Abstract

At the last Topical Meeting, Fraser ¹⁾ offered an explanation for the paradoxical fact that supernumeraries of the primary rainbow are regularly visible in rainshowers, although the obvious broad droplet-size distribution should prevent such. The reason of their appearance is the flattened shape of falling drops, which causes the rainbow angle to increase with increasing (equivalent) drop-radius r, at least for solar heights h between 0 and 42°. Since therefore the positions of the Airy maxima as a function of r have a minimum, the supernumeraries still show up in rainshowers. However, the angular spacing between then contains hardly information about the drop-sizes in the showers any more, as they are determined by the places of these minima: if the drop-size distribution is broad enough, the separation between the first and second supernumerary is fixed at 0.7°.

© 1986 Optical Society of America