From Wikipedia:

A circumhorizontal arc or circumhorizon arc (CHA), also known as a fire rainbow, is a halo or an optical phenomenon similar in appearance to a horizontal rainbow, but in contrast caused by the refraction of light through the ice crystals in cirrus clouds.

It occurs only when the sun is high in the sky, at least 58° above the horizon, and can only occur in the presences of cirrus clouds. It can thus not be observed at locations north of 55°N or south of 55°S, except occasionally at higher latitudes from mountains.

To be visible the sun must be at an elevation of 57.8° (90 -32.2°) or more and 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 phenomenon is quite rare because the ice crystals must be aligned horizontally to refract the high sun. The arc is formed as light rays enter the horizontally-oriented flat hexagonal 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 shine like a flaming rainbow.

A circumhorizontal arc can be confused with an infralateral arc when the sun is high in the sky; the former is however always oriented horizontally where the latter is oriented as a section of a rainbow, e.g. as an arc stretching upwards from the horizon.

About these circumhorizontal arc pictures

My family was about to leave for a day trip in August 2007, so I was bringing the new-ish camera along. As we were loading up the car, I saw this rainbow-like colourful patch in the sky. My first thought was that the weather conditions seemed to be rather odd to produce a rainbow, especially the complete lack of rain. Then I quickly realized that the bright spot was in the same direction as the Sun, completely impossible for a conventional rainbow. I didn't know what it was at the time, but at least knowing what it wasn't, I knew it had to be pretty rare, so I took some pictures. Of course, it was the circumhorizontal arc which you see in these pictures.

A shame that the camera was a somewhat new one, and I wasn't totally familiar with it yet. (Note the very low image file numbers.) If I had been, I probably would have taken more pictures, with different settings, and certainly some more zoomed-in ones. But at least I did get these.

As I recall, it was quite warm at ground level that day, certainly at least in the 90s °F, and rather dry. Obviously, it must have been considerably colder at high altitudes! Or the ice crystals responsible for forming circumhorizontal arcs couldn't have frozen.

In those labeled "enhanced", the only enhancement that was done was to decompose the picture in the GIMP into HSV layers, increase the values of the saturation layer using the levels tool, and recompose. In short, the net effect was, I increased the saturation.

Also see the recent Astronomy Picture Of the Day