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“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 24th, 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

“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 5th, 2013. The sunlight was blocked by a mast and its shadow fell on very thin clouds. Simultaneously there was a bright corona. (12345)

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.

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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

Duplicated cloud shadow

Sometimes it may happen that there are several cirrus layers above a building thunderstorm cloud. Only when the cumulus cloud has grown to a high level breaking through these cirrus layers, its top casts a shadow upon the cirrus layer.

If there are several cirrus layers, this multiple projection is caused by the perspective effect.

Also this photograph is rather old, it is still very impressive.

Camera: Nikon D40x; focus lenght: 55mm; exposure-time: 1/500 sec.; aperture: F/11; Date: 05.09.2009

Author: Michael Großmann, Kämpfelbach, Germany