Spring halos in Eastern Germany: 46°/supralateral splittings, tangent/Parry arc twins, a great pyramidal show, and biting cold

During the past months the sun was only rarely seen in Eastern Germany, and the number of observed halos was correspondingly low. Moreover, when everybody was hoping for the onset of spring, the winter regained its strength after March 10^th, and people were confronted with masses of snow and untypically cold days and nights for this time of year. But embedded in this belated winter period was a row of days (March 23^rd-28^th) with a remarkable outbreak of halo activity. This report will concentrate mainly on my own observations, though there is also more and complementary material available at the Meteoros message board (in German language).

Saturday, March 23^rd

In Hörlitz, Lower Lusatia (51° 31’ N, 13° 57’ E), the 22° ring and upper tangent arc (or upper part of the circumscribed halo, respectively) were visible from noon on, later to be joined with a suncave Parry arc for some minutes around 15:00 CET (15:01, unsharp masked) as well as a parhelion with a notable blue hue (15:08). From 16.00 to 16.45 the circumzenithal arc was also present. In the evening, the 22° ring, circumscribed halo, both paraselenae and the paraselenic circle appeared at the moon (19:34, USM). The further development is nicely illustrated by a time lapse video I took from 19:54 to 21:54. A weak 9° ring was also present, as visible in the filtered version of the frame from 21:04.

Sunday, March 24^th

Solar halos were again visible from noon on, but quickly changing as the cirrus clouds moved across the sky. I took a second time lapse video (13:23 to 14:40) from the same position as in the night before, showing the 22° ring and the upper part of the circumscribed halo. Note the increase in the wind velocity compared to the night before. This really “fresh” breeze from the East in combination with temperatures below 0 °C even at high noon was challenging for both the observer an the technical equipment. Though the video may suggest that the halo activity decreased during the afternoon, there were occasionally some colourful surprises embedded in the flow of cirrus patches (16:00).

Monday, March 25^th and Tuesday, 26^th (after midnight)

I continued my observations in the afternoon of March 25^th from the town of Dresden, Saxonia (51° 3’ N, 13° 46’ E). However, as I was later told, I already missed a parhelic circle segment that had been visible around noon. When I had the opportunity to look at the sky, all the halos seemed to reassemble slowly out of nothing (15:58, USM). This pattern of standard halos remained stable throughout the afternoon, and was joined by a photographically detectable supralateral arc at around 17:15. Its left wing became visible to the naked eye at around 17:35. Remarkably, a photo from 17:27 shows both the supralateral arc and the real circular 46° halo in the unsharp masked version, with the former touching the circumzenithal arc and the latter missing it; and both arcs merging at the left side at the spot where I later could see the “supralateral” arc by eye. Very likely this bright region was indeed not a pure supralateral arc, but a mixture with the 46° ring. An alternative way for halo image processing is the subtraction of the blue image channel from the red, which also yielded a convincing result here. Throughout the last months I had the opportunity to record this 46°/supralateral merging (or splitting) effect several times, though it never was clearly visible to the naked eye and could only be revealed by image processing.

At 18:10 (2° solar elevation) all halos had vanished for the naked eye, except for a bright upper tangent arc sitting on a weak 22° ring. Once more, unsharp masking revealed a surprise, namely a weak upper sunvex Parry arc looking like a shifted twin of the tangent arc (USM, R-B). This Parry arc had not been present in photos taken 8 minutes earlier.

