Fog bow

Another kind of rainbow can be observed when there is fog: the fog bow. This bow is white and it its about twice as wide as a normal rainbow. On its inner side there sometimes are interference bows. Its radius is about 42, but becomes smaller when the size of the droplets decreases.  Fog consists of very small water droplets. When the droplets are smaller, the about 50 micrometres, the rainbow angles of the different spectral colours overlap in a way that they add up to white light. When the droplets are smaller than 5 micrometres, the fog bow cannot he recognized because it then becomes very diffuse and faint That is why a fog bow is not always visible when the sun shines on a wall of fog.

The diagram above shows how colours and width of the primary rainbow change with the different sizes of droplets. So it is possible to draw direct conclusions from the sequence of colours 0f a rainbow on the size of the droplets. A bright red appears only when there are big raindrops. We also see that a rainbow becomes significantly wider and mainly is white when the droplets are small (fog bow). It is very interesting that the interference bows do not appear directly on the inner rim of the main rainbow when the droplets are small, but that we can find a rather wide gap there. This gap can be recognized well in the photograph below.

These phenomenons cannot be explained by pure refraction of light in drops of water. They are due to the wavy nature of light.  A fog bow can be seen more clearly when the observer stands in front of the wall of fog and there is no fog behind him or her. Otherwise the sunlight gets reduced too much by the fog. When there is ground fog, you can help yourself by positioning on a small hill and looking down on the fog.