Fogbow (aka: white rainbows, cloudbows or ghost of a rainbow) are formed in the same way as rainbows in that light is reflected inside tiny water droplets and emerges to form a large circle or arc of approximately 42°C centred on the antisolar point, opposite the sun.


However, there are major differences. Rainbows are formed by raindrops which are so large that rays passing through them follow well defined 'geometrical optics' paths. With the very finest droplets, such as those forming mist or fog, the light is no longer reflected and refracted within the drops, but is diffracted by them instead to produce a much broader and pale bow - the fogbow. In other words the fog and mist droplets are too small to refract light. Light hitting the tiny droplets merges into white, rather than being separated into rainbow colours. Fogbow colours are whiteish because the fogbows formed in each colour overlap considerably. Sometimes the inner and outer edges show faint bluish and reddish tinges. The actual colours are a result of the actual size of the water droplets.

When seen from within cloud or fog, the fogbow may be a partial or full circle. When seen from an aircraft cloud droplets might produce a similar bow. Fogbows are frequently seen over Arctic waters, but are also well known to mountain regions. The typical colourless fogbow above has been observed by Simon Caldwell (1999) at Glen Lyon in the Grampian Mountains.

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