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.