Several people sent me pictures of what they described as an upside down rainbow high in the sky around 5 PM today. Here are some examples:
What was causing this unusual occurrence? It is what is called a circumzenithal arc.
This arc are caused by the same general phenomenon that creates halos: the refraction of light in ice crystals. Circumzenithal arcs occur when the sun is relatively low in the sky (below 32 degrees above the horizon) in the presence of ice crystals with generally horizontal orientation.
Ice crystal have a hexagonal (six sided) structure and often are in the form of hexagonal plates (see figure). As they fall, they tend to have relatively horizontal orientation (not unlike leaves falling out of trees). Light passing through the top plate and then out the sides is bent (refracted) by roughly 46 degrees. The light (which initially has all colors is divided into the whole visible spectrum by this refraction--just like a prism--in a process known as dispersion)
The light of the C- arc is found about 46 degrees ABOVE the sun--with the arc (generally no more than 1/4 of circle) appearing to circle around the zenith of the sky.
Current research has not found any evidence for a pot of gold at the end of this rainbow. Apparently, it is too high for leprechauns to deal with.