Crepuscular rays from above and below

Yesterday there were observations of spread Crepuscular rays over Germany. The satellite image shows the origin of the long shadows: a powerful squall line over northwest Germany. The length of the shadows is about 400km – this is enormous!

Near Pforzheim in Baden-Württemberg Michael Großmann observed rays passing from the setting sun to the antisolar point. Rene Winter was in the district Gotha, Thuringia and saw crepuscular rays that were unusual intensively. Laura Kranich in Kiel wasn’t far away from the thunderstorms and had intense Crepuscular rays, too. There were single beams that ran across the entire sky.

Photo: Michael Großmann

Photo: Michael Großmann

Photo: Michael Großmann

Photo: Michael Großmann

Photo: Rene Winter

Photo: Laura Kranich

Photo: Laura Kranich

Photo: Laura Kranich

Crepuscular rays are rays of sunlight that appear to radiate from the point in the sky where the sun is located. These rays, which stream through gaps in clouds (particularly stratocumulus) or between other objects, are columns of sunlit air separated by darker cloud-shadowed regions. Despite seeming to converge at a point, the rays are in fact near-parallel shafts of sunlight, and their apparent convergence is a perspective effect (similar, for example, to the way that parallel railway lines seem to converge at a point in the distance).

The name comes from their frequent occurrences during twilight hours (those around dawn and dusk), when the contrasts between light and dark are the most obvious. Crepuscular comes from the Latin word “crepusculum”, meaning twilight.

Munich Süddeutscher Verlag Tower Sunrays in Fog and “Ghost Windows”

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

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.

“Shadow-Corona” observed on top of Fichtelberg and Zugspitze

It is not unusual that one can see some shadow rays in the sky due to clouds in front of the sun. One can also observe coronas in consequence of diffraction of the sunlight or moonlight by small waterdrops of thin clouds. But it’s a rareness to notice both phenomena at the same time.

It would be even more interesting to be at the top of a mountain with the clouds very close. So, thin wisp of clouds racking only a few meters over your head. Sometimes these wisps cause also beautiful coronas. If a building or a mast obliterate the sun, its superstructures can cast long shadows into the clouds.

Kevin Förster observed both phenomena on top of the Fichtelberg Mountain (Erzgebirge) on January 24^th, 2015. This time the sun was behind the tower of the weather station and the different appliances at the top of it afforded the shadows. The origin of the clouds was found in the “Böhmische Becken” situated at the southern slopes of the mountain range. Therefrom they drifted into the direction of the Fichtelberg Mountain. First it consisted of ice crystals and caused ice halos. Over the Fichtelberg there were widespread clouds of waterdrops, which caused a nice corona additional to the shadow rays.

“Shadow-corona” at the top of the Fichtelberg. (c) Kevin Förster.

A similar event was observed on Mount Zugspitze in the Bavarian Alps by Claudia Hinz on May 5^th, 2013. The sunlight was blocked by a mast and its shadow fell on very thin clouds. Simultaneously there was a bright corona. (1–2–3–4–5)

In both cases the sun was lower than the top of the tower so that the shadow of the tower was projected on the cloud layer above. This is a very uncommon phenomenon.

Crepuscular rays extended to (almost) 180° observed from Mt. Großer Zschirnstein, Elbe sandstone mountains, June 8th, 2014

Each year during the Pentecost holidays I undertake together with some friends a cycling tour to the Elbe sandstone mountains. This is usually a good opportunity to look for atmospheric phenomena, since we are out in the open the whole day. However this year we just had the sun shining from a plain blue sky most of the time. I feared that nothing interesting would happen, but I was wrong: In the evening of June 8^th, thunderstorms were active about 200 km or more to the northwest from our location (Großer Zschirnstein, 50° 51′ 23″ N, 14° 10′ 34″ E, 561m). The top parts of these clouds acted as apertures to cast crepuscular rays through the sky shortly after our local sunset. To the south the view from this mountain is fully unobstructed since the lookout point is located right above a 70 m high rock cliff. Our struggle to thrust the bicycles up there was rewarded by the beautiful sight of a bright, rosy coloured beam extending from the twilight sky in the northwest to the rising earth shadow in the southeast and passing just below the waxing moon.

