by 1 Cylivers
A circumhorizontal fire rainbow arc occurs at a rare confluence of right time and right place for the sun and certain clouds. Crystals within the clouds refract light into the various visible waves of the spectrum but only if they are arrayed correctly relative to the ground below. Due to the rarity with which all of these events happen in conjunction with one another, there are relatively few remarkable photos of this phenomena.
In areas of Northern Europe it can only be observed around the time of the Summer solstice. If cloud conditions are right it is seen along the horizon on the same side of the sky as the sun. It reaches its maximum intensity at a sun elevation of 67.9°.
The arc is formed as light rays enter the horizontally-oriented flat hexagon crystals through a vertical side face and exit through the horizontal bottom face. It is the 90° inclination that produces the well-separated rainbow-like colours and, if the crystal alignment is just right, makes the entire cirrus cloud appear to shine like an undulating rainbow.
