The Science Behind White Rainbows
Rainbows are not hard to spot. Find some flowing water, spend a minute finding the right angle to the sun, and you're in for a treat. But fog rainbows are another story.
Often called “fog bows,” or white rainbows, these arcs are much less vivid than the ones you're used to seeing on a rainy day. The science behind fog bows is the same as with regular rainbows: Sunlight bounces off of water particles produces the colorful prism. The water droplets are smaller in fog, accounting for the grey-ish color.
The contour of a fog bow may appear much looser than a regular rainbow's curvature. according to NASA: “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.”
You can even spot this natural phenomenon at night, when it's called a lunar fog rainbow.
To find one, you need to find the antisolar point—the spot exactly opposite from the sun from the viewpoint of the observer. To find the antisolar point, look away from the sun at a 40-degree angle. Your own shadow will mark the direction of the antisolar point, for easy reference.