Circumhorizontal arc

The Circumhorizontal Arc is one of the most colorful halos that resembles a horizontally extending rainbow.

Appearance

A Circumhorizontal Arc runs at a low altitude in a circle around the sky around the zenith, but only the maximum of the sun facing quarter of this circle is visible. The appearance of the shape of a rainbow colored bow piece and is located approximately 46 ° below the sun. Although the appearance is similar to a rainbow, it is not a rainbow and the colors are consistent with those of a rainbow agree only apparent.

Formation

The Circumhorizontal Arc formed as circumzenithal of floating sheets of ice which are aligned horizontally., The light enters into a vertical side surface and from the lower horizontal surface again. The Circumhorizontal Arc rises with the sun and reached at a solar altitude of 67.9 ° its maximum intensity. Under optimal conditions, the whole cloud can be colored like a rainbow.

The observed brilliant colors Zirkumzenital Zirkumhorizontalbögen and will be apparent from the comparison of the dispersion and the intensity distribution for individual colors. By refraction at a 90 ° - ice edge, the dispersion leads to a splitting of the halo between red and blue of approximately 2 °. Furthermore, the intensity profile of a single color is concentrated in an angular range of one -half degree. On the other hand, there are the color ranges no weakly decaying distribution of the intensity, as is the case with the rainbow and the sun dogs. Accordingly they overlap less and the colors are pure, that is, closer to the spectral colors and mixed colors less.

Occurrence and observability

A Circumhorizontal Arc can only occur when the sun is higher than 57.8 ° above the horizon. Therefore, he can not be in places further north than 55 ° N or south of 55 ° S observed. In Germany, where the sun can rise up to an altitude of 61 °, it is therefore very rare to see. This is only possible in the summer at midday, preferably by high mountains.

In contrast, a circumzenithal observed at low -lying sun. During the circumzenithal the blue color of the light spectrum has above, blue below the Circumhorizontal Arc.