Tuesday, May 30, 2006

Spider web reflections

In Germany October 2005 was very dry, calm and warm with no frosts. The weather gave rise to a large number of spiders and of course their webs. Towards the end of the month, Reinhard Nitze’s attention was drawn to an unusual light phenomenon on a ploughed field. The sun produced a silvery reflection like the glitter path seen on waves when the sun is low. He had seen the effect earlier on scythed grain and grass fields but never so conspicuous. The effects were produced by sunlight reflecting off threads of spider silk that happened to be perpendicular to the sun – eye line in the numerous webs in the fields (more images 1,2,3).

The effect is seen also in the lower left image where the web reflections in trees make concentric rings.

Bare and wet twigs produce similar effects as do support wires in vineyards. Perhaps observers have seen other reflection effects like these?

[Report: Reinhard Nitze]

Wednesday, May 24, 2006

Unusual Coronae & Sunsets - Finland

Very large coronae were observed in Southern Finland at the beginning of May 2006. Their radius was about 45° and they had a bluish disk and a brown ring. The sky was very clear and quite dark outside the corona. Also, very beautiful red sunsets were seen on those days.

The phenomena were visible on many days between May 2-11 and were possibly produced by smoke carried from Eastern Europe.

In the above corona image taken on 11th May a small segment of another photo has been added to show the size of 22° halo for comparison.

The sunset was photographed on 4th May.

Large coronae can be seen during very warm weather periods when the sky is usually hazy. This was the first large corona seen in Finland since the Bishop’s rings of 1991-1992. Smaller coronae (radius of about 5°) with a blue disk and brown ring have sometimes been observed.


[Text & Report: Jari Piikki, Juva Finland]

Monday, May 22, 2006

Fogbow with supernumeraries and Glory

This fogbow and glory was photographed by Ken Tape on May 12, 2006 while descending to a place called "Isachsen" located at 79 degrees North in the Canadian archipelago. The display and descent lasted for about 10 minutes, and the strength of the bows was fairly constant throughout. The intensity of the fogbow supernumeraries was strengthened by the use of a polarization filter. Note that the color sequence in the supernumeraries – blue outside – is reversed compared to that in the main maximum. The shadow in the glory is of a Twin Otter on skis. Nikon D70 with old 20mm lens (effective 30mm).
[Text: Ken Tape, edited by Günther Können]

Saturday, May 20, 2006

Reflected rainbow at a fountain

Alexander Haussmann searched specifically for a reflected rainbow at the fountain of the palace pond at Dresden. And with his precise look he found it – but it was faint. With a polarising filter he could increase its contrast. The reflected bow changed in brightness, occasionally it was obvious but then it became faint again. The visibility was most probably influenced by the wind and the smoothness of the water surface that was to some extent disturbed by the falling drops. This example shows that it is not difficult to find a (artificial) reflected rainbow with help of a fountain.

Friday, May 19, 2006

Alder corona with iridescence and cloud rays

The 27th of March was in a manner of speaking one of our first warm days this year. Owing to the long and cold winter together with the abrupt change the plants exploded into life and produced a very strong pollen concentration in the air. The humidity was very low and so the resulting pollen coronae stood out against the dry sky. The coronae originated from from alders which are very common in the area (Barsinghausen, near Hannover). Hazel were also flowering at that time but would have made a different form of corona.

That day Reinhard Nitze took very unusual pictures of the alder coronae. On the one hand the coronae were intense but appeared together with another phenomena, e.g. with iridescence clouds or cloud rays. Sometimes, however, Reinhard saw them "undecorated".

Pollen Corona produced by hop blooms

Here in Germany we see pollen coronae produced primarily by birch, spruce, pine, hazel and alder. However, by deliberately shaking the bushes to discharge clouds of pollen it is possible to generate other types of coronae. In this example Karl Kaiser photographed a coronae from male hop blooms. Other pollen coronae by Karl are here.

Sunday, May 14, 2006

Very large corona in Chile

This corona was photographed in December 2000 while camping at the El Tatio geyser field at 4300 meters elevation in the Chilean Andes. The corona radius is approximatly 50°. Blue inner disk extends to about 35° from the sun. In between the blue and red, a green band was seen visually, but it was not reproduced on film. Altocumulus clouds were developing from the stuff that made the corona. The corona may be related to the dust that is released in large amounts in the air from some mines in the area.

Cyan saturation was increased in the photo to make the inner blue disk better visible. Otherwise the photo is as scanned from the slide. See here for some more material. The degree grid is made with HaloSim by Les Cowley and Michael Schroeder.

