Have you ever seen a cloud that looked like a rainbow? You might have seen a circumhorizon arc! Some people call circumhorizon arcs “Fire Clouds” but that name is very misleading because they have nothing to do with fire. The conditions have to be just right for circumhorizon arcs to appear. This optical phenomenon occurs when the sun is very high in the sky, it has to be at least 58° above the horizon. The light is passed through a cirrus cloud, which must contain large ice crystals that are parallel to the ground. The light from the sun is refracted off the ice crystals inside the cloud.
Circumhorizon arcs look like this:
Circumhorizon arcs are often confused with cloud iridescence. Cloud iridescence also causes clouds to look like rainbows, but it is formed differently. This cloud rainbow is formed on top of a cumulus cloud after a thunderstorm. Cloud iridescence occurs through diffraction instead of refraction, like the circumhorizon arcs. This phenomenon is more common than circumhorizon arcs.
This is what cloud iridescence looks like:
Cirrus Cloud – a light and wispy cloud that occurs at a high altitude
Cumulus Cloud – puffy low-level clouds that look like cotton balls
Diffraction – occurs when a beam of light is bent around an object
Refraction – occurs when a beam of light changes direction, this happens most often when the beam passes from one object to another