From Glossary of Meteorology
(Also called fogbow, mistbow, white rainbow.) A large, faintly colored, circular arc formed by light (usually sunlight) falling on cloud or fog.
The apparent center of the cloudbow is the antisolar point (the shadow of the observer's head or possibly the shadow of the plane in which the observer is flying). The term rainbow is a general term for this phenomenon, a cloudbow merely being a rainbow formed in the smaller cloud or fog droplets. The broad whitish appearance of the cloudbow and slightly smaller angular radius certainly demonstrate that an explanation that treats light as a series of rays does not work very well for small droplets.