Adam Burton/Alamy
Natalie Roberts
Watford, Hertfordshire, UK
There are two different ways you can observe water droplets’ optical effects when facing away from the sun into the low clouds (fog and mist) caused by an inversion (when cool air becomes trapped under warm air). However, one involves an uncommon perspective on the cloud and the other is fairly unremarkable, so neither is common knowledge.
The Brocken spectre effect typically involves standing by the cloud edge, usually from just above it, so it is most commonly observed by climbers – although standing on a hill above valley fog would do it. Here, the tiny, uniform water droplets – typically less than 0.05 millimetres across – refract the light from behind you and cause a ring of repeating rainbow light to form around your shadow: a glory. This will always appear around your own shadow, no matter how many people are with you.
The other phenomenon is the fog bow, or white rainbow. This can be seen from within fog, so long as there is strong sunlight or moonlight behind you, and is for all intents and purposes a normal rainbow. However, here light diffracts through the small water droplets to blur out all the classical rainbow colours, leaving a mostly white bow with perhaps a faint red outer edge and blue inner edge. Because of this, the fog bow typically goes unnoticed. But, of course, that isn’t the same as not existing.
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