Having shut down the telescope and packed away my cameras, I left the dome and began walking down the hill to the accommodation block for some much-needed sleep. The sun had just risen and, while the sky above was azure, around me a light fog barely obscured my panoramic view of the Southern Alps. It was a great morning to be alive.
Walking towards the accommodation with my back to the sun, I noticed something rather lovely slowly appearing in front of me. A ghostly low-contrast achromatic arch curved across the sky from one telescope dome to another. I was witnessing the birth of something rather special: a fogbow.
After spending a few minutes photographing this beautiful phenomenon, I headed down towards the residence, determined to find out more about fogbows. Sleep would have to wait a little longer.
My research revealed that fogbows, which resemble pale white rainbows, are, unsurprisingly, sometimes seen in fog banks. But conditions have to be just right for you to witness one. The best chance is when the sun is low in the sky.
Fogbows are created in precisely the same ways as rainbows. Light from the sun passes through droplets of water and, after one reflection inside the droplet, is refracted and dispersed, forming the familiar spectrum of colour we see in a rainbow.
Unlike rainbows, fogbows are entirely devoid of colour. This is because the water droplets in fog are much smaller than those in clouds. It turns out that large drops are needed to form the vivid hues we see in rainbows.
Wave interference increases as the droplet size decreases, making colours overlap and wash out. A wise scientist has calculated that once a droplet size is less than 0.1mm, the human eye cannot detect colour. Fogbows lack colour because their droplets are much smaller than 0.1mm.
