"twinned bows" have been imaged,
mostly during heavy showers, but currently there is no agreed explanation
for them. They cannot be reflection
bows because at the time most were imaged, the reflection bow
from a horizontal water sheet would be in quite a different position.
They mostly do not resemble supernumerary bows
and attempts to model them with Mie or Airy
scattering models using spherical water drop populations
fail. They might form from a mixture of water drops and ice spheres.
Although diffuse ice-sphere bows might occur, ice balls of the required
sphericity and optical quality to produce the observed sharp twinned
bows are unlikely. A twinned secondary should also then be
observed and so far it has not.
A stronger possibility
is that non-spherical raindrops produce
one or both bows. Surface tension forces keep small raindrops fiercely
spherical but as they fall large drops are flattened by air resistance
or might even oscillate between flattened and elongated spheroids.
tracing by BowSim shows
that departures from sphericity by a mere 1-2% noticeably
distort the primary bow. We have to marvel that rainbows are
The BowSim computation at right is for a mixture of spherical
drops and oblate spheroids that have their vertical axis shortened
by 2.5%. This could for example be produced by two showers falling
through an observer's rainbow
cone, one of smaller spherical drops and the second of large
drops flattened by air resistance. The lower bow is from the oblate
spheroids. At the simulation solar altitude of 5º the secondary
bow is undistorted. This example is not to suggest that oblate
spheroids, merely convenient for the calculation, are required
to make twinned bows but rather that twinned bows could be
produced by some type of non-sphericity.
We need more observations of twinned bows to better know their
nature; close-up images at known focal lengths of the 'twins',
wider angle images around the bow and if possible the secondary
in order to measure the primary-secondary spacing and thus any
departures from circularity, exposures to show the detailed structure
secondary, all combined of course with the local weather conditions.