What the devil is a circumhorizon arc?

As we made our way from base camp for an acclimatisation hike on Ojos del Salado (6,893m) in the Puna de Atacama region of the Andes last month, we witnessed a strange phenomenon in the sky.

Above the adjacent peak of Cerro El Muerto (6,488m) a solid bank of cloud flashed all the colours of the spectrum. It was something none of us had ever seen before. We have all seen rainbows many times, but these are just narrow bands in a semi-circle. Here we had a whole band of dark grey cloud giving of...