Circumzenithal arc

The circumzenithal is one of the most colorful halos and often occurs together with a Sundog.

Appearance

A circumzenithal is circular around the sky zenith, but only the maximum of the sun facing quarter of this circle is visible. The appearance is similar to a " upside-down " Rainbow, whose closest to the Sun point 48 ° to the sun. A circumzenithal can only occur up to a solar altitude of 32 °, it is best at sun elevations of 15 ° to 25 ° visible.

If the sun is almost directly overhead, one can sometimes observe under her a Circumhorizontal Arc.

Formation

Similar to a sundog sunlight of horizontal floating ice crystals is broken into thin ice crystal clouds. It hits the circumzenithal but on the base surface on the ice crystal, is refracted and exits from the side face of the crystal again. The deflection of the beam amounts to about 48 °. This also means that the apex of the Zirkumzenitalbogens is at an angle of 48 ° to the sun results.

A sun position 32.2 ° leads to the jet outlet on the side surface is prevented due to total reflection. By rotating the crystal about its vertical axis, a circular arc is visible, because the side surfaces, through the outlet, the light beam can point in all directions. In this case, the deflection angle to the vertical stays the same, it only changes the default direction in the horizontal plane.

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 about 2 °. Furthermore, the intensity profile of a single color is concentrated in an angular range of one -half degree. On the other hand, there is for 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 less mixing colors.

Occurrence

Requirement is - as with all halos - the presence of suitable ice crystals in the atmosphere. They often occur in cirrus and cirrostratus clouds. You can also occur when ice fog and polar snow. They may also occur on virga, provided these are made of ice crystals.

Observability and frequency

A circumzenithal is observable in about as often as a rainbow. However, since it is close to the zenith sky, you will rarely pay attention to him, because this line of sight is rather unusual for a man. On days where an undesirable sun is visible, it is therefore advisable to direct the eye upward, because both phenomena often go hand in hand with each other. In a solar altitude of about 20 ° of circumzenithal reaches its greatest intensity and may have a greater luminosity than a rainbow.