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 the 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 ..
Fogbow at Skeiðarársandur, Iceland
Moonbows or lunar rainbows are rare natural atmospheric phenomena that occur when the Moon’s light is reflected and refracted off water droplets in the air. Moonbows are similar to rainbows, but they are created by moonlight instead of direct sunlight. Moonbows are rarer than rainbows because a variety of weather and astronomical conditions have to be just right for them to be created.
- The Moon has to be very low in the sky – no more than 42 degrees from the horizon.
- The Moon phase has to be a Full Moon or nearly full.
- The sky must be very dark for a moonbow to be observed – any bright light can obscure it.
- Water droplets must be present in the air in the opposite direction of the moon.