Note on Dew Bows

Extract

A dew bow differs from a fog bow in being formed by minute globules of water resting on the ground; but these globules may or may not have been produced by the usual process of dew formation. In the Proceedings of the Society (vol. vii., 1870), Clerk Maxwell has published a note on a bow seen on ice. With the exception of this record, we have been unable to find any distinct account of observations of the phenomenon, although no doubt it must have been occasionally observed soon after sunrise.

The peculiarity of the bows we observed was that they were seen by night, the sources of light being the gas lamps and electric lights of the city. It was on the evening of Friday, November 11, after several days of thick foggy weather. The tiny fog particles seem to have gradually settled down in the still air, the fog steadily clearing the whole time. These globules, in spite of their contact with the damp ground, must have retained their spherical form almost perfectly; for they were able to throw back to the eye, with a rainbow deviation of approximately 42°, the light incident upon them.

In the neighbourhood of every lamp where the surface of the ground had been undisturbed by traffic the bows were seen as bright streaks, which shifted position with the observer. Any disturbance of the surface, a wheelmark, a footprint, a finger drawn across the pavement, obliterated the bright streak; and it was not seen on the surface of puddles.