©Ian Goddard, shown with permission.
|Ian Goddard saw this rarity at Ashill in Somerset, UK on 7th
The rainbow orientation is as in the smaller
Purple and green fringes inside the primary bow are supernumeraries.
They are caused by interference between pairs of light
waves passing through rather small raindrops.
Supernumeraries are not always visible and when they are, only one
or two are usually seen. In this rare sighting there
are at least four in two fans where their spacing
increases with height in the sky.
The fringe spacing increases with decreasing drop size and it is
likely that the fans were produced by two localised rain showers
in which the drop size changed with altitude.
Rain showers usually have a distribution of drop sizes resulting
in supernumerary bows with a range of spacings which overlap
to blur out all but the first one or two fringes. Here the drops
at a given height must have been extraordinarily similar in size to
allow four fringes to survive.
A similar display was seen at Statesville, North Carolina.