Just like a rainbow, a fog bow appears opposite the location of the sun. Fog is made up of much smaller water droplets than rain is. We are getting down to a size where the wavelength of light is important and diffraction smears the colors. Therefore often all you can see is a white haze. I've see fog bows before but what I would like to see is a lunar fog bow - one occurring at night reflecting the light of the moon. Rare but doable.