Up to this point, the halo activity had already been much higher than what we get in average, but the definite climax was yet to come during the night. A weak 22° ring with a right paraselene (the view to left was obscured) was present around 21:00. At around 21:50 a weak 9° halo could be traced from the photos. At 23:30 the 9° ring was plainly visible, having a brighter spot at its bottom (i.e. the lower 9° plate arc). Due to this encouraging observation, I placed my camera on a cherry pit pillow at the balcony balustrade, and started an automatic time lapse series over almost 4 hours. Occasionally, I entered the balcony from inside to take a glimpse at the sky, but I did not want to disturb the fisheye photo recording by my presence. Hence my visual inspections were not carried out with full adaption to darkness. The 9° ring was very prominent until approx. 03:00, with a bright bottom and from time to time quite bright sides. The 22° halo was rather diffuse, which I took as a sign that further pyramidals might be hidden there. On its top something like a diffuse combination of  an upper tangent arc and a 23° plate or Parry arc was seen. Since the unusual quality and rareness of such an observation was immediately clear to me,  I was very excited what the time lapse video from 23:49 to 03:42 would reveal. The results did even exceed my expectations, especially in the unsharp masked version. In the following pictures (composites of each two neighbouring frames from the time lapse series for the sake of noise reduction) I labelled the halo species I could identify.

00.32.45, lunar elevation 35° 8’:
The distinction between the 23° plate arc and the Parry arc is difficult, but the presence of the other plate arc justifies the interpretation as the former effect. However, there is not enough detail in the bright region at the bottom of 22° ring to decide if more than an ordinary lower tangent arc, e.g. a  20° plate arc, is present. The circular 23° halo is either missing or masked by the outer intensity gradient of the 22° ring. It is however the only smaller halo that requires the prismatic top faces (or bottom faces, as being equivalent for random orientations) of the crystals, and hence it represents a special case. Against this view stands the presence of the 46° halo (at least 1 h later, see below), which requires such crystal faces as well, so the problem remains open.

A version of this photo without the labels is displayed as the title image of this report.

01.29.45, lunar elevation 29° 39’:
At this stage of the display, the bright regions at the sides of the 9° ring appear very prominent, corresponding to the visual impression. They can be associated with column arcs, however, I did not find traces of column arcs of the other halo families in the photos (yet).

01.36.45, lunar elevation 28° 52’:
A very strong unsharp masking reveals the additional presence of the 35° and 46° halos. The clear intersection with the paraselenic circle demonstrates that indeed the circular 46° ring and not an infralateral/supralateral combination is dominant. Note that this situation changes towards the final frames of the video, in which a clear supralateral arc without a 46° ring can be seen.

All radii have been checked by calculating the angular distance of several stars from the moon.

Tuesday, March 26^th and Wednesday, March 27^th (after midnight)

During the afternoon the halo activity rose again, until at around 14:00 both a complete 22° ring and 9° ring were visible again in rather structured cirrostratus clouds. Over the next hour, the clouds became more uniform, but also more dense (15:12). Unsharp masking and subsequent Red-Blue subtraction revealed also a weak 35° halo and 46° halo, both not being visible to the naked eye (USM, R-B). In the R-B picture, an additional ring-like feature is visible at about 12° distance from the sun, likely an artefact of this processing mode in connection with the camera and lens. It could be traced in later photographs (15:22, USM, R-B, composite of two images), maybe together with faint traces of the pyramidals near the 22° halo. As in the night before, the pyramidals faded over time, until a pattern of prismatic halos remained (16:35, USM).

Moon halos seemed at first unlikely due to the increasingly dense clouds, but after midnight once more the 22°/9° ring combination stood in the sky, however rather diffuse and less colourful than before (00:38, USM, composite of two images). A supralateral arc (or 46° halo) additionally appeared around 02:00 (02:12, USM, composite of two images).

Wednesday, March 27^th and Thursday, March 28^th (after midnight)

Around noon, a complete parhelic circle together with the 22° ring, circumscribed halo and both parhelia could be seen in the region of Dresden, though I personally missed this observation. When I began to look at the sky in the early afternoon (I was somehow a little afraid that this flood of halos would never end), the parhelic circle had lost most of its brightness, but was still detectable at the sunward side of the sky. No pyramidal halos showed up anymore, so maybe the most exotic halo species at this point was a small Lowitz arc reaching from the right parhelion to the 22° halo. However, the detection is difficult due to the presence of contrails and lower clouds, that produce artefacts in the image processing (13:34, USM). R-B subtraction also revealed a weak 46° halo.

