Rainbows are one of nature's most ephemeral beauties, but there's a related phenomenon that's even more ghostly.  These are the pale white, rainbow-like arches that are sometimes seen on foggy days or in dense fog banks.  What's responsible for these so-called "fogbows"?

Fogbows are created in much the same way as normal rainbows are.  Round droplets of water in the atmosphere can separate sunlight into different colors, just like a prism or a piece of crystal.  This is called "refraction."

People see a rainbow when the sun is behind them, but it's shining on raindrops in the sky in front of them.  Pure sunlight is whiteit contains all the colors of the spectrum--but each individual color is refracted out of the droplets at a slightly different angle.  A rainbow is the refracted light from millions of tiny raindrops.

A rainbow only appears when water droplets are small enough to be more or less round, but large enough to focus colors clearly.  The best droplet size for rainbows is about one or two fiftieths of an inch across. 

Larger drops tend to wobble as they fall, refracting the sunlight haphazardly.  Smaller drops can refract the light well enough, but they are too small to focus the colors clearly. 

The diameter of a fog droplet is only about one five hundredth of an inch, much too small for this sharp focus.  The light is refracted just like in a rainbow, but the colors all bleed into one another, and the fogbow appears a ghostly white.

Sources And Further Reading:

Byrd, Deborah. "What is a fogbow?" EarthSky. October 03, 2016. Accessed January 04, 2017.

Dennis, Jerry. It's Raining Frogs and Fishes. New York: Harper Perennial, 1993