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. This lack of color is a feature of a fog bow which distinguishes it from a glory, which has multiple pale colored rings caused by diffraction. When the droplets forming it are almost all of the same size the fog bow can have multiple inner rings, or supernumeraries, that 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.

Stunning picture of rare 'white rainbow' captured in Scotland as storms hit the UK   Mirror.co.uk - November 25, 2016
The beautiful 'fog bow' stunned onlookers with its magical aura shining against the bright blue sky. The rare white rainbow was captured on Rannoch Moor in Scotland. The image from Rannoch Moor in the west of Scotland was captured by Melvin Nicholson. Out walking on the moor, south of Glen Coe, he said the unbelievably beautiful white rainbow appeared before him. It is a colorless rainbow that is made up of tiny water droplets that cause fog.