Bizarre Natural Phenomena Vol. 58 - Rainbows Without Rain (Fogbows)

This series has discussed rainbows, moonbows, glories, but did you know that there is also a fogbow?

You mean like a rainbow made of fog?

I mean like a rainbow made of fog 😜

Image from flickr.com - Courtesy of: Ben - License: CC BY-ND 2.0 (updated license link)

Hey lady, this looks like no rainbow, even the moonbow looked more like a rainbow...

Fogbows are not the typical kind of rainbows. Just like normal rainbows that form when there are levitating raindrops in the air; fogbows form from smaller droplets of water like the ones in fog, mist or even within a cloud. The fog in this case must not be too thick as it would block the sunlight and there wouldn't be an arc to see. This size difference (10 to even 1000 times smaller than raindrops) is what makes a fogbow come out white and as a result gives it names like: white rainbow, cloudbow or ghost rainbow. [1, 2, 3]

Could you please elaborate?

Sure! If you remember how rainbows form:

We need the Sun to be low in the sky, behind the observer at an angle of about 30-40° from the ground. We also need water in the air in the form of droplets.

Each water droplet acts like a single prism, when they all combine the final result is a colorful circular band across the sky (with a pot of gold on its end and a leprechaun guarding it). Sunlight enters the suspending water droplets, the rays are refracted (bent due to the change in the medium), internally reflected (bounce back) on the walls of the droplet and then refracted again as they exit the water medium and return to the air; but the incoming ray won't be the same coming out, as some of it will be reflected once more at the time of the second refraction and will exit the droplet from the opposite side or keep reflecting inside the droplet (double refraction). [Bizarre Natural Phenomena Vol. 47 - Rainbows In The Night (Moonbows)]

Now, almost the same happens in a fogbow. Only, since the water droplets are significantly smaller (smaller than 0.05 mm), light is not reflected and refracted twice inside the droplets; they cannot work exactly like prisms. Light enters the droplet, gets refracted, then reflects once on the inner wall of the droplet and exits. The dominant effect here is diffraction, meaning that the light rays don't follow defined paths. For this reason fogbow colors fuse into white or may sometimes appear red on the upper parts and blue on the lower parts of the arc (the color depends on the size of the droplets). A broad bow for each color is formed and that overlaps with the others, giving the fogbow its characteristic washed out whitish tint. [4, 5]

Upon exit, light will "deviate" 135-150° from its original course (the path it was on before hitting on the droplet) and will end in a 30-45° radius circle around the antisolar point. If you think that for the rainbow these numbers are ~138° the angle of "deviation" and the final circle has a 42° radius, then you can say they are much too similar. [4, 5]

A fogbow may have multiple inner rings when the droplets that create it have the same size. These are called supernumerary bows and may occur in typical rainbows as well. [1]