Fogbow, a Colorless Rainbow, Appears in Midwest

Jon Erdman
Published: August 22, 2016

Fogbow in New Haven, Missouri, on Aug. 22, 2016. (Credit: Tammi Elbert/Twitter)

Have you ever seen what appears to be a colorless rainbow when it wasn't raining?
Chances are, then, you've seen a fogbow.
The photo above shows an example of a well-defined fogbow spotted in New Haven, Missouri, by Tammi Elbert on Monday. You can see the arc that you might have expected to be draped in a palette of colors, framing a misty area of ground fog.
This ghostly rainbow differs from its multi-colored cousin mainly in the size of water drops acting on incoming sunlight.
In the case of a rainbow, the raindrops are large enough to act like a prism, first reflecting a small fraction of sunlight within the drop, then refracting or bending white light's component colors on distinct paths back to your eye.
The suspended droplets within fog, however, are much smaller; at most, about 0.025 inches in diameter. These droplets are too small to reflect and refract sunlight as raindrops can.
Instead, the light is diffracted from these tiny droplets, clashing together and interfering in a haze of white or gray, rather than taking the neat, distinct, colorful paths from refraction in raindrops.
To see a fogbow, wait for a day with bright sunshine at your back, illuminating an area of dissipating or light fog in front of you. This can be in a field, a mountain valley, or at the coast or lakeshore.
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