A colorless or transparent rainbow recently seen and photographed over a number of towns in the American state of Missouri has reignited interest in this eerie rainbow without the seven colors of the rainbow.

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Technically, a colorless rainbow is called a "fogbow."

The well-defined fogbow over Missouri doesn't have a color because it appears as a bow in fog rather than rain. NASA said a fogbow's lack of colors is caused by the smaller water drops.

These suspended water droplets are smaller than 0.05 millimeters and this small size means these droplets are too small to reflect and refract sunlight as raindrops can when creating a rainbow.

A fogbow isn't totally devoid of colors, however. It does have colors but only very imperceptible ones because the small water droplets can't reflect that much light. Looked at closely, a fogbow has a red outer edge and bluish inner edge.

When the droplets are very small, however, a fogbow appears white. This type of fogbow is called a white rainbow.

A fog bow seen in clouds, especially by someone in an aircraft looking downwards, is called a cloud bow. Mariners sometimes call fogbows sea dogs.

Experts said that 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.

The U.S. National Weather Service defines a fogbow as a rainbow that has a white band appearing in fog that is fringed with red colors on the outside and blue on the inside.

NASA's more technical definition of a fogbow is one where the water drops are so small, "the quantum mechanical wavelength of light becomes important and smears out colors that would be created by larger rainbow water drops acting like small prisms reflecting sunlight."

TagsColorless Rainbow, fogbow, cloud bow, sea dogs, NASA