Circumzenithal arc

Taken 18May18 at around 1830 CEST. In this photo, one can notice the normal solar halo around the sun and an upper halo (having saturated colours) similar to an upside-down rainbow called the “Circumzenithal Arc”. According to the UK Met Office such arcs are formed when sunlight refracts through horizontal ice crystals at such an angle that the light enters the crystal through its flat top face and exits through a side prism face causing the distinctive upside-down rainbow effect. Althaugh it is said to be quite common, actually I have only seen this arc for the first time since I began photographing clouds and only through shear luck and clear eye detail, the visibility of such an arc to a ground observer requires a combination of atmospheric conditions. The height, depth and position of the ice clouds must be right as the cloud needs to be at a specific angle convex to the sun. The position of the observer is also important since its visibility can vary greatly over short distances.

As this photo was sent to for their analysis, it was commented that a section of the 22 degree ring and its upper tangent arc including some trace of 46 degs supralateral arc seemingly in contact to the circumzenithal arc were noticed lower in the photo. It was explained that circumhorizontal arcs occur when sun elevation is high hence appear low in the sky below the sun whilst this arc was seen high in the sky as the sun was setting when the photo was taken.

The first thumbnail is another view of the same phenomena. The high clouds being cirrostratus nebulosus developed at a height of around 8.3km to 10km at temperatures well below -30C making them primarily ice crystals and were associated with the edges of upper-level thunderstorms that developed in North Africa due to a heat low pressure system present over the region.

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