They look just like rainbows but lack the distinctive seven colours - in fact, fog bows appear entirely white.
They are another atmospheric optical phenomenon, where sunlight interacts with tiny water droplets suspended in the air contained in fog, mist and low cloud.
Unlike rainbows the water droplets are much smaller, some 10 to 1000 times smaller, measuring just 0.1mm across.
When rainbows form, sunlight enters raindrops and as it travels through the water the light splits into the seven colours. We call this refraction.
The light then gets reflected off the back of a raindrop and sent to your eyes. Hence for you to see a rainbow, the sun has to be behind you.
In a fog bow, a similar but slightly different process takes place. As the water droplets are much smaller, the sunlight is diffracted before being reflected to your eyes.
This makes the bow appear white and drained of colour.
This is why fog bows are sometimes referred to as "white rainbows".
In diffraction the beam of light is spread out far greater than refraction. This washes out the colours and thus the bow appears an eerie white.
To see a fog bow, the fog needs to be quite thin to allow light to travel through the droplets.
You can also see a similar atmospheric optical phenomenon from aircraft - although these are call cloud bows.
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