Fog bows are generally seen as an arc of dense fog along the edge of a fog patch. The physics is somewhat similar -- in that the bow is caused by sunlight refracting inside water droplets.
However, unlike rainbows, where the raindrops are large enough to refract sunlight into its individual colors, the water droplets in a fog bow are much smaller, and so the refractions aren't as precise.
Thus, the way the light scatters from a fog bow, you get a lot of color overlapping, and more of a hazy white bow instead of the colorful rainbow.
If you've never seen a fog bow, we have a photo of one posted on the Ask Steve Web site.
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