Even with a (full frame) fisheye lens it was hard to capture due to its extension of about 180°, so I decided to do panorama stitching from an image series (21:26 CEST: local solar elevation -1,5°). One should keep in mind that in reality crepuscular rays are straight lines and the curved shape in the photo is just a result of the cylindrical projection. Likewise it would have been possible to distort the horizon and make the crepuscular ray straight. Having a look at a panning video may be the best way to understand the geometry. Some minutes later (21:31 CEST: local solar elevation -2,3°) a second beam had appeared quite prominently above the first one, and even more might be detectable by image processing. Though all of them being parallel straight lines in 3D space, the mind is always tempted to interpret them as fanning beams like the emissions from a lighthouse.

Until 21.40 the rays disappeared almost completely apart from the foremost part in the northwest, which itself became quite bright at that time. Around 21.48 the cumulonimbus clouds themselves became visible for a while. This change in illumination and visibility must be caused by the increasing solar depression below the horizon which leads to more vertically inclined sunbeams, until the sun finally sets at around 52° N / 12° E (where the clouds might have been) in 10 km of altitude as well.

Reflected rays in windows

On 18th February 2014 at about 8:25 AM local time I observed a thick but shallow fog which streched to the altitude of 30-50 meters from the surface. I was photographing the fog from the top of an 11 story building when I noticed the strange reflections by some windows. (1 – 2 – 3 – 4)

The building where the phenomenom was formed was about 250 meters from me in the NW direction, just opposite the Sun which was SE, behind me at an altitude of 15 degrees, just above the fog layer.

The location was Budapest, Hungary.

Author: György Répás

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Another Post to this Topic: 

• Düsseldorf Leaning Tower and other Shadow Plays
• Reflected Crepuscular Rays
• Reflected sunray

Deformed Glory

Matěj Grék placed a strong halogen lamp somewhere around 20m away and take some photos from a fogbow. He noticed that the glory was deformed. The wind was strong in this night, the fog was moving quite fast, and with the fog of course also tiny water droplets. Maybe that’s why the glory is deformed in connection with divergent light. Images are taken with a polarization filter.

Camera: Nikon D60; F/6,3; f/30mm; t=30sec. at ISO 200

Author: Matěj Grék & Michael Großmann, Kämpfelbach, Germany

Shadows in sunward direction

Every day one can see shadows pointing away from the sun, but in rare cases there are also shadows visible in sunward direction. These shadows are projected against the sky. Seeing those spooky phenomena requires a low sun elevation, a high object which seems to tower above the sun, and a transparent “screen” of fog between the observer and the object. In the morning of September 11, 2013, Ulf Köhler observed such a phenomenon on the Hoher Peißenberg (998m). The shadow of the dome of the Mariä Himmelfahrt Pilgrimage Church appeared in passing wafts of mist. When Ulf Köhler changed his position within the shadow of the steeple, the shadow of the dome also changed its appearance. (1–2).

Another article on this topic: “Düsseldorf Leaning Tower and other Shadow Plays”

Double Contrail Shadows

Condensation trails are a common feature in the sky. But those which Richard Henkes saw on June 13, 2013, in Rheinstetten, are really unusual. A number of parallel contrails do not only cast their shadows onto the screen of high cirrus clouds, but also onto a narrow bank of stratocumulus clouds (1 – 2). Due to the different altitude of the two cloud layers, the angle of vision is different for each of the layers. This is why the shadows do not appear parallel to the contrails. Also their shadows in the atmosphere appear under a different angle as these are cast downwards. So they “stand” in the air, and their angle corresponds to the sun elevation at the moment the photo was taken. The shadows on the low clouds, however, are flat or only a little tilted according to the shape of the clouds. Both shadows, the one in the atmosphere as well as the one on the clouds, are seen laterally, which makes a sharp bend appear. And not all contrails and shadows are parallel because the airplanes which caused the contrails did not all fly parallely.

Authors: Peter Krämer und Claudia Hinz

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