A circumzenital arc runs in a circle around the sky zenith, but at most only the quarter of this circle facing the sun is visible. The appearance resembles an "upside down" rainbow , the closest point to the sun being 48 ° above the sun. A circumzenital arc can only arise up to a sun height of ~ 32 °, it is best visible at sun heights of 15 ° to 25 °.
When the sun is almost at its zenith, one can sometimes observe a circumhorizontal arc below it .
Similar to a sub-sun , the sunlight is refracted by horizontally floating ice crystals in thin ice crystal clouds. In the circumzenital arc, however, it hits the base surface of the ice crystal, is broken and emerges from the side surface of the crystal. The deflection of the beam is about 48 °. This also means that the apex of the circumzenital arc is at an angle of 48 ° to the sun.
A position of the sun above 32.2 ° means that the exit of the beam on the side surface is prevented by total reflection . By rotating the crystals around their vertical axis, an arc of a circle becomes visible, as the side surfaces through which the light beam emerges can point in all directions. The angle of deflection to the vertical remains the same, only the direction of failure in the horizontal plane changes.
The brilliant colors that can be observed in the circumzenital and circumhorizontal arcs are made clear by comparing the dispersion and the intensity distribution for individual colors. By breaking at a 90 ° ice edge, the dispersion leads to a split of the halo between red and blue of about 2 °. Furthermore, the intensity profile of a single color is concentrated in an angular range of only half a degree. On the other hand, there is also no weakly sloping distribution of intensity for the color areas, as is the case with rainbows and the suns below. Accordingly, they overlap less and the colors are purer, that is, closer to the spectral colors and fewer mixed colors.
As with all halos, the prerequisite is the presence of suitable ice crystals in the atmosphere. They often occur with cirrus and cirrostratus clouds . But they can also occur in freezing fog and polar snow . Furthermore, they can arise on falling strips , provided they consist of ice crystals.
Observability and frequency
A circumzenital arc can be observed about as often as a rainbow . However, since it is close to the celestial scene, one rarely becomes aware of it, as this direction of view is rather unusual for a person. On days when a sub-sun is visible, it is therefore advisable to look upwards, as both phenomena often go hand in hand. The circumzenital arc reaches its greatest intensity at a sun height of approx. 20 ° and can have a greater luminosity than a rainbow.
- Kristian Schlegel: From the rainbow to the northern lights. Luminous phenomena in the atmosphere . Spektrum Akademischer Verlag, Heidelberg 2001, ISBN 3-8274-1174-2 .
- Michael Vollmer: Play of light in the air. Atmospheric optics for beginners . Spectrum Akademischer Verlag, Heidelberg 2005, ISBN 3-8274-1361-3 .
- Circumzenital arc - description and origin
- Picture gallery
- Download photo album about circumzenital arcs
- GP Kann, page no longer available , search in web archives: Polarization and intensity distributions of refraction halos , J. Opt. Soc. At the. 73: 1629-1640 (1983) Chapter 8
- Archive link ( Memento of the original dated December 30, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.