FOG BOW, a white rainbow, about 4o° in radius. It has a reddish outer margin and an inner margin of bluish tinge, but the whole of the middle of the band is uncoloured. It occasionally accompanies fogs (q.v.) and its origin resembles that of the normal rainbow, but the smallness of the water drops (under o mm.) causes the colours to be mixed and the resulting band to be nearly white. (See HALO.)