Again, the clouds did thicken towards the evening, but this day before midnight a light snowfall set in. The series of halos seemed to have come to definite end. Nonetheless, during the night the upper part of the 22° halo appeared on the moon, just as to wave goodbye after an astonishing week full of surprises and challenges (02:11) and certainly one of the most remarkable periods in my 18 years of skywatching.

All images and videos from this report can be found here in chronological order. Any details concerning camera and lens type, focal length, precise time stamps etc. will be provided on request.

Author: Alexander Haußmann, Dresden, Germany

Where is the shadow?

The situation shown in the picture is often misinterpreted (Photo taken by Anja Hoff on 22-08-2012). Most people think that the shadow of the plane and the contrail cast on the thin cirrostratus cloud sheet must lay higher than the plane itself. This seems obvious, since the shadows are higher than the objects producing them. The low standing sun leads one to this conclusion – it is shown in the upper sketch:

The sun is perceived as low standing – lower than the clouds. The shadows, necessarily on the other side of the shadowing object, reach higher in the sky, and the illusion is perfect: the shadows must project upwards. But the actual circumstances are quite different. For any observer in the plane, the sun is above the same high over the horizon than it is for the observer on the surface. If he would see the shadow of his own plane, this would be underneath of him and the plane projecting towards the surface of the Earth.

The ground bound observer is a victim of the everyday perception. For him, the atmosphere is a three-dimensional volume, and the sun is located in it. But all the rays of the sun enter and cross the atmosphere parallel. This is shown in the lower sketch. From this it is evident, that the shadows can only be lower than the plane. Even at sunset/sunrise the shadows would not be cast above the plane. The single possibility, which I have had the opportunity to see once, is that the plane heads directly towards the sun eclipsing its own contrail. Another very interesting possibility is the eclipsing of the contrail from one side of the plane by the other, so that the one towards the sun is whitish-bright and the other grayish-dark – indeed a very spectacular view!

The two pictures below are from a series and can be used as a stereoscopic pair. If you look at the pair with crossed view, you will get a 3D impression of the scene – and you will notice that the top of the shadow peaks are much nearer to you than the clouds originating them.

Author: Christoph Gerber, Heidelberg, Germany

2012 in review

Our stats helper monkeys have been busy putting together a personalized report detailing how your blog did in 2012!

4,329 films were submitted to the 2012 Cannes Film Festival. This blog had 39,000 views in 2012. If each view were a film, this blog would power 9 Film Festivals

In 2012, there were 20 new posts, growing the total archive of this blog to 192 posts.

The busiest day of the year was August 3rd with 1593 views. The most popular post that day was Dark ring around the sun.

These are the posts that got the most views in 2012.

• 1 Dark ring around the sun January 2012
• 2 Interference Phenomena on Soap Bubble Surface May 2011
• 3 St. Elmo´s Fire on Mt. Hoher Sonnblick December 2012
• 4 Air mirage effects during transit of Venus June 2012
• 5 Full circle rainbow June 2011

Some of your most popular posts were written before 2012. These articles has staying power! Consider writing about those topics again.

We wish you a happy new year and furthermore many interesting observations!

Reconstruction of height and position of the green airglow from July 15th, 2012

It was an ironic situation when during the night from 14^th to 15^th of July 2012 (at a weekend) a high number of observers and photographers were looking for a predicted aurora borealis and instead were confronted with a remarkable outburst of structured (or banded) green airglow. This phenomenon is well known and explored by professional geo-scientists but seemed to have slipped the attention of most amateur observers, including myself, up to then. Though it first seemed likely that the geomagnetic storm may have somehow triggered this event, later observations (e.g. July 23^rd, 2012: http://www.polarlichter.info/airglow.htm) indicated that the traditional excitation mechanism (UV and X-Ray radiation from the sun) is capable of producing intense green airglow without the need for a geomagnetic anomaly.

