Single Secondary Rainbow imaged
by Michael Großmann. ©Michael
Großmann, shown with permission.
"The bow was seen June 15 2006 at Bilfingen, northwest of Pforzheim, Germany. The weather that day started clear and warm. In the afternoon a fresh stormy wind came up with a short shower. After 15 minutes the weather became clear again. It was the only shower that day"
Twinned bows are a doubling of the primary - not to be confused with supernumerary fringes. The twinning is often associated with violent showers and only lasts a short time.
We don't know for sure what causes them. The two bows might be produced by two types of raindrop, small spherical ones and large flattened ones. The lack of splitting of the secondary bow give some support to this.