pics. I checked out Les Cowley's Atmopsheric optics page and it seems that the closest thing I could find were supernumerary bows, but these seemed to be more spaced out than the examples on his website. So is that what these are? I have included two pictures (1 2) and two contrast enhanced b&w versions of the pictures to better show the details (1 2).
Picture details: Canon SD1000, ISO 200, 1/160 sec, f4.9

Author: Darryl Luscombe, Vancouver, British Columbia, Canada

Thursday, February 28, 2008

Aircraft Iridescence

While taking wildlife pictures in a nature reserve close to the Vienna, Austria, airport, Franz Kerschbaum noticed this 747 jet aircraft in a landing approach with a huge condensation cloud behind its wings. As it moved closer to the runway, he was evidently at just the right position for this shocking but beautiful artificial iridescent cloud to come into view. Iridescence and coronas are diffraction phenomena. The pastel or metallic colors result from deflection of sunlight about minute water droplets.

Photo details: Canon EOS 30D camera, 100-400 mm/4.5-5.6L lens at 400 mm and f/8.0

Wednesday, February 27, 2008

Unusually bright twilights over Western Europe

Between February 17 and February 20, 2008, large parts of Western Europe witnessed a series of unusually bright morning and evening twilights.
A few minutes after a quite normal sunset, the western skies began to burn in a strange yellow light which was bright enought to illuminate the landscape and giving a quite unreal touch to houses and trees.
Some minutes later, the yellow light in the west became surrounded by a brownish rim, turning into purple within some minutes. The yellow part of the sky slowly shrank towards the horizon, turning into orange and later into red and crimson. Some observers also reported of a dark, brownish-red light in the east which surrounded the whole horizon reaching up to 10° high.
The strange lights and colours in the sky were visible for up to about one hour after sunset. A similar “light-show” also appeared in the morning, starting with a crimson light over the eastern horizon and ending with the bright yellow light short before sunrise. The yellow illumination of the landscape could even be perceived through layers of low clouds (stratus) in some areas.
The phenomenon was reported from the British Isles, Scandinavia, The Netherlands and Germany, and there are even reports of unusual twilight observations from northern Spain.

These in some cases weird-looking twilights were probably caused by an outbreak of polar stratospheric clouds (PSC). These form at temperatures below -78°C in the stratosphere, at an altitude of about 20 – 25 km above the ground.
Soundings made at several stations showed that temperatures in the stratosphere really were unusually low over western Europe; up to -87°C (De Bilt) were measured, the lowest since measurements began in the 1980s. This makes the formation of PSC over a large area possible. Some photographs also show faint structures in the light, giving hints that they actually were caused by PSC.

Polar stratospheric clouds have never before been observed so far south. Normally, they can only be seen from Scandinavia, Canada and Alaska. Only in 1999 there was a confirmed observation of PSC from northern Germany.

Authors: Peter Krämer, Bochum & Claudia Hinz, Brannenburg

Sunday, February 17, 2008

Unusual corona around the moon

In the evening of February 14, 2008, an orographic cloud similar to this one formed above a sea of clouds under the influence of foehn wind above Mt.Wendelstein (1838m) in the Bavarian Alps. In this orographic cloud, an corona appeared around the moon which was more intensive and larger than any corona I had seen before. 4 systems of rings were clearly visible. The intensitiy leads me to the conclusion that all water droplets in the cloud were of the same size.

As I had a position very close to the cloud, the corona was extraordinarily large. A comparision to the constellation of Orion shows that it had a diameter of more than 20°. Below its lower part, the corona turned into a faint pink and green iridescence, indicating that the droplets were smaller towards the rim of the cloud. And it was also interesting that the thin cloud as well as the corona did not show greater changes in intensity and shape for about 4 hours. They only dissolved when the foehn wind broke down and the clouds of a masked upper level cold front came up.

Thursday, February 14, 2008

Shadows casting shadows

In the late afternoon of February 10, 2008, my friend and I went to a little pond on a field in Bochum, Germany, to take a few landscape photographs. It was a sunny day with a cloudless sky and no wind, so that the sun could be reflected very well from the smooth surface of the pond.
When we went along the pond, our shadows fell upon the water. As the sun was very low – it was about one hour before sunset – our shadows extended over the whole width of the pond, with the heads just beneath a bush positioned on the other side of the pond. Suddenly I noticed two fainter shadows just above the heads of our shadows, moving over the bush. When we stopped, these additional shadows appeared only as two faint stripes extending upwards across the bush from the heads of our shadows, as it can be seen in the wide-angle picture.

But when we moved, the phenomenon became quite obvious. So I did not only take a few photographs, but also made a short video with my digital camera. The video can be watched here.

What caused these additional shadows? First I thought that they were just the reflections of our shadows on the water, but when I later thought about my observation, I realized that it was a little bit more complicated:
The sun was shining on the water, and the sunlight was reflected from the smooth water surface. From the other side of the pond there could be seen a reflected picture of the sun in the water. This picture – or, better said – the reflected sunlight fell upon the bush, together of course with the direct sunlight. So the bush received direct light from the sun and also reflected light from the water surface.
When our shadows fell upon the water, the shadowed parts of the water could not reflect any more sunlight, so that the areas above the shadows received only the direct sunlight. So the parts of the bush which did not get the additional reflected light appeared less bright than the rest of it forming two slightly darker stripes extending upward from our shadows. So, what we saw were two secondary shadows, the shadows of our shadows. For a visual explanation of the phenomenon, I also drew a skech of the situation.