Due to the fascination I felt during my own observation, I got interested in using the many available photographs from the July 14^th/15^th night for a height and position reconstruction. However, as I later found out from literature, the airglow height of  87-95 km (i.e. a quite thin layer, comparable to the NLC layer around 83 km) is already well established by professional measurements. It is remarkable that this value can in fact be reproduced by comparing amateur photographs from various locations in Germany by an un-biased analysis, which I want to present here.

The first task to do was to contact other observes via the well-known communication boards about atmospheric optics to gather suitable photographic material. Of course I had my own images at hand and intended to use them for this process, so I already had a list of time slots to find synchronous counterparts for. Even though I could find several pictures taken within a tolerance of < 1 minute with respect to my own photos, I had to drop most of them since a coarse analysis of the viewing directions yielded no overlapping fields of view. But through discussing my idea with several other photographers, I was able to identify other matching pairs independent from my own material. Finally, I ended up with two data sets (image pairs), 1 and 2,  to work with:

1a) Frank and Sabine Wächter: July 15^th, 00:55 CEST, 51° 12’ N, 13° 35’ E, 189 m above sea level (Meißen, Saxonia): https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/1a.jpg

1b) Jens Hackmann: July 15^th, 00:55 CEST, 49° 29’ N, 9° 55’ E, 333 m above sea level (Weikersheim, Baden-Württemberg): https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/1b.jpg

2a) Franz Peter Pauzenberger: July 15^th, 02:02 CEST, 49° 00’ N, 11° 30’ E, 518 m above sea level (Beilngries, Bavaria): https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/2a.jpg

2b) Alexander Haußmann: July 15^th, 02:01 CEST, 51° 32’ N, 13° 58’ E, 110 m above sea level (Senftenberg, Brandenburg): https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/2b.jpg

For a detailed analysis, it is necessary to calibrate these photos, which means to precisely assign values for azimuth and elevation to each pixel. If the projection characteristics of the photographic lens are known, the positions of two stars in each picture are sufficient input for this purpose. However, the simple assumption of an ideal gnomonic (rectilinear) or equal-area projection (for ordinary and fisheye lenses, respectively) drastically limits the accuracy of the results. To overcome this, the projection characteristics for all four lenses were reconstructed by measuring the pixel distances of approximately 15 stars from the image center and compare these with the angular distance from the optical axis for each image.

After this calibration and assignment, longitude and latitude positions for each pixel can be calculated, allowing the projection of the photo onto a map if a certain height of the airglow layer is assumed. This method already proved to be very useful for the reconstruction of NLC positions (http://www.meteoros.de/php/viewtopic.php?t=8451). Since the goal is here to determine the layer height, this parameter is varied until the corresponding structures in both reconstructions of an image pair give the best fit. Indeed it was possible to find consistent height values for both data sets, 92 km for pair 1 (https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/1.gif) and 93 km for pair 2 (https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/2.gif). Here the traditional blink comparison technique was applied in a modern form using gif animations. It is fascinating to see how the airglow structures that look completely different in the original two photos of each pair coincide in the reconstruction on the map. Evidently, all non-airglow structures such as trees, background light, clouds, photographic violet aurora etc. have to be ignored in the reconstruction. It should be noted that more complex approaches (http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-51-7-963) are recently established in the professional field, allowing even to resolve finer structures within the thickness of the airglow layer.

Furthermore, these reconstructions show an undistorted view on the band structure of the green airglow layer. As already expected from the perspective view of the original photos, these bands are roughly aligned in the direction from West to East. Using the consistent height information obtained from the image pair comparisons, it is moreover justified to project a whole picture series from a single observation site onto the map in order gain insight in the airglow band dynamics. For this purpose I used a time lapse series that I took from July 14^th, 23.16 CEST to July 15^th,  01.20 MESZ at the Senftenberger See (51° 29’ N, 14° 01’ E), starting in the evening dawn and originally intended to capture the predicted aurora borealis (https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/3.avi). Due to the weak contrast of the airglow at this stage, strong image processing is needed to separate the bands from the background. Though this finally results in a rather poor signal to noise ratio, it can clearly be seen that the airglow bands move in northward direction (https://dl.dropbox.com/u/8849406/Forum/AirglowBlog/4.avi), illustrating the recombination and/or matter transport dynamics in the mesopause region.

