MEMPHIS, Tenn. (WMC) - Everyone knows about rainbows but did you that rainbows can form within fog? Have you ever noticed an arc in the clouds? If so, you likely spotted a fogbow. They are also referred to as a ghost rainbow or white rainbow because they are not as colorful as the traditional rainbow. They can also be referred to as cloudbows when they are seen from up above in an aircraft looking downward. Mariners sometimes call fog bows sea-dogs.
Fogbows do share some similarities with rainbows as they both form from sunlight and moisture but the biggest difference is the size of the water droplets. Fogbows oftentimes appear as an arc in the fog. The sunlight goes through the tiny droplets, bounces back and forms an arc.
Fogbows are caused by tiny droplets inside a fog or cloud rather than larger raindrops like that in which the rainbow forms. The very small size of water droplets that cause fog are smaller than 0.05 millimeters (0.0020 in). These tiny drops don’t produce as much color as opposed to that of a traditional rainbow that forms from bigger raindrops.
The lack of color is what sets it apart from the traditional rainbow. The fogbow can have multiple pale-colored rings caused by diffraction. According to NASA, when droplets forming it are almost all of the same size, the fog bow can have multiple inner rings, which are more strongly colored than the main bow.
The drops in a fogbow are so small that the wavelength of light becomes key but diffraction smears out colors that would be created by larger drops from the rain in traditional rainbows.
Diffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening.