A fog bow, sometimes called a white rainbow, is a similar phenomenon to a rainbow; however, as its name suggests, it appears as a bow in fog rather than rain. Because of the very small size of water droplets that cause fog—smaller than 0.05 millimeters (0.0020 in)—the fog bow has only very weak colors, with a red outer edge and bluish inner edge.
In many cases, when the droplets are very small, fog bows appear white, and are therefore sometimes called white rainbows. Along with its larger angular size, this lack of color is a feature of a fog bow that distinguishes it from a glory, which has multiple pale-colored rings caused by diffraction. When droplets forming it are almost all of the same size, the fog bow can have multiple inner rings, or supernumeraries, which are more strongly colored than the main bow. According to NASA:
The fogbow's lack of colors is caused by the smaller water drops ... so small that the wavelength of light becomes important. Diffraction smears out colors that would be created by larger rainbow water drops ...
A fog bow seen in clouds, typically from an aircraft looking downwards, is called a cloud bow. Mariners sometimes call fog bows sea-dogs.