Author: Alexander Haussmann

St. Elmo´s Fire on Mt. Hoher Sonnblick

On November 28, the webcam of the Meteorological Observatory on the 3106 metres high Mt. Hoher Sonnblick in the Hohe Tauern Mountains in Austria could catch this unbelievable St. Elmo´s Fire. This photographic webcam, a Canon EOS 1100D, was installed by radio hams and takes one picture every 10 minutes.

Weather observer Hermann Scheer describes the meteorological situation as follows: “The day the pictures were taken ist was snowing, and there was a stormy southwesterly wind with wind speeds around 60 kph. There was no thunderstorm near. I did not notice any discharges, but I could clearly hear the crackling noise on the tower outside, which is always a sign that there is a certain voltage applied. When I went to the platform with my camera the first time, I could not catch St. Elmo´s Fire quite clearly and did not really notice how strong it was. Later, during the second photo shooting, the camera on the tripod began to sparkle, and I also noticed the tension in my short hair. Then I saw St.Elmo´s Fire on the suntracker. The interesting phenomenon lasted for about one hour.”

A high field strength under conditions with falling snow and strong wind is not very rare, as these example diagrams from a day with similar weather conditions show (1-2). When there is enough tension generated by wind friction and high humidity, an electric current begins to flow between the charged air and the point of the instrument. The air becomes ionized generating a flickering, pale blue light that looks like a flame. St. Elmo´s Fire is probably reported so rarely because only very few people look for it on mountain tops under such weather conditions, and during thunderstorms there are no people there.

But even if St. Elmo´s Fire is very beautiful to see, it is in any case a warning. If you see St. Elmo´s Fire near, it is probably a hint that a discharge is imminent. So, if you see it, you should look for shelter immediately.

More pictures 1 – 2 – 3

Düsseldorf Leaning Tower and other Shadow Plays

On November 16, 2010, Hans Juergen Heyen had a rather “spooky” day in Düsseldorf. “A short time after noon I took a walk along the river Rhine and wanted to shoot some photographs. In the beginning, the sky was completely clear except some mist, but at about 14 hours some ragged clouds came up which lowered down to about 100 metres above the ground. The clouds were followed by a mixture of fog, sunshine and clouds which in this area (Lower Rhine region, about 45 metres above sea level) is very uncommon. Also the frequent change between a nearly closed layer of stratus clouds and blue sky was rather strange.
A great help for evaluating the situation was the Rheinturm, a TV tower which is about 240 metres high and has a restaurant with an inclined bank of windows at about 180 metres. This tower disappeared and reappeared between the fog and clouds causing some shadow plays and reflections which were rather irritating for the observer. Sometimes even phantoms of the tower appeared. The tower quasi became an actor in this weather phenomenon. Some time ago, I witnessed a similar phenomenon at the same place, which at that time had turned out considerably fainter. But this time, the phenomenon lasted for about two hours. It just seemed as if the Cllerk of the Weather had been sitting in a pub in Düsseldorf Altstadt and lost the control about his remote weather control while drinking the famous Altbier, a very popular kind of beer in Düsseldorf. For people interested in weather phenomena, it was a really gorgeous afternoon”.

Two main phenomena were visible: When the sun was beside the tower, it caused reflections in the bank of windows around the tower restaurant projecting shadow rays onto the wall of fog. This was a kind of reflected anticrepuscular rays (1-2-3-4).

Later, when the sun was almost behind the tower, another strange shadow play became visible. Contrary to what is usual, the shadow of the tower was not on the side of the tower which points away from the sun, but in direction towards the sun. This was because the observer was now postitioned in the diffuse shadow of the tower. Normally he should now see the shadow behind himself on the ground. But as the droplets of the wafts of mist formed a kind of screen in front of and below the top of the tower, the shadow of the tower was now displayed on the fog.