Never before I had thought that a shadow could also cast a shadow, and this observation was only possible because it was absolutely calm that afternoon. The slightest wind would have caused ripples on the water and thus blurred those secondary shadows.

Author: Peter Krämer, Bochum, Germany

Monday, February 11, 2008

Frost Optics

2008/02/10 Taken in Green Bay, Wisconsin, USA at temperature of -10 F. I looked out to see the sunrise refracting through frost crystals on our back window. The entire pane sparkled with these delicate, miniature creations, but only a few of them were postitioned so that I could see the entire color spectrum at once.

Saturday, February 09, 2008

Gree-blue Flash over the Canadian Rockies

This sequence of sunset images was taken from the Canadian prairies, looking ovcr the Rocky Mountains from a distance of about 100 km, in mid-December, 2007. The remaining solar limb shows distinct blue colour following green tints, but the sequence ends with a faint red band, which is probably caused by faint clouds on the horizon.

Friday, February 08, 2008

Diamond-dust Sundog

This very bright "localized" sundog was formed in ice fog among trees along the Bow River in Calgary on a very cold winter day recently. The remarkable feature of this sundog is that trees are profiled against it. The equivalent sundog on the opposite side of the Sun was almost as bright. These sundogs varied in brightness as ice-fog density varied. Individual ice crystals show up as transient bright points.

Saturday, January 26, 2008

Icy window corona

At the beginning of this winter season at a cold morning - dark outside - with a thin layer of hoarfrost on the inner side of the bus windows I've noticed that the light of the street lamps' light was not simply scattered on the windows, but having colorful corona around them. At that time I had no camera with me… All along the winter I was waiting for the same display each morning when some ice was on the bus windows, but for long weeks I haven't met with such spectacular phenomena again.This day was my day on 24th January. Very thin layer of frost was on the inner part of the windows, (with some frost ferns too by the corner of the window frame), mainly this layer was made of small uniform ice crystals made of frozen vapor droplets, we might call it an ice film, it was about 0,1 mm thin.All the lamps outside had a corona, no matter how far the lamp was.The street lamps with white light had the most beautiful color range, the orange colored light of sodium vapor lamps had a less distinct colors and they were a bit moved towards the reds. Not only the street lamps' corona was seen on the ice layer, but a much less spectacular corona around the reflection of the inner lamps of the bus too! At the part of the frost ferns pattern the corona became irregular in shape. When the lamp was near the window I could also see the bigger outer rings of the corona. Almost the same phenomena appears when simple water vapour is on the windows, but that one is really less poor in colors compared to the ice film corona.This was the first time when I could take photos of a corona produced by ice crystals. It's a pity, I could not take close-up photo of the ice layer itself (the bus was driving me to work so it was permanently moving).

Photos are collected here: The only afterwork was some noise reduction and size reduction. It was totally dark outside, so the pics were taken with ISO200 - ISO400; aperture F4 - F5,6; exp. 1/15 - 1/25 sec. auto white balance; without tripod or any kind of help to avoid moving.I wonder if the same phenomena would appear if the outer side of the windows would have a layer of hoarfrost on it. It's only weather's turn to show it up!

Author: Monika Landy-Gyebnar, Veszprem, Hungary

Thursday, January 10, 2008

"Subsun" upon ice crystal

While taking the photographs of the iridescent fissures in the ice, I noticed some beautiful ice crystal structures on one of the puddles. When I looked at them more closely, I saw intensive reflections of the sun in them. Animated by the hoar-frost halo I discovered before, I got the idea to photograph the reflection of the sun as a kind of “subsun”. To get the reflection completely into the picture, it was necessary to make a macro of the ice crystal. However, the crystal turned out ot be a lot more complex than I thought.

The reflection of the sun, however, was not a homogenuous surface, but appeared to be a slightly oval shaped, speckled white spot with a broad golden and also speckled rim. It is exactly this reddish-golden rim that gives me quite a headache, reminding me a lot of the reddish coloured fringes which sometimes can be seen at other haloes like parhelic circle or 120°-sundogs. The sequence of colours and the outward manifestation is that of a common aureole. However, in this picture it looks as if the colour came out of the inside of the ice crystals. Especially take a close look at the crystals marked with arrows.

About one week later at another frozen puddle, the reflection of the sun again had a reddish rim. But here the situation is quite different. This time it is a fragment of the frozen water surface. The wind has carried dust upon the ice which with the help of the sun had smolten innumerable tiny holes into the surface which originally had been smooth. Low temperatures then had made it freeze again. All in all, these processes have caused a rather permeable surface structure to develop, so the reflection of the sun is more like a diffraction. Especially in its upper part, there is an indication of iridescence. Who knows, on day somebody might succeed in taking photographs of “ice surface coronae”!

Last but not least: Of course you can also try to take a fragment of ice off a puddle and make some experiments with it. Maybe there will be confirmations of these sightings pointing also out to the possibility of a (additional) light refraction in the red rim.

Author: Reinhard Nitze, Barsinghausen, Germany