Blog Archives

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!

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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.

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.

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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

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

Alpenglow and Crepuscular Rays from above

In the morning of November 11, 2010, Anke Morbitzer from Gladbeck, Germany could enjoy the alpenglow from above during a flight from Milan over the Bern and Pennine Alps. Especially impressive were the shadows of the mountains being projected upward into the haze. Just before, also the upcoming twilight with its impressive colours had been very exciting.

Perspective of a crepuscular ray

We most always notice crepuscular rays as fingers of light and darkness that stretch toward us from the horizon.

However, on occasion it’s possible to observe them “sideways”. The photo above offering such a view was taken from Kämpfelbach (Germany), at sunset, as the cumulonimbus anvil at far right effectively blocked sunlight from reaching the low hills in the distance – middle and left.

Note that the wedge-shaped, blue-gray shadow from this towering cloud extends all of the way to the antisolar point. Photo taken on April 22, 2011

Author: Michael Großmann, Kämpfelbach, Germany

Shadows and lights of a flight

Many people find flights boring – but not all! If you are lucky to be seated by a window you can always find something interesting in the air beside or below the airplane. David Lukacs from Hungary took this picture on 2nd November 2009 on a flight from Rome to Budapest, about 15 minutes after the departure. A thin layer of haze was between the plane and the sea so the sun shining on the right side above the plane could cast radial shadows on the left below. The beams of shadow and light join at the antisolar point.

A bit later when the plane travelled above a cloud layer David also noticed a nice glory below them:

Even the shadow of the airplane appeared in the middle:

Posted by Noli

Thunderstorm shadows

In the evening of June 29, 2009, several thunderstorms formed unexpectedly over northwestern Germany, from the Ruhr area northward to southern Lower Saxony. They brought rainfalls up to 30 liters per square meter.

As the sun was almost setting, the shadows of the storm clouds reached a length of several hundred kilometers. The satellite picture taken at 19.15 UTC = 21.15 CEST shows the shadows extending even up to Thuringia and northern Bavaria.

Unfortunately, there are no reports on crepuscular rays from the area southeast of the thunderstorms.

Author: Peter Krämer, Bochum, Germany
Satellite image with kind permission of DWD (German Weather Office)

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

Shadow Rays above Pithead Rig

In the morning of December 21, 2007, a beam of shadow rays appeared above the pithead rig of the German Minery Museum in Bochum, Germany.

The picture was taken at about 10 a.m., one hour after sunrise. As the sun elevation was still very low, the shadow of the pithead rig was projected upwards and became visible as a beam of shadow rays in a thin layer of mist near the ground. A similar phenomenon can sometimes be seen above a pylon or tower, but there only one single dark ray appears. The shape of the pithead rig, however, made a beam of four shadow rays appear.

Author: Peter Krämer, Bochum, Germany

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.

Author: Peg Zenko, Wisconsin, USA