Sometimes the shadow was displayed on several layers of fog forming multiple images [1-2].
When the sun is positioned directly behind the tower, only the upward projection of the tower top is visible, being cast upon ragged clouds above the tower by the low autumn sun. More pictures 1-2-3-4-5-6

Wake vortices caused by planes

When air humidity is high, sometimes wake vortices can be observed on the wings of a plane. These are an accessory phenomenon of the ascending force, which needs a certain underpressure to be effective. This underpressure makes the air flow from beneath the wing to its surface for pressurization. As in these vortices there is an area of especially low pressure, the air cools down adiabatically here, often reaching temperatures below the dew point. This makes the water vapour in the air condensate to steam or fog, making the wakes visible.

In the morning of October 8, 2012, Renate Possiel could take a photograph of this phenomenon from the control tower of Munich airport. That day there were wafts of mist with different ranges of sight on the runways. More photographs: 1-2-3

Another reason for wake vortices to form is the downward acceleration of the air along the wings when the plane is ascending. At low temperatures and high humidity, also here visible condensation can occur. When a plane passes near the sun, sometimes an iridescence of the wakes can be observed, as showed in this photograph taken by Gabor Metzger.

More often iridescence can be observed on contrails, especially when they are very “young”.  Also these photographs (1-2-3) taken by Gabor Metzger.

Another articles to this topic:

• Shock wave refraction and iridescence over airplane wing
• Circular Wing Tip Trails

Air mirage over Munich

In clear air, the Alps are visible from Munich. In the morning of October 18, 2012, Frank Sievers observed this superior mirage of the Alps from the roof of a house behind the Perlach heat and power station. It was probably caused by layers of air of different temperature generated by the power station, which refract the light at different angles to the observer´s eye.

One day later, in the morning of October 19, 2012, the unusual mirage was visible again. This photo taken Stefan Rubach from the same Rooftop.

Zero Order Glow

When rain is falling with the setting sun behind it, there sometimes appears a golden or deep red glow in shape of a semicircle around the sun. Literature calls this glow “Zero Order Glow”. The name means that this glow is a zero order rainbow. This is because the light is not reflected within the raindrops once or twice, as it is the case in primary and secondary rainbows. In the case of a zero order glow, the light passes through the raindrops without being reflected, leaving them only a bit deviated. So there is no bow-shaped concentration of light, and the zero order glow appears in form of a diffuse shining area around the sun.

In normal cases, the phenomenon is visible only above the horizon. But when this photographs (2 -3) was taken, a fine drizzle fell into the valley from a very low layer of clouds, causing also a glow beneath the horizon. Due to the low sun elevation and the long way the sunlight had to travel through the atmosphere, together with the additional light diffraction on the small drizzle particles, there is only the red light visible.

Author: Claudia Hinz, Wendelstein (1835m), Germany

Algae corona

This photograph was taken by Hans-Jürgen Heyen from Meerbusch. “I discovered the glory effect in August 2012 at the pond of Hugenpoet Castle in Essen-Kettwig. It made me take a closer look at the water because one tends to think that there are mineral oil products on the surface. But there were gigantic carps in the pond and also some big golden fish which looked like koi-carps. And also a beautiful demoiselle flew over the water. And especially this species of dragonflies is very sensitive against environmental pollution. However, what really was there in the water were algae which caused a significant clouding of the water, and this obviously was the reason for the formation of the algae glory.”

These coronae are caused by light diffraction on very small particles. In most cases, they are caused by algae, but there were also such coloured rings observed around pollen which had landed on the water surface. The coronae are caused when a ray of light is split up into partial beams by such a small particle. These partial beams go on into different directions and interfere in the observer´s eye.

Just like pollen coronae, also algae coronae are not always round. When there are unusually shaped algae are involved, also their ring systems on the water surface can be ovally shaped or have bright spots.