|Place:||Bonn||Observer:||T. Credner, S. Kohle|
© Copyright by the observers
But if the crystals show a preferred orientation, some details in the outer halo are more pronounced. In the upper image we can see a brightening on top of the 22 degree halo and next to it arcs that deviate from the main halo. In all this is called the "upper tangent arc". It is the result of a preferred horizontal orientation of the crystal's axis of symmetry. Especially falling hexagonal needles favor this "lying" orientation. Above the sun, the minimum deviation of the horizontal needles remains 22 degrees, but it increases beside and thus leading to these extra arcs. Is the sun higher than 40 degrees above the horizon, a complete circumscribed halo appears. It touches the 22 degree halo at the upper and lower border, but is more extended at the sides.