The Gorch Fock in Stralsund in front of a fog bow, by Klaus Haase
The fog bow, also known as a white rainbow, is an atmospheric optical phenomenon in the form of a circular, bright white arc. It is a special type of rainbow and is caused by particularly fine droplets. This phenomenon is very rare.
Just like a rainbow, a fog bow is created by the refraction of light on water droplets in the air. The difference lies in the size of the water droplets. Large water droplets of more than 1.5 mm lead to a clear separation of colours. With a droplet size of less than 1.5 mm, the red colouring initially becomes weaker. In fog banks with droplet sizes between 10 µm (0.01 mm) and 50 µm (0.05 mm), however, diffraction effects overlap the refraction effects responsible for colour separation so strongly that colours (blue on the inside, yellow on the outside) are only visible at the edge of the bow. Essentially, the bow appears white.
The fog bow is always opposite the sun. Its radius can be up to 42°, but becomes smaller as the droplet size decreases. If the droplets are smaller than 5 µm, the fog bow can no longer be seen with the naked eye. To observe it, you must also stand in front of the fog wall with the sun behind you, otherwise the light will already be too dimmed by the fog.