Small and large coronas

How small is a pollen corona! On 14 May, a fine circular pollen corona re-appeared in clear sky blue over Deventer. It was magnified by zooming the camera lens, but actually real proportions of this small phenomenon are disguised by doing so. Here we see a fraction of a common water droplet corona in Fractocumulus too, left in the image. It is nice to see the differences in corona proportions, by considering pollencoronas, via common cloud coronas, to Bishops' ring....

Seawater & other bows

Günther Konnen has drawn attention to this famous image showing disconnected rainbows. J Dijkema imaged it in the Pacific Ocean.

The upper primary bow (with a fainter supernumerary) was formed by falling rain.

The lower bow was made by drops of seawater thrown up by waves against the ship’s side. The seawater bow has a slightly smaller radius (by about 0.8°). The difference would not normally be apparent but here it is obvious by comparison with the rain water bow.

Seawater, because it contains dissolved sodium, calcium and magnesium salts, is slightly denser than pure water and also has a greater refractive index. The lower diagram shows two minimum deviation rays going through spheres of different refractive index to form primary bows. Seawater and rainwater are too alike to show clearly distinguishable rays and so instead, ray paths are shown for a water sphere (n = 1.33) and a glass sphere (n=1.51).

As the refractive index increases, the incoming ray that forms the primary bow (minimum deviation) moves inwards to so that if it were undeviated it would pass closer to the drop centre. At a sufficiently large refractive index the ray actually passes through the centre, the deviation angle approaches 180°, and there is no longer a rainbow. Highly refractive substances cannot form rainbows.

Glass bead bow

While at work I was on break time I took some blast sand out of the sand blaster and scattered it on the ground and I happened to look down and saw a bright bow around the head of my shadow. The bow is like a rainbow but the higher refractive index of glass made a bow with a radius of 28 degrees rather than 42 for a water drop rainbow. Look for these glass bead bows on reflective signs and freshly painted black top.

Saturday, May 13, 2006

Pollen

During the very dry weather of the last week, predominantly faint circular pollen coronas were continuously visible from the Netherlands. On 12 and 13 May the pollen was considerably washed out by short rain showers and could easily be collected afterwards. The image shows some pollen grains taken with simple eqiupment, a small microscope and a digital camera pointed through the eyepiece. The diameter of the field is 1,0 mm. The pollen can be identified as that from pine-trees. After the first rain shower, the pollen corona appeared much better defined than before, being circular with threefold red.

Friday, May 12, 2006

Bishops' Ring after Pinatubo eruption

After the 1991 Philippinian Pinatubo eruption, a large diffraction corona around the sun became visible worldwide. It was seen for many months in 1992 and 1993. This corona phenomenon is known as a "Bishops' Ring". It appears when sunlight shines through stratosperic layers of volcanic dust. The pattern of light and color is the same as that of the common corona in cloud droplets, consisting of a blueish white aureole directly around the sun, surrounded by a reddish to brownish diffuse and broad ring. The radius, however, is large and comparable to that of the common halo, being typically about 25 degs. This photograph was taken by Peter-Paul Hattinga Verschure, this one with super wide-angle lens and his second with a 20mm, from Deventer in The Netherlands on 29th March 1992. It was a spring day with very transparant air conditions, the best circumstances to see this phenomenon.

Shadows during the annular eclipse



Günther Können took these pictures in Madrid during the annular eclipse of 3rd Oct 2005. They are solar images projected onto three mutually perpendicular surfaces: one horizontal and two vertical. During the annular phase (right hand image) the solar images were ellipses, with their long axes oriented in different directions on the two walls and the steps and ground. This was not unexpected. However, in the picture showing the pre-annular phase, one notices that the sun crescents on the ground and steps comprised the more pointed part of the ellipse, whereas on the walls they are the more rounded part of the ellipse. The key is in the third picture, where the shadow of the roof overhang kinks from one projecting plane to the other. In hindsight, all of this is understandable, but I have never seen a comment on this 'solar image puzzle'.

Sunday, May 07, 2006

Shadow Trident in Clouds

On April 15, I had the chance to observe a shadow trident above the pylon of our wind gauge on Wendelstein mountain (1835 m). When an observer is situated in the shadow of the pylon itself, there are regions above the pylon from where no light reaches the observer`s eye. If there is a “screen” of water droplets (clouds) or ice crystals (diamond dust) above the pylon, these regions can become visible as shadow rays. In my observation, cumulus clouds repeatedly passed rapidly over the top of the mountain and the shadow rays were briefly visible several times.

