A FogBow

A Fog Bow or a Cloud Bow in fog, most likely to be seen when one is in the mountains and looking down to a foggy valley, for example. Other places to look for it are when the sun is low near the horizon, and there is a tall, clearly defined bank of fog in the direction away from the sun. The fog bow is just an ordinary rainbow, but it appears white due to the small droplets and diffraction (wave-nature) of light.




Explanation: Is that white arch real? What is being seen is a fogbow, a reflection of sunlight by water drops similar to a rainbow but without the colors. The fog itself is not confined to an arch -- the fog is mostly transparent but relatively uniform. The fogbow shape is created by those drops with the best angle to divert sunlight to the observer. The fogbow's relative lack of colors are caused by the relatively smaller water drops. The drops active above are so small that the quantum mechanical wavelength of light becomes important and smears out colors that would be created by larger rainbow water drops acting like small prisms reflecting sunlight.

want more technical information about a Fog Bow?
http://www.gsfc.nasa.gov/scienceques2002/20030214.htm