Circumhorizontal arc
The circumhorizontal arc is one of the most colorful halos , resembling a horizontal rainbow .
Manifestation
A circumhorizontal arc runs in a circle around the sky at a low altitude , although at most only the quarter of this circle facing the sun is visible. The appearance has the shape of a rainbow-colored bow and is located about 46 ° below the sun. Although the appearance is similar to a rainbow, it is not a rainbow and the colors only appear to match those of a rainbow.
Emergence
Like the circumzenital arch, the circumhorizontal arch is created on floating ice platelets that are horizontally aligned. The light enters a vertical side surface and exits the lower horizontal surface. The circumhorizontal arc rises with the sun and reaches its maximum intensity when the sun is 67.9 °. Under optimal conditions, the whole cloud can be colored like a rainbow.
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 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.
Occurrence and observability
A circumhorizontal arc can only arise when the sun is higher than 57.8 ° above the horizon. Therefore it cannot be observed in places north of 55 ° N or south of 55 ° S. In Germany, where the sun can rise to an altitude of 61 °, it is therefore very rarely seen. This is only possible in summer at lunchtime, preferably from high mountains.
In contrast, a circumzenital arc can be observed when the sun is lower. While the circumzenital arc has the blue color of the light spectrum above, the circumhorizontal arc is blue below.
literature
- 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 .
Web links
- Circumhorizontal Arc - Description & Origin
- Atmospheric Optics - Circumhorizon Arc Gallery
- NOAA Gallery of the event June 3, 2006.
- Circumhorizontal Arc in Tokyo , April 22, 2001.
Individual evidence
- ↑ GP Can: Polarization and intensity distributions of refraction halos , J. Opt. Soc. At the. 73: 1629-1640 (1983) Chapter 8