Birch pollen corona

A couple of days ago birch started flowering in Southern Finland and pollen coronae appeared in the sky. I photographed this corona yesterday and here is a composite of six individual images taken in one F-stop intervals and merged together with the HDRI-technique. The photo was enhanced rather heavily by applying unsharp mask and hue adjustment.

More natural version of the same photo is here. Another HDR-image of birch pollen corona, taken by Timo Kuhmonen, is here. See also the pollen corona around the moon by Jari Luomanen.

Friday, May 05, 2006

Twilight Colours during the total Sun Eclipse

A total Sun eclipse occured on 29th March 2006. The weather was very good in all southern Turkey. The sequence photo was taken by Jukka Ruoskanen on a beach close to the town called Side in Turkey. Few minutes before totality high clouds came, and a halo was seen. The halo, of course, vanished with the sunlight and reappeared again after the total phase - the high clouds responsible for the halo can be seen in some of the photos. The sky colours were truly amazing with deep bluish hue towards the zenith and a great "sunset-like" appearence all over the horizon. The other noteworthy points were a significant temperature drop and the peculiar light some minutes before second contact. At that time the shadows were really sharp too.

The second (16 mm) wide-angle picture of the sky during the totality of the 2006 eclipse is taken by Günther Können in Colakli near Side at the south coast of Turkey, straight on the central line. The horizontal field of view is 135 degrees. At the 4 o'clock position fron the overexposed corona, Venus is visible. The limiting magnitude during totality is +3, about the same as during twiligt with the sun 7 degrees below horizon. The light of the sky occurs because of leaking of light via the horizon, from regions where the sun is not completely eclipsed.

Wednesday, May 03, 2006

St. Elmo's fire on Airplane

This bright St. Elmo's fire was photographed by Martin Popek from Czech Republic on 24.03.2006 during a flight to Antalya, Turkey.

St. Elmo's fire is a spark discharge seen typically from soaring buildings. It is generated by a high voltage between the ground and the air.

Condensation Trail Iridescence

Jurgen de Boer (site) imaged this aircraft and contrail near to sunset on 2nd May. The aircraft was 10-20° from the sun. The structured iridescence is interesting. Seven or more colour bands are visible. The water droplets or ice crystals of the condensation trail will have had similar formation histories. Perhaps this produced rather uniform sizes and mean size variation along the trail thus accounting for the uniform colour bands?

Reflection Rainbow at the Dutch isle Terschelling

It is not the brightness of this reflection rainbow that it makes special, but rather it is the almost complete absence of a normal rainbow: of the latter, only the base is visible. Just before this picture was taken, the photographer Günther Können saw both the reflection bow and the normal rainbow. By the time he got his camera ready the normal rainbow had gone. A cloud had blocked the direct sunlight to the rainshower. Reflected sunlight was still illuminating raindrops via a path under the cloud. The reflecting surface was a small lake about 200 m across and 1100 m from the photographer. The picture was taken on 18 Nov 2005 at the Dutch isle Terschelling, 16:05 CET. Solar elevation: 3 deg.

Tuesday, May 02, 2006

Cloud droplet phenomena


This image shows how light scattering by small cloud droplets produces multiple effects that are actually all part of the same phenomena. The scene was taken by Leigh Hilbert in Washington State in January ’06. The shadow of the descending aircraft is surrounded by a bright glory (1, 2,) centred just behind the wing where Leigh was seated. Much further from the aircraft shadow is a circular cloud bow (1,2), a form of fogbow (1, 2), produced also by scattering by cloud water droplets. The classical light paths producing it are those of the rainbow (1,2,3) but diffraction by the small droplets produces something much broader and almost lacking in colour. Inside the main cloudbow is a supernumerary arc that, characteristically for cloudbows and fogbows, has more colour saturation than the primary. The more distant clouds at the image top have produced a narrower cloudbow indicating that their droplets were larger.

Monday, May 01, 2006

Bright Fogbow with serveral supernumerary arcs

On October 3, 2005, on the Wendelstein mountain (1834 m) a very bright fogbow with several supernumerary arcs appeared during the partial solar eclipse. In the centre of the fogbow there also appeared the spectre of the Brocken in variable intensity and size, according to the distance to the clouds. The spectre was also surrounded by a bright glory. Using a polarization filter, Carolin Baumann made this impressive photograph.