Circumzenithal Arc 21.8.2008

At a pause during my praxis work I go out into the garden, at 17:43 p.m. For some reason, I look high up into the sky. There are white cirrus clouds advertising bad wheather (which occured a day later). In midst this cirrus cloud there is a light, short, coloured bow like a rainbow.

Its centre is averted from the sun (and from the ground), but it can be seen in the direction of the sun, as opposed to a rainbow, which can be seen with the sun in behind.

The centre of this circumzenithal arc in fact is the zenith - hence the name.

A Circumzenithal arc is arising from refraction of sunlight through non-terminated, horizontally-oriented ice crystals in certain clouds. It takes the shape of one quarter of a circle centered at the zenith and parallel to the horizon, on the same side as the sun. Its colors run from blue near the zenith to red towards the horizon; it is one of the brightest and most colorful halos. The light that forms the circumzenithal arc enters an ice crystal through its horizontal top face and exits through a vertical side face (the angle of separation must be 90). This means that it occurs only when the sun is at a lower altitude than 32.2 above the horizon. The circumzenithal arc is brightest when the sun is at 22 above the horizon (causing sunlight to enter and exit the crystals at the minimum deviation angle (taken from Wikipedia)

On http://www.bbs2-emden.de/photovoltaik/ you may ask for the altitude of the sun at the place, date and time. For the 21st of August at 5:43 p.m. central european summer time it showed out an altitude of 21,2 for Vienna (which is about 40 km apart), that means ideal visibility.

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