The circumhorizontal arc (abbr. CHA) corresponds in its colorfulness and brightness to the
circumzenithal arc. However, the sun must be very high for it to be visible. Only from a sun height of 57.8° (90° - 32.2°) does the arc appear as a colorful band on the sunny side of the horizon. With the sun, the circumhorizontal arc also rises and reaches its maximum brightness at a sun position of 67.9°. In Germany, the sun reaches a height of about 61° in summer. Therefore, the circumhorizontal arc can only occur here at midday on quite a few days of the year. It has only been observed a few times by us. If you travel to countries further south during the summer vacation, you have a better chance of seeing this impressive halo phenomenon. In countries north of the 55th parallel, the circumhorizontal arc cannot be seen because the sun is always lower than 57.8°. Thus, it is one of the few halo phenomena that cannot occur everywhere on Earth. However, if you climb a high mountain, a circumhorizontal arc is also possible in northern countries.
The following table shows when a possible circumhorizontal arc can be observed in which German city. The time periods are guidelines.
The circumhorizontal arc, like the circumzenithal arc, forms in
floating plates. In this case, however, the light enters a vertical side face and exits through the lower horizontal base surface.
