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.

3 photos in gallery
Click images for large photos